Sven Hofmann

Koen Van Gils

Steven Van Der Linden

Sylvain Iseni

Peter Bruggeman

Koen Van Gils

Time and spatial resolved optical and electrical characteristics of continuous and time modulated RF plasmas in contact with conductive and dielectric substrates

teaching and research institutions in France or abroad, or from public or private research centers.

Time and spatial resolved optical and electrical characteristics of continuous and time modulated RF plasmas in contact with conductive and dielectric substrates

Introduction

Cold atmospheric pressure plasmas (CAPPs) are nonequilibrium plasmas with a gas temperature as low as room temperature, while still maintaining an electron temperature of a few electron volt. This feature allows them to produce reactive chemistry at low gas temperatures and makes them applicable for treatment of heat sensitive materials [1].

In the past years cold atmospheric pressure plasmas have been shown to be a promising tool for applications in medicine such as inactivation of bacteria (e.g. Methicillin resistant S. aureus) or direct treatment of living tissue [2][3][START_REF] Kalghatgi | Plasma Assisted Decontamination of Biological and Chemical Agents[END_REF][START_REF] Mccombs | [END_REF][6][7][8][9][10][11]. Different types of configurations can be used to create CAPPs, ranging from DC, kHz-pulsed dielectric barrier discharges (DBDs) to nanosecond pulsed, radio frequency or microwave driven plasma jets [12][13][14].

Depending on the type of discharge and the application, the plasma is either in direct contact with the treated substrate or not. Except for surface DBDs and coplanar DBDs (see e.g. [15,16]), for most applications based on dielectric barrier discharges the treated substrate a Current address: Leibniz Institute for Plasma Science and Technology INP Greifswald e.V., Felix-Hausdorff-Str. 2, 17489 Greifswald, Germany b e-mail: pbruggem@umn.edu acts as one of the electrodes and is needed to create the plasma [15,17,18]. The disadvantage of those sources is that the plasma itself depends strongly on the characteristics of the substrate, such as conductivity, permittivity and smoothness of the treated area, which makes diagnostics of plasma parameters challenging [19]. In a plasma jet, however, the plasma is in most cases remotely created between two electrodes in a noble gas [12,13]. Due to the electric field and the gas flow, the plasma created species are then forced out of the plasma zone in the tube into the open air. The advantage with these devices is that the treated substrate is not needed in order to sustain the discharge, which means that the plasma can either directly (in contact) or indirectly treat the material.

It is commonly believed that the main difference between a direct and indirect treatment is that in the latter case, only long living species are able to reach the substrate, while in the first case also charged species, (V)UV and the electric field will interact with the substrate [20]. However, as in the case of a dielectric barrier discharge, the moment the plasma is close to or in contact with a substrate, the substrate itself will act as an additional electrode, changing the electric circuit and thereby altering the plasma properties itself. How much the plasma changes depends on the conductivity of the substrate and the configuration of the plasma source [7,[21][22][23]. In most cases the influence of the substrate on the plasma itself has been investigated using imaging. Results indicate that a plasma in contact with an insulator shows no significant change of the plasma while touching a conductive substrate does increase the intensity of the emission and in some cases the length of the plasma jet plume [24].

Simulations of a helium plasma needle show that particle fluxes depend strongly on the treated substrate. The reason for that is the altered configuration of the electric field which will change the flux of charged particles onto the substrate [22]. Experimentally these simulations have been compared with images of the visible plasma jet and power measurements as a function of the distance between the plasma and the treated substrate, in this case agar (a gel used for bacteria and fungi growth) [21,25].

One example of a diagnostic method which can influence the plasma is molecular beam mass spectrometry (MBMS). MBMS usually samples the plasma with a metal orifice and this will change the plasma properties completely when the plasma is in direct contact with the metal orifice, compared to the free plasma. The results obtained from these experiments have to be interpreted with care as observed and mentioned, e.g. in references [26,27].

As in most in vitro biological treatments water and saline-like solutions are present, understanding how the plasma behaves in contact with liquids is very important.

In this work we apply in situ measurements of the plasma dissipated power and the gas temperature, when the plasma is in contact with a substrate, i.e. water, glass and metal. The typical values of permittivity and conductivity of the interfaces used are shown in Table 1. The observed parameters have been obtained by time resolved ICCD imaging and time resolved plasma dissipated power measurements combined with spatially resolved optical emission spectroscopy. An RF argon plasma jet with two different electrode configurations and time modulated and continuous RF excitation has been used.

The paper is structured as followed. In Section 2 the plasma source and the diagnostics are introduced. Section 3 deals with the results starting with obtained CCD images, followed by electrical characteristics and ICCD images (Sect. 4) and gas temperature measurements of the plasma in contact with water (Sect. 5). In addition, these results are compared with measurements performed with a glass and an aluminium substrate (Sect. 6). In Section 7 a summary of the results is given and we discuss the consequences of the presented research for related studies.

Setup and methods

Figure 1 shows a schematic view of the setup. An RF voltage with a frequency of 13.56 MHz generated with a waveform generator (Agilent 33220A) and amplified by a power amplifier (Amplifier Research 75AP250) is applied to the needle electrode. The output of the amplifier is connected to an inductance, which is close to resonance with the jet at a frequency of 13.56 MHz. This simple network design has been chosen in order to reduce the amount of impedances in the circuit connected to ground. These need to be otherwise considered when correcting for the power which is dissipating in the matching network (see power measurements below and [START_REF] Hofmann | [END_REF]). The matching network is connected to the plasma source, which consists of a needle electrode (ø = 1 mm) in the centre of a dielectric enclosing. The needle is surrounded by a quartz glass tube, with an inner diameter of 1.7 mm and an outer diameter of 3 mm. An argon flow of 1 slm through the glass tube is controlled by a mass flow controller (Brooks 5850E calibrated for 0-3 slm of helium) connected to a control unit (Brooks 0254).

For the grounded electrode either a ring electrode surrounding the glass tube or a plate electrode in front of the glass tube is used. These configurations result in either an electric field perpendicular (cross-field configuration) or parallel to the gas flow (linear-field configuration) [33]. The ring electrode has a length of 5 mm, the end of the ring electrode is 2 mm behind the needle electrode and 0.5 mm before the end of the glass tube. The plate electrode is at a distance of 4 mm away from the needle electrode and 2 mm away from the end of the glass tube. The plate has a hole with a diameter of 5 mm to let the gas flow and the plasma penetrate through it. Note that the metal and glass substrates used in Section 6 are solid, having flat surfaces.

In order to reduce the gas temperature without increasing the gas flow and keeping a plasma length of a few millimetres we use a second waveform generator (Agilent 33220A) and apply a 20 kHz modulation of the RF voltage with a duty cycle of 20% when using the ring electrode configuration. The plate electrode is used either in continuous mode or with the same pulse modulation as with the ring electrode.

In order to compare the three sources at least one parameter needs to be comparable for all three configurations. Voltage and therefore power dissipation could not be used, since the power coupling between these devices depends on the electrode configuration which changes the matching. If the same power for the continuous plate electrode is used as for the pulsed ring electrode, either the plasma is too long and too hot in the former case, or the voltage is too low to sustain a plasma in the latter case. To resolve this issue the input parameters (i.e. amplifier gain and RF voltage to the amplifier) are chosen in order to obtain the same plasma length measured between the grounded electrode and the end of the visible plasma plume, i.e. 5-6 mm, while maintaining a gas temperature at the tip of the plasma of below 100 • C. However, for all three presented cases the input power is not changed between touching and non-touching conditions.

To measure changes in the plasma induced by a substrate, different materials have been placed below the plasma jet. The distance between plasma plume and substrate has been decreased until a clear visible connection between the plasma and the substrate is observed. To maintain a stable contact, the distance between the substrate and the jet is further decreased by 2 mm in all cases. The gain of the amplifier and the RF voltage supplied to the amplifier have been kept the same between the "free" plasma and the plasma in contact with a substrate. A CCD-camera (Sony DSC-R1) has been used to obtain images of the free plasma comparing it with a plasma in contact with a substrate.

Figure 2 shows a simplified equivalent circuit for the plate (Fig. 2a) and the ring electrode configuration (Fig. 2b). The amplifier is represented by an alternating voltage source and a 50 Ω resistor, which represents the output resistance. The matching network (a coil) is described as an inductance and a resistor. The latter includes the resistance of the wire and of the needle electrode. In the case of the plate electrode a plasma is created between the needle electrode and the substrate (if present). The plasma-electrode gap is approximated by a capacitor and a resistor in series. The substrate is represented by an impedance Z subs , which can be mainly resistive or capacitive, depending on the substrate (see Tab. 1). When no substrate is present, the plasma is capacitively coupled to the surroundings and Z subs is a capacitor.

Since there is no plasma between the needle and the plate electrode, the effect of the plate electrode can be approximated as a capacitor in parallel with the plasma. The main difference between the plate and the ring electrode configuration is that a plasma is not only created between needle and substrate but also created between the needle and the glass tube which is surrounded by the grounded ring electrode. This system is described as a resistor and a capacitor in series, which is in parallel with the plasma connecting to the substrate.

Power and phase shift measurements

A voltage probe (Tektronix P5100) and a current probe (Pearson 2877) connected to an oscilloscope (Agilent Technologies DSO1024A) have been connected between the power amplifier and the matching network to measure the voltage across and the current through both the matching box and the plasma source. Figure 3 shows an example of the voltage and current obtained for the ring electrode configuration in pulsed mode without a gas flow, i.e. without a plasma.

With P = 1 f t0+ 1 f t0 U (t)I(t)
dt, the average power over one RF cycle is then calculated. In order to obtain the power dissipated only in the plasma the delay induced by the voltage and current probes and the oscilloscope have to be corrected [34], as well as the power dissipated inside the matchbox. Details can be found in reference [START_REF] Hofmann | [END_REF]. Figure 4 shows an example of the instantaneous power, i.e. P (t) = I(t)U (t) and the power dissipation averaged over one RF cycle of the circuit without an applied gas flow (no plasma) calculated from the voltage and current signal from Figure 3. Two peaks in power are visible at the beginning and the end of the pulse. These peaks correspond to the increasing and decreasing part of the voltage amplitude which indicates that energy is stored in parts of the electrical circuit and released at the end of the pulse. Calculating the energy of the peaks results in an energy of 6.76 μJ and 4.10 μJ for the first and the last peak, respectively. The constant positive power offset in between the peaks originates from the resistance of the electrical parts, most dominantly from the wire of the coil in the matching network. The average power loss in the resistive part of the setup is (2.86 ± 0.12) W for the settings of Figure 4. Similar results are obtained with the plate electrode.

Next to the power dissipation the phase shift between the voltage and current signal has also been obtained to monitor the change in impedance of the circuit. The obtained phase shift is the phase induced by the coil of the matching network, the jet geometry, the plasma and any present stray capacitances.

For the continuous plate electrode the power measurements have been validated with power measurements performed with a power meter (AR DC2600A), showing that the calculated average power corresponds to the measured forward power subtracted with the reflective power.

Note that for the time resolved ICCD images a different power amplifier is used (E & I A-075) and the matching frequency shifted from 13.56 MHz to 13.96 MHz.

Time resolved ICCD images

An ICCD camera (Stanford Computer Optics 4Picos) has been used and synchronised with the kHz pulse modulation to obtain time resolved images of the plasma at different delay times during the 50 μs period of the modulated RF power. The exposure time of the images was set to 100 ns. The images are the accumulations of 1000 consecutive pulses at the same delay time. The images are synchronised with the pulse modulation but not with the RF frequency, meaning that each accumulation was obtained at a random phase of the RF cycle. This results in a measured plasma emission averaged over the RF modulation. In order to increase the spatial resolution of the image a camera lens (Tamron Japan 572D) and an additional lens with a focus of 30 cm have been placed between the ICCD camera and the plasma source. The images are wavelength integrated over the visible wavelength range.

Spatial resolved gas temperature

A 1 m-monochromator (Jobin Yvon HR 1000 M) has been used to obtain the molecular emission band of the OH(A-X)(0,0) transition. The monochromator is backed by a CCD camera (SBIG ST-2000XMI-UV, pixel size 8 μm). Two quartz lenses (f = 30 cm) have been aligned between the plasma jet and the monochromator to project a 1 to 1 image of the plasma jet on the entrance slit of the monochromator. An example of the emission spectrum can be seen in Figure 5 showing a normalised emission spectrum averaged axially over 30 pixels.

An example of the Boltzmann plot method corresponding to Figure 5 can be seen in Figure 6. Only the first five rotational numbers of the rotational emission band have been considered (corresponding to 6 different transitions), since it has been shown by Bruggeman et al. [35] that emission lines from higher rotational numbers do not This is the Accepted Manuscript version -post-print-of an article accepted for publication in The European Physical Journal D. This article may be downloaded for personal use only. Any other use requires prior permission of the authors and Springer Nature. This article may be found at doi:10.1140/epjd/e2014-40430-3. Springer Nature is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. necessarily follow a thermalised Boltzmann distribution corresponding to the temperature of the gas. In previous work [START_REF] Hofmann | [END_REF], the authors have validated this method with Rayleigh scattering. The error indicated in Figure 16 only takes into account the error of the least squares linear fit applied to the data.

The power shown in Section 4.3 and the temperature shown in Section 5 have been measured simultaneously. The CCD images have been captured at the same conditions but not during the same measurement series.

Appearance of the plasma jets

CCD-images

Figure 7 shows images of the plasma for the three different configurations. The first image 7a is the continuously driven RF plate electrode configuration. As seen here and previously [START_REF] Hofmann | [END_REF]36] the plasma is contracted showing either one stable column or one or more randomly moving filaments. As explained in [START_REF] Hofmann | [END_REF]37] losses dominated by dissociative recombination compared to losses due to diffusion are the main cause for the contraction of the plasma as the production of metastables in the dissociative recombination process leads to high densities of metastables and an efficient ionisation in the centre, with

Ar + 2 + e → Ar m + Ar (1) Ar m + e → Ar + + 2e (2)
The randomly created filamentation of the plasma as seen in Figure 7a is contributed to a relatively high absorbed power in the plasma which is then distributed into two or more randomly moving filaments rather than into one stable plasma column. However, the geometry and the high electric field at the needle tip and the presence of the dielectric glass tube do also play a role in the filamentation and the stability of the plasma. Filamentation in RF plasma jet sources have been also observed elsewhere and can exhibit self organized behaviour [38].

In Figure 7b the same electrode configuration is used but the RF signal is time modulated at 20 kHz and 20% duty cycle. In order to sustain the plasma and keep the length the same as in the continuous RF case, the power has to be increased in the case of the modulation as explained in Section 2. This leads to a higher instantaneous power in the latter case, but the average power and the gas temperature is lower than for the continuous RF driven jet (see Sects. 4. 3 and5).

The reason the plasma shows only one filament, when the voltage is time modulated, is probably due to the gas flow and the fact that the plasma is off every 50 μs for 40 μs, leaving time for the charge in the vicinity of the needle to significantly reduce and metastable species to be blown out. This leads to a reduction in the ionisation rate at the needle tip and a preferential direction guided by the gas (and metastable) flow. This has also been observed in the case of a pulsed dc excited plasma at similar frequencies in reference [39]. The advantage of the time modulation is that the relative high electron and reactive species density (typically in RF plasmas) is present, while having a lower gas temperature due to the low duty cycle (20%) (see also e.g. [40][41][42]).

The last image, Figure 7c shows the plasma created with the grounded ring electrode configuration. The modulation of the plasma is the same as for the grounded plate electrode configuration. With a gas flow of 1 slm and a strong discharge between the needle and the glass tube surrounded by the grounded ring electrode, the modulation is necessary to prevent the gas from heating too much inside the glass tube, while maintaining a plasma length of a few millimetres outside of the tube. Most of the plasma is created inside the glass tube in between the needle and the grounded ring, since the electric field is the highest in this region and drops rapidly outside the tube. Due to the gas flow this plasma is then blown out into the ambient air, resulting in a more homogeneous and diffuse plasma compared to the plasmas created with the plate electrode.

In the bulk of the diffuse plasma a bright core can be seen which is created, as in the previous two cases, due to the high electric field in the axial direction caused by the sharp needle tip. Note that, if the gas flow rate is higher and becomes turbulent, the filament disappears and the length of the diffuse plasma is also shorter, similar to reference [39] and other references therein. The low gas flow of 1 slm is chosen in order to perform treatment of liquid in small volumes with the plasma source and to keep a non-turbulent gas flow.

If a conductive substrate, such as water, is introduced to the circuit it can be seen that, if the substrate is close enough, the length of the plasma is increasing towards the substrate in all three cases and if the distance between plasma and substrate is even closer, the plasma gets in direct contact to the substrate as shown in Figure 8. For these images and the following measurements tap water with a conductivity of 250 μS/cm is used. The water is brought into contact with the plasma, by decreasing This is the Accepted Manuscript version -post-print-of an article accepted for publication in The European Physical Journal D. This article may be downloaded for personal use only. Any other use requires prior permission of the authors and Springer Nature. This article may be found at doi:10.1140/epjd/e2014-40430-3. Springer Nature is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. the distance between plasma and water while keeping the input parameters of the RF amplifier constant. Liquids ranging from distilled water to NaCl-solutions with a conductivity up to 14.2 mS/cm have been used as well, however for the investigated conditions of this work no significant dependence of the conductivity of the water in power dissipation and temperature have been observed. Figure 8a shows the plasma with the grounded plate electrode with continuous RF excitation in direct contact with the water substrate. Similar images have been also obtained for other plasma sources [24,38].

For the time modulated cases the plasma changes into a more intense filament (Figs. 8b and8c). In the latter case (modulated ring electrode configuration, Fig. 8c) a strong contracted discharge column is clearly visible between the needle electrode and the water substrate, while a diffuse discharge is observed without the liquid. Both filaments in Figures 8b and8c are stable and are not moving randomly on the substrate as in the case for the continuously driven plasma.

Figure 9 shows an emission spectrum of the plasma, obtained coaxially, created with the ring and the plate electrode configuration at the same peak dissipated power between 200 and 1000 nm. The setup and alignment used was the same as mentioned in reference [43]. The spectrum is normalized to the highest peak intensity and is not corrected for the wavelength dependent sensitivity of the spectrometer. Fig. 9. Comparison of the emission spectrum of the ring electrode plasma (pulsed) and the plate electrode plasma (pulsed and continuous) for the same peak power settings as used for the temperature measurements (free plasma). An intensity offset of 0.25 and 0.5 has been added to the spectra obtained from the plate electrode configurations at 20% and 100% duty cycle, respectively.

The emission spectrum in the UV range is dominated by N 2 (C-B) in the range from 300 to 400 nm and OH(A-X)(0,0) emission at 310 nm. The ratio between the This is the Accepted Manuscript version -post-print-of an article accepted for publication in The European Physical Journal D. This article may be downloaded for personal use only. Any other use requires prior permission of the authors and Springer Nature. This article may be found at doi:10.1140/epjd/e2014-40430-3. Springer Nature is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. 10. Time resolved power dissipation over one period of the RF modulation with and without direct contact with the water substrate at 6 μs. The jet configuration is with a grounded plate electrode.

nitrogen emission and the OH peak is higher in case of the plate electrode (and highest for the continuous RF). This is to be expected since the majority of the plasma for the ring electrode configuration is created inside the glass tube (less amount of air in the plasma plume). The plate electrode plasma is sustained outside the tube, where the air concentration in the argon effluent is higher [44]. This leads to a higher emission of the nitrogen bands. Below 300 nm rotational bands of the NO(γ)-system are visible in the case of the plate electrode configuration. In the range between 500 to 1000 nm atomic line emission from argon and the emission line from atomic oxygen at 777 nm are the most dominant peaks.

Electrical and optical characteristics

The main cause of the change of the plasma when a substrate is introduced is induced by a change of electric field (due to a change in geometry) and this results in a change in plasma impedance, altering the load of the amplifier and therefore changing the power delivered into the plasma. These effects are investigated in this section in detail by analysing the voltage, current, phase shift and power of the plasma.

Time resolved power and ICCD images

In Figure 10 the time resolved average power over one RF cycle is obtained for the time modulated plate electrode configuration with the plasma switched on. In this figure the power is shown for a plasma with and without direct contact with the water substrate. As explained above, the power is corrected for the power dissipation due to resistances in the electrical network. The power is not corrected for the reactive part (i.e. the power peaks during the on and off switching of the RF power).

For these measurements the input power and distance between plasma source and water substrate have been adjusted so that the plasma is in direct contact with the water substrate approximately half of the time. The time point when the propagating plasma reaches the water substrate is determined by taking time resolved ICCD images of the plasma (Fig. 11) and linking the time delay of the ICCD camera to the corresponding time of the currentvoltage waveforms. For these measurements the time at which the plasma reaches the water substrate is around (6 ± 0.5) μs after the start of the pulse as indicated in Figure 10 by the shaded area.

The time 0 μs corresponds to the start of the trigger to switch on the RF voltage as shown in Figure 3. The discharge ignites at the plasma tip at a delay of 1.5 μs. This is in correspondence with [45]. The voltage pulses switches off after 10 μs (see also Fig. 4 or Fig. 10). For the free plasma a weak emission is still visible up to 1 μs after the plasma is switched off.

Figure 11b shows the time resolved images of the same plasma setting, when in contact with the water substrate. Again, the plasma ignites around 1.5 μs after the start of the pulse and is switched off at a delay of 10 μs.

Looking at the dissipated power of the free plasma it can be seen that there is a linear increase in power over roughly the first 4 μs starting from 2.7 W with a slope of (0.82 ± 0.03) W/μs before the power stays constant at 5.7 W, for these settings. In the case of the touching plasma it is clearly visible that the moment the plasma is in contact with the substrate a second significant steep increase in power, up to a factor of 2.5, is observed. Before the plasma reaches the water substrate the power dissipation in the plasma is the same as in the case when no substrate is in front of the plasma source. Comparing this with the corresponding ICCD images a clear increase of intensity is also visible when the plasma is connecting to the water substrate (visible in Fig. 11

at 8 μs).

From the ICCD images in Figure 11 it can be seen that the recombination lifetime is longer, when the plasma is in contact with the liquid, since the plasma emission vanishes at around 12.5 μs instead after 11 μs as in the case of the free plasma. The higher intensity and the longer lifetime indicate that in these cases the density of electrons and/or the metastable species increase, when the plasma is in direct contact with the liquid.

Note that a long afterglow in the emission of NO(A-X) is observed in reference [45], due to the fact that the NO (A) state is produced by N 2 (A) + NO(X) → NO(A)+ N 2 (X). The decay is a measure of the metastable lifetime of N 2 (A) rather than the effective radiative lifetime of NO (A). A higher density of metastables, when the plasma is in contact with the liquid could thus also contribute to the strong emission and the longer afterglow as observed in Figure 11b.

In Figure 12 the power dissipation of the ring electrode configuration is shown. In order to get to a similar length in plasma as in the case presented in Figure 10 and the time at which the plasma touches the water equal to 6 μs, the power is chosen to be higher than in the case of the plate electrode.

When compared with Figure 13, it can be seen that the power of the plasma increases the moment the plasma reaches the water substrate. However this increase of dissipated power does not exceed a factor of 1.3 and is thus less pronounced compared to the plate electrode geometry.

Comparing the rise time of the time resolved power between the time modulated plate electrode and the pulsed ring electrode configuration, it can be seen that the power remains constant after 2 μs for the latter case, while in the case of the plate electrode geometry, the power increases up to 6 μs before reaching a steady state value. The difference in rise time can be attributed to the difference in gap size between needle and grounded electrode, which is approximately a factor 6 larger in the case of the plate electrode configuration.

Time resolved ICCD images have been obtained for the ring electrode configuration and are shown in Figure 13. The ignition of the ring electrode plasma happens 0.5 μs earlier than for the plate electrode configuration. However, the electric field is much higher in case of the ring electrode, due to the much smaller distance between the powered and the grounded electrode. Again, a large increase in intensity is visible, the moment the plasma connects to the water surface. The emission is also sustained for a longer time after the RF voltage is switched off compared to the free plasma.

Increase of plasma length during the pulse

From the ICCD images the propagation of the four different plasma configurations from the beginning of the pulse until the plasma is in direct contact with the liquid substrate, i.e. after a delay of 6 μs, has been calculated and are plotted in Figure 14.

Note that the velocity does not correspond to the velocity of the ionisation front during an RF cycle, which will be at least one order of magnitude faster [45]. Note that the gas flow is significantly slower and of the order of 10 1 m/s. The images therefore show the slow expansion of the plasma over many RF cycles. The increase of length of the plasma plume during the time modulation can be explained by pre-ionisation linked with the number of RF cycles at the specific time point during a single kHz period [46]. The plasma reaches the water substrate at 6 μs. The error on all velocities, which depended on the spatial resolution of the images, is around 0.8 km/s as indicated for one data point.

In the case of the plate electrode configuration it can be seen that the velocity in the first 3 μs increases when the liquid layer is in the vicinity of the plasma. The peak velocity is reached, both with and without the liquid, when the plasma is at the position of the plate electrode.

The velocity of the ring electrode configuration is in the first 4 μs a factor 2 to 3 lower than with the plate electrode. The higher velocity of the tip of the plasma is expected due to the electric field parallel to the gas flow. The velocity is higher for the case the plasma propagates towards the liquid substrate, compared to the free plasma.

Voltage, current and power during one RF cycle

To further investigate the influence of the conductive substrate on electrical parameters, voltage, current and phase shift between those signals have been compared for the different conditions.

Figure 15 shows one RF cycle of the voltage and current signals of the three plasma sources either in touching or non-touching mode.

The calculated dissipated power in the plasma and phase between the voltage and current are indicated in the graphs. The error of the power consists of the systematic error due to the calculation of the phase shift originating from the uncertainty in the time delay measurement of the cables and the U -I-measurement. The uncertainty of the cable delay is calculated to be in the order of 0.5 ns. The statistical error is calculated by calculating the power over around 20 RF cycles.

Since the power depends on the phase, the closer the latter gets to zero, the smaller is the error contributed from the cable delay. The phase is measured by determining the time difference between the zero crossings of the voltage and current for 20 RF cycles.

Figure 15a shows the voltage and current waveform for the continuous driven RF plasma with the plate electrode. The power and phase between voltage and current is indicated in the graphs. Comparing the voltage and current waveforms of the touching and non-touching mode, it can be seen that the signal is changing in voltage, current amplitude and phase-shift.

The apparent power, P app = U rms I rms is decreasing, when the plasma is in contact with the water substrate. But as the phase shift is decreasing, a more favourable matching between power amplifier and plasma source occurs, i.e. plasma + matching network impedance is closer to the impedance of the amplifier ((50 + 0i)Ω), and the power coupled into the plasma is higher. As expected, a similar behaviour is seen in the case of the pulsed plate electrode configuration (Fig. 15b).

Figure 15c shows the waveform of the ring electrode configuration. Comparing the signals of the three free plasmas, the lowest phase shift and therefore best matching is reached for the ring electrode configuration.

In this case the phase shift stays nearly constant. Due to the configuration and the strong discharge between needle and ring the load is well defined and does not change significantly with plasma conditions. The power is also increasing, which is not mainly due to the phase shift as in the two latter cases but due to the change of voltage and current amplitude, leading, contrary to the plate electrode cases, to a higher apparent power in this case and a larger power coupling to the plasma.

Comparing the ratio of the power dissipation in the touching and non-touching mode of the three investigated conditions the plate electrode is showing the biggest change, while the ring electrode shows the smallest relative change as also seen in the previous section (Tab. 2).

When the plasma is in contact with a conductive substrate, we assume that the main change is a drop in plasma resistance, possibly due to a higher electron density. This leads to a favourable matching leading to more power coupled into the load, resulting in a higher dissipated power and a reduction in phase-shift. This effect is less dominant for the ring electrode since the majority of the active plasma is created in between the electrodes, and is less affected by the surroundings.

Spatial resolved gas temperature

A change in power dissipation leads to a variety of changes of several plasma parameters. As the heat flux to a treatment substrate is a very important parameter for biological applications, the spatial resolved gas temperature of the three different cases presented in Figure 15 is shown in Figure 16.

The gas temperature is obtained by the rotational temperature of the OH(A-X)(0,0) rotational emission band. Figure 16 shows either the gas temperature without any substrate or the gas temperature when the plasma is in This is the Accepted Manuscript version -post-print-of an article accepted for publication in The European Physical Journal D. This article may be downloaded for personal use only. Any other use requires prior permission of the authors and Springer Nature. This article may be found at doi:10.1140/epjd/e2014-40430-3. Springer Nature is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. contact with water substrate at a distance of approximately 7-9 mm, depending on the configuration. The same input parameters for the RF amplifier have been used as in the previous section.

Figure 16a shows the spatial resolved rotational temperature for the continuous RF driven plate electrode configuration starting from the needle electrode on the left side of the graph until the position the plasma is touching the water substrate on the right side of the graph.

When the plasma touches the water, the gas temperature is consistently higher, compared to the gas temperature of the free plasma. In the latter case the temperature inside the glass tube is around 380 K and rises after it leaves the glass tube to around 440 K before it reaches a steady state at 380 K. The increase of temperature once the plasma leaves the glass electrode can be explained by the admixture of air, i.e. by adding molecules, more chemical reactions which transform excess energy from the plasma to the heating of the gas are added into the system increasing the thermal output. A less pronounced effect of this has been also observed by Zhang et al. [47]. Note that an overestimate of the rotational temperature due to argon ions and metastables is unlikely. If ions and metastables would influence the rotational temperature of OH(A) in this case, this effect would be more pronounced in the far afterglow of the plasma due to increased This is the Accepted Manuscript version -post-print-of an article accepted for publication in The European Physical Journal D. This article may be downloaded for personal use only. Any other use requires prior permission of the authors and Springer Nature. This article may be found at doi:10.1140/epjd/e2014-40430-3. Springer Nature is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. collisional quenching by air and a reduced effective lifetime of the excited state, as shown for a helium plasma by Chan et al. [48]. However, this is not the case in the used configuration, as the rotational temperature drops at the tip of the plasma. Furthermore, as mentioned above, only the lowest rotational numbers have been used for the Boltzmann-plot as has been shown by Motret et al. [49] and Bruggeman et al. [50], that at least transitions of the low rotational numbers are thermalised and corresponding to the gas temperature.

For the touching plasma, the temperature is around 50 K higher than in the case of the free plasma. This is to be expected since the plasma power is also higher.

The advantage of the time modulated plasma (shown in Fig. 16b) becomes clear from the gas temperature in the case of the undisturbed plasma. Although the instantaneous power is higher, the average power is reduced due to the modulation, leading to a lower gas temperature compared to the continuously driven grounded plate electrode configuration.

However, the advantage of modulation is changed the moment the plasma is in direct contact with the water substrate. The gas temperature increases by approximately 80 K in this case, from 350 to 430 K. The increase of the gas temperature and power is similar as in the former case, although the gas temperature difference is getting larger closer to the water electrode. This is consistent with the fact that the plasma is most influenced by the water substrate between the plate electrode and the water.

Figure 16c shows the spatial resolved gas temperature for the grounded ring electrode configuration. For both conditions a maximum of the gas temperature is found This is the Accepted Manuscript version -post-print-of an article accepted for publication in The European Physical Journal D. This article may be downloaded for personal use only. Any other use requires prior permission of the authors and Springer Nature. This article may be found at doi:10.1140/epjd/e2014-40430-3. Springer Nature is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. between 4 and 5 mm behind the glass tube and the ring electrode in the order of 440 K and 460 K for the undisturbed and the touching plasma, respectively. Afterwards, the gas temperature drops down to 340 K at the end of the plasma plume. The power and average gas temperature are higher than the modulated plate electrode configuration, indicating again that the discharge is the strongest inside the glass tube and only a small fraction is created behind the grounded ring electrode. A higher temperature when the discharge is in contact with the water substrate is also observed in this configuration, although the increase is smaller the temperature drops after 5 mm to the same level as with the free plasma.

Different materials in contact with the ring electrode configuration

In this section further examples for the ring electrode configuration are shown in contact with a dielectric material (glass) and metal.

Images of the two conditions are shown in Figure 17. These images illustrate, that the plasma does not change significantly when in contact with a dielectric. In contact with metal, the diffuse part of the discharge disappears completely and a plasma filament, similar to the modulated plate electrode geometry, is observed. This indicates that, if the distance between a conductive substrate and the needle electrode is close enough, the ring electrode does not play a significant role as electrode any more.

Figures 18 and19 show time resolved ICCD images and the corresponding time resolved power, for the plasma in contact with glass, metal and the free plasma. The images of the free plasma are shown in Figure 13a. As expected, the plasma touching the glass plate (which has a much lower electrical conductivity as tap water and aluminium) does not show any significant difference in power, compared to the free plasma. The plasma touching the metal substrate shows similar behaviour as in the plasma in contact with the water substrate, i.e. a higher power and higher emission, when in contact with the water (at 5 μs).

The electrical characteristics during one RF pulse for the three conditions are shown in Figure 20. When the plasma is in direct contact with the metal substrate a significant decrease in current is seen. It is believed that in this case the main current is going through the substrate and not the ring electrode. This is motivated by Figure 18b which shows a single filament and no diffuse plume superimposed in the effluent which is observed in the other cases and is due to the presence of a strong axial discharge between needle and ring electrode.

Figure 21 shows the spatial resolved gas temperature, corresponding to the electrical characteristics. As can be expected from the images, no significant change in electrical characteristics or in the gas temperature are observed if the plasma is in contact with a glass substrate, compared to the free plasma. If metal is used as substrate, power and gas temperature are increasing (with a maximum gas temperature approximately 1 mm before the metal substrate) as seen with the water substrate. The phase shift is slightly higher when the plasma is in contact with the metal substrate, compared to the other two cases, as well as the apparent power.

Conclusion

In this contribution measurements of the plasma dissipated power (time resolved) and the gas temperature (spatial resolved) of an atmospheric pressure RF driven plasma jet operated with three different configurations while in contact with various substrates of different conductivity and permittivity (water, glass, metal) are presented. It is shown that, if a conductive substrate is too close to the plasma plume, the power dissipation in the plasma and the gas temperature increases.

Time resolved measurements of the plasma morphology and the power indicated that the increase of power is only starting the moment the plasma touches the conductive substrate. An increase up to a factor of 3 in power has been observed for the plate electrode configuration, while a factor less then 1.5 is observed for the ring electrode configuration. The change in impedance of the plasma source leading to a more favourable matching is shown to be the main reason of the observed change in electrical characteristics. In addition when in direct contact with a conductive substrate, the gas temperature of the plasma jet increases in all observed cases with a maximum of up to 80 K.

The electrical properties of the material which is treated in direct contact with the plasma strongly influences the plasma (parameters) and diagnostics need to be applied for identical conditions. This also indicates, if conductive meshes are used, as for example to reduce the charge flux onto the substrate in biological experiments, it is likely that the plasma is changing (e.g. increase of the density of charged and reactive species) resulting in an increased flux of reactive species.

Table 1 .

 1 Typical electrical conductivity (σ) and relative permittivity ( r ) values of the substrates used in this work. [*] measured. Substrate σ [S/m]

Fig. 1 .

 1 Fig. 1. Experimental setup.

Fig. 2 .

 2 Fig. 2. Equivalent circuit of the two electrode configuration, i.e. plate (a) and ring (b) electrode.

Fig. 3 .

 3 Fig. 3. Voltage and current during the pulse modulation with the ring electrode configuration without plasma. A zoomed-in view of the voltage and the current is shown as well.

Fig. 4 .

 4 Fig. 4. Time resolved instantaneous power (black line) and the averaged power (red rectangle) without plasma, averaged over one RF cycle. A zoomed-in view of the instantaneous power is shown as well.

Fig. 5 .

 5 Fig. 5. Example of the spectrum from the OH emission band for the ring electrode configuration with no substrate approximately 5 mm away from the needle electrode. Plasma dissipated power = 5.2 W.

Fig. 6 .

 6 Fig. 6. Example of the Boltzmann plot from the OH emission band of the spectrum shown in Figure 5.

Fig. 7 .Fig. 8 .

 78 Fig. 7. CCD images of the three different plasma conditions not in contact with water (free plasma). The dashed line indicates the position of the grounded plate electrode. (a) Continuous RF driven needle with grounded plate electrode. (b) Modulated RF driven needle with grounded plate electrode. (c) Modulated RF driven needle with grounded ring electrode.

Fig. 11 .

 11 Fig. 11. Time resolved images of the time modulated grounded plate electrode configuration corresponding to the experimental conditions of Figure 10. (a) Free plasma. Exposure time 100 ns, accumulations = 1000, amplifier gain of ICCD = 760 V. (b) In contact with liquid substrate. Exposure time 100 ns, accumulations = 1000, amplifier gain of ICCD = 760 V.

Fig. 12 .

 12 Fig. 12. Time resolved power dissipation over one pulse cycle with and without direct contact with the water substrate at 6 μs obtained with the ring electrode configuration.

Fig. 13 .

 13 Fig. 13. Time resolved images of the pulse modulated grounded ring electrode configuration. The images correspond to the experimental conditions of Figure 12. Free plasma. (a) Exposure time 100 ns, accumulations = 1000, amplifier gain of ICCD = 760 V. (b) In contact with liquid substrate. Exposure time 100 ns, accumulations = 1000, amplifier gain of ICCD = 760 V.

Fig. 14 .

 14 Fig. 14. Velocity of the visible plasma tip as a function of time during one modulation cycle of the different plasma electrode configurations either with or without a water substrate.The plasma reaches the water substrate at 6 μs. The error on all velocities, which depended on the spatial resolution of the images, is around 0.8 km/s as indicated for one data point.

Fig. 15 .

 15 Fig. 15. Voltage and current waveforms of the three different investigated settings for touching and non-touching mode. (a) Continuous RF driven needle with grounded plate electrode. (b) Modulated RF driven needle with grounded plate electrode. (c) Modulated RF driven needle with grounded ring electrode.

Fig. 16 .

 16 Fig. 16. Spatial resolved gas temperature in axial direction as obtained from the rotational temperature of the OH(A-X)(0,0)emission band. The experimental conditions are the same as in figure 15. (a) Continuous RF driven needle with grounded plate electrode. (b) Modulated RF driven needle with grounded plate electrode. (c) Modulated RF driven needle with grounded ring electrode.

Fig. 17 .

 17 Fig. 17. CCD images of the plasma with the ring electrode configuration in contact with a glass (a) and a metal substrate (b). (a) Modulated RF driven needle with grounded ring electrode in contact with glass. Exposure time = 20 ms. (b) Modulated RF driven needle with grounded ring electrode in contact with metal. Exposure time = 6.25 ms.

Fig. 18 .

 18 Fig. 18. ICCD images of the plasma with the ring electrode configuration in contact with a glass (a) and a metal substrate (b). The dotted lines indicate the end of the ring electrode and the position of the glass and metal substrate, respectively. The power is shown in Figure 19. (a) Modulated RF driven needle with grounded ring electrode in contact with glass. (b) Modulated RF driven needle with grounded ring electrode in contact with metal.

Fig. 19 .

 19 Fig.19. Time resolved power, averaged over one RF cycle for a free plasma, a plasma touching a glass and metal substrate. The experimental conditions are the same as in Figure18.

Fig. 20 .

 20 Fig. 20.Voltage and current waveform for a free plasma and a plasma touching a glass or metal substrate. The plasma dissipated power and phase shift are also given.

Fig. 21 .

 21 Fig.21. Spatial resolved gas temperature for a free plasma, a plasma touching a glass or metal substrate. The experimental conditions are the same as in Figure20.

Table 2 .

 2 Plasma dissipated power, apparent power and phase shift of the six different conditions investigated.

	Electrode	Plasma power [W] Apparent power [W] Phase [ • ]
	Plate (cont.) free	1.1 ± 0.2	1 4 .4 ± 0.5	7 7± 1
	Plate (cont.) touching	2.9 ± 0.2	1 0 .1 ± 0.4	6 3± 4
	Plate (mod.) free	2.7 ± 0.2	1 3 .5 ± 0.5	6 3± 1
	Plate (mod.) touching	6.0 ± 0.2	1 0 .0 ± 0.4	3 4± 1
	Ring (mod.) free	5.2 ± 0.1	7 .6 ± 0.4	2 5± 5
	Ring (mod.) touching	6.2 ± 0.1	8 .5 ± 0.4	2 6± 7

The authors would like to thank Eddie van Veldhuizen and Ronny Brandenburg for fruitful discussions. P.J.B. acknowledges funding from the Dutch Technology Foundation (STW) and S.H. acknowledges funding from the Foundation for Fundamental Research on Matter (FOM).

This is the Accepted Manuscript version -post-print-of an article accepted for publication in The European Physical Journal D. This article may be downloaded for personal use only. Any other use requires prior permission of the authors and Springer Nature. This article may be found at doi:10.1140/epjd/e2014-40430-3. Springer Nature is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. This is the Accepted Manuscript version -post-print-of an article accepted for publication in The European Physical Journal D. This article may be downloaded for personal use only. Any other use requires prior permission of the authors and Springer Nature. This article may be found at doi:10.1140/epjd/e2014-40430-3. Springer Nature is not responsible for any errors or omissions in this version of the manuscript or any version derived from it.


S Reuter

J Winter

S Iseni

S Peters

A Schmidt-Bleker

M Dünnbier

J Schäfer

R Foest

K-D Weltmann

Detection of ozone in a MHz argon plasma bullet jet

This study for the first time confirms the presence of plasma bullets in a MHz argon atmospheric pressure plasma jet. Bullet characteristics are investigated by phase-resolved optical emission measurements. Regarding the jet's reactive component output, its ozone production rates are investigated by two independent diagnostic techniques yielding complementary results. The first method-UV-absorption spectroscopy in the Hartley band-determines space-resolved distribution of the ozone concentration in the jet effluent. The second method-quantum cascade laser-absorption spectroscopy in the mid-infrared spectral region-yields high sensitivity results of the average ozone concentration in a multipass cell, in which the effluent is directed. The results of both diagnostic techniques show excellent agreement.

Introduction

Due to progress in recent years in the development and research of non-equilibrium atmospheric pressure plasmas with high reactive species densities at low gas temperature, plasma treatment of sensitive surfaces has become possible. Most importantly, the possibility of treating living tissue promises frontier-breaking therapeutic approaches in the new field of plasma medicine [1][2][3]. For a thorough analysis of the interaction of cold plasmas with sensitive biological materials, a detailed quantitative analysis of the discharges dynamics as well as the reactive component output is required. Excellently suited for this analysis are non-invasive diagnostic techniques [4,5].

In this work a MHz argon atmospheric pressure plasma jet is investigated. Phase-resolved optical emission spectroscopy yields insight into its excitation dynamics and it can be shown for the first time that even at MHz operating frequency, plasma bullets are responsible for the effluent dynamics. Regarding the reactive species, this study focuses on the ozone output of the jet. The ozone density in the jet effluent is determined by UV-absorption spectroscopy in the Hartley band and by quantum cascade laser-absorption spectroscopy in the mid-infrared (IR) spectral region. UV-absorption spectroscopy can determine space-resolved ozone concentrations, while quantum cascade laser-absorption spectroscopy has a high sensitivity and determines the spatially averaged ozone output of the jet with high accuracy, so that even the ozone generation from ambient air oxygen diffusing into the plasma jet's effluent at zero oxygen admixture to the feed gas can be quantified. From the UV-absorption spectroscopy measurements a twodimensional map of the optical depth of the ozone absorption is recorded, from which a map of the ozone distribution is calculated via Abel transformation.

Previous approaches to operate the plasma in a controlled noble gas atmosphere in order to have a simplified environment for intricate diagnostics such as TALIF spectroscopy significantly change the chemistry in the effluent. In the present investigations, the plasma jet is operated under realistic conditions in ambient air.

Atmospheric pressure argon plasma jet

A concentric atmospheric pressure argon plasma jet (kINPen09 © ) is investigated. In the center of a ceramic capillary (inner radius r 0 = 0.8 mm) a pin-type electrode 1 This is the Accepted Manuscript version -post-print-of an article accepted for publication in Plasma Sources Science and Technology. This article may be downloaded for personal use only. Any other use requires prior permission of the authors and IOP Publishing. This article may be found at doi:10.1088/0963-0252/21/3/034015. IOP Publishing is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. (0.5 mm radius) is mounted (see figure 1). In the continuous working mode, a high-frequency sinusoidal voltage (1 MHz, 2-6 kV pp ) is coupled to the pin-type electrode [6]. The plasma jet is operated with a dry argon flux of 5 standard liters per minute (slm) and in this work with up to 2% molecular oxygen admixture. Additional technical details about the working conditions and the design of the jet have been presented in previous papers [7].

Phase-resolved optical emission measurements

Phase-resolved optical emission measurements can image complex plasma dynamics in the ns range and yield valuable information about the plasma chemistry [8]. As non-intrusive plasma diagnostic this technique can also provide access to quenching coefficients and excitation processes [9].

Experimental setup

The experimental setup consists of a high-repetition rate gated iCCD camera (LaVision PicoStar HR12) with 1370 × 1040 pixels and a mounted macro-objective, 25 cm in front of the plasma jet. The trigger of the camera's gate is-via a delay generator-connected to the trigger output of the plasma jet's 1 MHz power supply. Because of the setup's simplicity, a schematic is not shown. The system has a spatial resolution of approximately 30 µm. For phase-resolved measurements the iCCD camera is triggered synchronously to the excitation signal of the plasma jet. At a fixed phase position of the excitation signal, the light emission from the visible effluent is acquired with a gate width of 1 ns, as shown in figure 2. The emission from all cycles within a time frame of 700 ms was integrated.

After this time integration the phase is shifted by a defined time step of 25 ns with respect to the phase-locked trigger, so that one full cycle with a period of 1 µs is resolved into 41 equidistant time intervals. discharge dynamics. The transition from the illuminated tip of the outer ceramic capillary (z-positions below 0 mm) and the free effluent can clearly be observed in all contour plots as a stepwise increase of intensity. The remarkable characteristic, however, is the discontinuous effluent, indicating the presence of so-called plasma bullets, known in the literature for the case of kHz excitation [10][11][12]. These bullets become more pronounced for higher oxygen admixtures. The emission intensities in the first and second half-wave of the excitation cycle decrease dramatically to zero with increasing oxygen admixture in the case of the first half-wave.

MHz plasma bullets

For pure argon the plasma bullet evolves between 0.4 and 0.6 µs and detaches from the effluent. The black dashed line in figure 4 indicates the spatio-temporal development of the bullet and the lines slope represents the bullet velocity. The temporal position of the bullet t b (z) = t 1 t 0 dt tI z (t) with t 0 = T /4 and t 1 = 3 4 T is calculated for z 4 mm. I z (t) is the emission intensity normalized to unity. This yields the bullet trajectory z(t b ) from which the bullet velocity can be derived via linear regression. Bullet velocities for different oxygen admixtures are shown in figure 5.

In pure argon the bullet velocity reaches up to 7 × 10 4 m s -1 . Admixing only 5 standard cubic centimeter (sccm) oxygen, the bullet velocity is almost reduced to half of the initial value. With an oxygen admixture of 20 sccm the bullet velocity is reduced by about 80%. Toward higher oxygen admixtures, the bullet velocity converges to a level of approximately 1.1 × 10 4 m s -1 . These results agree with reported bullet velocities ranging from 10 3 up to 10 5 m s -1 for 'bullet-like' structures in other discharges [13][14][15][16][17]. Moreover, the velocity is not constant for the case of no oxygen admixture and for 5 sccm O 2 admixture at the end of the effluent, in contrast to the other slopes for higher oxygen admixtures. This observed non-linearity can be due to a factual change of the bullet velocity or it can be an overlying feature of a front bullet and an occurring back bullet, as seen in a kHz driven plasma needle [18]. This back bullet can assumedly be seen in the round-shaped structure at 8 mm z-position in the contour plot without oxygen admixture. For an elucidation of back bullets in the MHz plasma jet, future investigations with a higher temporal resolution will be performed. As shown in figures 6 and 7, both the length and the maximum intensity of the effluent emission drop with increasing oxygen admixture.

Ozone detection by UV-absorption spectroscopy

Experimental setup

The spatially resolved absorption signal of ozone in the jet effluent is determined by the UV-absorption setup presented in figure 8. Here, the commonly used absorption of ozone at the wavelength of 253.7 nm is taken as a measure for the ozone concentration. A deuterium lamp (DH-2000, Ocean Optics) is used as background source. This lamp emits a broadband spectrum of UV light in the range from 200 to 300 nm. To ensure that only photons with a wavelength of around 254 nm are applied for the absorption measurements an interference filter with a central transmission wavelength of 254 nm and full width at half maximum (FWHM) of 10 nm is inserted into the setup between the lamp and the plasma jet. A UV beam sheath is generated by means of an aperture, biconvex lenses, and a cylinder lens and focused onto the entrance slit (400 µm) of a 0.5 m imaging spectrometer (Shamrock 500i, grating: 2400 lines mm -1 , blazed at 300 nm, Andor). An iCCD camera (iStar, Andor) is connected to the spectrometer as a detector. This setting provides a sensitive and spectrally resolved detection of UV light around 254 nm. The exposure time of the camera is 100 ms.

The plasma jet is positioned orthogonally to the UV beam sheath on a two-axis stepping motor system. Hence, the jet can be precisely moved in the axial and vertical directions. The spatial resolution in the axial (z) and vertical directions (y) is determined by the sheath-thickness and the resolution of the iCCD camera and amounts to 0.5 mm and 0.2 mm, respectively. Using an imaging spectrometer, the spatially resolved absorption signal in the vertical direction (along the spectrometer's entrance slit) can be detected instantaneously while the axial distribution has to be determined by moving the z-axis motor stepwise. The axial step size is set to 2 mm. The starting point (z = 0 mm) is the tip of the jet nozzle and the largest distance from the nozzle was 20 mm. The y-axis stepping motor is only used for a radial adjustment of the jet. Prior to the absorption measurement, the plasma jet is driven for 20 min with a gas flow mixture of 5 slm of pure argon (99.999%) and a small admixture of 25 sccm of pure oxygen (99.999%) to provide stable conditions. For the same reason the UV lamp is switched on several hours before the measurement. To avoid lamp switching during the measurement the internal lamp shutter is used to interrupt the UV beam.

Spatial distribution of ozone

In order to obtain a reliable optical depth profile generally four separate measurements are required. First, the background light emission I b has to be determined. This is carried out by measuring the stray light intensity with the jet and the UV lamp switched off. In the second measurement the emission I p of the jet effluent is determined without UV-lamp emission. In the third measurement the absorption intensity I is detected, where both, the plasma and the UV lamp are switched on. In the final measurement, the intensity of the UV lamp is determined without the plasma jet. In order to reduce measurement error an average of 5000 acquisitions were performed for each of the four intensity measurement steps.

From the four measurements the line-of-sight optical depth profile τ (y, z) can be calculated according to

τ (y, z) = -ln I (y, z) -I p (y, z) -I b I 0 (y, z) -I b . ( 1
)
This optical depth calculation results in the map displayed in figure 9. Additionally, a photograph of the jet with its effluent is included in the figure to indicate the geometrical relations. Due to the expanding gas flow in the radial direction and the diffusion of ozone from the center of the effluent in the radial direction a funnel-shaped optical depth profile forms. In the axial direction the highest line-of-sight optical depth was detected at the initial measurement position z = 4 mm at a distance of 4 mm from the jet nozzle. Its value on the axis at y = 0 mm amounts 8 × 10 -3 . For increasing distances z The absorption measurement directly inside the visible effluent region (below z = 4 mm) turns out to be difficult since plasma-induced temperature variations lead to a refractive index change and in consequence to a distraction of the UV beam. In fact, the cylindrical visible effluent shape acts as a convex lens that strongly refracts UV light from the center to the outer regions. This diagnostic problem is emphasized in figure 10, where the radial-dependent line-of-sight optical depth τ (y) measurement is shown for a nozzle distance of z = 2 mm. In the outer radial regions negative optical depth values are detected. This means that the absorption intensity measured at these radial positions must be higher than the background intensity emitted from the deuterium lamp. Furthermore, around r = 0 mm an additional dip is obtained. Both observations are in agreement with the above-discussed lens effect. However, the correction of this refracting effect is sophisticated and intended for further investigations. In this paper, the data obtained for nozzle distances below z = 4 mm are not used for evaluation.

Outside the visible effluent region (z 4 mm) reasonable optical depth profiles were measured. Two exemplary profiles are shown in figure 11 for nozzle distances of z = 8 mm and z = 20 mm. As already discussed for the spatial optical depth map (figure 9) the profile obtained at a smaller nozzle distance is narrower and has a higher amplitude than the profile obtained at z = 20 mm.

The relation between the optical depth τ and the ozone concentration is given by Beer's law

τ (z) = σ L 0 [O 3 ](r, z) dr, ( 2
)
where L is the total absorption length, [O 3 ](r, z) is the ozone concentration in molecules per cubic centimeter and σ = 1.14 × 10 -17 cm 2 is the ozone absorption cross section at a wavelength of 253.7 nm [19]. Under the assumption of an axisymmetric absorption profile, which is reasonably fulfilled for the presented measurement, the radial and axial ozone concentration [O 3 ](r, z) is calculated numerically using Abel transformation

[O 3 ](r, z) = - 1 π • σ R r dτ (y, z) dy dy y 2 -r 2 , ( 3
)
where r is the coordinate in the radial direction and R is the distance from the axis at that point where the ozone concentration declines to zero (set to R = 7 mm in the calculation).

Since numerical Abel transformation is sensitive to signal noise, the measured profiles were approximated by Gaussian profile functions as is shown in figure 11. Subsequently these fitted Gaussian functions are Abel transformed instead of the originally measured profiles. The Abel transformation result is displayed in figure 12. The ozone concentration [O 3 ](r, z) is maximal on the axis and declines in the radial direction. Similar to the line-of-sight optical depth profile τ (y, z) the concentration profile has a funnel shape.

Considering the axial dependence (figure 13) a maximum concentration value of 6.5×10 15 cm -3 was found at z = 4 mm. From this the concentration declines with increasing nozzle distances to a value of approximately 1 × 10 15 cm 3 at z = 20 mm.

Detection of ozone by IR-absorption spectroscopy

Experimental setup

The experimental setup for the ozone concentration measurements by absorption spectroscopy in the middle IR range is presented in figure 14. For these measurements, a commercial absorption spectrometer (Q-MACS, neoplas control [20]) is used, with a specification of the spectral resolution of 0.004 cm -1 , which offers a higher resolution than a more commonly used Fourier-transformed IR spectrometer. The IR source is a 2.5 mW pulsed quantum cascade laser (Alpes Lasers). The center frequency is tuned through temperature variation controlled by a Peltier element. The full accessible range is from 1024.5 to 1029.9 cm -1 . The IR beam is collimated with an off-axis parabolic mirror and guided to a 1 m focal length spherical mirror, which focuses the beam on the entrance of a 60 cm multipass White cell. Thus the total absorption length in the multipass cell is 16.80 m for 28 paths of the beam. The exit beam is focused on a detector (Q-MACS IRDM-600A) by a further off-axis parabolic mirror. In order to reduce noise and increase efficiency, the detector is cooled by a Peltier element and an external water-cooling unit. In order to align the IR beam, a 5 mW red laser diode was used. It is assumed that the red and the IR beams follow the same path according to the two apertures set between the laser and the multipass cell. The number of paths is controlled via an adjustable mirror within the cell roughly aligned through the red laser beam. An accurate control is performed with an oscilloscope connected to the detector to measure the delay between the emitted and the detected IR laser pulse. The number of passes can be adjusted between 2 and 28 paths. The multipass cell is at atmospheric pressure and in ambient air to allow the plasma jet guided into the cell to be operated under the same conditions as in open air. A glass tube of 57.5 cm length is set in the middle of the multipass cell in order to control the atmosphere around the plasma. An inlet of 2 cm diameter is situated in the middle of the tube in order to insert the jet 2 cm inside the cell. The sides of the tubes are closed by two plastic disks. Each of these lids has a rectangular cut-out of 15 cm 2 as a gas outlet. The argon feed gas flow is fixed at 5 slm. In order to avoid saturation of argon within the tube and thus change the ambient conditions, four inlets distributed each 90 • along a 4 cm diameter circle centered on the plasma jet axis were added. Through these inlets, an additional air gas flow fixed at 10 slm provides a good homogeneity of air surrounding the jet.

The gas was flowing for a considerable time and the plasma jet was switched on at least 1 h before the first measurement in order to assume dry feed gas and stable conditions. Particular attention was paid to reduce the humidity in the feed gas due to its considerable influence on the jet chemistry. In order to study the influence of oxygen admixture to the feed gas on the production of reactive species, the quantity of 5 to 100 sccm of oxygen (0.1 to 2%) was added to the argon gas flow.

Detection of ozone

It is well known that ozone has strong absorption bands in the IR region [21]. Figure 15 presents the line strength of ozone molecules versus the wavenumber. The probability of absorption is directly proportional to the line strength. Detailed information about the concept of line strength applied here can be found, for example, in [22]. The excitation frequency centered on 1027 cm -1 has been chosen, since the strongest absorption within the spectral range of the measurement setup can be found there.

The laser frequency is calibrated using a reference gas cell filled with a mixture of 95% argon and 5% carbonyl 6 sulfide (OCS). The cell is 15 cm long and closed by two circular potassium bromide (KBr) windows of 0.5 cm thickness. The pressure in the cell is set to 10 mbar in order to provide a narrow OCS absorption peak. Figure 16 shows a transmission spectrum of OCS. For this measurement, the gas reference cell is inserted into the multipass cell at 28 paths, which corresponds to 4.2 m absorption length. It should be noted that the baseline, which corresponds to no absorption, exhibits a wavy structure. It can be assumed that this is due to the KBr windows, which absorb a part of the IR beam and are also a source of interference. This distortion of the signal does not appear during the ozone measurements, as the IR beam for these measurements passes through the 15 cm 2 outlets of the glass tube. The calibration of the wavenumber scale is carried out via a 1 inch germanium (Ge) etalon which acts as a Fabry-Perot interferometer.

Figure 17 shows the transmission spectrum of ozone generated by the plasma jet with a mixture of argon and 2% of oxygen admixture. On the same graph a simulation of the ozone absorption spectrum is plotted for the same The simulation of the absorption profile is based on the compilation of experimental and theoretical data from the HITRAN database.

The algorithm computing the transmittance spectrum T s is based on Beer-Lambert's law. Equation ( 4) corresponds to the calculation of the transmittance T 1 of only one ro-vibrational transition, where I 1 (ν) is the intensity of this single transition, N is the density of the species, L is the absorption length, S 1 is the transition line strength and f 1 (ν -ν 0 ) is the line shape or function profile of this transition. The implementation of the Doppler broadening-which is very small at atmospheric pressure-as well as the pressure broadening is included in this function profile. According to the concept of line strength, which assumes the Boltzmann distribution, the measure of only one transition enables us to reach the absolute density. However, mainly because of the pressure broadening we cannot measure the transmittance of a single transition. The measured spectrum T m is actually the sum of many transmittances from all the transitions, which are in the range from 1027.18 to 1026.80 cm -1 . In this case, the result of all physical effect is convoluted with the instrumental profile h(νν 0 ); see equation (6).

T 1 (ν) = I 1 (ν) I 0 (ν) = exp[-NLS 1 f 1 (ν -ν 0 )] (4)
T (ν) = n T n (ν) (5) T s (ν) = T (ν) ⊗ h(ν). ( 6
)
During the acquisition of the spectrum T m , the simulation is running and a good fit to the measurement is achieved by optimization of the concentration value. The good agreement of T m and T s shows that the jet in fact produces ozone molecules. This is in contrast to the overview FTIR absorption measurements performed by Pipa et al on a similar, but hotter plasma jet [23] which did not show any sign of ozone.

Determination of the ozone concentration

For the measurement of the concentration, the fitting processing is updated every 5 s and the value of the concentration is recorded. Due to the atmospheric pressure, the broadening of the absorption lines is so significant that the IR radiation is absorbed along the whole observation band. For this reason, the maximal intensity without any absorption is saved in order to be set as baseline for the fitting procedure. The variation of the baseline is less than 0.5% after 1 h, which ensures a good accuracy of the measurements.

In order to reduce the error of the measurement and to investigate the stability of the jet, the concentration was measured over the time period of a few minutes. The data were statistically processed. Figure 18 shows a complete measurement cycle, which contains three phases: during the 2.5 first minutes the jet is not set into the tube, which provides a background measurement with only the residual ozone from the air. Subsequently, the jet is inserted in the cell, resulting in a significant rise of the ozone concentration. After a few minutes, the jet power supply is switched off but the feed gas is kept running in order to keep the same ratio of argon to air inside the tube. This is also an additional control to check the stability of the laser intensity. The concentration is determined as the average of all the measurements (from 3 to 9 min). This average is corrected by the background measurement, which corresponds to the average of the concentration without the jet (from 0 to 2:30 min). The error is then deduced by the sum of the deviations, i.e. the deviation of the background sample and of the measurement sample. For this configuration, the concentration is (93 ± 3.5) × 10 13 cm -3 .

With the above-described setup the influence of oxygen admixture is investigated. Phase-resolved measurements showed that the oxygen admixture causes a reduction of the visible jet length. However, figure 19 shows that the generated ozone concentration increases significantly with increasing oxygen admixture. The production of ozone seems to follow an exponential law and to reach a saturation regime. This will be the topic of a further investigation by a chemical reaction. By the IR-absorption technique we are able to detect and measure that ozone is also produced with a pure argon gas flow into the jet. We have ensured that there is no humidity in the argon flow, which leads to the fact that the only source of oxygen is the surrounding air. The ozone detected in the cell must therefore be a result of dissociation of indiffusing air oxygen. Further investigations, especially of the diffusion mechanism, should be undertaken in the future.

Discussion

Plasma dynamics and reactive species generation

The envisaged use of the investigated plasma jet for biomedical applications makes it necessary to consider the question of correct handling and treatment procedure. Biological material is most sensitive to changes under plasma treatment conditions, therefore an adequate distance measure of the plasma jet from the biological sample is frequently discussed in this context. From a practical point of view the visible jet effluent is often used intuitively as a measure of distance and plasma jet efficacy. However, considering the above results of the ozone concentration and the effluent length in dependence of the molecular oxygen admixture a different picture emerges. When the oxygen admixture is increased the plasma length decreases and the visible effluent intensity drops (see figures 6 and 7). In contrast to this declining behavior the ozone concentration increases. This converse characteristic can be explained by the manifold ways in which molecular species can get excited and form a significant energy or electron sink. In consequence the density of excited atomic argon species, which typically dominate the visible appearance of the visible effluent, rapidly decreases. Since a significant energy fraction is channeled into the molecular oxygen, formation of ozone as a reaction of atomic and molecular oxygen is enhanced. The results of our investigations with phase-resolved optical emission measurements and ozone absorption spectroscopy techniques distinctly show that the presence of reactive species is not necessarily connected to brightness and length of the jet's effluent.

Comparison of the ozone detection techniques

In this study two different ozone detection techniques were employed: UV-absorption spectroscopy using radiation with a wavelength of about 254 nm and tuneable laser-absorption spectroscopy in the mid-IR spectral range at a wavenumber of 1027 cm -1 . In both methods the ozone concentration is obtained by classical absorption technique and subsequent application of Beer's law. In contrast to the UV-absorption measurement, using the IR laser yields a significant higher spectral resolution. A better spectral resolution results in a more accurate measurement of the ozone concentration since a spectrally resolved absorption profile is yielded. This profile can be fitted with a theoretical profile to deduce the concentration. However, in the mid-IR region the absorption coefficient of ozone is much lower than in the UV-spectral range, which complicates absorption measurements on objects with short absorption length and small absorber concentration, as is the case in the jet's effluent. To bypass this disadvantage, the plasma jet was inserted inside a glass vessel and a multipass cell. The first leads to a higher amount of ozone in the direction of the laser beam and the latter significantly increases the absorption length. Together with the high spectral resolution discussed above a very accurate concentration measurement can be performed with this measurement technique. Even if no oxygen is admixed the ozone generation due to oxygen diffusing into the effluent from ambient air is detectable. In this case the measured ozone concentration amounts to (7.7 ± 1.1) × 10 13 cm -3 . This advantage in accuracy, however, goes along with a lack of spatial information, since the measured ozone concentration is the average concentration value inside the glass vessel. Nevertheless an ozone production rate R O 3 of the jet plasma can still be calculated taking into account the total gas flow rate into the glass vessel. Along with the assumption that ozone is not chemically destroyed while it is in the vessel and that the measured average concentration [O 3 ] IR is equal to the concentration at the vessel outflow the ozone production rate in a stationary situation is

R O 3 = [O 3 ] IR • . ( 7
)
The result of the ozone production rate determined from the IR-absorption measurement in dependence on the oxygen admixture is shown in figure 20. In contrast to the length of the effluent and the visible plasma intensity, the ozone production rate increases with increasing oxygen admixture. The highest value is found to be 2.3 × 10 17 s -1 .

The ozone absorption measurement in the UV-spectral range yields space-resolved concentrations. This is the major advantage of this method compared with the used IRtechnique. Since no artificial absorption length enhancement was performed in the UV setup good accuracy was yielded by accumulating over a long time period. However, with the given setup, significant absorption can only be measured when oxygen is admixed to the jet. In order to quantitatively compare both techniques the production rate R O 3 is calculated also for the UV results: the measured ozone concentration [O 3 ] UV (r) at z = 4 mm is multiplied with the velocity profile v(r) of the gas flow. This velocity profile at z = 4 mm was determined The concentration of ozone molecules in the radial direction corresponds to the velocity profile of the gas flow. Mostly only inside the gas flow channel of the jet are an appreciable number of ozone molecules present. This leads to the conclusion that at least for this short nozzle distance the ozone molecules are mainly created inside the plasma region and transported outward by the applied gas flow of the jet.

With both the velocity profile and concentration profile, the production rate

R O 3 = 2π • ∞ 0 v(r) • [O 3 ] UV (r) • r dr (8)

Figure 1 .

 1 Figure 1. Schematics of the argon plasma jet (kINPen09).

Figure 3 Figure 2 .

 32 Figure3shows a side view of the propagating argon effluent as spatial intensity distribution at a specific time of the excitation cycle. The tip of the outer ceramic capillary is illuminated by the discharge and the horizontal white dotted line indicates the position where the emission of the effluent is analyzed phase resolved. Accordingly, all deduced axial intensity profiles of the emission are arranged as color-coded 2D plots over time, shown in figure4. These non-interpolated raw data reveal the

Figure 3 .

 3 Figure 3. Space-resolved wavelength integrated emission profile. The image is false color-coded. Zero denotes the tip of the nozzle. The jet is operated with 5 slm argon and no oxygen admixture. Integration time is 700 ms.

2 Figure 4 .

 24 Figure 4. Phase-and space-resolved wavelength integrated emission profiles in the visible spectral range for different oxygen admixtures over one excitation period T.

Figure 5 .

 5 Figure 5. Plasma bullet velocity dependent on O 2 admixture.

Figure 6 .

 6 Figure 6. Variation in effluent length dependent on O 2 admixture.

Figure 7 .

 7 Figure 7. Dependence of the maximum emission intensity of the effluent on O 2 admixture.

Figure 8 .

 8 Figure 8. Experimental setup for the spatial-resolved ozone concentration measurement. LS: UV-light source, L: lens, AP: aperture, CL: cylinder lens, IF: interference filter.

Figure 9 .

 9 Figure 9. Spatial line-of-sight optical depth of ozone in the vicinity of the jet effluent resulting from the UV-absorption measurement. Due to plasma-induced UV beam distraction no reliable data are obtained for nozzle distances below 4 mm. On the left-hand side of the figure a scaled photograph of the jet and the effluent is shown.

Figure 10 .

 10 Figure 10. Radial profile of the line-of-sight optical depth measured at a nozzle distance of z = 2 mm.

Figure 11 .

 11 Figure 11. Radial profile of the line-of-sight optical depth measured at a nozzle distance of z = 8 mm and z = 20 mm. Both profiles were fitted with a Gaussian function.

Figure 12 .

 12 Figure 12. Spatial ozone concentration in the vicinity of the jet nozzle as a result of Abel transformation.

Figure 13 .

 13 Figure 13. Axial ozone concentration distribution at r = 0 mm.

Figure 14 .

 14 Figure 14. Experimental setup. QCL: quantum cascade laser, Pa.M: parabolic mirror, Pl.M: plan mirror, S.M: spherical mirror.

  This is the Accepted Manuscript version -post-print-of an article accepted for publication in Plasma Sources Science and Technology. This article may be downloaded for personal use only. Any other use requires prior permission of the authors and IOP Publishing. This article may be found at doi:10.1088/0963-0252/21/3/034015. IOP Publishing is not responsible for any errors or omissions in this version of the manuscript or any version derived from it.

Figure 15 .

 15 Figure 15. Ozone line strength values retrieved from the HITRAN database [21].

Figure 16 .

 16 Figure 16. Identification and wavenumber calibration using an OCS gas reference cell together with a spectrum measured with a germanium etalon.

Figure 17 .

 17 Figure 17. Detection of ozone molecules. Measured spectrum and simulation by Q-MACSoft HT [20] based on HITRAN database [21].

Figure 18 .

 18 Figure 18. Record of a full measurement procedure. The jet is operated with argon and 2% of oxygen admixture.

Figure 19 .

 19 Figure 19. Ozone concentration measured for different ratios of oxygen to argon.

Figure 20 .

 20 Figure 20. Ozone production rate of the investigated plasma jet in dependence on oxygen admixture determined with the IR-absorption technique (open circles) and UV-absorption technique (full circle).

Figure 21 .

 21 Figure 21. Radial gas velocity profile and ozone concentration profile at a nozzle distance of z = 4 mm.

Acknowledgments

The authors thank Dr Andrei V Pipa for the discussions about IR spectroscopy and Dipl. Ing. Henrik Zimmermann for the technical support concerning the computational process and the Q-MACS implementation. The authors gratefully acknowledge the funding by the German Ministry of Education and Research (BMBF, grant number 03Z2DN12).

This is the Accepted Manuscript version -post-print-of an article accepted for publication in Plasma Sources Science and Technology. This article may be downloaded for personal use only. Any other use requires prior permission of the authors and IOP Publishing. This article may be found at doi:10.1088/0963-0252/21/3/034015. IOP Publishing is not responsible for any errors or omissions in this version of the manuscript or any version derived from it.

Plasma Sources Sci. Technol. 21 (2012) 034015 S Reuter et al is calculated. The result is shown as a full circle in figure 20 for the applied oxygen admixture of 0.5%. Reasonably good agreement is found when comparing the value resulting from the UV-absorption measurement to that obtained from the IRabsorption technique. The ozone production rates, which are supposed to be equal for both experiments, differ only by a factor of 1.3. The reason for lower production rates in the IR-absorption measurement may be the destruction of ozone molecules at the vessel wall, which are not considered in the calculation of R O 3 . This would be an additional loss term for ozone and in consequence lead to a lower measured production rate.

Summary

A cold atmospheric pressure argon plasma jet (kINPen) was investigated by optical and spectroscopic methods. Phaseresolved optical emission spectroscopy showed that the argon plasma jet is in fact a (1 MHz) bullet plasma jet. This is the first time that plasma bullets have been reported in a MHz argon atmospheric pressure plasma jet.

The time-and space-resolved measurements showed that the jet effluent length decreases with higher oxygen admixture, as can be expected. It is noted that the bullet velocity strongly depends on the oxygen admixture. The traveling velocity of the bullets in the order of 10 4 m s -1 decreases with rising oxygen content in the feed gas.

The ozone density of the plasma jet was investigated by two complementary techniques: UV-absorption spectroscopy with high special resolution and mid-IR absorption spectroscopy with high sensitivity. Although the dynamic measurements show that the visible effluent length and intensity decrease, the obtained ozone production of the jet increases up to the maximum investigated molecular oxygen admixture of 2%. A correlation of both complementary diagnostic techniques via the ozone production rate of the jet shows excellent agreement.