Introduction: the Non Thermal Plasma boundaries

In 1927 Irving Langmuir introduced for the rst time the term "plasma" to describe the way an electried uid carries electrons and ions. As such one can consider him as the rst "plasma physicist". Langmuir became a pioneer of plasma physics together with his colleague Lewi Tonks while investigating how to increase the life time of tungsten-laments light bulbs. While doing so, they established the rst description of plasma sheath which later on will have huge development for plasma processing technique especially for manufacturing integrated circuits. This is a good illustration that from its very early age, the physics of Non Thermal Plasmas (NTP) develops in close connection with industrial applications. It also shows that studying NTP almost always requires to take into account the surfaces in contact with the plasma (the sheath forming on the tungsten lament in the case of Langmuir).

Non Thermal Plasmas (NTP) are versatile energy converters, transforming electrical energy into either radiation, heating or chemical energy. The non equilibrium nature of NTP is the key for tuning the way the energy is transformed by adjusting simple parameters such as the reactor design, the gas mixture or the power source for instance. Based on this property many industrial applications have taken advantage of the production of highly reactive species with a limited energetic cost thanks to the minimization of gas heating. As a result, the eld of NTP physics is strongly driven by applications and it is very common to dene the research activity of plasma physicists through their industrial objectives. However the physics underlying their work is often transverse to many dierent technologies.

Furthermore the fundamental questions addressed while studying NTP are often essentially multidisciplinary, involving atomic and molecular physics, condensed matter, spectroscopy, quantum mechanics, chemistry, statistical physics, thermodynamics, uid dynamics, etc... The richness of Non Thermal Plasma Physics lies in the coupling of numerous phenomena. The exibility of NTP for various industrial processes comes from the combined eect of strong electric eld, temperature, photons, charged particles, radicals and excited molecules, most of the time in interactions with surfaces. This strong multi-physic/chemistry coupling is a gift and a curse, that opens many opportunities but makes the understanding of fundamental mechanisms very challenging in a "real" system. Most of the time an accurate description of such complex multi physics system is still beyond our current computing and experimental capabilities. Therefore the plasma physicist always needs to nd a delicate balance: investigating fundamental mechanisms with simplied system while keeping the horizon of engineering. In turn the development of "real" systems often reveals new elds of research.

Besides the variety of possible industrial processes based on NTP technologies, the diversity of mechanisms induced by NTP is also leading many dierent scientic communities such as biologist, chemists, solid state physicists or planetologists to use NTP in their eld of research. This is a chance to expand the science of plasma physics but the risk is that plasma is used only as a tool, more or less treated as 1 a black box without any real understanding. To prevent the pitfall of NTP physics disappearing in other scientic elds, as well as for learning good practice of other elds for which plasmas are useful, close collaborations with other communities is an absolute necessity. As a researcher in experimental plasma physics,I therefore try to combine three component in my research strategy: seek for innovative industrial processes based on NTP technologies and be aware of the engineering constraints associated to them be curious of other elds of science where NTP can be of interest Always focus on fundamentals of plasma physics even through the two rst items Since my PhD defense in november 2006, these three goals are underlying my whole research.

Research strategy 2.1. General orientations

The common thread of all my work is the plasma/surface interaction at medium and high pressure (from 1 to 1000 mbar).

Most of the studies carried out on plasma/surface interaction have been done at pressure below 0.1 mbar with the motivation of material processing. Our work on the contrary has initially been motivated by atmospheric pressure plasmas. In keeping with my PhD work, our motivation was at rst plasma/catalyst coupling for indoor air treatment.

About 20 years ago, it was commonly believed that surfaces had only minor inuence on plasma development at atmospheric pressure because of the very short life time of ions and radicals. However plasma/catalyst coupling implies the use lamentary discharges developing in direct contact with high specic surface materials. It is now clear that plasma is capable not only to induce chemistry on the catalyst, but also modies the surface properties. The other way around, the catalyst can strongly aect the plasma dynamics. The work on indoor air treatment has been initiated at LPP by Antoine Rousseau in 2003 at the beginning of my PhD. Our work together on indoor air treatment has contributed to evidence some of the plasma/catalyst interaction mechanisms. The general principles of indoor air treatment by plasma are now well established in the community and reported in several review papers [5964] including our own one [START_REF] Thevenet | Plasmacatalyst coupling for volatile organic compound removal and indoor air treatment: a review[END_REF]. This has allowed the development of devices for indoor air cleaning already on the market. If one would draw a "hype curve" [START_REF] Fenn | Mastering the hype cycle: how to choose the right innovation at the right time[END_REF] for applied research topic, indoor air cleaning by plasma would therefore be currently on the second rising slope. In spite of this level of technological maturity, the understanding of plasma/catalyst coupling is still very little predictive. For every new pollutant, or new surrounding atmosphere composition, new trial and error studies are needed to assess the appropriate catalyst combination and reactor design. The eld is therefore still progressing a lot with empirical approach because of the lack of deep understanding of fundamental mechanisms at stake. The gathering of large amount of data with various pollutant/catalyst/reactors can of course lead to the emergence of interesting processes. It is one path that we are also following in collaboration with a spin-o company, Air-Serenity funded by a former PhD student of the group (Joseph Youssef ). However it is also clear that no real breakthrough can be achieved in plasma/catalysis without addressing several fundamental scientic questions separately.

In the rst place, the role of radicals in surface reaction, especially on complex material surfaces has to be understood. This is a very challenging task and the originality of our approach consists in the utilization of a low pressure ( 1 mbar) dc discharge (a Pulsed Glow Discharge PGD) used in contact with "real" complex catalytic materials that are also studied in atmospheric pressure DBDs. The PGDs make it possible to use numerous diagnostics to obtain a detailed characterization of the gas phase kinetics in contact or not with catalytic surfaces, during the plasma or in the post discharge. The data obtained are particularly interesting for the validation of 0D kinetic models, which in turn allow a better understanding of modications induced by complex surfaces. We have initially applied this fruitful approach for the study of N 2 /O 2 plasmas with Daniil Marinov (PhD 2009-2012) who is the rst PhD I have formally co-supervised together with Antoine Rousseau. Our investigation were primarily devoted to the surface reactivity of N and O atoms. Later I have chosen to adapt this approach for investigating CO 2 conversion by plasma, and concomitantly pure O 2 plasmas in collaboration with Jean-Paul Booth (see below).

Obviously surface reactivity under plasma exposure can not be understood without a good understanding of the gas phase kinetics interacting with the surface.

A signicant part of our work is therefore dedicated to gas phase plasma kinetics. I have developed a strong collaboration since 2009 with Pr Carlos Pintassilgo and Pr Vasco Guerra from IST in Portugal to model both gas phase and surface kinetics and compare these models to our measurements. The gas temperature (T g ) is a crucial parameter for gas phase kinetics because of its direct inuence on rate coecient of chemical reactions, but also because of its consequences on the value of reduced electric eld (E/N) via the change of gas density. The development of various techniques to accurately measure T g in molecular gases plasmas in dierent pressure range has become a constant concern of my work.

For reasons that will be explained in section 2.2, in the case of indoor air treatment, the reactivity of radicals on surfaces is certainly more relevant to investigate than ions or excited molecules reactivity. However over the last 5 years, a new possible application of NTP has emerged with the recycling of CO 2 . NTP could be the most ecient way to convert CO 2 molecules in other added value molecules by taking advantage of vibrational excitation. Plasma/catalyst coupling has still a key 3 2.1 General orientations role to play in this eld but with a completely dierent challenge since the molecules to be converted are not any more very diluted. It becomes therefore essential to understand the role of vibrationally excited molecules on surface reactions.

A very large part of the energy deposited in a NTP ignited in a molecular gas is transferred to vibrational excitation. Indeed the typical energy threshold of vibrational excitation often matches the average electron energy. Despite this well-known fact, the vibrational kinetics of many molecules in plasma is still poorly understood, and the interaction of these excited molecules with surfaces is even less so. This is especially true for CO 2 despite all the work done for CO 2 lasers because of the complexity of energy transfer mechanisms between the three vibrational modes of this molecule. A very large research plan is now opened on this topic, starting from a detailed description of vibrational kinetic of CO 2 containing plasma, their interaction with surfaces, and the possible use of excited molecule in catalytic reactions.

Another unexplored question in plasma/Catalysis is the impact on catalytic reactions of surface electric eld and charge deposited by plasma. The adsorption energy of reactive species could be strongly aected by the intense electric eld (several tens of kV/cm) induced by ionization waves, and therefore their reactivity could be considerably modied. In a broader perspective, the interaction of ionization waves with surfaces is the key for many applications such as surface treatment or biomedical applications but it remains poorly understood. In spite of the experimental work from Allen in the 70s [START_REF] Nl Allen | Streamer propagation along insulating surfaces[END_REF][START_REF] Nl Allen | Corona propagation and charge deposition on a ptfe surface[END_REF] or more recently the modelling work from Babaeva or Pechereau et al [68,69], direct measurements on targets under lamentary plasma exposure are too scarce. Therefore we have initiated a long term project dedicated to the interaction of ionization waves and dielectric surfaces with a special focus on surface electric eld and deposited charge determination. This research has started during the post-doc of Ana Sobota at LPP in 2013 which turned now into a collaboration since her nomination as associate professor at TU/e. The measurement of electric eld in atmospheric pressure lamentary plasma is very challenging. Atmospheric pressure plasma jets (APPJ) in noble gases excited with kHz range power supply are the most reproducible source of ionization waves. Therefore as a rst step, we are developing advanced diagnostic techniques to study APPJ impacting surfaces as a model of lamentary discharge interaction with targets.

The development of advanced diagnostics, for ionizations waves development, gas temperature determination or in situ time resolved kinetic studies is a recurrent part of my work as an experimentalist. A summary of dierent diagnostic techniques I have been using with their main characteristics is given in appendix ??.

To summarize, my research activity was at rst dedicated to plasma/catalyst coupling for indoor air treatment. It has now evolved in two main axes: (i) the propagation mechanisms of atmospheric pressure plasma jet on surfaces, and (ii) the conversion of CO 2 by plasma. These two main themes can be subdivided into the following topics:

1. fundamentals of plasma/catalyst interaction (5 articles)

2. ionization waves interaction with targets (13 articles) In sections 2.2, 2.3 and 2.4, the context of the three main elds of application of my research (indoor air treatment, APPJ and CO 2 conversion) is briey described.

The general strategy chosen for each topic, as well as quantitative data concerning the funding obtained, the related published work, and the students with whom I have been working are presented. However, the main scientic results will only be presented in section 3, articulating them around 3 axes: the surface reactivity of O atoms (section 3.1), the inuence of surfaces on vibrational excitation of N 2 and CO 2 (section 3.2), and nally the development of a helium plasma jet above, and on surfaces (section 3.3).

Indoor air treatment

"The smallest alteration in atmospheric constitution, in which we are condemned to live, leads to a disorder in our functions and can become the seed of our diseases." and also because they oer the easiest way to place a catalyst in direct contact with the plasma with the so called "packed bed" reactor conguration.

The coupling between a plasma and a catalyst can be achieved in dierent ways:

the catalyst can be placed inside the plasma (in situ coupling IP) or after the plasma zone (post-discharge coupling PC). In PC only stable molecules, meaning mostly O 3 can reach the surface of the catalyst while in IC conguration, the short life species can also interact with the catalytic surface. Therefore the IC seems a priori to provide more reaction pathways to remove VOC's but the overall energy eciency of the system is not necessarily higher than with PC. Moreover when using a DBD in air at atmospheric pressure, considerable amount of O 3 are produced (ozone production is the rst use of DBD by Siemens already in 1857 [75] ) and can not be released in the breathable air. Therefore even an ecient IC conguration requires a catalyst downstream the reactor to convert the left over of O 3 back to O 2 . Instead of being a problem, the surface dissociation of O 3 can be benecial for oxidizing further the VOCs coming out of the plasma reactor.

The catalytic material for indoor air treatment by plasma has therefore multiple utilities: it promotes the oxidation of the VOCs activated by the plasma and it removes the O 3 from the exhaust, but its main purpose is in fact to increase the probability for the VOCs molecules to encounter an oxidizing specie.

Indeed the low concentration of the molecules to be treated makes very improbable the dissociation of VOCs by direct electron impact, or even by reactions with ions.

Radicals such as O atoms or OH have a longer life time and a very high reactivity that makes their interaction with the VOCs more feasible. However even these radicals have a much higher chance to recombine into O 3 or H 2 O than reacting with the targeted VOCs. A decisive advantage of using a catalyst is therefore, above all, to achieve surface densities of pollutants that are suciently high inside the plasma region in order to increase the likelihood of reactions with the radicals produced by plasma. With appropriate materials, the radicals themselves can be adsorbed and their life time on the surface increased, promoting also a higher reactivity towards

VOCs. The material in contact with the plasma for air treatment must then be a good adsorbent of VOCs in the rst place, but also preferably of O and OH radicals.

Until now only the catalytic material used in regular thermal catalysis are used with plasmas but these catalysts are designed to reduce the energy barrier of a given reaction by producing a reactive specie on the surface. A plasma is already producing very reactive species, making the main action of usual catalyst at best redundant, or completely useless. Catalytic materials really dedicated to the synergy with a plasma still need to be invented. Towards an optimal process for VOC abatment with plasma From a process point fo view, The PhD work of Christelle Barakat (that I have cosupervised with Antoine Rousseau) in collaboration with Al-Ko therm has completely changed our approach for air treatment. This work has evidenced the gain of energy eciency when using a "sequential method". The idea is to have a two steps process with rst the trapping of the pollutant on an adsorbent material like in a normal lter, and then a regeneration phase of the surface by plasma. Filters have usually a limited eciency due to the saturation of the surface for a given compounds,and the competitive adsorption of several molecules that results in release of pollutants in the gas phase. These problems can be avoided if the surface coverage by the pollutants is always kept signicantly lower than the saturation level, letting enough free adsorption sites for capturing completely the ow of pollutants. This can be done by adjusting the frequency of regeneration phases according to the pollution level. The energy consumption is then optimal because the plasma is switched on only when the surface coverage requires it, and the lter keeps the eciency of a new one at any time.

We have initially validated the concept of plasma regeneration of adsorbent materials on simple test molecules (isopropanol, acetone [START_REF] Barakat | Oxidation of isopropanol and acetone adsorbed on tio 2 under plasma generated ozone ow: Gas phase and adsorbed species monitoring[END_REF][START_REF] Thevenet | Plasmacatalyst coupling for volatile organic compound removal and indoor air treatment: a review[END_REF]). The sequential method has been transferred to Al-Ko Therm and the air treatment unit from Air Serenity are also based on this idea.

Most of the publications devoted to air treatment by plasma are using relatively high pollutant concentration (typically 10-1000ppm). This is due to the diculties related to control and measurement of very low concentrations (1-100 ppb) which would be more realistic for indoor air pollution conditions. An important part of our work has focussed on lowering down the detection limit of pollutants and products measurements to assess the eciency of our plasma reactor in realistic conditions to remains below toxicity threshold. As an example the careful design of gas lines installed with Paul Gravejat during his post-doc allowed us to go down to a detection limit of 10 ppb of CO 2 by FTIR with a 10m gas cell. Along the same line, an ANR project Apolin'R ended in 2013 was dedicated to the detection of formaldehyde and formic acid (two markers of the degradation of numerous VOCs in air) with a detection limit at 100 ppt! This has been done with a platform of optical feed back cavity enhanced spectroscopy (OF-CEAS) running with a diode at 1769 cm -1 Investigating surface mechanisms under plasma exposure the sequential approach described above (pollutants adsorption on the catalyst followed by plasma regeneration of the surface) is very benecial from a process point of view, but it is also convenient for fundamental studies. Indeed the adsorption phase allows to determine quantitatively the amount of molecules adsorbed on the surface before following their oxidation under plasma by simply measuring a breakthrough curve. The oxidation products generated during the plasma regeneration phase can then be quantied in the exhaust. The originality of the work we carried out (Christelle Barakat's thesis and post-doc by Paul Gravejat) consisted in measuring directly on the surface the species adsorbed by infrared spectroscopy. As an example of this experimental method, the oxidation of isopropanol (IPA) has been monitored on a surface of TiO 2 [39]. The use of infrared absorption through surfaces directly under plasma exposure gives valuable information but one should never forget that species detected are mostly the ones accumulating on the surface. Therefore they are often species poisoning the surface and not the most reactive intermediary species. Nevertheless one of the key result obtained in [START_REF] Barakat | Oxidation of isopropanol and acetone adsorbed on tio 2 under plasma generated ozone ow: Gas phase and adsorbed species monitoring[END_REF] has conrmed the key role of O atoms in VOCs abatment on catalytic surfaces, as we had already suggested in previous works [START_REF] Guaitella | Plasma-photocatalyst interaction: Production of oxygen atoms in a low pressure discharge[END_REF][START_REF] Rousseau | Photocatalyst activation in a pulsed low pressure discharge[END_REF]. A detailed investigation of recombination mechanisms and surface reactivity of radicals on complex surfaces was therefore an obvious necessity, however dicult to perform at atmospheric pressure. This motivated a series of successful experiments in a pulsed DC discharge at reduced pressure (few mbar), the main results of which will be described in section 3. As interesting as these results obtained at lower pressure are, they do not allow a direct understanding of the development lamentary discharges interacting with surfaces. I chose to address this topic by studying the simplest source of ionization wave: the atmospheric pressure plasma jets.

developped

Atmospheric pressure plasma jets (APPJ)

In the midst of the rushing waters it happened that, when there was a hurricane, suddenly a divine lantern was seen shining at the masthead, and as soon as that miraculous light appeared the danger was appeased. A major trend in our community over the last 20 years has been the considerable increase in applications of cold plasma at atmospheric pressure (see gure 1). ing through "streamer" breakdown mechanism (or "ionization waves"). These streamers are occurring randomly in time and space, lasting only a few nanoseconds, and having a typical diameter of few hundreds microns. Their interaction with complex surfaces is therefore extremely challenging to study and papers addressing this issue are rare as it can be seen from the recent and extensive review on DBDs from R.

The air treatment described above

Brandenburg [78]. The plasma laments are often used to modify material surfaces but it is evident that in turn, the properties of material to be treated are strongly aecting the plasma dynamic as well. However beyond this general statement, the mechanisms at stake in the interaction between an ionization wave and surfaces are still poorly understood. The best uid models describing streamer development often treat the surface only with a dielectric constant and an eective secondary electron emission coecient. The nature, the life time and the mobility of charge adsorbed on dielectric material, the modication of gas dynamic above a target, the potential re-distribution induced above a oating conductive target, the role of micro and nano-porosity of the material are just a few examples of the many questions that remain unanswered to date.

Obviously, the study of plasma/surface interaction requires not only characterizing the eects induced in the treated material with surface diagnostics, but also simultaneously the properties of the plasma in contact with the surface. In order to investigate fundamental properties of ionization waves and obtain basic parameters such as electric eld (E f ield ), electron density (n e ), electron temperature (T e ) or gas temperature (T g ), compromises need to be done.

It is possible to force the ignition of a streamer at a given time and a given place by using a pin to pin geometry with a fast rise time of the applied voltage. This approach provides very reproducible streamers that allow for signal accumulation as it is necessary for cross correlation spectroscopy for instance [7880]. This method is very interesting and provides with a very high time resolution (tens of ps) the evolution of electric eld and charge density. It is a technique based on emission of excited states and therefore the absolute values obtained rely on the accuracy of the kinetic model used for describing the population of excited states considered. However the pin to pin conguration is a strong constraint that do not allow for studying the interaction between streamers, or the development over complex surfaces.

Another possibility to generate reproducible ionization waves is to use atmospheric pressure plasma jets (APPJ). These plasma sources have become the subject of intense research activity over the last 10 years due to their potential for biomedical applications [START_REF] Park | Atmospheric-pressure plasma sources for biomedical applications[END_REF] or the treatment of surfaces, such as polymers, which are temperature sensitive [START_REF] Oleksiy V Penkov | A review of recent applications of atmospheric pressure plasma jets for materials processing[END_REF][START_REF] Surowsky | Interactions of non-thermal atmospheric pressure plasma with solid and liquid food systems: a review[END_REF]. The name "APPJ" correspond in the literature to many dierent types of plasmas from corona to microwave, or radiofrequency discharges [START_REF] Winter | Atmospheric pressure plasma jets: an overview of devices and new directions[END_REF]. However one of the most common source of APPJ are DBDs powered with sinusoidal or pulsed voltage in the kHz frequency range, with the feed gas owing inside a capillary tube (in the following APPJ will refer only to this conguration). Using this conguration with a ow of noble gas, very reproducible ionization waves can be generated over several cm or even tens of cm in a tube of few mm in diameter.

The role of the capillary tube is to keep a charge density high enough for developing the discharge through a streamer mechanism over long distance away from the electrodes. These APPJ are cheap and their reproducibility allow to implement feedback loop to control their eect on a target [START_REF] Gidon | Model-based feedback control of a khz-excited atmospheric pressure plasma jet[END_REF]. As a consequence an abundant literature is dealing with their application in various elds but our interest for these sources has not been focus on applications.

We have started to use these APPJ in 2013 in collaboration with Ana Sobota (TU/e), as a model for detailed study of ionization waves and their interaction with various surfaces. APPJ generate ionization waves with longer time scale, larger dimension, and much higher stability than any streamer in molecular gas. They can easily be applied on any targets, dielectric, metallic, liquid etc... They are reproducible even when ignited with a low frequency sinusoidal voltage, meaning that no over-voltage is required to study them in a stable regime. Our approach is therefore to take advantage of these APPJ to perform complementary diagnostic techniques providing E f ield ,n e , T e , T g to compare these data with results of uid models describing the growth of ionization waves. The necessary use of noble gases however implies that gas mixing at the output of the capillary tube plays a major role in the discharge development, and this additional diculty needs to be addressed.

During the rst years of this project we have been developing diagnostic techniques that are now giving interesting results as it will be shown in section 3.3. In addition to the commonly used iCCD fast imaging and Schlieren imaging, we have been measuring the maximum of E f ield along the plasma jet propagation with Stark polarization spectroscopy [START_REF] Sobota | Electric eld measurements in a khz-driven he jet: the inuence of the gas ow speed[END_REF][START_REF] Goran | Electric eld measurement in the dielectric tube of helium atmospheric pressure plasma jet[END_REF] and in the surrounding with a commercial probe based on pockels eect. The n e and T e are obtained by Thomson scattering and the gas temperature with optical probe. All these methods are applied in the jet expanding freely in surrounding atmosphere but also above dielectric, metallic or liquid targets.

The eect induced on target is also monitored directly with Mueller polarimetry which allow the measurement of E f ield induced in the target by deposited charge by using the Pockels eect [38, [START_REF] Slikboer | Time-resolved electric eld measurements during and after the initialization of a khz plasma jet: from streamers to guided streamers[END_REF][START_REF] Slikboer | Charge transfer to a dielectric target by guided ionization waves using electric eld measurements[END_REF] but also other eect such as surface temperature (T surf ) [56] or etching depth. We are entering the most exciting part of the project since all these complementary measurements can now be performed in various gas mixture and above complex surfaces (polymers, dielectric with controlled patterns, biological tissues, etc...) and nally be compared with uid models as it will be discussed in section 3.3.

The electric eld in the front of an ionization wave is usually very high (up to several tens of kV /cm) which can be benecial when seeking to promote the dissociation of molecules by direct electronic impact. However for some applications in molecular gases it is more interesting to keep lower electron energy and take advantage of the natural tendency of NTP to store energy in vibrational excitation of molecules. This has motivated a completely dierent project devoted to the recycling of CO 2 by plasma, and more specically the vibrational kinetic of CO 2 , and the plasma/catalyst coupling at reduced pressure.

Plasma conversion of CO 2

A great deal has been written on the inuence of the absorption of the atmosphere upon the climate. Fourier maintained that the atmosphere acts like the glass of a hothouse, because it lets through the light rays of the sun but retains the dark rays from the ground. The selective absorption of the atmosphere is according to the researches of Tyndall, Lecher and Pernter, Röntgen, Heine, Langley, Ångström, Paschen, and others, of a wholly dierent kind. It is not exerted by the chief mass of the air, but in a high degree by aqueous vapour and carbonic acid, which are present in the air in small quantities. .

The challenge is at an inconceivable scale: the values can change depending on the model but CO 2 emission are expected to increase from 34 GT/y nowadays to 50 GT/y in 2040 [START_REF]Organisation for Economic Co-operation and Development[END_REF]. Not one single solution will solve this problem. The priority is to reduce the emissions by means of carbon-free energy sources. CO 2 capture is then the most advanced technology in terms of tons of CO 2 removed from the atmosphere.

Even combining the performance of all CO 2 recycling technologies currently imple- mented, only about 0.4% of the CO 2 emitted is recycled. A full panel of sustainable solutions need then to be developed (only 4% of CO 2 recycling project are already at a commercial stage) to be able to choose the most suitable recycling technology for each CO 2 emission source. It also means that CO 2 emission is an almost innite reservoir of carbon to be functionalized to build platform molecules for the chemical industry. The common issue to any recycling technique is the source of energy to be transferred to activate the CO 2 molecule.

Renewable energies powered by the sun including photovoltaic, concentrated solar or wind driven sources provides a partial solution to this challenge. However, the geographically and temporally ill-matched supply and demand characteristics as well as the absence of useful storage solutions cause signicant problems. The energy storage into chemical energy is very ecient thanks to the high energy density of hydrocarbon-based chemical fuels (HBF) that could be methanol or ethanol. An attractive course of action is therefore the power-to-gas represented on gure 2, where the excess electrical power produced by solar or wind generators is used to convert CO 2 into hydrocarbon fuels, thus replacing fossil HBF by renewable HBF also called solar-fuels.

One signicant advantage of this scheme is that, as the technology remains rooted on hydrocarbon fuel, it is possible to use the existing infrastructure for gas and oil storage, transport and distribution.

In the energy storage chain covering capture, conversion, separation and production of fuel, the weakest link is the conversion. CO 2 dissociation is the most dicult step. Indeed splitting CO 2 into CO and O is a strongly endothermic process, which makes it hardly ecient with conventional thermal and catalytic reaction techniques.

Many technologies are currently investigated to circumvent the major obstacle of CO 2 dissociation (Bio-reforming, electrolysis, thermo-chemistry, catalysis. . . ) [88].

Worldwide a lot of research focuses on photoelectro catalytic processes. Classic electrolysis reaches an energy eciency of 70-80% [START_REF] Zuofeng | Splitting co2 into co and o2 by a single catalyst[END_REF], but makes use of precious metals as a catalyst, such as Platinum, which drives up cost. Avoiding scarce materials in alternative photo-and electro-catalytic schemes presents a challenge [START_REF] Lewis | Powering the planet: Chemical challenges in solar energy utilization[END_REF]. Solid

Oxygen Electrolysis Cells based on layers of rare earth metal composites operate at high pressure and high temperature that remains an engineering challenge. Thermal dissociation based on heat generated with concentrated solar power by means of nano-structured metal oxides reaches an energy eciency of less than 5%. Most ecient metal oxides are based on rare earth metals such as Cerium.

Compared to other techniques, Non Thermal Plasmas are still studied very little for CO 2 dissociation. Yet NTP provide a unique way to break CO 2 molecule with the minimal energy of 5.5 eV corresponding to the bond energy C=O because of their non equilibrium nature. NTP are therefore very promising as the rst step towards solar-fuels and/or platform molecules synthesis.

Having this context in mind, in 2014 I have spent one year in Eindhoven in the group of R. Engeln (TU/e) to work on CO 2 conversion in atmospheric pressure discharges.

After this initial approach I have decided to develop a dierent but complementary strategy at LPP by using reduced pressure discharges coupled with catalyst. I obtained a funding from ANR on this topic and this project is now the core of a strong collaboration with TU/e (R. Engeln and A. Sobota) and IST Lisbon (V. Guerra).

2.4.2. Motivations for using mid-range pressure non thermal plasmas (MP-NTP) to dissociate CO 2

Non Thermal Plasma are a non equilibrium medium meaning that temperature of the electron (T e ), vibrational temperature of molecule (T v ) and temperature of the gas (T g ) can be very dierent. Depending on the plasma source used, the energy transferred to the gas can be concentrated mainly on CO 2 excitation processes while keeping T g as low as possible. Hence, NTP provide an entirely dierent environment for chemical reactions as compared with the typical processes occurring under thermal equilibrium conditions.

CO 2 dissociation in a non-thermal plasma can proceed mainly through three mech- anisms: direct electron impact dissociation (reaction 1), vibrational pumping (reaction 2) or reaction between excited CO 2 molecule and O atoms (reaction 3)

e -+ CO 2 ( 1 Σ + ) → CO(a 3 Π) + O( 1 D) (1)
e -+ CO 2 ( 1 Σ + ) → CO v * 2 ( 1 Σ + ) → • • • → CO(a 3 Π) + O( 3 P) (2) CO v * 2 → CO + O 2 (3)
The direct electron impact (mechanism 1) is the result of few electrons having the highest energy. It is not the most energy ecient process because it produces an atomic oxygen in an excited state.

The vibrational pumping of CO 2 (mechanism 2) is the most ecient mechanism. It occurs when low energy electrons (T e around 1 eV) excite the asymmetric stretching mode of the CO 2 molecule [91]. If the density of vibrationally excited molecules is high enough, the vibrational quanta can be pumped up to the dissociation limit during the relaxation process, due to non-resonant vibration-vibration energy exchanges similarly to what has been described in nitrogen [START_REF] Guerra | Kinetic modeling of nitrogen discharges and post-discharges[END_REF]. In addition, the third mechanism 3 with O atoms could improve the eciency of the dissociation by reacting with CO v * 2 . It must be stressed here that vibrational excitation can also lower the energy threshold of direct electron impact dissociation as it has been shown for N 2 [93].

The contribution of O atoms can however also reduce the dissociation rate if the gas temperature is too high, because of the back reaction from CO to CO 2 . The control of O atoms recombination into O 2 or O 3 at low gas temperature is one of the key for an ecient CO 2 dissociation.

The energy eciency η for splitting CO 2 into CO and O is dened as the ratio of dissociation enthalpy and energy spent in the plasma per CO molecule produced.

η = ∆H diss E CO (4)
The dissociation enthalpy ∆H diss = 2.9eV correspond to the average of the dissociation enthalpy with reaction 2 (∆H 2 = 5.5eV ) and the enthalpy of reaction 3 (∆H 2 = 0.3eV ).

The feasibility of ecient plasma decomposition of CO 2 was rst reported in the former USSR [START_REF] Friedman | Plasma chemistry[END_REF], with energy eciencies of about 90% reported for non-equilibrium microwave plasmas at pressures in the range 50-200 Torr in supersonic ows [94].

Similar experiments recently performed at DIFFER institute in The Netherland attempt to reproduce these energy eciency but until now they have reached 60% in the best case [START_REF] Goedel | Production of solar fuels by co2 plasmolysis[END_REF]. Few more results become available from recent experiments [START_REF] Brehmer | Co and byproduct formation during co2 reduction in dielectric barrier discharges[END_REF][START_REF] Silva | Optical characterization of a microwave pulsed discharge used for dissociation of co2[END_REF] and a rst modelling attempts have been done [START_REF] Kozák | Splitting of co2 by vibrational excitation in non-equilibrium plasmas: a reaction kinetics model[END_REF].

Using appropriate non-thermal plasma sources with a low mean electron energy (1-2 eV), typically RF and MW discharge at medium pressure range, very high energy eciency η can be reached but at the cost of a low dissociation rate α (dened as the ratio of CO molecules produced and initial CO 2 molecules). The challenge is to nd a way to improve the dissociation rate α while keeping high energy eciency η. However an ecient CO 2 conversion is not necessarily obtained in pure CO 2 : a vibrationally excited CO 2 molecule could become more reactive with other molecules (CH4, H 2 O, H 2 , etc...) in the gas phase or on surfaces. The main goal of the plasma would then be to "activate" chemically CO 2 , without needing to fully dissociate it via electron processes.

To summarize, in order to develop an eective process for the conversion of CO 2 by plasma, several key points need to be well managed: optimization of the vibrational excitation of CO 2 in various gas mixture control of the recombination of O atoms to prevent back reaction with CO minimization of the gas heating Surfaces in contact with the plasma could be very benecial. First of all, plasma/catalyst coupling could help promoting O atom recombination into O 2 . It could also promote reactivity of excited CO 2 with other adsorbed reactant (such as H 2 on Nickel surface for instance). In addition the interaction of vibrationally excited molecules with surfaces can be much more complex than a simple loss of one vibrational quanta as it is often considered in kinetic models. A lot of work has been done in chemical physics to show how a molecule excited in dierent vibrational modes can have an enhanced reactivity with other molecules in the gas phase [?], but also on solid [99101] and even liquid surfaces [START_REF] Gilbert M Nathanson | Molecular beam studies of gas-liquid interfaces[END_REF]. These results are usually obtained from pump/probe experiments with a population of molecule excited in one single vibrational mode. In more realistic conditions for plasma exposure, the surface mechanisms involving vibrationally excited molecules are rarely considered except for vibrational de-excitation with more or less unknown accommodation coecients. A few papers mainly dealing with re-entry plasma are considering the recombination of atoms leading to vibrationally excited molecules, especially N 2 and O 2 [103]. Even in the gas phase, the reactivity of M v * is rarely known. For instance the production of NO from N v * 2 and O is sometime considered to have two dierent rates, for v < 13 or v > 13 because above v = 13 the reaction becomes exothermal [START_REF] Guerra | Non-equilibrium coupled kinetics in stationary n2-o2 discharges[END_REF]. The modication of the vibrational distribution function of molecules excited in a MP-NTP due to the interaction with a complex surface like a catalyst has never been reported.

The work on CO 2 plasmas that I initiated at LPP in 2015 is organised around three main lines:

provide experimental data to establish a detailed description of CO 2 vibrational kinetic investigate the reactivity of CO v * 2 with other molecule (in particular CH4) and on porous surfaces (rst of all silica, and then more complex catalysts) design pulsed plasma sources with higher density (pulsed RF-discharges for instance) and use various duty cycle ratio to optimize vibrational excitation while keeping minimum gas temperature

The rst studies dedicated to CO 2 vibrational kinetic and the inuence of porous material are performed in a pulsed glow discharge which is also used for studies of atoms recombination and molecule formation at the wall as it will be described in the following sections.

Main scientic results

The scientic results presented in this habilitation thesis are articulated around three main themes: 3.1 the surface reactivity of oxygen atoms which is essential to the three elds of application described in the section 2, 3.2 the rst investigations on the eect of surfaces on vibrational excitation of N 2 and CO 2 , and 3.3 the rst measurements characterizing both a plasma jet and its eects inside a dielectric target.

Investigating atoms reactivity on surfaces

Any laboratory plasma is interacting with walls. This interaction is often the key for industrial applications of NTP and this has motivated a lot of studies mostly at very low pressure (below 10 Pa, 0.1 mbar). At higher pressure the increase of collision frequency makes a priori the gas phase processes become dominant over surface processes. However reaction rates on surfaces, especially on complex materials with high specic surface, often unknown, can be orders of magnitude higher than in gas phase. Consequently the relative importance of surface and gas phase processes is not necessarily clear. This is especially true for radicals such as O or N atoms produced from dissociation of molecular gases because of their relatively long life time compared to ions, and their high reactivity. The chemistry induced by NTP at medium and high pressure (from 1 mbar to atmospheric pressure) can therefore be strongly inuenced by surface recombination and reactivity of atoms. Surprisingly there is still a lack of data for adsorption, recombination probability or reactivity of atoms on dielectric surfaces as simple as silica or Pyrex, not to say about real high specic surface catalyst. Yet, oxygen containing plasmas are crucial for many applications, from re-entry plasmas, surface processing, assisted combustion to indoor air cleaning and CO 2 conversion for instance. A better understanding of surface mech- anisms under direct plasma exposure involving radicals and in particular O atoms is thus essential. This has motivated an important part of my work during the last ten years and that work is still ongoing. Before presenting our main results, the next subsection present a brief summary of what was known in the literature before we start working on the topic.

Usual description of radical reactions on surfaces

This section provides some of the basic concepts of surface mechanisms description mostly to illustrate how the relevant parameters crucial for this description are most of the time unknown.

The general picture of gas/surface interaction is described in many textbooks [START_REF] Georey | Heterogeneous catalysis[END_REF].

A "surface mechanism" correspond to a succession of elementary steps including (i) adsorption, (ii) surface reaction and (iii) desorption of the products. The adsorption step is generally described by considering two sub cases: physisorption and chemisorption.

The physisorption corresponds to Van der Waals collective interaction with the atoms of the surface with typical desorption energy E d = 0.01 -0.2eV . It is characterized by the surface density of physisorption sites [F ], the fractional coverage of physisorption sites θ F , the energy of desorption E d and diusion E D (typically E d /E D = 0.5 [START_REF] Barrer | The measurement of surface area of nely divided solids by the ow of gases through them[END_REF]), and the characteristic frequencies of desorption ν d and diusion ν D , as well as the sticking coecient s = s 0 (1 -θ F ) where s 0 is the sticking coecient on clean surface. A good illustration of this interaction is to consider a periodic potential wells both in perpendicular and parallel direction to the surface, in which the particle is moving with characteristic residence time of desorption and surface diusion depending on ν d and ν D with an exponential dependence with the wall temperature T wall .

τ d(D) = 1/ν d(D) exp(E d(D) /kT wall ) (5)
[F ] coincides typically with the number of atoms on the surface (typically ∼ 10 15 cm -2 ). In the models s 0 is often considered equal to 1 even though this assumption is probably getting wrong for high energy impinging particles [START_REF] Guerra | Analytical model of heterogeneous atomic recombination on silicalike surfaces[END_REF] The chemisorption corresponds to the formation of a chemical bond (with typical binding energy E chem d ∼ 0.4 -8 eV ) which therefore can occur only at specic sites on the surface. In the case of O atoms on silica surface, the unpaired electrons of O may eciently form a chemical bond but the chemisorption sites have not been directly identied and are only supposed to be either non-saturated Si atoms, or impurities, or surface defects. This lack of available data explains that reported ratio of chemisorption [S] over physisorption [F ] sites densities on silica is scattered from 10 -3 to 10 -1 [107111]. As the nature of chemisorption site is not known, the fractional coverage of chemisorption sites θ S and the desorption energy E chem d are generally also unknown. In models atoms chemisorption sites are usually considered to be trapped with high E chem d (for instance 3 eV in [START_REF] Young | Recombination of oxygen, nitrogen, and hydrogen atoms on silica: kinetics and mechanism[END_REF]) and therefore no thermal desorption or diusion are considered. the coexistence of "mobile" physisorbed atoms and "trapped" chemisorbed atoms implies the possibility of two surface reaction mechanisms: i) Eley-Rideal (E-R) involving a gas phase specie impacting on an adsorbed specie, and ii) Langmuir-Hinshelwood (L-H) requiring diusion of specie on the surface, therefore needing physisorbed specie. L-H becomes more important at lower wall temperature when the physisorbed specie spend more time on the surface. The recombination of atoms at the wall require to provide an activation energy E rec to break the bond with the surface. O and N atoms on silica are strongly bound and therefore E rec is estimated to be relatively high (E rec ∼ 0.2 eV according to [START_REF] Young | Recombination of oxygen, nitrogen, and hydrogen atoms on silica: kinetics and mechanism[END_REF]). In models E rec is usually supposed to be the same for both L-H and E-R and E rec determines the temperature dependence of the reaction rate. From a thermodynamic point of view one can understand that for a reaction to happen, the adsorption energy of O ads should be lower than the bond energy in the product molecule.

Plasma/surface interaction can be modelled with dierent levels of complexity. A description at macroscopic level rely on eective probabilities γ for surface processes that are usually unknown and serve as tting parameters to account for experimental measurements [112114]. On the other hand, a microscopic description at atomic level can be obtained with molecular dynamics calculations using interaction potential obtained from density functional theory (DFT) [115118]. This very interesting approach provides calculated values of recombination probabilities or accommodation energy but they are obtained on a limited number of atoms simulating a well ordered surface and the extrapolation to "real" disordered surfaces is questionable.

In addition, surface processes in plasmas correspond to characteristic time scales that can range from microseconds to several seconds, incompatible with MD calculations even if methods exist to extend MD simulation timescales [119,120]. An intermediary approach is the description at a mesoscopic level based on the parameters introduced above. These models describe the kinetics of surface reactions with the fractional coverage of active sites given by a set of dierential equations, and the elementary steps (adsorption, desorption, diusion, reaction) with given rate coecients [107111]. It is also possible to have a more microscopic description at mesoscopic level with Monte-Carlo [121] especially with the Kinetic Monte Carlo model algorithms in which the individual surface processes are treated as discrete jumps between system states avoiding the time loss of simulating exact trajectories of atoms [START_REF] Marinov | Deterministic and monte carlo methods for simulation of plasma-surface interactions[END_REF]123].

The mesoscopic approach seems to be a good compromise to describe and predict reliably surface reactivity in plasma. However the exact values of parameters such as [S], E D , E d , E rec , ν D , ν d are usually not known. Even when data are available, large discrepancies can be found in literature. As an example the values reported for recombination probabilities of O (γ O ) and N (γ N ) on silica materials may vary of several order of magnitude [124,125]. This is due to the fact that recombination process depends on the surface pretreatment history, the surface temperature, obviously the material but also the morphology of the surface, and the conditions of plasma exposure. The pressure and gas mixture certainly inuence γ but values are also very dierent if the measurements are done in spatial post discharge (prole of atom density in the gas ow after the plasma zone [START_REF] Young | Recombination of oxygen, nitrogen, and hydrogen atoms on silica: kinetics and mechanism[END_REF]112,126129]) , in temporal post discharge (decay of atom density right after plasma exposure [START_REF] Cartry | Atomic oxygen recombination on fused silica: modelling and comparison to low-temperature experiments (300 k)[END_REF]111,[START_REF] Cartry | Atomic oxygen surface loss probability on silica in microwave plasmas studied by a pulsed induced uorescence technique[END_REF]131]), in stationary discharge (deduced from gradient of atom density in front of the studied surface [START_REF] Gordiets | Surface kinetics of n and o atoms in discharges[END_REF][START_REF] Bedra | Atomic oxygen recombination on quartz at high temperature: experiments and molecular dynamics simulation[END_REF]132135]) or with the original spinning wall technique [START_REF] Vm Donnelly | Critical review: Plasma-surface reactions and the spinning wall method[END_REF]137].

The dierent values of γ are sometimes attributed to the "cleaning" of active site by ions bombardment leading to higher values under direct plasma exposure than in the post discharge. It illustrates that even for simple heterogeneous atomic recombination, there is a lack of deep understanding about surface elementary processes. This is even worse concerning molecule production at the wall such as N x O y from N and O, O 3 from O and O 2 or CO 2 from CO and O that could signicantly contribute to the loss of atoms on surfaces but are still poorly documented. The diculty to have reliable data is due to the multiplicity of reactions paths in mixtures of gases, both in gas and adsorbed phase. The reported rates of surface reaction are then obtained from the best agreement between gas phase products measurements and kinetic models with precision limited by the accuracy of the gas phase model itself [112,138140].

The lack of data available in the literature even for systems as simple as pure O 2 plasma interacting with silica surfaces motivated the work we started more than 10 years ago and which is still ongoing today. Our goal was at rst to answer questions such as Can atoms be chemisorbed on SiO 2 ? What is the surface coverage of chemisorbed atoms? What is their role in recombination processes and molecule formation? Our goal was then to design simple experiments to isolate as much as possible each elementary processes on non-ideal surfaces.

Experimental approach

The aim of this document is not to describe thoroughly the experimental setup we have used for studying plasma/surface interaction. Details can be found in already published works [21, 31] and only the important characteristics needed to explain the results shown in the following sections are given here. The originality of our approach is to use a low pressure pulsed DC discharge to study elementary surface processes on SiO 2 but also complex catalytic materials. The production of NO 2 starts immediately after the introduction of NO corre- sponding to Eley-Rideal mechanism. For high lling pressure the production of NO 2 reaches a saturation around 3 • 10 14 cm -3 requiring a surface coverage with O ads around 2.5 • 10 14 cm -2 ie less than 10% of a monolayer. The most interesting result is shown on g.4c. After the same O 2 pre-treatment as for g.4b, 6 successive llings of the tube with the same amount of NO molecules (introduced with 1%NO in Ar at p=0.045) are made with pumping between each lling. The decay time of NO reduces with each lling of the reactor. Yet the total amount of NO introduced during the 6 injections corresponds only to ∼ 4 • 10 13 cm -2 ie less than 15% of the total estimated coverage of O ads . Therefore the reduction of NO decay rate is not due to a lack of O ads but most probably to the existence of several types of chemisorbed atoms with dierent binding energy resulting in a distribution of reactivity. A simple surface kinetic model has been used in [START_REF] Guerra | No oxidation on plasma pretreated pyrex: the case for a distribution of reactivity of adsorbed o atoms[END_REF] to explain these experimental data and a distribution of not less than 7 dierent adsorption sites is necessary to reproduce the measurements in all the conditions. Our results are in agreement with the data from [137] showing similarly that strongly bonded O atoms exhibit a distribution in binding energy and are not ecient for Langmuir-Hinshelwood recombination. The traditional description with one binding energy for physisorbed atom and one for chemisorbed atom seems already signicantly wrong. Strongly adsorbed O atoms are capable of oxidizing NO but the question remains of their role in the O atom recombination in O 2 .

Role of chemisorbed atoms in O atom recombination

A similar pre-treatment/probing procedures as described in previous section can be used to investigate O atoms recombination by using isotopic exchange 16 O ↔ 18 O. All the results of this section are obtained for gas pressure low enough (P = 0.53mbar) to have negligible recombination in the gas phase and all the reasoning can be done considering O atoms loss only at the wall. A schematic of the experimental procedure is shown on g.5a.

Fig. 5 Isotopic exchange on the wall: a) experimental procedure, b)Evolution of the coverage of 16 O desorbed from the surface of silica discharge tube under 36 O2 plasma exposure (Step 1, "pre-treatment"). On the second step 18 O was desorbed from the surface by a discharge in 32 O2 during 240s, c) Comparison of the kinetics of desorption of atoms from the surface under plasma exposure, either desorption of 14 N under 30 N2 plasma exposure from silica surface pretreated by 28 N2 discharge or desorption of 16 O from silica under 36 O2 plasma

The same RF discharge is used as in section 3. 16 O detected after only the Ar cleaning (like on g.5b) and after argon cleaning followed by 20 minutes of 32 O 2 plasma exposure (not shown on g.5) have indicated that approximately 5•10 14 cm -2 additional 16 O can be grafted on the wall by the 32 O 2 . This value is remarkably similar to the amount of chemisorbed O atoms probed with NO conversion into NO 2 (see section 3.1.

3)

The red curve on g.5b shows the amount of 18 O extracted from the surface during 240s of exposition to 36 O 2 plasma. The maximum amount of 18 O extracted is 5 to 10 times larger than the amount of chemisorbed atoms added onto the surface by O 2 plasma. Nevertheless these atoms were strongly bond to the surface since they remained on the surface in spite of the pumping. This can be explained by exchange of lattice oxygen atoms from the silica network. In view of these results, the image of SiO 2 surface under plasma exposure is that of an essentially dynamic object. The plasma is not only adsorbing atoms with more or less strong binding energy, it fully reshue the atoms from the material surface itself. This process continues over very long time scale as can be seen from g.5c. showing the amount of atoms extracted from the surface as a function of plasma exposure duration.

Similar isotopic exchange experiments have been performed also in nitrogen plasma.

Surprisingly the exchange of atoms with the material appears to be done with similar rate under O 2 or N 2 plasma exposure with the same power. One would expect the N atoms density to be lower than O atoms density because of the energy cost of N 2 dissociation [START_REF] Eletskii | Dissociation of molecules in plasma and gas: the energy[END_REF]. Therefore g.5c suggests that ions bombardment is responsible for the observed atoms exchange with the surface.

The distribution of reactivity of chemisorbed atoms shown in section 3.1.3 could then be a consequence of constant reorganization of surface structure under ion bombardment. Another important observation is that the rate of atoms from the surface released in the gas phase is too slow to account for the recombination of all the atoms produced during the plasma. We have measured with short discharge pulses that ux of atoms reaching the surface is about 10 times larger than the amount of strongly bonded atoms extracted from the surface shown on g.5c (these results are published for N atoms in [START_REF] Marinov | Adsorption and reactivity of nitrogen atoms on silica surface under plasma exposure[END_REF]). This suggests that physisorbed atoms play a key role in surface reactivity and especially in atoms recombination. However the multi-steps experimental method described so far do not allow to investigate physisorbed atoms.

Other strategies are described in the next sections.

Role of weakly bonded atoms in molecule formation on complex surfaces

In order to follow the recombination of weakly adsorbed atoms in situ diagnostics with good time resolution are required. In this rst example we are using two photons laser induced uorescence to measure the O atoms density during the post-discharge of short plasma pulses and time resolved UV absorption spectroscopy to measure the corresponding ozone formation from the Hartley band between 220 and 300 nm.

The recombination of O atoms is not necessarily leading to O 2 . The production of O 3 can also be a signicant sink of O atoms depending on the conditions. At low pressure O 3 is often neglected because its formation requires a three body reaction O + O 2 + M -→ O 3 + M where M is a molecule absorbing the excess of energy. However the rate of this reaction with O 2 acting as a third body (k O3 = 6.3 • 10 -34 • (300/T g ) 2 cm 6 s -1 ) shows that already at 5 mbar ozone production in the gas phase is signicant. The possibility that surface plays the role of third body and enhances ozone production is still little investigated. However a few papers mention that surface ozone production could even be the main O atoms loss mechanisms at pressure from 1 to 50 mbar [134,[START_REF] Janssen | Isotope evidence for ozone formation on surfaces[END_REF]. The increase of ozone production with high specic surface materials has also been reported in DBD discharges at atmospheric pressure [START_REF] Guaitella | Plasma-photocatalyst interaction: Production of oxygen atoms in a low pressure discharge[END_REF][START_REF] Jodzis | Eect of silica packing on ozone synthesis from oxygen-nitrogen mixtures[END_REF][START_REF] Jodzis | Ozone synthesis under surface discharges in oxygen: application of a concentric actuator[END_REF]. This material (called bers in the following) is described in [START_REF] Guaitella | Plasma-photocatalyst interaction: Production of oxygen atoms in a low pressure discharge[END_REF] and is made of silica bers of ∼ 10µm in diameter partly covered of SiO 2 nanoparticles. This material has a large specic area (BET)

and the amount placed on the inner wall of the reactor corresponds to roughly 100m 2 added in the reactor.

The O 3 production with the bare tube has very similar characteristic time as the O atoms loss τ 0 . The lifetime of O atoms due to ozone formation estimated from the gas phase production of O 3 (k O3 given above) is about ∼ 60ms at 300K, matching very well the measured τ 0 . The characteristic time of O atom loss is much shorter with the bers (τ 0 = 5.5ms) than without (τ 0 = 65ms). Considering the O atoms diusion coecient given in [134], the diusion time in these conditions is estimated to be ∼ 4ms. It means that all the O atoms diusing to the wall are lost on the bers. Production of O 3 is much faster and higher than with the bare tube in the early post-discharge with a characteristic time of 9ms that can not be explained with any volume processes. This result gives a direct evidence for surface production of ozone. We have estimated that production of ozone on the bers account for ∼ 25% of the O atoms loss (see details in [29]). However on the bare tube surface vs gas phase relative contribution to ozone formation is more dicult to determine. A kinetic model described in [35] allowed us to show that gas phase production is dominating in our conditions but that O 3 production can not be explained without considering an intermediary plasma but it is also of primary importance for the understanding of CO 2 plasma (see section 3.2.2). The principle of these measurements is to use a continuous plasma with small square modulation of the current around an average value (∼ 15%). As a result the surface of the reactor is under an almost constant ions bombardment and atoms ux but the time variation of O atoms density can be measured to deduce their life time (see g.8).

Actinometry is a convenient technique to perform quickly a parametric study with the modulated discharge but the absolute O atoms density has also been conrmed with TALIF. The laser used for these TALIF measurements has been developed by Cyril Drag recently arrived at LPP and is described in [START_REF] Booth | Gas temperature measurements in oxygen plasmas by high-resolution two-photon absorption laser-induced uorescence[END_REF]49]. The special feature of this laser is that it achieves much higher spectral resolution than a conventional dye laser and therefore allows the gas temperature to be obtained from the Doppler broadening of the O atoms TALIF signal. This temperature is used as an input of actinometry together with the measured Ef ield to determine the excitation coecients of O and Ar lines. We have applied this technique with the DC discharge in a tube with a controlled wall temperature in the range T w = 240 -320K. becomes important at high dissociation rate in CO 2 plasma. However even in this "high pressure" regime, the importance of surface recombination could easily reach 10 or 20% and a detailed kinetic model of the gas phase is necessary to give accurate values.

In the low pressure regime, the recombination is dominated by surface mechanisms.

According to [111], in the range of surface temperature reached in our conditions the recombination occurs probably with E-R mechanism. The loss probability with E-R mechanisms depends on the ux of O atoms to the wall and the surface coverage of O atoms that can be either on physisorption or chemisorption sites. The absolute density of atoms in the gas phase is lower at lower gas pressure. Weakly bonded atoms have probably a fast sorption balance (equilibrium between atoms ux and desorption rate) and therefore their surface coverage should reduce at lower pressure.

The increase of loss frequency seems to points towards an increase of the amount of strongly bonded atoms. The number of adsorption sites with strong chemical bonds could increase because of stronger ions bombardment. However even at the lowest pressure considered here (p = 0.2T orr) the sheath of dc discharge is still collisional and the population of ions having enough energy to break bounds in the SiO 2 network might not be high enough to explain such a strong increase of O atoms loss. Moreover strongly bonded O atoms were shown in section 3.1.4 to not be the main specie for O atoms recombination... The role of energetic neutral species should also be investigated.

We are still working on the analysis of these results. Other measurements are currently being done to obtain densities of other species of importance both in O 2 and CO 2 plasmas. Measurements campaign on the VUV line of synchrotron SOLEIL are being done to investigate in particular O 2 (a 1 ∆ g ) and O 2 (b 1 Σ + g ) metastables. Vibrationally excited O v * 2 can also be measured with broadband UV absorption like in [START_REF] Foucher | Highly vibrationally excited o2 molecules in low-pressure inductively-coupled plasmas detected by high sensitivity ultra-broad-band optical absorption spectroscopy[END_REF]. However all these data will provide more understanding only when compared to detailed kinetic models of the gas phase and description of the surface mechanisms. In spite of the fact that we are working on this topic for more than 10 years, rst with A. Rousseau and D. Marinov, then with JP Booth, Abhyuday Chatterjee and Ana-Soa Morillo-Candas, and from the beginning with models developed by V. Guerra and C. Pintassilgo, a lot still need to be done and this work will be continued in the coming years. Among other things it includes a need for a more accurate description of the energy stored in vibrations of molecules.

Vibrational kinetic and surfaces

It has already been mentioned in section 3.1.5 that vibrationally excited ozone plays a key role in O 3 formation. In general vibrationally excited molecules act as a large energy reservoir in NTP and vibrational kinetic description often suer of a lack of experimental data. The interaction ov vibrationally excited molecules with surfaces is even less studied. Accurate measurement of vibrational temperature is especially important for CO 2 conversion by plasma as explained in section 2.4. In the following subsections the work started on CO 2 vibrational kinetic in 2015 will be described but vibrational de-excitation of N v * 2 is rst used to illustrate the inuence of various surfaces on vibrational kinetics. The reported values of γ N 2 lies in the range 10 -4 -10 -2 . These low values are attributed to the fact that relaxation mechanism would proceed in two steps with rst physisorption of N 2 and then transfer of vibrational energy to phonons of the surface. However the values of γ N 2 on surface under direct plasma exposure could be very dierent.

The diculty to obtain values of γ N 2 is the detection of N v * 2 . A simple and orig- inal technique rst used in [START_REF] Vi Egorov | On the mechanism of heterogeneous relaxation of vibrationally excited nitrogen molecules[END_REF] is to take advantage of the resonance between the vibrational levels of CO 2 and N 2 . The rst level of the asymetric CO 2 mode is very close in energy to the rst vibration level of N 2 (2349 cm -1 and 2331 cm -1 respectively). By adding small amounts of CO 2 to a N 2 plasma, the vibration temperature of CO 2 will quickly equilibrate with that of N 2 , giving a chance to make a titration of N 2 v by measuring CO 2 with infrared absorption spectroscopy. Similar resonance exists also with vibrational levels of CO and N 2 O. Three dierent Quantum Cascade Lasers (QCL) have been used to perform this titration of N v * 2 in situ, as a function of time in the afterglow of a single plasma pulse. The details of this method have been published in [START_REF] Marinov | Surface deactivation of vibrationally excited n2 studied using infrared titration combined with quantum cascade laser absorption spectroscopy[END_REF].

Similarly to previous sections, a pulsed dc discharge is used with a pre-treatment phase and then measurement during and after a 5ms single plasma pulse in N 2 with small admixture of the titrating molecules. The measured absorbance follows the Beer Lambert law and is proportional to the dierence of population of the lower and the upper state of the transition (see equation 6)

ln( I ν (L) I ν (0) ) dν = -α (N 0 -βN 1 ) (6)
where N 0 and N 1 are the densities of lower and upper vibrational levels, α is obtained from a calibration, β depends on the chosen transition (the dierence of rotational energy of the levels) and the gas temperature. An example of measured absorption signal obtained with CO 2 admixture in N 2 is given on g.10a.

The dierence of absorption signal before the plasma pulse and 200ms after is due to the dissociation of the titration molecule in the plasma. The observed relaxation time during the post-discharge could be due either to variation of the rotational distribution induced by gas heating, or to vibrational excitation. The characteristic time of heating and cooling down in the same N 2 plasma is in the range of few ms only [START_REF] Cd Pintassilgo | Study of gas heating mechanisms in millisecond pulsed discharges and afterglows in air at low pressures[END_REF][START_REF] Carlos | Modelling of the temporal evolution of the gas temperature in n2 discharges[END_REF] and therefore can not explain the observed relaxation time. The g.10b shows that for each titration molecule the inverse of the relaxation time τ ef f is proportional to the admixture of CO 2 , CO or N 2 O but the extrapolation for zero admixture gives similar values for the extrapolation in pure N 2 . A simple vibrational kinetic model is required to obtain γ N 2 from τ ef f as described in details in [START_REF] Marinov | Surface deactivation of vibrationally excited n2 studied using infrared titration combined with quantum cascade laser absorption spectroscopy[END_REF]. This method has allowed us to obtain values of γ N 2 in the range 5 • 10 -4 -30 • 10 -4 depending on the material of the surface (Silica, Pyrex, TiO 2 ...), but also on the pretreatment of the surface. For instance γ N 2 on silica surface is measured to be twice larger after a pure N 2 pre-treatment than after a pure O 2 pre-treatment, which could be due to a better accomodation of the excited molecule with an adsorbed one as suggested for CO 2 in [153]. The material of the walls, their roughness, but also the molecules adsorbed on the surface can signicantly aect the vibrational excitation of the molecules in the gas phase. This could have important consequences for the use of plasma for CO 2 conversion, where it is critical to optimize the vibrational excitation.

CO 2 vibrational kinetic

The exact pathways of CO 2 plasma dissociation are far from being understood. A lot of work has been done on CO 2 plasma in the 70's for the development of CO 2 lasers, atmospheric re-entry plasmas, and the early stage of plasma-chemistry mostly led by russian and italian groups [START_REF] Friedman | Plasma chemistry[END_REF][START_REF] Sergeev | Vibrationally excited molecules and mechanisms of chemical and physical processes in non-equilibrium plasmas[END_REF][START_REF] Capezzuto | Contribution of vibrational excitation to the rate of carbon dioxide dissociation in electrical discharges[END_REF]. In spite of the available literature there is no predictive model capable of evaluating the dissociation rate of CO 2 , mainly because V-V and V-T transfers are not well captured and because the electron kinetic is strongly inuenced by vibrational superelastic and electronically excited collisions [START_REF] Capitelli | Fundamental aspects of plasma chemical physics: thermodynamics[END_REF]. In fact, even dissociation through direct electron impact on the ground state could lead to the production of an excited O atoms (around 7eV) or a excited CO (around 10.5 eV) and cross sections from the literature can have a factor 10 dierence in magnitude [157159]. It is even worse for the other dissociation mechanisms. Depending on the number of vibrational levels considered, on how the climbing along the vibrational lader is treated [START_REF] Armenise | State-to-state models for co 2 molecules: from the theory to an application to hypersonic boundary layers[END_REF], or how the superelastic collision are estimated, very dierent results are obtained [START_REF] Kozák | Splitting of co2 by vibrational excitation in non-equilibrium plasmas: a reaction kinetics model[END_REF][START_REF] Bultel | Elaboration of collisionalradiative models for ows related to planetary entries into the earth and mars atmospheres[END_REF]. Until now the uncertainty on data for elementary processes is too large to build a predictive model from the theory.

In order to obtain ecient CO 2 conversion by plasma one should manage either to dissociate it directly into CO, or at least to make CO 2 reactive with other species (CH4, H 2 O, H 2 , etc...) which is a priori achieved with ecient vibrational excitation.

In both cases, the recombination of O atoms must be controlled to prevent oxidation back to CO 2 . Appropriate materials coupled with the plasma could be benecial both for reaction of CO 2 with other molecules, and for recombination of O into O 2 . However the primary requirement remains: CO 2 must be eciently activated to initiate chemical conversion. Before studying the inuence of any surface it is necessary to get a deeper understanding of CO 2 vibrational kinetic.

Our approach is therefore to measure as many parameters as possible, in situ and time resolved in a pulsed glow discharge to provide constraints to self consistent 0D kinetic model developed at IST which couple electron, vibrational and chemical kinetic as well as calculation of the gas heating. Only low energy excitation regime will be reached with a pulsed glow discharge (PGD) in CO 2 but this initial validation step is necessary before being able to have similar approach with RF or microwave discharges. It is also necessary to have a detailed and accurate description of the gas phase before being able to understand the role of surfaces as it will be illustrated by the rst measurements done with catalysts in section 3.2.3. The rst results of the work on CO 2 are providing T v , T g , O, CO v * 2 , CO v * , O v * 2 , CO, CO 2 , O 3 in a pure CO 2 PGD. The most challenging measurement is to obtain the population of the vibrational levels of CO 2 and CO.

In the following quantum numbers ν 1 , ν 2 and ν 3 designate respectively the symmetric stretch, doubly degenerate bending and the asymmetric stretch of CO 2 . In order to get insight on energy transfers between these three modes, it is necessary to use a measurement technique capable of detecting the population of levels from all three modes. Only the symmetric stretch mode of CO 2 is Raman active which invalidate Raman scattering and coherent anti-Stokes Raman scattering methods used in [162165]. On the other hand the three modes are strongly infrared active.

Infrared emission spectroscopy requires taking into account the optical thickness of the plasma to determine accurate densities [START_REF] Depraz | Infrared emission spectroscopy of co 2 at high temperature. part i: Experimental setup and source characterization[END_REF][START_REF] Depraz | Infrared emission spectroscopy of co 2 at high temperature. part ii: Experimental results and comparisons with spectroscopic databases[END_REF]. IR absorption spectroscopy had already been used by mean of tuneable laser diodes to determine densities in CO 2 laser [168, 169] but the narrow spectral width of IR lasers makes it inconvenient to get information of ro-vibrationnal absorption structures spread over several hundreds of wavenumbers. FTIR spectrometers on the contrary allow to obtain spectra over a very broad spectral range. The method we have developed is based on the use of FTIR with the IR beam going through the plasma reactor where the PGD is ignited. The homogeneity of the positive column of a glow discharge is an advantage to determine accurate densities from a line of sight measurement like IR absorption. Details of the experiment can be found in [START_REF] Blm Klarenaar | Time evolution of vibrational temperatures in a co2 glow discharge measured with infrared absorption spectroscopy[END_REF]. Fig. 11 gives example of transmittance spectra obtained at dierent time during and after the plasma pulse for a pure CO 2 plasma at 6.7 mar and 50mA.

(ν 1 , ν l 2 2 , ν 3 ) -→ (ν 1 , ν l 2 2 , (ν 3 + 1)
) where l 2 is the angular momentum of ν 2 . It makes the tting of absolute densities of each level more complex but it also gives a chance to obtain information about the population of the three vibrational modes of CO 2 (and also the vibrational excitation of CO) simply by measuring the wavenumber range

1975 -2400cm -1 .
In order to t the data, we have developed an algorithm in collaboration with B.

Klarenaar (PhD student at TU/e under supervision of R. Engeln, and my own cosupervision) and Ana-Soa Morillo-Candas capable of calculating absorption spectra without thermal equilibrium. The details of this algorithm are given in [START_REF] Blm Klarenaar | Time evolution of vibrational temperatures in a co2 glow discharge measured with infrared absorption spectroscopy[END_REF]. It makes use of the HITEMP-2010 database [START_REF] Ls Rothman | Hitemp, the high-temperature molecular spectroscopic database[END_REF] that includes transitions for asymmetric mode up to ν 3 = 6 → 7. It calculates rovibrational state distributions assuming the same rotational temperature for all vibrational levels, line strengths of all transitions, temperature and pressure broadening, and nally a transmittance spectrum including the prole of instrumental broadening. This spectrum calculation algorithm is included in a loop to t the experimental spectra using 7 tting parameters: the rotational temperature (T rot ), the bending and symmetric mode temperature that are in Fermi resonance (T 1,2 ), the asymmetric mode temperature (T 3 ), CO vibrational temperature (T CO ), the rotational temperature in the part of the gas along the line of sight that is at thermal equilibrium (T th ) (about 26% of the full line of sight), the dissociation rate of CO 2 into CO (α), and the pressure (p). The coloured spectra on g. 11 correspond to the transmittance spectra of each individual transition of one quanta in the ν 3 mode for all other states of the ν 1 and ν 2 .

The tting parameters presented above are temperatures. It should be mentioned that the calculation of the transmittance spectra can be done letting each single vibrational level independent, without assuming any distribution. A Treanor distribution is assumed (using T 1,2 , T 3 , T CO for scaling the harmonic part and T rot for scaling the anharmonic part) only to speed up the tting algorithm, especially for time resolved measurement series.

Fig. 10 gives an example of time evolution of vibrational temperature obtained with this method. During the rst ms of the plasma pulse, T 3 and T CO increases very fast while T rot and T 1,2 are still low. T 1,2 remains always nearly thermalized with T rot . When T rot and T 1,2 increase, T 3 and T CO are pulled back to lower values probably because of more ecient V-T transfers at elevated gas temperature. In the post discharge the gas thermalize very quickly and all the temperatures are following the same relaxation. The vibrational levels population densities are then too low, and the pressure already too high to keep a non equilibrium during the post-discharge.

In order to promote the excitation of ν -3 that is supposed to be benecial for CO 2 conversion, it seems particularly important to maintain a low gas temperature. This has been conrmed by modelling in collaboration with the group of Pr Guerra [START_REF] Guerra | The case for in situ resource utilisation for oxygen production on mars by non-equilibrium plasmas[END_REF]. Gas temperature is generally a very important parameter in plasma because of the dependence of reaction rates with T g and because it aects also the E/N . Various techniques have been developed to measure the gas temperature (emission spectroscopy, Doppler broadening of absorption line, etc...), but all have signicant disadvantages. In our case, determining accurately T g appears to be critical because of the consequences on vibrational kinetic and chemistry, but also because we wanted to conrm the accuracy of the tting procedure of IR spectra described above. Indeed the number of tting parameters and the convolution of several tens of transitions on the same spectral band could make the results questionable. Therefore we have compared three dierent measurements techniques of T g in the same glow discharge reactor: 1) T rot from FTIR data as described above, 2) T rot from Raman scattering measured at TU/e in the same reactor, 3) T g deduced from Doppler broadening of TALIF signal of O atoms as described in [START_REF] Booth | Gas temperature measurements in oxygen plasmas by high-resolution two-photon absorption laser-induced uorescence[END_REF]49] and in section 3.1.6. Fig. 13 shows the comparison of measured temperature with these three methods.

Raman scattering and TALIF are both measuring at one particular point of the reactor while FTIR is a line of sight measurement. TALIF is probing the temperature of the ground state of O atoms, while Raman scattering gives the rotational temperature of the ground state of molecules. Nevertheless, all three techniques are perfectly matched for the whole pressure and current range investigated. It gives us condence in the validity of the tting of FTIR data.

parametric studies are then possible for many dierent plasma conditions (duty cycle ratio, pressure, current, etc...), dierent gas mixtures, with and without catalytic materials and of course with other plasma sources expected to induce higher densities of high vibrational levels. This work is currently being done in close collaboration with Pr. Engeln's group in Eindhoven and Pr. Guerra's group in Lisbon and is the core of the PhD of Ana-Soa Morillo-Candas. Already g.14 shows the evolution of T 3 , T 1,2 and T rot at t = 1.5ms during a 5ms plasma pulse for dierent discharge currents and gas pressures. The temperature of the asymmetric mode T 3 is always higher than T 1,2 and T rot . However in order to maximize T 3 it is clear from g. 14 that T rot must be kept as low as possible: at 0.7ms the gas temperature has not yet raised and T 3 is higher than other temperatures for all conditions, however at 4ms at high pressure the gas temperature is already high and T 3 is almost thermalized with T 1,2 and T rot . Low pressure and high current are the most favourable conditions in a dc discharge and pulsing the plasma could be interesting to excite vibrational states while keeping low gas temperature. These conclusions were already known from CO 2 laser littera- ture however it is the rst time that measurements of the dierent temperature are performed time resolved in a pulsed plasma.

As it will be discussed in section 4, the advantage of low gas temperature and low pressure could even make plasma technologies a viable solution for production of O 2 from Mars atmosphere for fuel production or human mission to the red planet [START_REF] Guerra | The case for in situ resource utilisation for oxygen production on mars by non-equilibrium plasmas[END_REF].

However industrial applications on Earth would not be viable with too low pressure.

The use of CO 2 /N 2 mixtures to keep non equilibrium at higher pressure is one of the promising routes since the exhausts of industrial plants requiring CO 2 recycling are often containing N 2 . Moreover, as discussed in section 3.2.1, rst vibrational level of N 2 is resonant with asymmetric stretch of CO 2 and could enhance T 3 . The coupling with catalytic materials is another path of investigations.

rst measurements with specic materials

In section 3.2.1 we have shown that surfaces are playing an important role on vibrational de-excitation of N v * 2 and that dierent surface states can change the de- excitation probability. The material introduced in section 3.1.5 made of silica bers can be placed on the inner surface of the discharge tube to increase signicantly the specic surface area without changing the chemical nature of the walls. One would expect that using this material in a pure CO 2 plasma would increase the vibra- tional de-excitation and lower the vibrational temperatures. Fig 15 is showing the rst measurements of vibrationtal temperature by infra-red absorption as described previously, performed in a continuous CO 2 glow discharge for various pressures.

The vibrational temperatures are not decreased with the bers. On the contrary T 1,2 and T CO are signicantly higher with the SiO 2 bers while T 3 is slightly increased and T rot remains unchanged. This eect is more pronounced at low pressure when surface processes becomes dominant. The introduction of the silica bers is aecting the whole plasma dynamics and one should certainly not conclude from this preliminary data that vibrational de-excitation at the wall is lower with this material. First of all the increase of charge loss at the wall results in an enhancement of the electric eld for the same plasma current. However the small E f ield increase is probably not sucient to account for the observed vibrational temperature change.

We have already shown in a pure CO 2 plasma that SiO 2 bers are drastically reduc- ing the O atoms density, similarly to what is shown in pure O 2 on g 7a. Thus we suppose the increase of vibrational temperature observed is a consequence of a lower quenching by O atoms. The role of porous materials, catalysts and membranes on CO 2 conversion will be an important part of our investigations in the near future, especially during the thesis of Ana-Soa Morillo-Candas.

Ionization waves interaction with surfaces

The interaction of lamentary discharges with surfaces is studied for decades and yet, no clear description is made of it. From the early theory of ashover [START_REF] Watson | Pulsed ashover in vacuum[END_REF], to the numerous studies on plasma actuators [START_REF] Thomas C Corke | Dielectric barrier discharge plasma actuators for ow control[END_REF], or theoretical and experimental work on corona discharge interacting with targets [173175], an extensive literature exists on this topic. Nevertheless the nature of the charge deposited or induced on/in a target, the inuence of the roughness on the discharge propagation depending on the orientation of the eld with respect to the surface, the role of the surface in the pre-breakdown phase are among the questions remaining un-answered. As explained in section 2.3 lamentary plasmas are now the focus of many research groups because of the development of numerous emerging applications dealing most of the time with treatment of targets. More and more complex reactor congurations are investigated in order to full applications requirements with electrodes deposited on exible polymers for biomedical applications [START_REF] Bkhl Boekema | A new exible dbd device for treating infected wounds: in vitro and ex vivo evaluation and comparison with a rf argon plasma jet[END_REF], arrays of microplasmas for air treatment [START_REF] Tachibana | Current status of microplasma research[END_REF], cascaded DBD or roll to roll conguration for polymer and textile treatment for instance [START_REF] Heise | Sterilization of polymer foils with dielectric barrier discharges at atmospheric pressure[END_REF][START_REF] Mirko | Generation of a highdensity highly non-equilibrium air plasma for high-speed large-area at surface processing[END_REF]. Among the atmospheric pressure plasma sources, the plasma jets (APPJ) have motivated a lot of studies mostly because of their high reproducibility. Like for any other atmospheric plasma, the design of sources is very diverse and tend to increase in complexity with industrial motivations like in [START_REF] Peeters | The inuence of partial surface discharging on the electrical characterization of dbds[END_REF].

However, just as we chose the simplest possible plasma discharge to study the surface reactivity of excited atoms and molecules (PGD in previous sections), we chose one of the simplest plasma jet conguration to study the interaction of an ionization front with complex surfaces.

Since 2013, in collaboration with the group of Ana Sobota in Eindhoven, we are investigating a coaxial APPJ powered with kHz pulsed or sinusoidal voltage. The interaction of this APPJ with various surfaces can not be understood if the jet itself is not well characterized rst.

Characterization of the APPJ freely expanding

The APPJ used throughout our work is described in several papers [38,[START_REF] Sobota | The inuence of the geometry and electrical characteristics on the formation of the atmospheric pressure plasma jet[END_REF][START_REF] Guaitella | The impingement of a khz helium atmospheric pressure plasma jet on a dielectric surface[END_REF][START_REF] Sobota | Electric eld measurements in a khz-driven he jet: the inuence of the gas ow speed[END_REF]. A metal tube acts simultaneously as high voltage electrode and gas inlet inside a Pyrex capillary tube. A ring shape ground electrode is placed outside the tube forming a coaxial DBD conguration. The gap distance between the electrodes, the width of the ground and the length of the tube after the ground can all be varied but what is called in the following our "reference conguration" corresponds to a gap of 5mm, a ground of 3 mm thickness and a tube length after the ground of 20 mm with a glass tube of 4 mm outer diameter and 2.5 mm inner diameter. With this geometry and a helium gas ow of 700 sccm, the plasma is expanding of a few mm outside the capillary in the surrounding air as can be seen on part A of g.16. Three parts of the setup are distinguished in the following discussion: the "source" (space between the two electrodes), the "tube" (segment of tube after the ground), and the "plume" or "jet" (the part of plasma outside the tube). With this conguration and a sinusoidal When the front reaches the nozzle, it starts propagating in the surrounding air with a conical shape imposed by the gas mixing between helium and air (iCCD images (B.e), (B.f ), and colour picture of gas mixing simulation E). The development of the discharge in the tube and the plume corresponds to the propagation of a streamer connected to the high voltage electrode by a conductive channel (the inuence of the ground is mostly shielded by the deposited charge). It could be compared to a streamer in a corona discharge but this particular streamer is rst conned by the capillary tube, and then strongly inuence by the proportion of air entrained in the helium ow (lower than 1%). The plasma jets can therefore be called "guided ionization waves". The plume gives a good illustration of the necessity to include the surrounding in the study of a non thermal plasma. Picture A on Fig. 16 shows that the dimensions of the plasma plume visible with emitted light (∼ 10mm)is very small compared to the characteristic size of phenomena driving this APPJ (more than 70mm length). This is clear from parts C to F on Fig. 16 all shown with the same scale as the iCCD imaging on part B. On part C the axial eld measured with a commercial probe at time t = 0.6µs corresponding to image (B.c) shows strong eld values about 3 kV.cm -1 few cm around the tube and the plume which could be important for applications. On parts E the Schlieren imaging of the gas ow mixing (700 sccm He) shows that buoyancy force is limiting the vertical extension of the He ow at a position where gas ow simulation (part D) estimates only 0.9% of air is mixed into He. However when the plasma is ignited, the laminar ow region is extended further down up to ∼ 70mm which is also the distance at which the maximum gas temperature of 45°C is measured (see temperature map on part F). The reason why the plasma is expanding the laminar ow on loonger distance is still controversial in litterature and could either be a consequence of ionic wind (or Electro-Hydro Dynamic force EHD) or due to gas heating [181184].

Attempt were done to calculate the relative contribution of gas heating and EHD but the gas heating mechanism itself is not yet understood. For instance in [START_REF] Pk Papadopoulos | Interpretation of the gas ow eld modication induced by guided streamer (`plasma bullet') propagation[END_REF] the maximum temperature is considered right at the exit of the nozzle which is not corresponding to measurements of part F in our case. Exothermic chemical reactions could be responsible for this heating away from the nozzle. It is in any case clear that the proportion of air entrained in the gas ow determine the length of the plasma plume [START_REF] Sobota | Electric eld measurements in a khz-driven he jet: the inuence of the gas ow speed[END_REF][START_REF] Karakas | Correlation between helium mole fraction and plasma bullet propagation in low temperature plasma jets[END_REF]. Therefore understanding the 10mm of plasma plume require to describe accurately the 100mm of gas dynamic around! Until now uid models reproducing the development of ionization waves are not solving simultaneously the uid dynamic and the chemistry induced [START_REF] Bourdon | Numerical and experimental study of the dynamics of a µs helium plasma gun discharge with various amounts of n2 admixture[END_REF] even if recent works are addressing this challenge [START_REF] Lietz | Plasma-induced ow instabilities in atmospheric pressure plasma jets[END_REF]. Besides the complexity of the coupled phenomena, the validity of the models is often dicult to assess since the experimental data available on APPJ are most of the time focused on the chemistry induced which is not the main output of uid models due to the limited kinetic schemes included in these models. In [START_REF] Bourdon | Numerical and experimental study of the dynamics of a µs helium plasma gun discharge with various amounts of n2 admixture[END_REF] in order to bridge the gap between models output and experimental data, a calculation of electronic excited states of He and N 2 is included for comparison with emission spectroscopy. In the same paper a comparison is made with the electric eld measured with a commercial probe placed aside the capillary tube. This type of probe can be used to map the electric eld in the surrounding of the APPJ setup as it is done in Fig. 16 but the dimension are too large to insert it inside the plasma. Moreover these probes are calibrated for an homogeneous eld which is very dierent from the situation of an APPJ. There is an obvious lack of experimental data on parameters that are direct output of uid models such as Electric eld E f ield , electron density n e and electron temperature (T e ). Our approach is therefore to develop and apply experimental methods capable of providing basic parameters of the plasma (E f ield , n e , T e ) in a reproducible APPJ source applicable later on various targets. Fig. 17 gives example of measurements obtained with Stark Polarization spectroscopy (SP)

and Thomson Scattering (TS) with the jet vertical pointing downward.

SP is based on the detection of the emission of He(2 1 P -4 1 D) allowed line at 492.19 nm and the forbidden He(2 1 P -4 1 F ) line nearest to it as described in [START_REF] Sobota | The inuence of the geometry and electrical characteristics on the formation of the atmospheric pressure plasma jet[END_REF][START_REF] Cvetanovi¢ | Electric eld measurement in gas discharges using stark shifts of he i lines and their forbidden counterparts[END_REF]. Since the signal detected comes from the light emission of the plasma, the E f ield measured is dominated by the peak value in the ionization front. The E f ield as a function of position shown on Fig. 17 is therefore an image of the time evolution of the maximum eld along the propagation of the discharge. The results obtained for a sinusoidal power source with 2 kV amplitude and a pulsed power source with 6 kV amplitude are shown on Fig. 17. Inside the tube, E f ield is decreasing for both power supply probably because of the charge losses on the wall of the capillary tube (in [START_REF] Slikboer | Charge transfer to a dielectric target by guided ionization waves using electric eld measurements[END_REF] we have estimated this loss to ∼ 7.5pC • mm -1 ). The eld is stronger with the pulsed power supply which could be simply a consequence of a higher voltage amplitude (6kV instead of 2kV ). However it could also results from a weaker "memory eect" due to remaining charge and/or metastable states density from one ignition to the next one since the pulsed jet is ran at 5kHz instead of 30kHz. The gas ow has little inuence on the eld inside the capillary tube (which seems contradictory with Fig. 17 Electric eld measured along plasma propagation inside the capillary tube and in the plume for both sinusoidal (opened symbols) and pulsed (solid symbols) power supply and dierent gas ows (700, 1000, 1500 sccm). The electron density and electron temperature measured by Thomson scattering is shown on the right axis for the case at 1500 sccm and pulsed power supply the possible role of "memory eect" on the comparison pulsed/sinusoidal jet) . On the contrary, after the nozzle the inuence of the gas mixing, and therefore the gas ow, on E f ield is evident. A linear increase of E f ield is observed after the nozzle for all ows. The conical shape of the plume imposed by the structure of the helium ow expanding in air that is more dense, increase the charge density by reducing the volume of the ionization front [START_REF] Sobota | The inuence of the geometry and electrical characteristics on the formation of the atmospheric pressure plasma jet[END_REF]. This is conrmed by the measurement of n e by TS which showing indeed an increase along the plume axis. As far as we know only one paper [START_REF] Hübner | Electron properties in an atmospheric helium plasma jet determined by thomson scattering[END_REF] reports n e measured with TS in a helium APPJ giving similar values as in our case with n e around few 10 19 cm -3 . Fluid models are often giving lower values of electron density which might be a consequence of cumulative eect over several period of the power supply in experiments.

When the gas ow is increased, the slope of the E f ield reduces simply because of the extension of the length of the pure helium region as shown by Schlieren imaging (see part E g 16). It is worth noticing that in spite of a higher eld amplitude with the pulsed power supply, the slope of the peak E f ield in the plume is the same as with the sinusoidal voltage for a given gas ow (1500 sccm in this case), showing that the gas mixing is the dominant parameter structuring the plume of the APPJ.

The description of the APPJ development is still phenomenological but to the best of our knowledge, it is the rst time that measurements of peak E f ield , n e and (T e ) are successfully performed in the same plasma source. It opens the door to detailed comparison with uid models but it also gives us an excellent tool for checking the

APPJ above surfaces

We are only starting to investigate the inuence of targets on the development of our APPJ source in the frame of the PhD of Marlous Hofmans (cosupervision with Ana Sobota TU/e). As a rst step to identify the main inuence of a surface on the APPJ, very dierent targets are compared: a metal (copper plate), an insulator (glass or PVC plate) and a liquid (water). We have performed SP and TS in the jet impacting the surface at 90°vertically (see g.18). The target is placed at 10mm

from the nozzle and the jet is powered with a pulsed voltage of 6 kV amplitude.

The left graph shows TS results obtained with the laser focus 1mm above the surfaces as a function of time. In this case t = 0 corresponds to the time at which the ionization front monitored with iCCD camera passes at the focal point of the laser. A small decay of T e is obtained after the ionization front arrival but the n e remains almost constant over 120ns as expected in the conductive channel left behind a streamer. Surprisingly a dielectric target do not seem to aect E f ield nor n e , T e . From Schlieren imaging it is clear that at least the gas mixing is modied when any target, including dielectric target, is facing the gas ow. The results of g.18 probably means a gas ow of 1500 sccm is large enough to have an almost pure helium gas reaching the surface at 10mm distance in spite of ow modications induced by the target. However water surface and copper plate, both not grounded, enhance E f ield , n e and T e for the same distance. After the impact of the ionization front on the surface, a channel with high charge density is established from the high voltage electrode to the conductive target and higher current can be pulled in the plume. The electric eld is enhanced not only in the vicinity of the target surface but all over the gap between the nozzle and the target, and even inside the capillary tube.

As the eld detected with SP is only the maximum eld in the ionization front, it is not yet clear how the eld enhancement occurs before the ionization front reaches the target. When water or copper surface are used, several "re-strike" of the discharge are observed after impact like in [START_REF] Darny | Analysis of conductive target inuence in plasma jet experiments through helium metastable and electric eld measurements[END_REF][START_REF] Norberg | Helium atmospheric pressure plasma jets touching dielectric and metal surfaces[END_REF]. More measurements will be necessary with dierent gas ows, distances, targets conductivity in order to fully understand the inuence of targets on the APPJ. In any case it is clear from these measurements that no industrial application of APPJ can be developed without considering how the substrate that need to be treated will modify the properties of the plasma. In biomedical applications it is sometimes claimed that electric eld induced by the plasma on tissues can inuence electroporation of cell membrane. As the target itself modies the eld in the plasma, a good control of the process would require a better understanding of the interaction depending on the nature of tissues treated.

There are very few method measuring directly under plasma exposure quantitative data of the eect induced by plasma in/on a target. Most of the time material processing studies make use of diagnostics to characterize modication of materials after plasma exposure. These measurements gives information only on the material structure without direct connection with the plasma properties. It is however possible to measure the electric eld, the temperature gradient and possibly other modication induced in a dielectric target by using polarimetry techniques.

Measures on dielectric surfaces with Mueller polarimetry

APPJs can modify the surface of a material in dierent ways, especially chemically, but we have decided to focus our work primarily on determining fundamental parameters of the plasma spreading over the surface, similarly to what we have done for the propagation in the gas phase. Determining the electric eld induced on a surface E surf and in the bulk of a target E bulk as well as the surface charge deposited (Q surf ) by the plasma have been our rst goals before investigating possible chemical etching or other surface processes.

There are not many options to measure surface eld under plasma at atmospheric pressure [START_REF] Na Othman | Charge distribution measurement of solid insulator materials: A review and new approach[END_REF]. It is possible to use capacitive probes to determine surface charges but the spatial resolution is limited by the deconvolution of the signal coming from dierent location on the surface, and an image of the distribution of charge require to scan step by step the whole surface [START_REF] Wang | Improved algorithm of surface charge density distribution and electric eld distribution on insulating materials[END_REF]. Instead we have chosen to use optical method based on electro-optic eects (and in particular the Pockels eect) induced in specic birefringent crystals.

The Pockels eect corresponds to a variation of refractive index of a material when an electric eld is applied. When a polarized light is shined through a crystal sensitive to Pockels eect, a retardance is induced that can be measured to deduce the E surf .

Measurements of electric eld induced by various plasma sources based on Pockels eect were rst used with corona discharges for ashover studies [START_REF] Hidaka | A new method of electric eld measurements in corona discharge using pockels device[END_REF]. In [START_REF] Sobota | Experimentally obtained values of electric eld of an atmospheric pressure plasma jet impinging on a dielectric surface[END_REF] we crystal by the APPJ source described in section 3.3.1 using a Sénarmont setup. This type of conguration measures solely the retardance in one optical coordinate system which has some limitation as it will be discussed later. Nevertheless with this rst approach a parametric study on the E surf induced Q surf deposited with our APPJ source was done at the begining of the thesis of Elmar Slikboer (co-supervision with A. Sobota, TU/e and E. Garcia-Caurel, LPICM, Ecole polytechnique) [START_REF] Slikboer | Charge transfer to a dielectric target by guided ionization waves using electric eld measurements[END_REF]. It is important to note here that it is not straight forward to relate the measured value of electric eld E bulk to the amount of charge deposited Q surf and surface eld E surf .

Indeed the retardance detected is directly rerlated to the eld across the thickness of the crystal. The typical thickness of BSO crystals used is 500µm. If the eld applied on the crystal is homogeneous over dimensions much larger than the crystal thickness, then a simple approximation of a parallel plate capacitor would give a good value of Q surf . Unfortunately the spreading of the plasma jet over the surface has characteristic dimensions in the mm range and a correction procedure taking into account the penetration of the eld inside the crystal need to be done, as proposed in [START_REF] Slikboer | Charge transfer to a dielectric target by guided ionization waves using electric eld measurements[END_REF].

In the frame of the PhD of Elmar Slikboer the diagnostic has been extended further by developing an imaging Mueller polarimeter shown on g.19

The Mueller matrix M is a general tool to describe the optical properties of a material as described in [START_REF] Jorge | Polarized Light and the Mueller Matrix Approach[END_REF]. A Mueller matrix is a 4x4 matrix that describes both the behaviour of polarized light (like Jones matrices) and depolarized light which will be useful in future work with complex surfaces. The new polarimeter developed in collaboration with E. Garcia-Caurel from LPICM, consists of two polarizers (polarization state generator PSG, and polarization state analyser PSA) each composed of two ferroelectric liquid crystals. By varying the voltage applied on the four liquid crystals, 16 dierent polarization states of the light passing through the crystal can be imposed and detected on an iCCD camera. Knowing the Mueller matrix of the PSA and PSG, the matrix M of the crystal can be deduced. This matrix contains all the optical properties but the informations are entangled. The procedure to analyse M is given in [START_REF] Slikboer | Electric eld and temperature in a target induced by a plasma jet imaged using mueller polarimetry[END_REF]. The nal result is the non-depolarization matrix L m containing diattenuation and birefringence (see eqn. 7)

L m =     0 β γ δ β 0 µ ν γ -µ 0 η δ -ν -η 0     -→   Depolarization = 0 Diattenuation = β γ δ Retardance = η ν µ   (7)
The elements η and -η gives the linear birefringence in the (0/90°) coordinate system, while ν and -ν show the linear birefringence in the diagonal system (45/135°). The right part of g.19 shows an example of experimental matrix L m obtained after impact of the plasma jet on a BSO crystal. By expanding the light source beam and detecting on the chip of an iCCD camera, each element of the matrix L m is an image of the crystal. Therefore all the optical properties can be deduced at every point of the image.

The elements η, -η, ν and -ν on g.19 exhibit a background structure as well as a white "circle with a tail" in the middle of η and -η. This small white structure appears only after impact of the ionization front and is due to the charge deposited by the plasma on the crystal. The background structure visible on the four elements is on the contrary much more stable in time. When the plasma is ignited it takes a few minutes to stabilize and then remains constant. This structure is the result of surface temperature gradient. We have been able to use this background structure to determine the pattern of the temperature gradient at the same time as the eld measurement. A maximum of surface heating about 11°has been deduced from this background signal demonstrating high sensitivity of the detected signal to small temperature variations [START_REF] Slikboer | Electric eld and temperature in a target induced by a plasma jet imaged using mueller polarimetry[END_REF].

As the background structure induced by temperature is stable in time, it can easily be substracted to analyse properly the eld induced by the plasma jet. The rst row of Fig. 20 shows an example of the element η and ν from matrix L m on BSO crystal after substraction of the temperature background.

Only the element ν in the diagonal system (45/135°) shows the eld induced by the plasma. The signal on this element is proportional to the electric eld strength along the z direction (ie perpendicular to the crystal surface). The maximum eld strength obtained is about 6kV /cm averaged throughout the 500µm thickness of the crystal which corresponds roughly to ∼ 10kV /cm on the surface exposed to the plasma.

The second row of Fig. 20 is measured in the same way but with another crystal (F e : LiN bO 3 "Felinbo"). The symetry point group of Felinbo is dierent from BSO and as a result its optical response on the linear birefringent part is sensitive to the radial component of the eld E x and E y instead of E z for BSO. Both crystals have The coupling of Mueller imaging with FTIR in transmission will give us for the rst time a possibility to correlate the electric eld induced at the surface of a catalyst with the chemical species adsorbed on the surface and the corresponding surface reactivity.

These two applications are only examples of the variety of information that can be obtained on plasma surface interaction with Mueller polarimetry. This instrument will therefore be a corner stone of our research in the coming years.

Conclusions and perspectives

Plasma/surface interaction at atmospheric or medium range pressure (1-1000 mbar)

is the common thread running through my research. The use of energy stored in the form of vibrational excitation of molecules, or the study of ionization waves on surfaces are in fact two aspects of the same quest: to highlight how cold plasmas can oer new way of molecule synthesis and even reinvent catalysis with the use of short life species. This general idea naturally nds its application in CO 2 recycling.

Optimizing CO 2 conversion by using vibrational excitation and catalytic materials In a broader view, the role of vibrational excitation and surface catalytic reactions can go far beyond the single CO 2 conversion. Ammonia formation in N 2 /H 2 plasmas for fertilizing industry is another example of potential eld of investigation. Interestingly, both CO 2 and N 2 /H 2 plasmas have relevance in planetology in addition to their industrial potential.

It has been mentioned in section 3.2.2 that Mars atmosphere present the ideal conditions to promote vibrational excitation. This is a great opportunity to use plasma for oxygen production from in situ ressources. The CO 2 vibrational kinetic is also pertinent for studying consequences of lightning in Mars atmosphere [START_REF] Carlos I Calle | The electrostatic environments of mars: Atmospheric discharges[END_REF].

Concerning N 2 /H 2 , Audrey Chatain has started a PhD in 2017 (co-supervision with Nathalie Carrasco LATMOS) to simulate the ageing of aerosols from Titan atmosphere. The complex chemistry happening in Titan leads to the formation of complex organic molecules in a partially ionized atmosphere in a large range of pressure. Good analogues of dust particles detected in Saturn's moon atmosphere, called "Tholins" can be synthesized with CH 4 /N 2 /H 2 plasmas [199]. Studying tholins with

Teaching and research networking

As a research engineer from Ecole polytechnique, I don't have any teaching duties.

However I have chosen to get involved in two yearly summer schools. Since 2013 I am lecturer and I organized practicals during the summer school organized by LabeX plas@par every august. Since 2014 I am also lecturer of the "international summer school on low temperature plasma" that is organized for more than 20 years by the university of Bochum in Bad Honnef (Germany). In 2017 Bochum university asked me to be the international chair of this summer school together with Anne Bourdon and I will still be chair for 2018.

I am also member of the steering committee of the "réseau plasma froids" for 5 years now. The goal of this network is to bring together the french community of plasma physicists, but also all users of plasmas such as researcher from material science for instance.

Curriculum vitae

3

  . molecule formation and atom recombination on dielectric (9 articles) 4. gas phase kinetic and gas heating mechanisms (10 articles) 5. kinetics of vibrationally excited molecules and interaction with surfaces (3 articles) the number of article corresponds to the papers I am one of the co-author published for each topic after my PhD defense (after 2006). A very complex case of plasma/surface interaction is that of plasma in contact with liquids. Even if this has not been the core of my research, this is a side topic on which I have also been working (6 articles since 2010) and that could have future development for CO 2 conversion (see section 2.4) My list of publication can be found in the rst 55 references of the bibliography. After my PhD, I have been strongly involved in the work of 5 PhD without being ocial supervisor (2 of them during my one year secondment at TU/e in 2014). I have been formally co-supervisor of 2 PhD who already graduated, and I am currently co-supervisor of 4 PhD students. I have been also working with 4 post-docs and I supervised more than 20 master students.

François-

  Vincent Raspail, Health Manual Directory, 1894 recent fundings as PI: AirClean project: ASTRE nancial support from Essonnes (2013-2016), 80k¿ co-supervisor of PhD students: Daniil Marinov (PhD, 2009-2012), Christelle Barakat (PhD, 2011-2014) other co-workers PhD students and post-docs: Joseph Youssef (PhD, 2007-2010), Binjie Dong (Post-doc 2011-2012), Paul Gravejat (Post-doc 2012), Eugenio Fasci (Post-doc 2012)2.2.1. Context of indoor air treatment with plasma/catalysisNew energy-saving standards promote the construction of buildings that are better thermally insulated. The renewal of indoor air is then limited. As a result various toxic compounds are accumulating in the indoor air. In particular the concentration of Volatile Organic Compound (VOCs) released by various sources, paintings, furnitures, carpets, etc... is increasing in our new living spaces. We spend on average 80% of our time being exposed to this pollution in conned spaces. The typical concentration of indoor air pollutants (most of the time below ppm range) could seem very low and benign. However as we are exposed to these pollutants everyday, it is now undeniable that the eect of these pollutants on long term is harmful to our health. The technological challenge is therefore to invent a way to remove very diluted molecules from air with a low energy cost.Non thermal plasmas allow to achieve signicant abatement of VOCs without loosing energy in gas heating. In addition NTP oer the possibility to switch on and o the treatment instantaneously depending on the level of pollution to treat. Plasma technologies are not only trapping the VOCs but they are converting them into CO 2 and H 2 O by oxidative mechanisms. However the oxidation of VOCs by NTP in air leads to the production of undesired by-products that are dicult to control. The selectivity of VOC oxidation into CO 2 and H 2 0 has therefore been the rst motivation for coupling plasma with catalytic materials already back to the 1990's[7074]. Dielectric Barrier Discharges (DBD) quickly became the most used plasma source for indoor air treatment because of their simplicity, low cost, safety,

  Moreover a catalyst aims at reducing the activation energy of a reaction which partly depends on the adsorption energy of the reactant. However the adsorption energy as well as the surface mobility of adsorbed species could be signicantly affected by the surface electric eld induced under plasma exposure. Surface electric eld might therefore signicantly change the surface reaction pathways. The possible eect of charge deposition and surface electric eld in adsorption processes and reactivity is completely unknown and only very recent work starts considering it in modelling[64, 76]. On one hand the work we have done on air treatment aimed at investigating the best approach to achieve high energy eciency in realistic conditions of air treatment from a process point of view. On the other hand several experiments far from air treatment conditions are still currently being developed to start clearing 2.2 Indoor air treatment the complex plasma/catalyst interaction mechanisms and the possible role of surface electric eld.2.2.2. Methodology and summary of the work done on air treatmentThe lack of fundamental understanding of plasma/catalyst interaction is still too great to clearly dene the characteristics of a material that would be truly optimized to use the reactive species produced in a plasma. Nevertheless very promising results can already be obtained with commercially available catalysts. From an application point of view, in addition to the energy cost of the treatment, the diversity of pollutants to be treated is an important issue to take into account. Since 2007, these two observations have led us to follow two complementary approaches in parallel: a) a more application oriented strategy in collaboration with two companies (Al-Ko Therm in Germany and Air Serenity which is a spin o funded by a previous PhD student of the group, Joseph Youssef ), and b) the study of surface reactivity of radicals and the measurement of surface electric eld induced on dielectric.

Fig. 1

 1 Fig. 1 number of publication recorded per year on Isi web of knowledge about the topic "atmospheric pressure plasma"

Arrhenius, philosophical magazine

  Fig. 2 The ideal carbon cycle

Fig. 3

 3 Fig. 3 picture and schematic of the low pressure experiment that can be powered both with pulsed DC or with RF power supply.(1) interchangeable discharge tube section, (2) buer volume, (3) high voltage electrode, (4) grounded electrode, (5) pneumatic valves, (6) mass ow controllers

  Fig. 6 MEB picture of the silica bers material used to increase the specic surface

Fig. 7 O

 7 Fig. 7 O atoms conversion after a short plasma pulse of 1 ms at 6.7 mbar: a) decay of O atoms in temporal post-discharge, b) corresponding ozone formation with and without SiO2 bers material on the inner wall of the reactor

Fig. 9

 9 Fig.9 shows the loss frequency of O atoms obtained in pure O 2 and in pure CO 2 plasma for various pressure and current (central value of the modulation).

Fig. 9

 9 Fig. 9 Loss frequency of O atoms as a function of pressure for dierent average currents in a) pure O2 plasma and b) pure CO2 plasma

3.2. 1 .2

 1 Vibrational de-excitation at the wall: example of N v * For many molecules the cross section of excitation of the rst vibrational levels by electron impact have low energy threshold. As a result the electron energy distribution function is strongly aected by vibrational excitation. At reduced pressure the main de-excitation channel for vibrationally excited molecules (M * v ) is often the de-excitation on the walls. Therefore the probability of relaxation of a vibrational quanta on the surface γ v is an important parameter driving the energy balance in molecular NTP. Nevertheless only few papers report about N v * 2 relaxation on sur- faces and they are usually providing data in the owing post-discharge [149151].

Fig. 10

 10 Fig. 10 titration with CO2, CO and N2O of N v * 2 b during and after a 5ms and 50mA plasma pulse at p=1.33mbar: a) example of absorption signal for dierent admixture of CO2, b) inverse of the relaxation time obtained with the 3 titrating molecules.

Fig. 11

 11 Fig. 11 Example of measured and tted transmittance spectra obtained at dierent time during a 5ms plasma pulse and during the 10ms post-discharge in pure CO2 with 50 mA

Fig. 13

 13 Fig. 13 Gas temperature measured with Raman scattering, Doppler broadening of O atoms TALIF signal, and FTIR absorption in the same plasma conditions in pure CO2

Fig. 14

 14 Fig. 14 Values of T3, T1,2 and Trot at t = 1.5ms during a 5ms plasma pulse for dierent discharge currents and gas pressures. The top row is taken at 0.7 ms and the bottom row at 4 ms during the pulse. Left and right graphs are showing the same data from dierent angle for visibility

Fig. 15

 15 Fig. 15 comparison of vibrational temperatures of CO2 and CO T3, T1,2 and TC O as well as Trot as a function of pressure with and without glass bers material in a continuous glow discharge at 40mA measured by infrared absorption

  applied voltage with 2 to 3 kV amplitude, an ionization wave is developing only during the positive half period of the voltage. The growth of the ionization front can be seen on the iCCD images on g.16 taken with 100 ns exposure time. A plasma is rst ignited in the source in a glow like regime (image (B.a) on g.16). When the charge deposition in front of the ground electrode is large enough to induce a eld higher than the breakdown eld, an ionization front develops beyond the ground electrode inside the capillary tube (images (B.b),(B.c) and (B.d)).

Fig. 16

 16 Fig. 16 comparison of measurements performed on the same APPJ positioned on the gure with the same spatial scale: A. setup and long exposure image, B. fast iCCD imaging (a to f) with 100 ns exposure time, C. surrounding eld obtained with commercial probe (scale [0-3kV/cm] from blue to yellow), D. simulation of gas mixing without plasma (pure He in blue, 0.9% Air in He in yellow), E. Schlieren imaging without and with plasma, and F. gas temperature measurement obtained with GaAs optical ber probe (scale [18 -45°C from blue to red]))

Fig. 18

 18 Fig. 18 measurements performed with the same APPJ as in g. 16 and g. 17 in freely expanding jet and above glass, metal and water targets placed at 10 mm. a) Results of electron density and temperature obtained from Thomson scattering, b) Electric eld values from Stark polarisation spectroscopy

Fig. 19

 19 Fig. 19 optical setup developed to obtain the sixteen components of the Mueller matrix M of the crystal under plasma exposure, and the corresponding non-depolarization matrix LM deduced from M after impact of the APPJ on a BSO crystal

Fig. 20

 20 Fig. 20 axial and radial eld component on surfaces of two dierent crystals of Bi12SiO20 and Fe:LiNbO3 exposed to the same APPJ source

  is at the same time a case study of molecule conversion in plasma at mid-range pressure, and a major societal challenge. I have started working on this topic in 2015 at LPP and it will be the core of my work in the near future. The research on plasma/catalysis for CO 2 conversion at pressure 10-100 mbar has been supported by an ANR JCJC SYCAMORE running until 2020. In order to develop further the eld of plasma catalysis for molecule synthesis, I am the scientic coordinator of an ITN-EJD named PIONEER submitted in january 2019 for the second time. This project is dedicated to plasma/catalyst coupling for CO 2 recycling. This project aims at educating 15 PhD student all across Europe to form a new generation of inter-disciplinary experts in both plasma physics and catalysis to invent technologies of CO 2 recycling.

Fig. 21

 21 Fig. 21 Titan the satellite of Saturn (to the left) and Mars (to the right) gives two examples of the interest of non thermal plasma for studying planetary atmosphere evolution, or for space missions

  1.3. After 30 min of argon plasma cleaning, pre-treatment of the Pyrex tube is made in closed reactor with a pure36 O 2 plasma in order to graft18 O on the wall. This pre-treatment can be done with dierent durations from 0.1 to 1000s. The probing phase consists in a pure 32 O 2 plasma of variable duration. The proportion of 32 O 2 , 34 O 2 and 36 O 2 are then analysed by mass spectrometry. A statistical distribution of the three isotopologues is quickly established in the gas phase but the proportion of atoms desorbed from the surface can still be deduced from the signals of the three masses. The surface density of atoms is deduced from the known gas pressure in the closed reactor and the surface/volume ratio. However the diculty is that Pyrex or silica surfaces already contains 16 O. The black curve on g.5b shows the amount of 16 O extracted from the surface under 36 O 2 plasma exposure. These 16 O atoms show the reactivity of atoms from the silica crystalline network of the tube itself. Additional measurements comparing the amount of