Flux & Energy Dependence of Methane Production From Graphite Due to H + Impact

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1 0.9^07090 Canadian Fusion Fuels Technology Project Flux & Energy Dependence of Methane Production From Graphite Due to H + Impact AUTHORS: J.W. Davis, A.A. Haasz, P.C. Stangeby University of Toronto Institute for Aerospace Studies CFFTP PREPUBLICATION CFFTP Report Number June 1986

2 The Canadian Fusion Fuels Technology Project represents part of Canada's overall effort in fusion development. The focus for CFFTP is tritium and tritium technology. The project is funded by the governments of Canada and Ontario, and by Ontario Hydro. The Project is managed by Ontario Hydro. CFFTP will sponsor research, development and studios to extend existing experience and capability gained in handling tritium as part of the CANDU fission program. It is planned that this work will be in full collaboration and serve the needs of international fusion programs.

3 VVhX. & ENERGY DEPENDENCE OF METHANE PRODUCTION FROM GRAPHITE DUE TO H+ IMPACT CFFTP - B June 1986 C - Copyright Ontario Hydro, Canada Enquiries about Copyright and reproduction should be addressed to: Program Manager 2700 Lakeshore Road West Mississauga, Ontario L5J 1K3

4 FLUX AND ENERGY DEPENDENCE OF METHANE PRODUCTION FROM GRAPHITE DUE TO H + IMPACT Report No. CFFTP-B June 1986 by J. W. Davis, A. A. Haasz and P. C. Stangeby University of Toronto Institute for Aerospace Studies Fusion Research Group CFFTP PREPUBLICATION This document is intended for publication in the open literature. It is made available on the understanding that it may not be further circulated and extracts or references may not be published prior to publication of the original, without the consent of the Program Manager, CFFTP. Enquiries about Copyright and reproduction should be addressed to: Program Manager, CFFTP 2700 Lakeshore Road West Mississauga, Ontario L5J 1K3 Report No. CFFTP-B CFFTP PREPUBLICATION

5 Prepared by: Jx J. W. Davis P. C.Vst*rtgeD, University of Toronto Institute for Aerospace Studies Fusion Research Group Reviewed by: K. Y. Wong, Manager Canadian Fusion Fuel nology Safety.nology Project Approved by: T. S. Drolet, Program Manager Canadian Fusion Fuels Technology Project Report No. CFFTP-B-86O17

6 ACKNOWLEDGEMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada and the Canadian Fusion Fuels Technology Project. Report No. CFFTP-B-86O17 ii

7 SUMMARY Carbon is in widespread use for limiter surfaces, as well as first wall coatings in current tokamaks. Chemical erosion via methane formation, due to energetic H + impact, is expected to contribute to the total erosion rate of carbon from these surfaces. Experimental results are presented for the methane yield from pyrolytic graphite due to H + exposure, using a mass analyzed ion beam. H + energies of kev and flux densities of ~5*10 13 to H + /cm 2 s were used. The measured methane yield (CH^/H" 1 ") initially increases with flux density, then reaches a maximum, which is followed by a gradual decrease. The magnitude of the maximum yield and the flux density at which it occurs depends on the graphite temperature. The yields obtained at temperatures corresponding to yield maxima at specific flux densities also show an initial increase, followed by a shallow maximum and a gradual decrease, as a function of flux density; the maximum occurs at ~10 15 H + /cm 2 s. Also presented are results on the methane production dependence on ion energy over the range 0.1 to 3 kev, and graphite temperature dependence measurements. Report No. CFFTP-B iii

8 1.0 INTRODUCTION 2.0 EXPERIMENT 3.0 RESULTS AND DISCUSSION TABLE OF CONTENTS 3.1 H + Flux Dependence of Methane Yield 3.2 H + Energy Dependence of Methane Yield 3.3 Temperature Dependence of Methane Yield 4.0 CONCLUSIONS 5.0 REFERENCES FIGURE CAPTIONS Figure 1: Constant temperature flux profile measurements for 3 kev H 3 beam (i.e., 1 kev/h + ) at normal incidence: V 700K, A 750K, o 800K, D 850K. The lines are drawn through constant temperature points. Figure 2: Maximum CH^ yield, measured at T m, as a function of flux density for beam at noijinal incidence: o 3 kev H 3 (1 kev/h + ) and O 9 kev H 3 (3 kev/h + ). Figure 3: CH H yield as a function of incident ion energy for beam at normal incidence: o 2xio 12t H + /cm 2 T ffl = 750K, exio 1 * H + /cm 2 T = 775K, A ~10 16 H + /cm 2 T m = K, A ~10 16 H + /cm2s (3 T m = K (sample floating at +2700V and +2100V respectively to decelerate beam), * 2X10 1 ** H + /cm 2 s e 800K (off T m ). 10 ~J Figure 4: CH,, yield as a function of sample temperature, at ~J0 H + /cm 2 s and beam at normal incidence: * 300 ev H 3 (sample floating at +2700V); A 900 H 3 (sample floating at +2100V); o 3 kev H 3 ; D 9 kev H Report No. CFFTP-B-86017

9 1. INTRODUCTION The first walls and limiters of fusion reactors will be subjected to large fluxes of H + ions and charge-exchange neutrals of various energies, as well as other hydrogenic and non-hydrogenic species. Graphite limiters and wall protection tiles are in widespread use in current tokamaks, e.g., about 2000 kg of graphite in TFTR and >20 m 2 surface coverage in JET. Carbonization of all internal surfaces has also been used in TEXTOR and JET [1,2], It is therefore important to determine the extent of the chemical erosion of carbon at the various fluxes concerned. Limiter H fluxes are generally H + /cm 2 s; walls receive energetic charge-exchange neutral fluxes of H cx /cm 2 s. In both cases energies are 10's to 100's ev. Since the total flux of charge-exchange neutrals to the walls is about equal to the total ion flux to the limiters, and the energies are similar (10's to 100's ev), these two impurity sources are of approximately equal strength [3]. The wall-released carbon will generally be a less important source for contamination of the central plasma because of the screening action of the scrape-off layer [4]; this source, however, dominates the edge plasma itself and contributes to edge radiation effects, presumably including MARFE's (Multifaceted Asymmetric Radiation From the Edge). Furthermore, the gas removed from the torus at the end of a discharge is strongly contaminated with hydrocarbons, presumably created from the wall carbon as the plasma confinement is lost. In D-T devices, much of the tritium will be subject to this mechanism, increasing tritium recovery problems. It has frequently been suggested that in an actual reactor environment, the high H + flux density at limiter surfaces will lead to saturation of the chemical erosion process, resulting in much lower CH^/H + yields. Initial support for this prediction was provided by Smith and Meyer [5], who found that the collected experimental results of various investigators [6-10] fit closely to the downward trend predicted by the model of Erents et al [6], This conclusion may be questioned, however, because the effect of bombarding H + energy (<1 kev to 20 kev) has not been accounted for. It has since been shown that differences in H+ bombarding energy have a far stronger influence than expected by Smith and Meyer [5] (see, for example, the recent review by Auciello et al [11] and present results). Report No. CFFTP-B-86017

10 More recently, results by Bohdansky and Roth [12] have shown a similar decrease in yield at high flux densities (>, H + /cm 2 ). Also, recent results from the JET and DITE tokamaks indicate that chemical erosion of graphite at limiter surfaces may not be significant in actual tokamak discharges [3, 13], While controlled laboratory experiments with mass-analyzed ion beams are limited to fluxes corresponding to the first wall (viz, ~10 16 H + /cm 2 s at 100's ev energy), it has been possible to demonstrate a decreasing trend in the methane formation yield at high flux densities. The primary objective of the present study is to investigate the possible existence of saturation effects in the chemical erosion, via methane formation, of graphite exposed to various H + flux densities in the kev energy range. 2. EXPERIMENT All experiments were performed in an UHV system, bakeable to 500K, with typical base pressures <10~ 9 Torr, mainly H 2. The pyrolytic graphite sample used was an 8.5 mm wide strip of ~0.25 mm thickness and 50 mm length. It was held at both ends by stainless steel grips, which allowed resistive heating of the sample to 2200K. Sample preparation normally consisted of baking for 3-4 min at 2200K in a separate UHV chamber. The hydrogen ions used in the experiments were produced by a low-energy, high-flux, mass-analyzed ion accelerator. To obtain the highest possible flux densities, the sample was positioned normal to the beam, about 7 mm away from the 3 mm diameter beam aperture. Allowing for some beam divergence, the beam spot size on the sample was estimated to be ~0.1 cm 2. All of the results reported here were obtained with an H 3 beam; thus a beam current of 53 M-A H 3 corresponds to a flux density of ~10 16 H + /cm 2 s. For most experiments, the sample was biased at +20V with respect to ground, in order to suppress secondary electrons produced at the target by the incident ions. The exceptions to this were some of the low energy H 3 bombardment experiments, which were performed with the sample floating at positive high voltage (3 kev H 3 beam and sample at or V) in order to decelerate the beam, and thus produce high fluxes of low incident energies at the target. Some difficulty was encountered in measuring the true beam current under these conditions, and therefore only data with reliable Report No. CFFTP-B-86017

11 current measurements for the target deceleration cases are presented here; such data are identified on the plots. The methane produced during the hydrogen-carbon interaction was monitored in the residual gas via the mass 15 signal with an Extranuclear quadrupole mass spectrometer housed in a differentially pumped chamber. After each experiment, the quadrupole sensitivity was calibrated using a known CH 4 leak rate. The H + bombardment of the graphite samples was performed with the test chamber backfilled with H 2 to 3.6X10" 1 * Torr in order to maintain a constant total pressure in the differentially pumped quadrupole chamber. This was necessary to ensure that the quadrupole sensitivity to CH 4 remained constant when H + fluxes were varied. Under these conditions the H 2 0/H 2 partial pressure ratio was <10" 1 *. Several experiments were performed to confirm that the H 2 molecules had no effect on the H + -induced methane production. Apparent differences noted in an earlier study [14] are attributed to quadrupole sensitivity changes. 3. RESULTS AND DISCUSSION 3.1 H + Flux Dependence of Methane Yield The flux profiles of the CH^ yields from pyrolytic graphite due to 1 kev H + impact (using 3 kev H 3 ions) for constant sample temperatures, reveal a very distinctive decrease in the erosion yield at both low and high flux densities, see Fig. 1. However, when the maximum yield, Y m is considered, the variation with flux density is much less pronounced, although a decreasing trend at both low and high fluxes is still observable, see Fig. 2. It has been previously shown [e.g., Refs. 5, 6 and 15] that t.ie temperature, T m, at which the maximum erosion yield, Y m, occurs, varies substantially with flux density. For the experiments reported here, the T m varied from ~725K to 850K, corresponding to flux densities of 5*10 13 and 1.7xlO 16 H + /cm 2 s, respectively. Should the constant-temperature yield profiles (see Fig. 1) and the increasing trend in T m prevail for higher fluxes, viz, for expected fluxes at limiters (~ H + /cm 2 s), chemical erosion of graphite limiters in fusion devices may not be observable unless temperatures exceed ~900K; such an effect might account for the DITE graphite probe limiter results [13]. At the reactor walls, on the other hand, where flux densities are of the Report No. CFFTP-B-86O17

12 order of H + /cm 2 s, low levels of methane production could be attained if these surfaces are maintained at temperatures below ~600K. 3.2 H+ Energy Dependence of Methane Yield As has been shown in several previous studies [11], the CH 4 production yield depends on the energy of the incident ions. In Fig. 3, an energy profile is presented for flux densities of 2xlO 11+, 6xlO 14 and ~10 16 H + /cm 2 s at T m. For each flux case, the profiles are very similar, with a maximum occurring at ~600 ev/h +. A further profile at 2xlO 14 H + /cm 2 s is shown for a case where the sample was kept at a constant temperature at 50K above T m (i.e., 800K). The profile has become somewhat distorted, with a peak closer to ~400 ev, as was observed previously (see Fig. 6a in Ref. [11]). The change in the energy profile for the off-t m case results from the shapes of the temperature profiles in Fig. 4. The flatter temperature profiles at low energy result in essentially the same value of Y at T m and at T m ± 50K. At higher H + energies, on the other hand, the temperature profiles become fairly steep, so that being 50K away from T m would produce a relatively larger reduction in the yield. Therefore, the large variations in previously reported yield results [11], as a function of ion energy, could at least be partially due to variations in graphite sample temperatures. A further reason for the variation of results could be due to the fact that CH^ is not the only reaction product, especially at low energies (see, e.g., Haasz et al [16] and Yamada [17]). The effect of different temperature dependences of heavier hydrocarbon yields [17] could influence the comparison of CH 4 yields with total yields measured by weight loss [12]. 3.3 Temperature Dependence of Methane Yield The temperature dependence for the production of methane by H + bombardment is shown for four energies in Fig. 4, for flux densities of ~10 16 H + /cm 2 s. The shape of the temperature profiles is strongly affected by the H + energy, especially at energies <1 kev. For the 1 and 3 kev/h + cases, there is very little difference in the shape of the temperature profiles; however, when the beam energy is reduced to 300 ev/h +, the low temperature slope becomes less steep, while the high temperature slope remains about the same as the higher energy cases. When the beam energy is further reduced to 100 ev/h +, both high and low temperature slopes are substantially altered. Similar results have been observed by Yamada et al Report No. CFFTP-B-86017

13 for H + ions-only impact [18, 19], and by Haasz et ai [14] for simultaneous sub-ev atomic hydrogen and H + -ion bombardment. Temperature profile experiments performed for flux densities in the range 10 u to 10 1 * H + /cm 2 showed that flux density had very little effect on the shape of the profiles. Both high and low temperature slopes remained approximately constant, while the T m and Y m values were dependent on the flux density. We have also observed hystereses in the temperature profiles, similar to those reported by Roth [20], In order to minimize the hysteresis effect, for most of the experimental results reported here, a minimum fluence of ~10 17 H + /cm 2 was used, following each 25K temperature increment between consecutive measurements. This appeared to be more than sufficient to ensure that steady state methane production levels were reached. The exceptions to this are the two lowest flux cases for 3 kev H 3 bombardment (Figs. 1 and 2), where very long exposure times were required. This may account for the larger-than-expected yield measurements for these cases. 4. CONCLUSIONS It has been demonstrated quite clearly that for a constant graphite temperature, the methane production process saturates, leading to reduced erosion yields at high H + ion flux densities. When the maximum erosion yield (corresponding to specific H + flux densities) are considered, the saturation effect is less pronounced; a shallow maximum at ~10 15 H + /cm 2 s, with decreasing yields both above and below this flux are observed. If the observed trends in the constant temperature data (see Fig. 1) were to be projected up two orders of magnitude in flux density (i.e., H + /cm 2 s, typical of fluxes at limiters), the effects of chemical sputtering might not be observable unless temperatures exceed ~900K. For the flux densities expected at the walls of a tokamak, H + /cm 2 s, and at possible operating temperatures of K, the exact conditions are met for the maximum possible erosion yield. Substantial reductions in CH^ production are implied for operation of graphite-protected walls at a lower temperature, say < 600K. The energy dependence of methane yield, at T m, is characterized by a maximum at -600 ev/h +, with yields at 100 ev/h + and 3 kev/h + being about 1/3 and 3/4 of the maximum, respectively. The energy dependence of the yield, Report No. CFFTP

14 measured at constant temperature, off T m, differs from that obtained at T m due to the fact that the temperature profiles of the yields vary with ion energies. In fact, the broad temperature profiles at low energies (~100 ev/h + ) might, at least, partially explain the absence of significant temperature dependence of carbon erosion in the DITE graphite probe limiter experiment [13]. REFERENCES 1. J. Schulter, E. Graffmann, L. Konen, F. Waelbroeck, et al., Proc. 12th European Conf. on Controlled Fusion and Plasma Physics, Budapest, Part II (1985) J. P. Coad, G. M. McCracken, S. K. Erents, et al., Proc. 12th European Conf. on Controlled Fusion and Plasma Physics, Budapest, Part II (1985) M. F. Stamp, K. H. Behringer, M. J. Forrest, et al., Proc. 12th European Conf. on Controlled Fusion and Plasma Physics, Budapest, Part II (1985) G. M. McCracken and P. C. Stangeby, Plasma Phys. and Controlled Fusion 27 (1985) J. N. Smith and C. H. Meyer, J. Nucl. Mater., 76 & 77 (1978) S. K. Erents, C. M. Braganza and G. M. McCracken, J. Nucl. Mater. 63, (1976) K. Sone, H. Ohtsuka, T. Abe, et al., Proc. Int. Symp. on Plasma-Wall Interaction, Julich, 1976 (Pergamon Press) N. P. Busharov, E. A. Gorbatov, V. M. Gusev, et al., J. Nucl. Mater., 63 (1976) B. Feinberg and R. S. Post, J. Vac. Sci. Technol. 13 (1976) J. Roth, J. Bohdansky, W. Poschenrieder, et al., J. Nucl. Mater. 63 (1976) Aucielio, A. A. Haasz, and P. C. Stangeby, Rad. Effects, 89 (1985) J. Bohdansky and J. Roth, Rad. Effects, 89 (1985) C. S. Pitcher, G. M. McCracken, D. H. J. Goodall, et al., Nucl. Fusion (submitted). 14. A. A. Haasz, 0. Auciello, P. C. Stangeby, et al., J. Nucl. Mater., 128 & 129 (1984) 593. Report No. CFFTP-B-86017

15 15. J. Roth, J. Bohdansky and K. L. Wilson, J. Nucl. Mater. Ill & 112 (1982) A. A. Haasz, J. W. Davis, 0. Aucielio, et al., J. Nucl. Mater, (to be published). 17. R. Yamada, J. Nucl. Mater, (to be published). 18. R. Yamada, K. Nakamura, K. Sone, et al., J. Nucl. Mater. 95 (1980) R. Yamada and K. Sone, J. Nucl. Mater. 116 (1983) J. Roth, "Chemical Sputtering", in Sputtering by Particle Bombardment II, Ed. R. Behrisch (Springer-Verlag, 1983). Report No. CFFTP-B-86017

16 K + X 0.07 I 006 ^0.05 O , 0 l I I 10 l6 FLUX DENSITY (H + /cm 2 s) CFFTP-B FIGURE 1 8

17 o 3 UJ o i i l l 10 l4 10 l5 FLUX DENSITY (H + /cm 2 s) I I I L 10 l6 CFFTP-B FIGURE 2 9

18 0.10 o o -I UJ H + ENERGY (ev) CFFTP-B FIGURE 3 10

19 O O o ' ' TEMPERATURE (K) 1000 CFFTP-B FIGUF;: 4

Synergistic Methane Formation on Pyrolytic Graphite Due to Combine H + Ion and H Atom Impact

v.. '. ;:. o M 07- Ha 1 is 11 iii! a Canadian Fusion Fuels Technology Project TP Synergistic Methane Formation on Pyrolytic Graphite Due to Combine H + Ion and H Atom Impact AUTHORS: A.A. Haasz, J.W. Davis,