Etienne Girel

Amandine Cabiac

Alexandra Chaumonnot

Michele Besson

Alain Tuel

Selective carbon deposition on γ-alumina acid sites: towards the design of catalyst supports with improved hydrothermal stability in aqueous media

Keywords: γ-alumina boehmite transformation, hydrothermal decomposition, sorbitol, adsorption, pyrolysis, carbon

γ-alumina, a widely used industrial catalyst support, undergoes irreversible transformation into various aluminum hydroxides under hydrothermal conditions, resulting in strong modification of its intrinsic properties. Most of the strategies that have been proposed to prevent or at least minimize its transformation into oxy-hydroxides consist in covering the alumina surface by a hydrophobic carbon layer, making it less sensitive to modifications induced by water. However, such methods necessitate high carbon contents, which significantly modifies structural and chemical properties of alumina. Here, we propose a new method based on a series of adsorption/pyrolysis cycles using sorbitol molecules previously adsorbed on specific hydration sites of (110) faces of γ-alumina crystals. Those sites, which are responsible for the dissolution f γ-alumina crystals in water, are thus selectively protected by carbon clusters, the rest of the surface being totally exposed and accessible to adsorbates. Under hydrothermal conditions (10 hours in water at 200°C), the formation of hydroxides is almost totally suppressed by covering less than 25% of the surface with only 7 wt. % carbon, which is far below the amount necessary to get similar results with more conventional carbon deposition methods.

 INTRODUCTION

Molecules derived from existing biomass treatment processes such as fermentation and hydrolysis (sugars, alcohols, polyols, carboxylic acids, etc.) are platform molecules for bio-based products manufacture. They consist of oxygen-rich, highly functionalized and generally water soluble molecules and reactions to convert them into valuable bio-products in aqueous media may require relatively high temperatures and sometimes high pressures. These particular conditions, which combine water and temperature, are called hydrothermal conditions (HT) and they represent a serious challenge in the field of today's heterogeneous catalysis. [START_REF] Fiorentino | Chemicals from Biomass: Technological versus Environmental Feasibility. A review. Biofuels[END_REF][START_REF] Xiong | Hydrothermally Stable Heterogeneous Catalysts for Conversion of Biorenewables[END_REF] Indeed, conventional catalysts used in the petroleum refining industry generally consist of an active metal phase deposited onto an inorganic support and they may not be appropriate for applications involving water as solvent under such HT conditions. For examγ-alumina, a widely used industrial catalyst support, undergoes irreversible transformation into various aluminum hydroxides by a dissolution/re-precipitation process when placed in a water-rich medium at moderate temperature, resulting in strong modification of its intrinsic properties. [START_REF] Abi Aad | Chemical Weathering of Alumina in Aqueous Suspension at Ambient Pressure: A Mechanistic Study[END_REF][START_REF] Carrier | Transformations of Gamma-alumina in Aqueous Suspensions 1. Alumina Chemical Weathering studied as a function of pH[END_REF][START_REF] Ravenelle | Structural C f γ-Al2O3 -Supported Catalysts in Hot Liquid Water[END_REF][START_REF] Mironenko | Effect of γ-Al2O3 Hydrothermal Treatment on the Formation and Properties of Platinum Sites in /γ-Al2O3 Catalysts[END_REF][START_REF] Réocreux | Structuration and Dynamics of Interfacial Liquid Water at Hydrated γ-Alumina Determined by Ab Initio Molecular Simulations: Implications for Nanoparticle Stability[END_REF][START_REF] Ngouana-Wakou | An Atomistic D f γ-Alumina/Water Interface revealed by Ab Initio Molecular Dynamics[END_REF] The stability of alumina materials in water has been a matter of great interest during the last years and many strategies have been proposed to prevent or at least minimize their transformation into hydroxides. The approach generally consists in modifying the alumina surface in order to make it less sensitive to the modifications produced by water under hydrothermal conditions. The benefits of organic additives (such as carbon [START_REF] Pham | Improved Hydrothermal Stability of Mesoporous Oxides for Reactions in the Aqueous Phase[END_REF][START_REF] Xiong | Graphitic-Carbon Layers on Oxides: Toward Stable Heterogeneous Catalysts for Biomass Conversion Reactions[END_REF] ), inorganic additives (such as silicon [START_REF] Abi Aad | Inhibition by Inorganic D f γ-Alumina Chemical Weathering under Hydrothermal Conditions: Identification of Reactive Sites and their Influence in Fischer-Tropsch Synthesis[END_REF][START_REF] Liu | Silica Deposition as an Approach for Improving the Hydrothermal Stability of an Alumina Support during Glycerol Aqueous Phase Reforming[END_REF] or phosphorus [START_REF] Van Cleve | Enhanced Hydrothermal S b f γ-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings[END_REF] ) and metallic additives [START_REF] Ravenelle | Effects of metal precursors on the stability and observed reactivity of /γ-Al2O3 catalysts in aqueous phase reactions[END_REF] on the HT stability of alumina have been reported. However, the main parameters that govern the dissolution of alumina remain poorly known, which makes difficult the development of rational methods to improve the stability of this material. Ravenelle and coworkers first noticed that alumina transformation did not occur when polyols were present in the aqueous medium during a hydrothermal process. [START_REF] Ravenelle | b f /γ-Al2O3 Catalysts in Model Biomass Solutions[END_REF][START_REF] Jongerius | Stability of /γ-Al2O3 Catalysts in Lignin and Lignin-Model Compound Solutions under Liquid Phase Reforming Reaction Conditions[END_REF] The effect is supposed to result from the chemisorption of the organic molecules on alumina, which avoids direct contact between water molecules and the surface and significantly improves water resistance. [START_REF] Copeland | Surface Interactions of Glycerol with Acidic and Basic Metal Oxides[END_REF] This result can be compared with the work of Pham et al. who increased alumina stability in water by covering its surface with an oxygenated carbonaceous layer resulting from sucrose pyrolysis at 400°C. [START_REF] Pham | Improved Hydrothermal Stability of Mesoporous Oxides for Reactions in the Aqueous Phase[END_REF] They showed that 10 wt. % of carbonaceous deposit on the alumina surface was enough to prevent the transformation of the solid into boehmite. In another study, the same group used methane as a carbon source to produce graphitic carbon on alumina surface by gas phase carbonization. In this case, a higher amount of carbon was required to effectively protect alumina (35 wt. %), leading to a complete modification of its original properties (textural properties and surface chemistry). [START_REF] Xiong | Graphitic-Carbon Layers on Oxides: Toward Stable Heterogeneous Catalysts for Biomass Conversion Reactions[END_REF] Despite a growing number of articles dedicated to the improvement of alumina based materials in hydrothermal conditions, rationalization of predominant parameters that control hydrothermal stability remain unclear. In a previous work, we proposed a mechanism of the first steps of alumina dissolution based on experimental and theoretical approaches. [START_REF] Réocreux | Reactivity of Shape-Controlled Crystals and Metadynamics Simulations Locate the Weak Spots of Alumina in Water Nat[END_REF] This mechanism involves some specific surface sites located on basal (110) planes of alumina platelets which are particularly reactive towards liquid water. We also showed that sorbitol and xylitol preferentially adsorb on these specific sites of the alumina surface, which explains the effect observed by Ravenelle and al. when alumina was treated in the presence of polyols in solution. [START_REF] Ravenelle | b f /γ-Al2O3 Catalysts in Model Biomass Solutions[END_REF][START_REF] Jongerius | Stability of /γ-Al2O3 Catalysts in Lignin and Lignin-Model Compound Solutions under Liquid Phase Reforming Reaction Conditions[END_REF] Herein, we propose /γalumina composites using adsorbed polyol molecules as carbon source. The synthesis route makes it possible to control and optimize some of the critical parameters that affect the transformation of alumina in water. We discuss the importance of carbon surface coverage and its localization on the alumina surface on the stability of the solid. The advantages of this new synthesis route are demonstrated by comparing the obtained composites with those obtained using more conventional methods of the literature. As a proof of concept, this preliminary work mainly concerns the stability of the support, which is the basement for the preparation of stable supported catalysts.

 EXPERIMENTAL SECTION

Material synthesis γ-alumina was obtained by calcination under air of a commercial boehmite (Sasol PURAL SB) at 600°C for 4 h with a heating rate of 5°C/min. b /γ-alumina composites were synthesized using 3 different methods: incipient wetness impregnation/pyrolysis of sucrose or sorbitol, chemical vapor deposition of ethanol and adsorption/pyrolysis of sorbitol. Incipient wetness impregnation: materials were synthesized following the protocol described by Lin and al. [START_REF] Lin | Uniformly Carbon-Covered Alumina and its Surface Characteristics[END_REF] An aqueous solution of sucrose or sorbitol with various concentrations was impregnated on 2 g of alumina powder, the material was dried at ambient temperature overnight and heated at 400°C or 600°C under nitrogen for 1 h with a 5°C/min heating rate. CVD of ethanol: 8 g of alumina powder were placed in a tubular reactor equipped with an ethanol saturator. After 15 min at 600°C under pure nitrogen flux (50 mL/min), the gas flow was passed through the ethanol saturator maintained at 30°C (see scheme in ESI). After a given reaction time, the reactor was cooled down under nitrogen and the solid was recovered. Adsorption/pyrolysis: 15 g of alumina were dispersed in 100 mL of an aqueous solution of sorbitol (75 g/L). After 2 h at 200°C under autogenous pressure (14 bar) and vigorous stirring, the autoclave was cooled at room temperature and the solid phase was recovered by centrifugation, dried in an oven at 100°C overnight and heated at 600°C under nitrogen for 1h with a 5°C/min heating rate. This process was then repeated using the same material, sorbitol concentration was adjusted with the mass of material treated (see main text for details). Hydrothermal test. The hydrothermal stability of the different materials was evaluated through an on purpose hydrothermal b . T 2 f γ-alumina were dispersed in 50 mL of deionized water. The mixture was heated at 200°C for 10 h under mechanical stirring and autogenous pressure in a 100 mL stainless steel autoclave. After cooling at ambient temperature, the powder was recovered by centrifugation and dried à 100°C overnight. The same procedure was adopted for the sorbitol adsorption experiments except that aqueous solutions of sorbitol with different concentrations were used in-stead of pure water. In this case, the liquid phase was systematically analyzed after each treatment to determine the final sorbitol concentration. Solution Analysis. Sorbitol concentrations were determined by HPLC analysis using a Shimadzu LC20A instrument connected to a differential refraction detector (Shimadzu RID10A) and equipped with a Rezex RXM-Monosaccharide Ca2+ 8% column (7.8 mm x 300 mm heated at 60°C). The amount of polyol adsorbed on the alumina surface was calculated by the difference between initial and final concentrations w x m f γ-alumina (% polyol adsorbed = mpolyol adsorbed/malumina * 100).

Characterization techniques

Powder X-ray diffraction (XRD) analyses were performed ff m ( AN X O M ) Kα . ff mm w obtained within the 4-74° 2θ range with a step of 0.033° and 5 s per step. The fraction of boehmite phase in a solid was estimated by integrating the peak at 14.5° corresponding to the (020) reflection of AlOOH in the corresponding XRD pattern. A calibration curve was first γ-alumina/boehmite mechanical mixtures (See Figure S1 Textural properties of materials were determined by N 2 sorption studies at -196°C using a Micromeritics ASAP 2000 instrument. The BET method was applied to determine the specific surface area. Prior to the analysis, samples were dried at 150°C for 6 h under primary vacuum.

Transmission electron microscopy (TEM) images were obtained on a JEOL 2010 LaB6 microscope operating at 200kV. A dispersion of the sample crushed in ethanol was deposited on standard holey carbon-covered copper TEM grids.

The nature of the carbon deposits in sorb /γ-alumina materials was studied by [START_REF] Van Cleve | Enhanced Hydrothermal S b f γ-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings[END_REF] C solid-state nuclear magnetic resonance (NMR) on an Avance II 500 WB spectrometer (B0 magnetic field of 11,7 T). Magic angle spinning (MAS) spectra were recorded at 10 kHz using a single pulse sequence with 60 s recycle delay. 1 H- [START_REF] Van Cleve | Enhanced Hydrothermal S b f γ-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings[END_REF] C CP/MAS spectra were acquired at the same spinning speed using a classical cross-polarization (CP) technique with 2 ms contact time and 10 s delay.

TGA analyses were performed using a Mettler TGA851 thermobalance. The carbon content was calculated by integration of the mass loss between 200 and 750°C corrected with m f γ-alumina (see Figures S2 andS3, ESI). Raman spectra were acquired on a Renishaw spectrometer equipped with a laser emitting a 532 nm.

The proportion of f w b /γalumina composites was estimated by ethanol (EtOH) adsorption followed by TGA analysis. This characterization method was adapted from the work of Caillot and al [START_REF] Caillot | Quantification of Brønsted Acid Sites of Grafted Amorphous Silica-Alumina Compounds and their Turnover Frequency in m -Xylene Isomerization[END_REF] and of Kwak and al. [START_REF] Kwak | f f γ-Al2O3: The Active Surfaces for Alcohol Dehydration Reactions[END_REF] The amount of EtOH that remains adsorbed on the composite after surface saturation followed by desorption under inert atmosphere corresponds to EtOH on free alumina since the interaction of the alcohol with carbon is very weak. In contrast, alumina surface strongly adsorbs EtOH and most of the molecules adsorbed cannot desorb under a simple flow of inert gas without increasing temperature (Figure S4, ESI).

 RESULTS AND DISCUSSION

Stability of γ-alumina in water. T k f γ-alumina transformation under hydrothermal conditions was first investigated. Figure 1 shows the percentage of AlOOH at 200°C for different periods, estimated from the corresponding XRD patterns using the relationship plotted in Figure S1 (see ESI). The transformation was complete after 90 min, which is considerably less than the duration reported by Ravenelle et al. [START_REF] Ravenelle | Stability of Zeolites in Hot Liquid Water[END_REF] (10 h) but in agreement with a more recent work of the same team [START_REF] Van Cleve | Enhanced Hydrothermal S b f γ-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings[END_REF] (Figure S5,ESI). This difference may result from the different nature of the γ-alumina material and from the conditions used for the treatm ( m f …). Abi Aad et al. have shown that the kinetics of alumina dissolution depends on its specific surface area and crystal structure (allotropic form). [START_REF] Abi Aad | Chemical Weathering of Alumina in Aqueous Suspension at Ambient Pressure: A Mechanistic Study[END_REF] Despite those differences, the duration of the HT treatment was kept at 10 h, which is long enough to estimate the stability of pure and modified alumina materials.

The specific surface area was much affected by the HT treatment: it first increased from 212 to 272 m²/g during the first 15 minutes and then drastically decreased to 45 m²/g (Figure 1). These observations are once again in good agreement with previous works. [START_REF] Ravenelle | Structural C f γ-Al2O3 -Supported Catalysts in Hot Liquid Water[END_REF][START_REF] Abi Aad | Inhibition by Inorganic D f γ-Alumina Chemical Weathering under Hydrothermal Conditions: Identification of Reactive Sites and their Influence in Fischer-Tropsch Synthesis[END_REF] The initial evolution has been attributed to an increase of the roughness of the alumina surface due to the formation of AlOOH micro domains. As these micro domains grow, the morphology of the particles changes as it can be observed on TEM pictures (Figure 2) and the f . W f γ-alumina is composed of aggregated nano platelets with an average size of 8 nm, AlOOH crystallites are much larger and they can reach several hundreds of nanometers. This modification in size and morphology corresponds to the textural modifications discussed before, in particular the evolution of the BET surface area. The same phenomena were also observed by Jun Cheng and al. using MEB analysis. [START_REF] Jun-Cheng | Effect of Hydrothermal Treatment on the Acidity D b f γ-Al2O3 Support[END_REF] U k b m → m transition upon calcination, the γ-m → b m sformation is not topotactic, i.e. the crystal lattice of boehmite does not show any relationships to the crystal lattice of γalumina. [START_REF] Margulies | Molecular Mobility of Sorbitol and Maltitol: A 13 C NMR and Molecular Dynamics Approach[END_REF][START_REF] Rohrbaugh | Desciptions of Molecular Shape Applied in Studies of Structure/Activity and Structure/Property Relationships[END_REF] The mechanism of this transformation has already been discussed by Carrier and al. [START_REF] Carrier | Transformations of Gamma-alumina in Aqueous Suspensions 1. Alumina Chemical Weathering studied as a function of pH[END_REF] and Abi Aad et al. [START_REF] Abi Aad | Chemical Weathering of Alumina in Aqueous Suspension at Ambient Pressure: A Mechanistic Study[END_REF][START_REF] Abi Aad | Inhibition by Inorganic D f γ-Alumina Chemical Weathering under Hydrothermal Conditions: Identification of Reactive Sites and their Influence in Fischer-Tropsch Synthesis[END_REF] It was proposed to be a dissolution/recrystallization process, which is in good agreement with our observations. Other textural properties were also modified during the HT treatment. The shape of hysteresis loops in N 2 sorption isotherms changed, revealing a deep modification of the porous structure, in particular the BJH pore diameter distribution (Figure S6, ESI). The porous volume decreased from 0.55 to 0.32 mL/g and the mesopore population with a mean diameter around 8 nm vanished. [START_REF] Ravenelle | b f /γ-Al2O3 Catalysts in Model Biomass Solutions[END_REF] . They attributed it to a strong multidentate and irreversible adsorption of the polyol, creating a protective organic layer on the alumina surface. In our work, γ-alumina was first treated with a solution of sorbitol (25 g/L), then recovered by centrifugation and dried. As expected, the corresponding XRD patter w f γalumina material, thus confirming the protective role of sorbitol molecules (Figure 3). The solid was then submitted to a second hydrothermal treatment in pure water. Weak additional peaks corresponding to traces of AlOOH were observed by X b /γ-alumina interaction since a part of sorbitol adsorbed was not removed from the surface even when no molecule was present in solution. However, the appearance of a small fraction of boehmite indicates that adsorption was not fully irreversible and that the material would likely become less and less stable upon multiple HT treatment in pure water. After any HT treatments in the presence of sorbitol, sorbitol was the only product detected in the liquid phase, indicating that the molecule remained intact and did not transform at 200°C. This was confirmed by [START_REF] Van Cleve | Enhanced Hydrothermal S b f γ-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings[END_REF] C solid-state NMR: the spectrum of the solid after sorbitol adsorption showed two bands at ca. 64 and 73 ppm with an intensity ratio of 1:2 (Figure S7, ESI). [START_REF] Margulies | Molecular Mobility of Sorbitol and Maltitol: A 13 C NMR and Molecular Dynamics Approach[END_REF] Chemical shifts and relative intensities are the same as those observed for sorbitol, indicating that the molecule retains its molecular structure upon adsorption. [START_REF] Margulies | Molecular Mobility of Sorbitol and Maltitol: A 13 C NMR and Molecular Dynamics Approach[END_REF] The relatively broad signals indicate that sorbitol molecules are immobilized on the surface of alumina with a reduced mobility. T m m m b b γalumina was estimated by performing a series of HT treatments in solutions with concentrations in the range of 0.25-15 g/L. The amount of boehmite formed decreased as polyol concentration increased and a total stability of the alumina was obtained for concentrations between 2 and 4 g/L (Figure S8, ESI). [START_REF] Réocreux | Reactivity of Shape-Controlled Crystals and Metadynamics Simulations Locate the Weak Spots of Alumina in Water Nat[END_REF] The amount of sorbitol adsorbed on the solid (determined by HPLC analysis of the equilibrated solution) first increased rapidly with the concentration to reach about 2 wt. % at C = 2 g/L (Figure 4). In this region, there is a direct correlation between the amount of sorbitol adsorbed and the amount of AlOOH formed. Above 2 g/L, the amount of sorbitol adsorbed increased more slowly to reach 3wt. % at C = 15 g/L. This suggests that primary adsorption sites become saturated at concentrations between 2 and 4 g/L and that sorbitol adsorbed at higher concentrations is no more necessary to fully protect alumina from dissolution. Assuming that sorbitol molecules adsorb in their linear conformation with OH groups in threo configuration directed towards the alumina surface, the projected surface area of one molecule is estimated to 60 A² (calculated with Molecular Shadow software using the methodology developed by Rohbaurd and al). [START_REF] Rohrbaugh | Desciptions of Molecular Shape Applied in Studies of Structure/Activity and Structure/Property Relationships[END_REF] A concentration of 2 wt. % of sorbitol adsorbed (at C = 4 g/L) corresponds to a density of 0.33 sorbi-/ m² f b 2 % f f f γalumina crystals. Since boehmite is not formed under such conditions, it seems that the surface does not require to be fully covered with sorbitol to efficiently protect alumina from dissolution. This suggests that sorbitol adsorbs preferentially on specific adsorption sites, which are those whence dissolution process starts. Identification and description of those specific adsorption sites have been discussed in our previous work. [START_REF] Réocreux | Reactivity of Shape-Controlled Crystals and Metadynamics Simulations Locate the Weak Spots of Alumina in Water Nat[END_REF] We showed that polyols adsorb on two neighboring octahedral aluminum atoms localized on (110) basal surface planes of alumina platelets, in good agreement with the work of Copeland and al. regarding glycerol adsorption. [START_REF] Copeland | Surface Interactions of C2 and C3 P w γ-Al2O3 and the Role of Coadsorbed Water[END_REF] This result can be compared with the work of Watling and al. who discussed the adsorption of polyols on gibbsite crystals and suggested a selective adsorption on specific surface sites. [START_REF] Watling | Gibbsite Crystallization Inhibition 2. Comparative Effects of Selected Alditols and Hydroxycarboxylic Acids[END_REF] New design of C/alumina materials. A b /γalumina composites are stable under HT conditions, they cannot be used as catalyst supports for long-term reactions, in particular if sorbitol is absent from the reaction medium (Fig-

). b /γ-alumina cannot withstand high temperature treatments (calcination or pyrolysis) because of polyol thermal degradation. To overcome those limitations, we m b /γ-alumina composite by pyrolysis of polyol molecules selectively adsorbed on sites of γ-alumina involved in dissolution mechanism. Assuming that carbon is formed locally where polyol molecules are adsorbed, the thermal treatment (pyrolysis) would lead to a composite in which carbon is located on -or at least close to -the hydration sites. Thus, we expected to produce a material with carbon deposited specifically on alumina primary hydration sites. The /γ-alumina composite, obtained by pyrolysis at 600°C of a sample prepared by adsorption of sorbitol using a highly concentrated solution (75 g/L) contained 1.5 wt. % C, showed very low hydrothermal stability under HT conditions with more than 95% of boehmite formed after the treatment. The pyrolysis process is accompanied with the release of oxygen atoms (mostly as H 2 O molecules). Carbon formation certainly involves condensation of several sorbitol molecules, so the surface occupied by the resulting pyrolytic carbon cannot be as high as the one occupied by sorbitol. As a consequence, some of the adsorption sites that were protected by sorbitol molecules become exposed again after the pyrolysis treatment and the resulting material shows a poor stability. The amount of carbon, and therefore its surface coverage, was increased by repeating the adsorption/pyrolysis process several times on the same material. Between two consecutive cycles, the experimental process was adapted to keep the sorbitol/alumina ratio constant and particular attention was paid to saturate all adsorption sites after each process by keeping sorbitol concentration higher than the minimal required for a full protection of alumina. TGA derivative curves of sorbitol/alumina composites obtained from a series of successive cycles before and after pyrolysis show two major weight losses: a first one between 200 and 400 °C corresponding to the degradation of adsorbed sorbitol into CO 2 + H 2 O and a second one between 400 and 550°C attributed to the combustion of carbon formed upon pyrolysis (Figure 5). The amount of carbon formed increases with the number of cycles as one of adsorbed sorbitol decreased. After 7 adsorption/pyrolysis cycles, the amount of carbon formed became high enough to completely prevent sorbitol adsorption (figure 5, cycle 8) and the carbon content no longer increased (Figure 6). This solid showed a very low sorbitol adsorption capacity, indicating that most of the adsorption sites had been covered with carbon.

Figure 6

Carbon content of materials prepared by successive sorbitol adsorption/pyrolysis cycles Both [START_REF] Van Cleve | Enhanced Hydrothermal S b f γ-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings[END_REF] C NMR and Raman spectroscopies showed that the carbon formed after 10 adsorption/pyrolysis cycles is semigraphitic, not functionalized with oxygen and most likely chemically inert. [START_REF] Ferrari | Resonant Raman Spectroscopy of Disordered, Amorphous, and Diamond-like Carbon[END_REF] The NMR spectrum of the solid showed a single signal at 125 ppm characteristic of aromatic carbons (Figure 7-a). The G (graphitic) band of the Raman spectrum, which arises from the elongation vibration of sp 2 carbons atoms and is located around 1580 cm -1 in pure graphitic carbon, is shifted to 1600 cm -1 , indicating the presence of aromatics, polycyclic or/and olefinic species (Figure 7-b). The D (Default) band results from vibrations of defects in the graphitic structure (like sp 3 carbon atoms). [START_REF] Ferrari | Resonant Raman Spectroscopy of Disordered, Amorphous, and Diamond-like Carbon[END_REF] The intensity ratio of both bands (ID/IG) is often used to characterize the chemical structure of carbon materials: the highest the ratio, the less crystallized the carbon. In the case of carbon formed after 10 adsorption/pyrolysis cycles, a value of 0.68 was obtained, which clearly characterizes a semi-graphitic carbon. [START_REF] Xiong | Graphitic-Carbon Layers on Oxides: Toward Stable Heterogeneous Catalysts for Biomass Conversion Reactions[END_REF][START_REF] Ferrari | Resonant Raman Spectroscopy of Disordered, Amorphous, and Diamond-like Carbon[END_REF] Figure 7 [START_REF] Van Cleve | Enhanced Hydrothermal S b f γ-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings[END_REF] C NMR (a) and Raman (b) spectra of a C/γ-Al 2 O 3 composite obtained after 10 adsorption/pyrolysis cycles. Asterisks in the NMR spectrum indicate spinning side-bands This contrasts with data from Pham et al. who showed that the carbon formed from sucrose pyrolysis at 400°C is still highly functionalized with oxygen and remains reactive during a HT treatment at 200°C in liquid water. [START_REF] Pham | Improved Hydrothermal Stability of Mesoporous Oxides for Reactions in the Aqueous Phase[END_REF] The average pore diameter, surface area and pore volume of composites prepared by sorbitol adsorption/pyrolysis at 600°C decreased with the carbon content and seemed to stabilize after 7 cycles (Table The small increase of the BET specific surface area could be attributed to the formation of AlOOH micro domains in agreement with Ravenelle et al. observations. [START_REF] Ravenelle | Structural C f γ-Al2O3 -Supported Catalysts in Hot Liquid Water[END_REF] The percentage f b m f m f HT m f /γ-alumina composites obtained by sorbitol adsorption/pyrolysis cycles decreased with the carbon content to a limit of 12-13% after 7 cycles (Table 2, Figure 7 and Figure S11, ESI). Aqueous solutions recovered after treatments did not contain any trace of organic species, indicating that the stability of the solids was not due to sorbitol molecules desorbed in solution.

Moreover, the instability could not be attributed to the fraction f f γ-alumina surface. Indeed, no correlation could be found between the amount of AlOOH formed and the free alumina surface, samples being very stable despite a high proportion of surface accessible to EtOH (Table 3). A solid prepared using a unique impregnation/pyrolysis step and hav-ing similar carbon content and free accessible surface gave 85 wt. % boehmite under the same conditions (Table 3 and Figure 7). This shows the importance of the multi-step process to obtain a material with a specific location of carbon species on γ-alumina surface and thus a high HT stability.

Comparison of C/γ-alumina composites obtained following different preparation methods.

Impregnation/pyrolysis of sucrose. /γ-alumina composites have been prepared by impregnation of sucrose solutions followed by pyrolysis. Two main synthesis parameters have been considered: the pyrolysis temperature that influences the chemical nature of the carbon deposit and the sucrose concentration that determines the final carbon content. Concerning the pyrolysis temperature, Pham et al. have reported that alumina could be protected under hydrothermal conditions by addition of 10 wt. % carbon formed by pyrolysis of sucrose at 400°C. At this temperature, the carbon phase formed remained highly oxygenated and functionalized. [START_REF] Pham | Improved Hydrothermal Stability of Mesoporous Oxides for Reactions in the Aqueous Phase[END_REF] Other works related to the synthesis of carbon/alumina composites used higher pyrolysis temperatures but these studies were not devoted to the stability of materials in aqueous phase. [START_REF] Caillot | Quantification of Brønsted Acid Sites of Grafted Amorphous Silica-Alumina Compounds and their Turnover Frequency in m -Xylene Isomerization[END_REF][START_REF] Zheng | Carbon-Covered Alumina: a Superior Support of Noble Metal-like Catalysts for Hydrazine Decomposition[END_REF][START_REF] He | Effect of Carbon Addition on the Pt-/γ-Al2O3 Catalyst for Long Chain Paraffin Dehydrogenation to Olefin[END_REF] In the present The second parameter concerns the amount of carbon formed at the surface of the alumina. In a first step, a minimal carbon content was determined considering a theoretical monolayer of graphite deposited on the entire surface of the alumina. This content was estimated to 0.16 g of carbon per gram of alumina (ie 14 wt. % of the composite). This theoretical content represents an ideal case where carbon would be in graphitic form and homogeneously distributed over the entire surface of the alumina.

In a second step, another set of samples was prepared using twice the concentration to achieve a higher carbon content. Samples are denoted CA-X-Y where CA means carbon/alumina, X is the pyrolysis temperature (in °C) and Y the carbon content after pyrolysis (in wt. % of the composite).

Their chemical and structural properties are listed in Table 4. For samples CA-400-11 and CA-600-9 prepared with a sucrose concentration equivalent to a theoretical graphite monolayer (14 wt. %), the resulting carbon content is always lower than expected, suggesting that part of the impregnated carbon is released during the pyrolysis step, probably in the form of light hydrocarbons and/or CO 2 /CO mixtures. The same trend is observed for samples prepared with twice the amount of sucrose but the final carbon content is always above 14 wt. % which indicates that the surface would be totally covered. For the two concentrations, the carbon content decreases with the pyrolysis temperature. The f m f /γ-alumina composites f ff f m f γ-alumina support (Table 4). In contrast, a significant decrease of the porous volume is observed for both pyrolysis temperatures. At low carbon contents, N 2 adsorption/desorption isotherms are m f γ-alumina and characteristic of mesoporous materials (Figure S12, ESI). For higher contents, a hysteresis loop with closure at p/p0 = 0.45 is observed, indicating that large pores are partially blocked and accessible only via entrances smaller than 4 nm. 33 The presence of an additional micro/mesoporosity (Table 4) suggests that the carbon formed upon pyrolysis is porous and partially blocks the large mesopores of γ-alumina. This is clearly evidenced by the almost complete disappearance of the original pore population at 8-9 nm in pore size distribution curves (Figure S12, ESI). [START_REF] Van Cleve | Enhanced Hydrothermal S b f γ-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings[END_REF] C solid-state NMR and Raman spectroscopies confirmed that carbon formed at 400°C still contained oxygenated species, resulting from an incomplete degradation of sucrose (Figures S13 andS14, ESI). In contrast samples prepared at 600°C showed all characteristics of semi-graphitic type carbon.

CVD of ethanol.

b /γ-alumina composites are also very often prepared using a light gas phase precursor as a carbon source, for example ethanol. This precursor molecule has already been used in the literature to synthesize carbon/zeolite and carbon/titanium oxide composites, but not carbon on alumina to our knowledge. [START_REF] Kim | Ethanol-Based Synthesis of Hierarchically Porous Carbon using Nanocrystalline Beta Zeolite Template for High-Rate Electrical Double Layer Capacitor[END_REF][START_REF] Janus | Carbon-Modified TiO2 Photocatalyst by Ethanol Carbonisation[END_REF] A series of five samples with a carbon content ranging from 2 to 29 wt. % was prepared at 600°C by varying the reaction time (1, 2, 4, 6 and 12 hours respectively). They are denoted CA-CVD-X, where CA stands for carbon/alumina, CVD refers to the preparation method and X is the carbon content in the composite (wt. %) (Table 5). The carbonization yield, which corresponds to the mass of carbon deposited compared to the quantity of vaporized ethanol (reduced to the mass of carbon contained in ethanol), is between 4 and 6% depending on the sample, indicating that the major part of ethanol present in the effluent gas does not react with alumina or does not lead to carbon formation. However, this synthesis route enables an easy control of the final carbon content in the materials since the mass or carbon formed is directly proportional to the duration of the reaction (Figure 8). Both the specific surface area and pore volume significantly decrease with carbon content (Table 5). Changes become particularly important for the highest carbon content (CA-CVD-29) for which the specific surface area drops by 40% and the pore volume is decreased by a factor of 2.5 as compared to alumina. For this sample, the nitrogen desorption and average pore diameter distribution curves suggest a strong pore clogging with the appearance of the artifact at 3.5 nm already observed in samples prepared by sucrose pyrolysis but containing much less carbon (Figure S15, ESI). It is also noted that no microporosity was detected in these samples. The chemical nature of the carbon phase is quite similar to that obtained after pyrolysis of sucrose at 600°C, the Raman spectrum of CA-CVD-29 being characteristic of a semi-graphitic carbon (Figure S16, ESI). Composites prepared by CVD do not show any additional porosity, as it was the case for compounds obtained from sucrose pyrolysis. The carbon phase formed by CVD is therefore either non-porous or more uniformly deposited on the surface of the alumina, thus limiting the formation of carbonrich zones or aggregates. The second hypothesis is preferred because it is in agreement with the results of Ilinich et al. regarding C/alumina composites formed by ethylene carbonization. [START_REF] Ilinich | Mesoporous Alumina Infiltrated with a very thin and complete Carbon Layer[END_REF] The average pore diameter decreases much more rapidly with the carbon content than for samples prepared by impregnation/pyrolysis (Table 5 and Figures S12 andS15, ESI). This difference reinforces the hypothesis of a less homogeneous covering in the case of composites prepared by impregnation. Indeed, covering the pore surface with a regular carbon layer continuously decreases the pore diameter without pore blocking. In contrast, if porous carbon "clogs" the pores of alumina, the average pore diameter should remain practically unchanged with a sharp drop in intensity and a peak should appear at 3.5 nm in the pore size distribution curve, corresponding to the closure of the hysteresis at p/p0 = 0.45.

Table 5. Carbon content and structural properties of C/γalumina composites prepared by CVD of ethanol

Sample

Accessibility of the alumina surface determined by EtOH adsorption followed by ATG. Materials prepared by pyrolysis of sucrose at 600°C and those prepared by CVD at the same temperature show the same evolution of the free alumina surface with the carbon content (Figure 9). For samples prepared by CVD, the accessible alumina surface decreases from 80% at 4 wt. % carbon to 14% at 29 wt. % carbon. For a content of 14 wt. % corresponding to a theoretical graphite monolayer, only 50% of the surface of alumina is covered when carbon is prepared by impregnation/pyrolysis of sucrose whereas this value increases to ca. 75% when composites are prepared by CVD. The difference confirms that CVD promotes a more homogeneous carbon deposition than impregnation/pyrolysis, particularly at high carbon contents. However, f γ-alumina surface covered by carbon are very high compared to a material prepared by adsorption/pyrolysis of sorbitol. Indeed, even after 7 adsorption/pyrolysis cycles, only 23% of the surface was effectively protected by carbon (Table 3).

Figure 9

Fraction of accessible alumina surface as a function of the carbon mass content for samples prepared by impregnation/pyrolysis of sucrose and CVD Stability of C/alumina composites. The protective effect of carbon formed by pyrolysis of sucrose at 600°C is quite low: despite a high carbon content (15 wt. % C in CA-600-15), the solid contains 73% of boehmite after a HT treatment (Table 6 and Figure S20, ESI). Solids obtained at lower pyrolysis temperature (400°C) seem to be more stable, with only 45% of boehmite formed in CA-400-11. However, the method does not enable a complete protection of alumina, contrasting with data reported by Pham et al. [START_REF] Pham | Improved Hydrothermal Stability of Mesoporous Oxides for Reactions in the Aqueous Phase[END_REF] The main difference between samples prepared at 400 and 600°C concerns the chemical nature of the carbon phase. When pyrolysis is carried out at 400°C, the degradation of the sugar is incomplete and the carbon phase remains highly oxygenated and functionalized.

Table6. Fraction of free alumina surface and percentage of boehmite formed after HT treatment of various C/alumina composites. The evolution of the BET surface area upon HT treatment is also reported. S19, ESI). The carbon phase is unstable under hydrothermal conditions and part of it decomposes in the aqueous phase, generating acidic organic oxygenated compounds in solution. In order to understand the f f m b f γ-alumina, a sample was impregnated with sucrose (0.41 g sucrose/g of alumina; final carbon content = 14 wt. %), dried and directly tested without pyrolysis (see paragraph 2.5.1., ESI). Results indicate that sucrose effectively prevents alumina transformation since only 12% of boehmite are formed under HT conditions, much less than for a sample with similar carbon content but pyrolyzed at 400°C (45% boehmite formed for CA-400-11, Table 6). Characterization of the solid and liquid phases after test confirm that oxygenated organic products resulting from the decomposition/polymerization of sucrose under HT conditions (5-HMF, levulinic acid, humins, acetic acid, etc.) can adsorb on the alumina surface and form a protective layer, as already observed for sorbitol. Samples prepared by CVD also show a low hydrothermal stability. For example, 68 % of CA-CVD 12 containing 12 wt. % C are transformed into boehmite, which is far from the "total" protection expected for such a carbon content (Table 6 and Figure S21). Nonetheless, the protective effect seems to be more pronounced for samples prepared by CVD than for samples prepared by impregnation of sucrose and pyrolyzed at 600°C. Indeed, for an equivalent carbon content of about 15 wt. %, the proportion of boehmite formed is 52% in the case of CVD whereas it is 73% in the case of impregnation/pyrolysis of sucrose at 600°C (Table 6). By extrapolation, the carbon content required to achieve total protection of alumina by CVD is estimated to be around 35 wt. %. Such result is consistent with the observations of Xiong et al. who stabilized alumina with 35%wtcarbon deposited by CVD of methane. [START_REF] Xiong | Graphitic-Carbon Layers on Oxides: Toward Stable Heterogeneous Catalysts for Biomass Conversion Reactions[END_REF] From a preparation point of view, the deposition of 35% of C would necessarily lead to significant changes in the textural properties of the starting alumina (clogging of pores, drastic reduction of surface area and pore volume, etc.) and is not an ideal solution. The theoretical carbon content required for complete protection by CVD is therefore much greater than the value corresponding to a theoretical monolayer (14 wt. %), which suggests either the formation of carbon clusters or a particular texture of γ-alumina (surface roughness, platelet agglom-…) m b w m b a flat surface. T b f /γ-alumina composites obtained by the different methods are summarized in Figure 10. Whatever the preparation method, the amount of boehmite formed after a HT treatment decreases almost linearly with the carbon content in the composite. However, the amount of carbon necessary to ensure an almost complete protection of the support strongly depends on the preparation method. In particular, the synthesis route that consists in successive sorbitol impregnation/pyrolysis cycles enables a much better protection than any other conventional method: an almost complete protection is obtained with less than 8 wt. % carbon. For comparison, only materials prepared by hydrothermal carbonization of sucrose show a similar protection, but with twice the amount of carb . T mb w b f γalumina under hydrothermal conditions can be drastically improved by neutralizing only a small fraction of surface sites by carbon deposition. Non-selective methods, which randomly cover the surface with carbon are less effective and significantly modify the structural characteristics of the pristine alumina surface.

Sample

 CONCLUSION

Conventional methods of carbon deposition such as impregnation/pyrolysis or V m b f γ-alumina in hot liquid water. However, the amount of carbon necessary to prevent significant dissolution and formation of boehmite is sometimes so high that it totally modifies the porous structure and surface chemistry of the pristine crystals. We have shown f f γalumina crystals with carbon to effectively reduce their dissolution in hot water. Crystals can be almost completely protected by neutralizing specific sites of the (110) basal planes. These sites, which were previously identified as neighboring octahedrally coordinated Al atoms in the vicinity of an Al tetrahedron are involved in the alumina dissolution. When sorbitol molecules are present in the hydrothermal solution, they adsorb selectively on the two Al octahedra and block the γ-m → b m f m b of the third unsaturated Al site. This selective adsorption has b b /γ-alumina composites using sorbitol as carbon source. The method is not straightforward and it necessitates a succession of adsorption/pyrolysis cycles to neutralize progressively all sites potentially involved in the dissolution process. This new route allowed us to synthesize composites in which carbon was located exclusively on -or at least close to -. m b /γ-alumina composites obtained by more conventional methods the amount of carbon necessary to stabilize the support is low, typically less than 8 wt. %, which represents approx. 20% of the total surface of alumina crystals. As a consequence, most of the original surface remains accessible to molecules such as ethanol and solids retain textural and chemical properties of γ-alumina, even after a hydrothermal treatment in w . T m w γalumina is used as support for metallic nanoparticles because the corresponding catalysts can be stable while preserving metal-support interactions. Even if the amount of carbon certainly depends on the morphology of the starting alumina, in particular the relative proportion of (110) faces in crystals, it is always small compared to a complete coverage. Clearly, the present method can hardly be applied at industrial scale because the multi-step preparation is long and necessitates a great number of adsorption/pyrolysis cycles. However, it is a proof of concept that paves the way to the preparation of hydrothermally stable catalyst supports with optimized surface composition and controlled properties.

  , ESI). The AlOOH phase f alumina for 10 h at 200°C. All solids were tested just after drying at 100°C overnight.

Figure 1

 1 Figure 1 Percentage of AlOOH formed (left axis) and specific f ( x ) f γ-alumina after HT treatments at 200°C in water for different periods

Figure 2

 2 Figure 2 T M f γ-alumina before (a) and after (b) a 10 hour HT treatment at 200°C in water

Figure 3

 3 Figure 3 XRD patterns of (a) alumina after a HT treatment (10 h; 200°C) in a 25 g/L aqueous solution of sorbitol and (b) the same solid after a second conventional HT treatment in pure water

Figure 4

 4 Figure 4 Evolution of the proportion of AlOOH and of the amount of sorbitol adsorbed after 10 h HT treatments at 200°C in aqueous solutions of sorbitol with different concentrations

Figure 5

 5 Figure 5 TGA f b /γ-alumina materials prepared by adsorption/pyrolysis cycles before (top) and after (bottom) the pyrolysis step. Conditions: air 25 mL/min, temperature ramp: 10°C/min)

Figure 7 Table 3 .

 73 Figure 7 Fraction of AlOOH formed upon HT treatments as a function of the carbon content in compounds prepared by sorbitol adsorption/pyrolysis cycles and treated for 10 h at 200°C (red points). The blue point represents a sample containing 6.7 wt. % C obtained by a single impregnation/pyrolysis step

a

  Determined with TGA analysis b Pore diameter (Dp), pore volume (Vp) and micro/mesoporous surface area (Smic) determined from N2 sorption isotherms

Figure 10

 10 Figure 10 Percentage of boehmite formed after a hydrotherm m (w 2 ° ) f /γ-alumina composites obtained following different methods

Table 1 . Chemical composition and structural characteris- tics of C/γ-alumina composites obtained by successive sorbitol adsorption/pyrolysis cycles

 1 FigureS10, ESI). After the HT treatment, the characteristics of the sample obtained after 7 cycles were not significantly modified (Table1, cycle 7 data before HT are not available but we can assume that they are similar to the cycle 6 ones).

			T ( C)
	150	250	350		550	650	750
						Cycle 1 + S Cycle 1 + sorbitol
						Cycle 2 + S Cycle 2 + sorbitol
	(a.u)					Cycle 7 + S Cycle 7 + sorbitol Cycle 5 + S Cycle 5 + sorbitol
	DTG					Sorbitol/alumina Cycle 8 + S Cycle 8 + sorbitol
			T ( C)
			350	450	550	650	750
							Cycle 1
							Cycle 2
	(a.u)						Cycle 3 Cycle 4
	DTG						Cycle 6 Cycle 8
							Cycle 9

and a BET surface area (S BET ), pore diameter (Dp des, measured on the desorption branch of the isotherm) and pore volume (Vp) estimated from N 2 adsorption/desorption isotherms

Table 2 . Carbon content and percentage of boehmite formed after a hydrothermal treatment of 10 h at 200°C in water

 2

	Sample	C (wt. %)	AlOOH (wt. %) a
	Cycle 1	1.5%	> 95%
	Cycle 2	2.8%	> 95%
	Cycle 3	3.8%	> 95%
	Cycle 4	4.6%	61%
	Cycle 5	5.1%	n.d.
	Cycle 6	5.9%	33%
	Cycle 7	6.9%	12%
	Cycle 8	7.4%	15%
	Cycle 9	7.2%	13%
	a Estimated from XRD patterns of the treated solids using the
	calibration curve in Figure S1 (ESI)

Table 4 . Properties of C/alumina materials prepared using impregnation/pyrolysis of sucrose

 4

	Sample	C (%wt) a	b (m²/g) S BET	Dp Des b (nm)	Vp b (mL/g)	Smic b (m²/g)
	γ-alumina	0	212	8.8	0.55	9
	CA-400-11	11	203	7.9	0.36	0
	CA-400-19	19	205	7.9	0.25	33
	CA-600-9	9	194	8.5	0.4	21
	CA-600-15	15	240	x	0.34	47

a Determined with TGA analysis b Pore diameter (Dp), pore volume (Vp) and microporous surface area (Smic) determined from N 2 sorption isotherms

  Carbon content and surface area of samples prepared by CVD as a function of reaction time.

	Figure 8
		C (%wt) a	b (m²/g) S BET	Dp Des b (nm)	Vp b (mL/g)	Smic b (m²/g)
	γ-alumina	0	212	8.8	0.55	9
	CA-CVD-2	2%	202	7.4	0.47	0
	CA-CVD-4	4%	187	7.8	0.45	0
	CA-CVD-12	12%	195	6.5	0.38	0
	CA-CVD-16	16%	157	6.0	0.30	0
	CA-CVD-29	29%	122	-	0.18	0

ASSOCIATED CONTENT

Supporting Information. Determination of the boehmite content by XRD; determination of the carbon content in C/alumina composites by ATG; determination of the free alumina surface by ethanol adsorptio f w b ATG; b f γ-alumina w b X ; b f γalumina in the presence of sorbitol studied by [START_REF] Van Cleve | Enhanced Hydrothermal S b f γ-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings[END_REF] C NMR and XRD; [START_REF] Van Cleve | Enhanced Hydrothermal S b f γ-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings[END_REF] C NMR spectra, Raman spectra and N 2 adsorption/desorption isotherms of C/γ-alumina composites obtained from sorbitol adsorption/pyrolysis cycles; characterization by nitrogen adsorption, [START_REF] Van Cleve | Enhanced Hydrothermal S b f γ-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings[END_REF] C NMR and Raman spectroscopy of solids prepared by impregnation/pyrolysis of sucrose; characterization by nitrogen adsorption and Raman spectroscopy of solids prepared by CVD of ethanol; hydrothermal stability of C/γ-alumina composites obtained by different methods.

AUTHOR INFORMATION Corresponding Author

* E-mail: alain.tuel@ircelyon.univ-lyon1.fr (A.T.)

ORCID


Dynamic Localization of Vapor Fraction in Turbo

Pump Inducers by X-Ray Tomography Walid Hassan, Samuel Legoupil, Dominique Chambellan, and Stéphane Barre

Abstract-An experimental study of the instantaneous local behavior of cavitation in turbo-pump inducers is presented in this article. Experiments held on a hydrodynamic facility equipped with an Ariane 5 inducer permitted achieving the aim. Cavitation is attained by reducing the pressure in a turbo machine having an inducer rotating at 4000 rpm. An x-ray tomography system developed specially for this aim, was used to measure the cavitation. The system employed an industrial x-ray generator and scintillation detectors. The generator/detectors system was fixed while the inducer was rotating. Vapor fraction was determined instantaneously, which confirms the applicability and the precision of the method in such type of measurements despite the constraints imposed by the geometry and the rotation speed. The dense composition of the system components introduced difficulties in the measurement especially at the inducer axis. However, prior information concerning the vapor regime as well as its stationary behavior in certain time intervals helped overcome this problem. Consequently a quantitative and qualitative evaluation of the vapor fraction is obtained. Results show a cavitation regime mostly synchronous with the rotation of the inducer thus permitting the use of tomographic reconstruction for the localization of vapor in the machine. An algebraic reconstruction algorithm (ML-EM) was used to achieve image reconstruction. Index Terms-Cavitation, industrial X-ray generators, tomography, turbo pumps.

I. INTRODUCTION

S PACE exploration motivates the space industry to develop launchers with more and more efficiency so that they can transmit loads to space. In order to deliver combustibles to the combustion chamber during flight, launchers engage high flow turbo pumps (e.g: 0.5 m3/s of liquid Hydrogen). Being constrained by size, the inducer-located in the upper stage of the launcher-should rotate at high speeds to maintain a constant supply of fuel (Oxygen and Hydrogen). The high rotational velocity, coupled with the inherently low pressure in the propellant tanks cause cavitation. Many research groups have attempted to illustrate the behavior of air bubbles and cavitation pockets in turbo pumps. [START_REF] Stutz | Analyse de la structure diphasique et instationnaire de poches de cavitation[END_REF]- [START_REF] Hassan | Dynamic vapor fraction measurement in turbopump inducers by X-rays[END_REF]. These methods used gamma-rays from radioactive sources to perform tomographic measurements of the pumps. However, the radiation flux possible from any reasonable source is too low to provide the instantaneous evaluations desired in this application. Recently Stutz et al. [START_REF] Stutz | Measurements within cloud cavitation by means of x-ray attenuation device[END_REF] adapted an ionizing radiation technique to describe and measure the vapor fraction in a pipe with two-phase flow. An x-ray generator and a set of detectors were employed to achieve the measurement. The efficiency and robustness of the method support its application to explore more complex systems as well as turbo pump inducers. However the use of x-ray generators-especially the industrial types-is still not well reputed in this domain. An important reason for such a bad reputation is that these generators show sharp instabilities in the energy and the number of x-ray photons of the emitted beam [START_REF] Hassan | Industrial x-ray generators measurements in multi-phase flows[END_REF].

A similar work was carried out by Coutier-Delgosha et al. in 2003 [START_REF] Coutier-Delgosha | Internal structure and dynamics of sheet cavitation[END_REF]. The aim of their work was to measure the vapor fraction in cavitation pockets produced by water flow on a hydrofoil. Their measurements were verified via numeric calculations obtained by coupling Navier-Stoke equations to a simplified cavitation model.

A research work was carried out at CEA-FRANCE in 2000, where a gamma tomograph was developed to measure the void fraction in a fixed bed petrochemical reactor. The system contained a 300 mCi Cesium source and a group of BGO scintillation detectors. The filtered back-projection method was used to reconstruct images of the distribution of vapor in the reactor [START_REF] Boyer | Measurement of liquid flow distribution in trickle bed reactor of large diameter with a new gamma ray tomography system[END_REF].

A tomographic system, including both a gamma tomograph and a capacitive tomograph, was developed by Johansen et al. in 1996 [12]- [START_REF] Abro | Determination of void fraction and flow regime using a neural network trained on simulated data based on gamma ray densitometry[END_REF]. This system was used to visualise a three-phase flow containing oil, water and gas. The capacitive tomograph was used to distinguish between the oil and the water, which could not be done with the gamma tomograph due to the small difference between their densities.

Prasser et al. [START_REF] Prasser | Gamma and X-ray tomography for transient two-phase flows and other instrumentation developed by ROSSENDORF[END_REF] in 2004 worked on a gamma ray tomograph with a source of 5 Ci of Cesium for the visualisation of two-phase flow (liquid and air) in the inducer of an axial pump. Images were reconstructed with the filtered back-projection algorithm, from measurements averaged over 7 minutes.

Methods using gamma rays showed good results in measuring and visualizing multi-phase flows. However, due to the fact that these methods can't perform high speed measurements, we report in this work the use of an x-ray generator to perform tomographic measurements on a turbo pump. Indeed the system takes into account the instabilities in the x-ray beam, the 3D geometry of the inducer, the high rotation speed, and the dynamic nature of the vapor structures.

II. NOMENCLATURE

• : Vapor fraction.

• : The cavitation parameter or the similarity parameter for cavitation [START_REF] Lecoffre | La Cavitation: Traqueurs de Bulles[END_REF]. -pw: Housing walls.

-mi: Inducer metal.

-W: Water.

-v: Vapor.

-d: Detector number -: Angular position of the inducer.

III. METHOD

Tomography was achieved by immobilizing the source-detector measurement system while rotating the inducer. Projections were acquired by 10 detectors for a total of 75 angles of incidence (Fig. 1). Measurements could not be acquired in the center of the machine due to the presence of the mounting axis holding the inducer, which constitutes 35 mm of iron. This quantity of iron is capable of absorbing all incident X-ray photons in the energy range in which we are working.

The x-ray absorption technique is used to perform integral measurements of the vapor volume in the inducer. We define the vapor fraction, , as the ratio of the volume of vapor to the total volume of measurement :

(1)

The vapor fraction could be estimated from the three different measurements and as listed below :

(2)

where .

(

) (4) 3
where . Because is negligible compared to we can write :

Solving the equations for beta we get:

(5)

In fact, the thickness of metal varies with the rotation of the inducer. Thus the calibration measurements should be performed for all angular incidences of the inducer. Hence is also a function of the angle and is given by: [START_REF] Boitel | Effet Thermodynamique Dans un Inducteur Cavitant[END_REF] where indicates the angular position of the inducer.

A. Test Facility and Measurement System

Experiments were performed on the water test bench of the Centre de Recherche et d'Essais de Machines Hydrauliques de Grenoble [CREMHyG]. The bench is equipped with a special Aluminum inducer designed by Snecma, the company in charge of the Ariane space manufacturing.

Cavitation is produced by reducing the inlet pressure of the pump, while maintaining the inducer rotation speed at 4000 rpm. The cyclo-stability of cavitation structures is required in order to perform image reconstruction later. The angular position of the inducer is provided by a position sensor situated at the housing of the machine. This sensor provides a signal between 0 and 10 volts corresponding to angular positions between 0 and 360 degrees. Cavitation measurements were performed in three axial positions at the machine, with a section thickness of 20 mm for each axial position.

A Philips M160 X-ray generator and 11 NaI scintillation detectors functioning in current mode were employed to perform integral measurements. The X-ray tube high voltage supply was fixed at 80 kV, 30 mA for and measurements while is performed at 80 kV, 5 mA to avoid saturating the detectors. Detector positions were selected based on the opening angle of the beam from the x-ray tube. Unfortunately, due to the presence of support equipment for the pump, the same detector source spacing could not be used for all of the detectors.

Technical parameters of the system (high voltage, current, detection area, etc.) were optimized to ensure a good signalnoise ratio and thus to reduce the measurement error. Simulation codes like VXI [START_REF] Freud | Fast and robust ray casting algorithms for virtual X-ray imaging[END_REF] and Macalu 7 [START_REF] Tola | Logiciels d'aide à la conception et à l'interprétation de mesures basées sur la détection d'un rayonnement ou X[END_REF] were used for this purpose. Practically the measurement error is inversely proportional to x-ray intensity (defined by the tube current) and the measurement time. Because the high rotational speeds limit the measurement time, we used the maximum tube current at the lowest acceptable tube high voltage to maximize the contrast.

Signals issuing from X-ray detectors as well as signals from the position sensor, and pressure sensors situated at the housing of the machine were acquired simultaneously. The acquisition was performed with a National Instruments [NI] acquisition card (PCI 6033) using Labview. The acquisition frequency is 5000 Hz at each detector, thus permitting to acquire 75 projections during one complete rotation of the inducer. This reflects a measurement time in the order of 200 sec per projection.

Fig. 1 shows the presence of a reference detector situated between the X-ray tube and the machine. The purpose of this detector is to monitor the X-ray beam, and the acquired signal is used for treating instabilities in the x-ray beam. These fluctuations are related to the high voltage generator of the x-ray tube.

The procedure to eliminate the fluctuations has been previously described [START_REF] Hassan | Dynamic vapor fraction measurement in turbopump inducers by X-rays[END_REF]. The main idea is to perform a double calibration in order to determine the vapor fraction : one for the angular position , and another for the variations of energy and flux provided by the reference detector. Thus can be rewritten as follows:

(7) Therefore, the void fraction measured at a time , in an angular position , by a detector is given by: (8)

B. Experimental Validation

The experimental validation of the measurements was performed by analysing two different parameters. The first is concerned with finding and the other is for the determination of the void fraction. The thickness of the measurement volume seen by each detector can be determined via the calibration measurements and as follows:

The measurement can be written as: [START_REF] Hassan | Industrial x-ray generators measurements in multi-phase flows[END_REF] while The measurement is expressed as follows: [START_REF] Coutier-Delgosha | Internal structure and dynamics of sheet cavitation[END_REF] Solving for the water thickness from the two equations we get: [START_REF] Boyer | Measurement of liquid flow distribution in trickle bed reactor of large diameter with a new gamma ray tomography system[END_REF] Note that in this equation we did not consider due to the fact that and could be averaged with respect to the angular position for a long time and thus we eliminated the fluctuation effect.

However in the equation were measured experimentally by determining the attenuation of a 10 mm sheet of Plexiglas, knowing that this is equivalent to 12 mm of water in terms of attenuation. Fig. 2 shows the measurement volume thickness obtained experimentally compared to that obtained by simulations. Values seen by the 10 detectors at a fixed angular position are also shown.

The second validation procedure is to compare frequency analysis (spectrum) of the measured vapor fraction to the spectrum of the pressure sensor situated at the housing of the machine, provided that the pressure sensor is sensible to vapor structures produced in the machine.

Fig. 3 shows an example of spectra obtained from both the pressure sensor and the vapor fraction measured by an X-ray detector. The spectra in the figure correspond to a super synchronous cavitation regime. In this regime usually we find two essential forms of cavitation : pockets rotating at the inducer frequency and pockets rotating at a superior frequency . The two sensors show well the spectral content of this cavitation mode. The super synchronous term ( , where Hz is the inducer rotation frequency) could also be seen by the two sensors. The X-ray sensor is still sensitive to small structures alternating at that could not be seen by the pressure sensor.

The analysis of the X-ray detector spectrum always showed a small factor of 100 Hz caused by the fluctuations of the X-ray beam. However, this was caused by the measurement , rather than by a defect in the treatment method. As mentioned above, is performed at 5 mA while and were performed at 30 mA in order to avoid saturation of the detectors. The compensation is done by multiplying by a factor (6) assuming linearity in the number of emitted photons as a function of the tube current. This hypothesis is true in case we don't have instabilities in the X-ray beam, especially it has been shown [START_REF] Hassan | Dynamic vapor fraction measurement in turbopump inducers by X-rays[END_REF] that fluctuations are more intense at 30 mA than at 5 mA. A thorough analysis of this problem shows that the fluctuations are more related to the energy rather than the number of photons, which permits such a compensation to take place.

In the mean time the 100 Hz factor is accepted since the amplitude of these fluctuations is too small, to affect the measurements. Furthermore the image reconstruction method is still robust while the noise level (beam fluctuation) is lower than 15% in the measurements. This was proved by performing reconstruction on simulated data and by adding random noise to this data. Fig. 4 shows the mean square deviation of the recontructed image as a function of the noise level in the simulated data. Here a simulated data representing an image with 4 vapor pockets is reconstructed while adding different levels of random noise to these data and then calculating the mean square deviation between the reconstructed image and the real one.

As can be seen in the figure, the reconstruction method is robust upto a noise level of 15 % in the data to be reconstructed.

C. Void Fraction

The void fraction is measured at three axial positions on the inducer. At each position, three cavitation cycles corresponding to three different flow rates were performed. During each cycle, 10 to 15 pressure points were tested. At each pressure point, signals are acquired during 10 sec.

A statistical study is performed on the measurements to verify the statistical convergence of the void fraction. Fig. 5 shows the mean void fraction obtained from N samples divided by the average value obtained from the total number of samples ( sec). The importance of this test is that it defines the threshold of convergence, i.e., the number of samples necessary to ensure the stability of void fraction in vapor structures, noting that stability is necessary to perform the tomographic reconstruction.

Assuming a 3% uncertainty, we found that the means are convergent after 5000 samples, which corresponds to a 1 sec acquisition time.

IV. TOMOGRAPHIC MEASUREMENTS

In order to realize tomographic reconstruction of a section of an object, it is necessary to have projections (or integral measurements) in different angles of incidence. In our case the object to be identified (vapor structures) is dynamic having a behavior that might not be related to that of the inducer. So we might have vapor structures rotating faster (super synchronous) or slower (under synchronous) than the inducer or at the same speed (synchronous). Thus we need to reconstitute the sinograms of measurements.

A. Constitution of Sinograms

The procedure of the constitution of the sinogram depends on the cavitation mode. So for synchronous modes, there is no specific treatment needed to reconstitute the sinogram. The 75 projections measured by the 10 detectors during a rotation of the inducer are taken completely. While for non synchronous modes two cases could be showed. The first case is that if the periodicity of structures is known from literature, the sinogram could be reconstituted from a certain number of rotations, by taking one projection line per rotation (Fig. 6). The position (delay) of the line to be taken is defined throughout the periodicity.

In the other case, where the periodicity is unknown, we would refer to the spectral analysis of the measured vapor fraction or that of the pressure sensor to determine the periodicity.

B. Reconstruction

The nature of data contained in the sinograms play an important role in the choice of the reconstruction algorithm. As mentioned above, detectors were placed in a manner to avoid the mounting axis of the machine. Therefore sinograms are truncated at the middle, thus representing an important discontinuity in the projection function. This discontinuity did not permit the use of classical reconstruction methods (analytic ones). For this reason, the well adapted Maximum likelihood expectation maximization (MLEM) statistical method is used [START_REF] Shepp | Maximum likelihood reconstruction for emission tomography[END_REF], [START_REF] Lange | EM reconstruction algorithms for emission and transmission tomography[END_REF]. Fig. 7 shows two reconstructed images corresponding to two different sigma values or cavitation conditions and (synchronous regimes) and to a nominal flow . In both of the reconstructed images we could distinguish the four structures attached to the blades of the inducer. For , usually we find cavitation pockets at the boundaries of the blades attached to small marginal structures. However this is well reflected by the reconstructed image. In the center of the image we could notice small vapor structures that are attached to the mounting axis of the inducer. In fact we are considering these figures because the reconstructed image refers to data measured in the first axial position or the outermost section of the inducer. In order to verify these results we compare in Fig. 8 a 2D representation of the reconstructed image to a photographic image taken under the same cavitation conditions.

The values of the vapor fraction in the image are re-dimensioned (multiplied by a constant factor). In the photographic image we note the existence of vapor at the walls (marginal cavitation). This is due to the fact that the photos are taken along the axis of rotation, and the vapor that appears in this region of the photo is out of our section of measurement.

V. C ONCLUSION

The vapor fraction in the machine was measured despite technical difficulties related to the complex geometric design of the inducer and its high rotation speed. Adequate procedures were developed to treat fluctuations in the measurements due to X-ray beam instabilities.

Integral measurements of void fraction achieved at 75 angles of incidence, permitted to determine via tomographic reconstruction the distribution of vapor in the machine. Reconstructed images show good agreement with photographic images taken in identical cavitation conditions.

Improvement of the axial resolution by increasing the number of axial sections to be tested would permit us to reconstruct the vapor fraction distribution in vapor pockets attached to the blades of the inducer.

  W. Hassan, S. Legoupil, and D. Chambellan are with the Commissariat a lénergie atomique, 91191 Gif sur Yvette, France. S. Barre is with the LEGI-CNRS-INPG, 38 041 Grenoble, France.

Fig. 1 .

 1 Fig. 1. The measurement system.

Fig. 2 .

 2 Fig. 2. The water thickness obtained. The blue markers are the values determined by simulation and the red ones are those measured.

Fig. 3 .

 3 Fig. 3. The spectral analysis for measured vapor fraction and the pressure sensor.

Fig. 4 .

 4 Fig. 4. The mean square deviation as a function of the noise level.

Fig. 5 .

 5 Fig. 5. Statistical convergence of mean void fraction.

Fig. 6 .

 6 Fig. 6. Constitution of sinograms. For synchronous modes the 75 projections acquired by each detector during on rotation of the inducer are taken into account. For non synchronous modes one projection line per rotation is taken depending of the periodicity of structures.

Fig. 7 .

 7 Fig. 7. Reconstructed images corresponding to two different cavitation conditions (Left =0 :0356) and (Right =0 :0116).

Fig. 8 .

 8 Fig. 8. The reconstructed images for =0:0356 compared to a photographic image taken in the same cavitation conditions.

ACKNOWLEDGMENT

The authors would like to thank the CNES and SNECMA motors, Rocket Engine Division, the financial supporters of this project, and special thanks to Dr. G. Albano (CNES) and Dr. P. A. Lambert (SNECMA) for the management and the support of this research.


Greg Henning

Using containers for data analysis

this paper describes how one can use container (in particular with docker) to perform data analysis, distribute analysis code,

The issue of having a unified environnement for data analysis inside a collaboration between teams, labs, ... arises naturally from this situation.

Even for a simple few-people team, computing environnement can be diverse between the experimental setup, user laptop and desktop computers, and grid infrastructure to run large scale analysis.

In the following paper, I'll discuss mostly the situation of a user running linux software on a windows computer, as it qualifies as the worst combination.

The two objective aimed here are:

• running linux softwares on a non-linux system • having a defined, reproducible and distribuable environnement for development and execution of an analysis code.

Virtual machine

A virtual machine (such as VMware or Virtual Box) emulates the full computer system on top of the running operating system. This solution allows the user to define the available memory, CPU, ... of the virtual machine. Any system can then be installed as on a real computer.

This solution offers the possibility of a very precisely defined and reproducible environnement as the virtual machine and installed OS can be chosen. However it is not very efficient for several reasons:

• the whole computer has to be emulated, which costs a lot of memory and CPU on the host • File exchange can be tricky between the host and the virtual machine • The virtual machine system takes up to several tens of Giga-Bytes of space on the disk • It is not possible (unless putting in place specifi strategies for that) to restart the virtual machine to its initial state after each run, neither it is simple to run several instance of the same virtual machine in parrallel.

Windows Subsystem for Linux

The Windows Subsystem for Linux (WSL) is available on Windows 10 and offer a linux kernel for the windows OS (natively running for WSL2, emulated in its first version).

This allows the Windows user to run linux distributions with ease and minimal overhead compared to a virtual machine. Some out-of-the-box distributions are available in the windows store, such as Ubuntu, SUSE, debian, Kali, ... However, some disadvantages of virtual machines exist also: the installed distribution behave like the installed system on a virtual machine: only one instance can be runned and it can't be reset to an intial state.

Docker and containers

Many of these drawbacks are solved by the use of containers.

What is a container ?

A container is an environnement that runs a specific set of codes, with a given set of libraries and parameters.

The basis for a container is an image that contains all the code, its dependencies and settings needed. However it does not contain the underlying running system as it is the case for a virtual machine.

When the inage is run by an engine, the container runs on top of the operating system, but isolated from it (hence the name container). This means that the container will have its own environment and run only within it, while drawing on the computer capabilities of the host.

The image is the starting point and can be run multiple times in parrallel. Every new instance of the container will be separated from the others and from previous execution. Because of that, containers are widely used in web application backends.

As the image contains only the code of interest and not the OS ones (which are taken care of by the engine), the image file is smaller than a full virtual machine disk. Still, it can contains parameters, libraries, ... and therefore all the needed files to reproduce the system on any instance of the engine. Thanks to that, it allows a great deal of uniformity to execute the code even on a diverse set of underlying infrastuctures.

Docker and other container engines

Docker is a widely used container engine (or runner). It does all the necessary tasks to run the image with the appropriate containerization. One task of a container engine is to allow network or file exchange with the host, so that the running application within the container con be used with external control.

Other container engines exist, such as Singularity or LXD, but we will discuss only docker here. Most of the examples given below can be transposed in some way to another container runner.

Rather than giving a detailed documentation of Docker, I will present a few use cases.

Use cases

Now that containers have been introduced, let us discuss some use-cases in order to illustrate the interest of using such tool.

L A TEX

Installing L A TEX on windows can be tedious (not outrageously so, but still more complex than apt install latex on Ubuntu). But one can easily use a docker image ready for latex and simply run it with docker to convert .tex files to pdf on demand.

We actually don't need to get inside the container, we just pass a command to docker and it is exectuted within the container, then the container simply exits.

dir src docker run --rm -v %cd%\src:/media/usb -w="/media/usb" texlive/texlive-full l atex -interaction=batchmode document.tex docker run --rm -v %cd%\src:/media/usb -w="/media/usb" texlive/texlive-full l atex -interaction=batchmode document.tex docker run --rm -v %cd%\src:/media/usb -w="/media/usb" texlive/texlive-full d vipdfm document.dvi dir src

(replace %cd% with `pwd` when using a bash-like shell instead of windows' batch).

The command is divided in several parts:

• docker run calls docker and tells it to run a container.

• --rm tells docker to clear (remove) the container after running (otherwise, its state is saved and one can go back to it later).

• -v %cd%\src:/media/usb mounts the host directory %cd% (i.e. the current working directoty) onto the container's /media/usb point • -w="/media/usb" tells docker that the working directory on the container will be /media/usb

• texlive/texlive-full is the name of the image to fetch from the image repository and run in the container • latex -interaction=batchmode document.tex and dvipdfm document.dvi are the command to run in the container.

The first time the image is used to run a container, docker will fetch it from the docker hub. The iamge is fetched by parts, because the images are built by adding layers on top of each other. We will see in the next example how we can create our own image by adding some libraries to an existing one.

Note that the command docker run ... latex document.tex is called twice because that's the needed number for latex to proplery build section indexing.

The latex compilation files appear directly and instantly in the directory, since there's no virtualization layers between the container and the host: latex runs directly on the files.

Test this in the latex directory of the examples

Fortran

It can happen that you have to use some tools written in Fortran. As for other languages, installing and running a fortran compiler on Windows can be tricky. The easiest way to compile such software is therefore to start a container dedicated to fortran compilation. Once the code is compiled it can be run from inside the container.

In this example, we will use the ruler program provided by tha IAEA as part of the ENSDF Analysis and Utility Programs.

We will also create our own fortran-dedicated image to be run in a container. To that end, we start by writing a Dockerfile: We then create the docker image with docker build -t myfortran:001 .

The image is built (i.e. the commands are executed and the resulting image is kept to be launch in a container on demand) and saved under the name and tag myfortran:001.

We can now launch the image in a interactive container (i.e. we well be able to type commands in the shell).

> docker run --rm -ti -v %cd%:/home/fortran/work myfortran:001

And now, we can download, compile and run the fortran software: In general, it is better if you or your own collaboration designs its own image with the specific tools needed, by building on top of existing ones.

~ $ cd

Geant4

Like Root, there is no official image provided. An unofficial image for trainning purpose exist at https://hub.docker.com/r/dplatten/geant4-educational with the issue of visualisation solved thru the use of an RDP server running in the container to which you connect from outside. It's an elegant solution, but makes the image quite large.

In the following, we will write our own docker file for a simple geant4 image.

Writing the Dockerfile

The Dockerfile will contain the following lines:

• FROM ubuntu:18.04 indicates that we are using ubuntu version 18.04 as basis for our image.

• RUN apt-get update &&\ apt-get install -y libxerces-c-dev qt4-dev-tools freeglut3-dev libmotif-dev tk-dev cmake libxpm-dev libxmu-dev libxi-dev wget tar &&\ apt-get clean : this updates the package list for the distribution, install the needed libraries to build Geant4 and clean up. It is important to put all the commands into one RUN line because each call to RUN creates a new layer in the image and potentially increase the size of the final image. If we were to clean in a separate RUN line, the previous layer would still contain the files cached by apt and the size of the final image will not be reduced by the subsequent RUN apt-get clean call. • CMD ["/bin/bash"]: Sets /bin/bash as the entrypoint of the image, i.e. the command that is run when starting a container.

The actual Dockerfile can be found here. It contains a few additionnal commands to make the compilation easier.

The image building then is done by calling, from the directory where the dockerfile is, docker built -t geant4:001 .

The different calls of RUN will then be executed. In particular, the compilation of the geant4 library will be done. It will take some time.

Once the image is built, you can verify it by calling We see that the size of the image is about 2.87 GB, most of it comes from the reactiond ata bases used by Geant4.

Using the image

Now, we will start up an container using our new image and run an example of Geant4.

Start docker with

docker run --rm -ti geant4:001

We can now type the following commands in the bash terminal : mkdir example wget http://cern.ch/geant4-data/releases/geant4.10.06.p01.tar.gz tar -xzf geant4.10.06.p01.tar.gz cp -vr geant4.10.06.p01/examples/basic/B2 ./example/ rm -rf geant4.10.06.p01.tar.gz cd example mkdir B2-build cd B2-build cmake ../B2 make cd B2a ls -lsh ./exampleB2a run1.mac This will download, prepare, compile and run an example script for Geant4. If it runs correctly, that means our distribution is ready to be used to run our simulations.

Obviously, you may want to add to this Dockerfile, for example to add support for the Root library...

Using a software distributed via linux package

Containers can also be a simple solution to use a software distributed via only one channel (such as debian repository) on an a priori uncompatible system.

For example, the faster acquisition system comes with some analysis tools that are distributed uniquely as debian packages. That prevents anybody running Windows, MacOs or a non-debian based flavor of Linux to use them... unless using containers.

It is very easy to create an image on which the package is installed and can be runned from: The image is built using docker build -t faster:001 . and then used as following for example.

Start the container with

docker run -ti --rm -v %cd%:/home/faster/work faster:001

and type the following commands:

Publishing your own application by distributing binaries without the source code

Finally, using images can be useful wif you want to distribute your software (with all its dependencies) but not the source code. Evidently, it is strongly recommended to distribute data and analysis source code openly as part of an Open Science startegy. However, there are various good reasons why fully open distribution of your code is not possible (because you use restricted distribution dependencies, part of the code is not open, ...) in that case, distributing the final binary is a good alternative.

Building an image, one can compile a code in a controlled environnement and remove the sources from the final image, effectively distributing a garantee-to-run binary without being concerned with users not having the necessary libraries, ... (since they are all in the image).

In this example, we will compile a C++ code written to read faster files and extract data in ascii formatted files and distribute it as an image. The code we use is not restricted in access,

although not yet openly distributed because it has to go thru additionnal testing before release to the public. You see there that at the end of compilation, we rm -rf /setup to remove all source files.

The image can be used with docker run --rm -ti -v %cd%/work:/home/user/work distrib:001 (from the examples/distrib directory), and launch as a test: cd work faster2h --conf=etc/the.config --map=etc/the.map --input=data/test_0001.fast And it should work perfectly...

Discussion

Now that we looked at a few example of how containers can be used to run your projects, let's considere the pros and cons.

Containers allow a controlled environnement to be run in a consistent way one different platforms (Windows, MacOs, Linux, ...) with all the dependincies and needed libraries contained in the image. It's a great opportunity for open science, open source (even with the source code, having a ready-made environnement where to run the code is nice). It is likely to be the basis of distributed grid job submission in the future.

On the other hand, one still have to install the proper container runner on the host machine, which may require special permission that normal users don't have. Using plenty of images and containers may accumulate junk (dead container, unused images, ...) on the host computer. Also, the average user may not know how to write aproper Dockerfile for his needs. Additionnaly, there are several container runners avaialble (docker, singularity, ...) and even if they have some cross compatibility, they are not used the same way.

Perspectives

One thing to look forward runners close future is the distirbution of Window's WSL2 (Windows subsystem for linux 2). As the first version relied on an emulated linux kernel, WSL2 will provide a native Linux kernel on the Windows host. This will speed up the execution, faciliate file access, ... Large computing infracstructures such as Open Science grid and EGI are starting to integrate containers running in their job submission (more or less easily). It is certain than in the future, any job launch on a grid will include the name of an image to run the job on.

Finally, people developping code in Github or a Gitlab use docker images to run their CI/CD (Continuous Integration/Continuous Deployment) jobs.

Conclusion

Containers offer the ability to define and distribute a consistent and uniform software environnement accross teams and machines in order to facilitate research work. The container images can be tailored to your own need and include all necessary software, libraries and dependencies needed to run the analysis work associated. Containers are ideal for deployement of code in multiple, parallel tasks that need to run the same environnement.

FROM

  alpine:3.11.5 RUN apk add gfortran wget RUN adduser --disabled-password --gecos '' fortran USER WORKDIR /home/fortran/ CMD ["/bin/sh"]

FROM

  ubuntu:18.04 RUN apt-get update &&\ apt-get install -y wget software-properties-common &&\ apt-get clean RUN wget -O -http://faster.in2p3.fr/distribution/fasterv2/fasterv2_repo_pgp_ pub_key.asc | apt-key add -&& \ apt-add-repository 'deb http://faster.in2p3.fr/distribution/fasterv2/ubun tu/ bionic non-free' && \ apt-key list && \ apt-get update && \ apt-get install -y fasterac &&\ apt-get clean RUN adduser --disabled-password --gecos '' faster USER faster WORKDIR /home/faster/ CMD ["/bin/bash"]

  This allows you to use the faster tools on your local files as if you were on an Ubuntu 18.04 system.

  Creates a user geant and setup the image to start running with the working directory /home/geant and the user geant

	•	RUN mkdir /geant4.setup && cd /geant4.setup && \
		wget http://cern.ch/geant4-data/releases/geant4.10.06.p01.tar.gz && \
		tar -xsf geant4.10.06.p01.tar.gz && \
		mkdir geant4-build && mkdir /geant4-install && \
		cd geant4-build && cmake -DCMAKE_INSTALL_PREFIX=/geant4-install -
		DGEANT4_INSTALL_DATA=ON /geant4.setup/geant4.10.06.p01 && \
		make install && \
		cd / && rm -rf /geant4.setups
		this series of command (again, put together to reduce the layer size) download the
		geant4 installation package, unpack it, configure it, compile it, install it and clean up
		the setup files.
	•	RUN useradd -m geant
		USER geant
		WORKDIR /home/geant/

•

RUN echo "cd /geant4-install/bin; source geant4.sh" >> /home/geant/.bashrc &&\ echo "clear;" >>/home/geant4.bashrc : Sets up the bash environnement for the user geant so that it loads the PATHS needed for geant4 when starting a new bash session.

  The Dockerfile looks like this:

	FROM gcc:9.3.0
	COPY faster2h-c /setup
	RUN ls -lhs && cd setup && ls -lhs &&\
	make binaries && ls -lhs &&\
	cp -v bin/faster2h.`hostname` /usr/bin/faster2h &&\
	cd / && rm -rf /setup
	RUN adduser --disabled-password --gecos '' user
	USER user
	WORKDIR /home/user/
	CMD ["/bin/bash"]

ghenning@iphc.cnrs.fr

...

9955]

~/work $ unzip ruler-src-2019-01-24.zip Archive: ruler-src-2019-01-24.zip creating: ruler-src/ inflating: ruler-src/howto.txt inflating: ruler-src/Makefile inflating: ruler-src/nsdflib95.f inflating: ruler-src/ruler.ens inflating: ruler-src/ruler.f inflating: ruler-src/ruler1.rpt-0 inflating: ruler-src/ruler1.tti inflating: ruler-src/ruler1.tto-0 inflating: ruler-src/ruler2.out-0 inflating: ruler-src/ruler2.prob-0 inflating: ruler-src/ruler2.rpt-0 inflating: ruler-src/ruler2.tti inflating: ruler-src/ruler2.tto-0 inflating: ruler-src/ruler3.ens inflating: ruler-src/ruler3.out-0 inflating: ruler-src/ruler3.prob-0 inflating: ruler-src/ruler3.rpt-0 inflating: ruler-src/ruler3. Program completed successfully ~/work/ruler-src $ diff ruler1.rpt ruler1.rpt-0 ---ruler1.rpt +++ ruler1.rpt-0 @@ -1,5 +1,5 @@ -RULER Version 4.1a [19-Nov-2017] RUN The commands are listed in the commands.sh file of the example directory.

Dedicated softwares and libraries

Science and data analysis often comes with many dedicated softwares and libraries. Keeping up with the versions, ... on a single machine can be tedisou. It's even harder -as mentionned in the introduction -in a large collaboration where many team members have to use the same environnement.

That's when docker (or containers) come in play and make everything simpler. Members of the team just have to use the same image for their work and the homogenieity of software and librabries environnement is assured.

Cern's Root

The largely used Root library has several versions with sometimes no backward compatibity between major ones. Docker can help for both cases: either to keep uptodate with the latest version without having to reinstall or upgrade your own computer, or keeping one static version.

Unfortunately, there has not been any support for official root image for a while: https://hub.docker.com/u/rootproject A few basic images exist and can be used, but their intereste stays limited as a working image. However, they can be used as basis for your own images.

If you want an image that can work for you out of the box, you can try unofficial ones. The image akalinow/root-fedora31 works well and offer jupyterlab support, solving the issue of display with a container (a container is intended to work as command line only or over http connection).