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Surface modification of 300 series stainless steel by a deuterium plasma
439
Journal of Nuclear Materials 103 & 104 (1981) 439-444 ~rth-Ho~~d ~~~~g Company
SURFACE EDIFICATION
OF
300 SERIES STAINLESSSTEEL BY A Q~UTERIUM PLASMA* R. A. Kerst
Sandia National Laboratories Livermore, California 94550 USA
Samples of 300 series stainless steel (SS) and MO have been ex osed to 200 eV and 20 cm- s-l. The eV 03 ions from a deuterium plasma with fluxes up to 1.7 x 10 17 D exposed surfaces were examined by scanning and transmission electron microscopy. Although the highest fluxes produced evidence of erosion, no blisters were seen, The same exposure given to MO, however, produced extensive blistering, The experimental results are compared with theoretical computatjons of hydrogen transport in SS and interpreted in the context of a model for hydrogen blistering. l
1.
INTRODUCTION
Modification of the first-wall and limiters by the edge plasma of a fusion reactor has considerable impact on many areas of reactor operation. Erosion, whether by sputtering or by blistering and exfoliation, has a deleterious effect on wall lifetime and reactor Q-value [l]. Microstructural changes, such as bubble and microchannel formation, may affect helium ash removal as well as tritium inventory and tritium permeaIt is important to determine which of tion. numerous demonstrated and proposed mechanisms for surface modification are actually relevant to fusion reactor operation. Among the proposed mechanisms is the production of "metal snow," [Z-6] which may occur when a surface is exposed to an energetic flux of hydrogeni~ ions or atoms. The incident flux may, if the surface recombination rate is low, lead to a high gas pressure in preexisting If the pressure in the resultcracks or voids. ing blisters were to exceed some critical value, they would burst, releasing metal into Evidence of blister formathe vacuum chamber. tion on stainless steel surfaces exposed to an atomic hydrogen atmosphere has been reported previously [7f.
In the present work, samples of 300 series stainless steel (SS) were exposed to a deu~erium discharge in a piasma-materials interaction simulation device to determine what surface modifj~ation effects should be expected when SS is exposed to the edge plasma of a fusion In particular, the likelihood of proreactor. ducing metal snow is addressed. 2.
APPARATUS
The experimental apparatus consisted of a Lisitano coil discharge [S] produced in a SS 304 vacuum vessel, with an applied axial magnetic *This work was supported Department of Energy.
by the U. S.
field of 150 G for an RF frequency of 400 MHz; the plasma drifted along the magnetic field lines, forming an afterglow column outside the Lisitano coil. With an RF power input of 100 W, a plasma column of 6.0 cm diameter was produced in deuterium, with an electron density of 3.9 x 109 cm-3 and an electron temperature of 6.8 eY, as measured by a Langmuir probe located 30 cm from the Lisitano tail. The neutral gas pressure was controlled by a flow measurement system which operated a piezoelectric leak valve, the pressure being determined by the pumping speed of the system, In these experiments, the deuterium pressure ranged from 3 to 6 mforr. The plasma composition, as measured by a quadrupole mass spectrometer 41 cm from the Lisitano coil, was 11% Dt, 25% 03, and 62% Df, for a deuterium pressure of 4.0 mTorr. These numbers varied with pressure and should be taken as approximate, In most cases, ~70% of ionized D was in the form of D$. Energettc D was provided at the surface of a sample by a bias voltage, V . Samples were mounted on a movable holder whit $ had all parts except for the sample surface electrically insulated from the plasma by a glass-ceramic shroud. This holder was inserted into the afterglow at a point 10 cm from the Lisitano coil, where the plasma density was a factor 21.6 greater than at the 30 cm point. The energy of D striking a sample surface is taken as T
E = + e(V
where mined which 8.4 x 3.
P
- Y,),
f’f
Y is the plasma space potential, deterb$ the Langmuir probe. The vacuum chamber, was unbaked, achieved a base pressure of 10-8 Torr.
CALCULATIONS
The basis of the theory, described originally by Pinchuk et al. [5] and developed more recently bv Ali-Khan et al. [2-31. is that H which is implanted into the nearz&rface region of a metal results in a depth profile of the H concentration c(x,t) which depends upon the depth profile of the implantation T(x), the diffusion
coefficient D, the recombination constant K at the surface (x=0), and the implantation flux multiplied by the sticking probabiljty tx$(t). If a void exists at some depth x, then the H pressure in the void px(t) is determined by Sieverts' law: c(x,t) = co P,(t))* where co is the solubility. When pxft) reaches some critical value, determined by the size of the void, bubble growth may occur which leads to blistering and/or exfoliation. Thus the necessary conditions for the blistering and exfoliation are: (1) the material must contain preexisting voids,‘preferably of large size; (2) the gas must diffuse in the material rapidly enough to reach the bubble growth sites during implantation (but not so rapidly that c is significantly lowered at bubble growth sites by diffusion into the bulk); and (3) the effective flux a$ must be large enough to induce the In terms of physical paracritical pressure. meters, the requirements are high a+, low co, some optimum range of D, and large rfr=characteristic size of voids).
Considering typical values of plasma density and and scrape-off layer temperature, n and T thicknesses 6 ? or con f' emporary tokamaks, and assuming a 4.5, an upper bound for a$ at the wall is 5 x 1017 cm-2s-1 [9]. It is further assumed that the SS wall is at temperature T = 300 K and that D = 0.17 exp(-Ol$jl eV/kT) exp (-0.091 (cm2fs) [lo] an4 co =*7.7 x 10 ev/kT) (H * cm- atm-z)fll]. Baskes 1121 has calculated the recombination rate constant as
C
*2
where y is the surface free energy. Taking y ~250 N/m for SS [x6], the critical pressure for r = 1 nm is p ~5 x 104 atm, Using the solubility and temperature given above, the critical D concentration is c uik,;;;,4,,” is exceeded by c . able to search f??xblisters on IS surfaces which have been exposed to high fluxes of energetic 0. 4.
EXPERIMENTAL
RESULTS
Initially, an experimerit was done to test the accuracy of the DIFFUSE code in modeling lowenergy D implants. With the gas flow into the discharge chamber regulated, the plasma-wall interaction was manifested by fluctuations in the neutral D2 pressure Ap:
AP = b,,
(5)
where a+ is the net Cl flux into the wall (positive or-negative),
To be determined is whether this mechanism is valid for a fusion reactor wall made of 300 series SS under incident fluxes characteristic of the first wall environment.
l/2 c1 cl expC(Es
required to cause growth of a bubble having linear di~nsion r is [15]
and k is a constant
Inositivel.
Figure 1 shows a typical case where the discharge was operated in steady state for ~57 s, then turned off. The global influx of D to the wall was co stant during the discharge f@ ~6.7 x 1014 cm-%- P ), so that the changes in QI_+were due to recycling. Also shown in the figure is the prediction of ap from DIFFUSE. To realistically account for the finite time response of the pumping system, the output from DIFFUSE was used to program the gas source with no discharge running. The agreement is good, although the code slightly overestimates the recombination rate, as seen by the saturation level.
- ED)/kTl (3)
0
where cl = 2.6 x 10z5 molecules - K1j2 . AMUl/2/ atm s s * cm2, Es and ED are the activation energies for solubility and diffusion respectively, co* is the pre-exponential factor for the solubility, and M is the molecular weight of the gas molecules. The DIFFUSE code [13] was used to compute the evolution of c(x,t) for the above values of The effective D flux and materials parameters. time duration was to = 3600 s, although c(x,t) for the first 17 nm reaches a steady state after -1 s. Although it is important to insert the incident flux-beneath the surface [14], the detailed profile is not critical. In these computations, the depth of the implanted D was 0.1 rim. The peak concentration is found to be Cmax(X,t) 22.b X lo-' H/metal ratio at x zO.2 rim. The half-width of the profile is '40.2 pm. In the simplest approximation, the pressure
Samples of SS 304 and 316 were mechanically polished or electropolished and exposed to various deuterium discharge conditions. After plasma exposure, the samples were examined with optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) for evidence of blisters, bubbles, or exfoliation. Table 1 summarizes the parameters and results for several exposures. Df ions were accelerated to samples 3-7 and the c&rent was measured to determine the flux. Nuclear reaction analysis (NRA) of carbon paper samples exposed in this apparatus showed the assumption of Df as the main ion species to be valid. The NRA also showed that the current measurement underestimates the D flux by a factor ~2.2, due to eneraetic neutrals produced by recombination or charge exchange. Secondary electron emission was not significant. This correction factor was included in the flux values given in Table 1. Samples 1 and 2 were electrically floating in the plasma, so a current measurement was not possible. The flux to these samples was estimated as the same as that to the
R.A. Kerst /Surface modification of 300 series stainless steel
~~ . 6.0
0
too
50 TimeW
Figure
1.
Pressure changes induced in the discharge chamber by plasm-wall interaction vs. time. The solid curve is experimenta? data, and the dashed curve is computed from DIFFUSE [13].
wall (described above). This value may be low, as the samples were placed well into the plasma column.
In some cases, the same region of the sample was examined before and after the plasma exposure. . _ __ In all cases, a distinct exposure boundary existed, caused by a mask placed over each sample during exposure. The general result of post-exposure analysis was that SEm showed no morphological change in the surface due to plasma exposure. Figure 2 shows the surface of sample 3 both inside fa) and outside lb) the exDosed reaion. There ii no essential difference between ihe two
441
regions, all of the features in both cases being associated with the ~chanical polishing. The TEM of sample 5 also showed no bubbles down to a size of ~3 nm. Figure 3 shows the exposed region of sample 4 (similar to that of sample 6). The eroded or etched appearance of the overall surface is similar to that reported previously for H sputtered SS [17]. This effect was observed to be progressive in the case of a sample which was removed from the discharge and examined at A negative result with a regular intervals. lower flux of H ions on SS 304 was also reported previously by Clausing et al. [18]. A MO sample was exposed to the same plasma conditions as sample 4. The as-received material was foil, produced by powder metallur~ and zone In this case, the surface was extenrefined. sively covered by blisters, ranging in size from 22 to 30 pm, as shown in Figure 4. There was no exfoliation, however. An attempt was made to observe exfoliation from the walls of the plasma chamber. A grounded piece of In foil was placed in the bottom of the machine where it was not directly reached by the plasma, but had a vertical line of sight to the wall. After being in this position during ?,43 hr. of plasma discharges, the In surface was examined by Auger electron spectroscopy (AESj and SER. The AES showed no elements pertaining to any interior component of the machine, including The SEM, combined with X-ray analythe SS wall. sis, showed the surface to consist en irely of . The area of exoosed In was ~8 cm m!i In.
Figure 2.
5.
SEMs of sample 3, SS 304 03 energ 320 eV, flux = 2.6 x lo16 Dvcm-2sfluence = 1.9 x 1020 D-cm-2. (a) Un: exposed region. (b) Exposed region. All features seen are due to mechanical polishing. There is no eviden:e= of blistering.
Figure 3.
SEM of sample 4, SS 304 D< energy = 310 eV, flux = 1.7 x 1017 ti.cm-2s-l (max.), fluence = 3.2 x 1020 D.cme2. The enhanced erosion on the righthand side is due to crystallographic orientation.
Figure 4.
SEM of sample 7, MO, D' energy = 315 eV, flux = 1.6 x 1217 D*cm-2s-1 (max.), fluence = 3.0 x 1020 D.cm-2. The blisters seen range in size from s2 to 30 urn.
DISCUSSION AND CONCLUSIONS
In general, the SS samples exposed to D plasma in this experiment showed no evidence of blistering. In one instance only (sample 4), some features appeared on SS which could plausibly be identified as blister remnants. Even here, though, the surface was so eroded that such identification would be conjectural at best. Indeed, this fact is telling in itself. If hydrogen blistering does occur but only in conjunction with erosion from other sources, then it is not likely to be an important cause of high-Z impurity influx into fusion plasmas. Blisters did form on MO, however, as reported previously [5,6], which tends to substantiate the theory of Ali-Khan et al. [2-31. The difference between MO and SS with regard to blister formation may be due to the brittle character of the former. Since preexisting voids or cracks are required for exfoliation to occur, the relative ductility of SS tends to render it more immune. It is concluded from this work that brittle materials are susceptible to blistering in a hydrogen plasma, and possibly to exfoliation. Fio limiters,for example, which receive a high flux of ions, may introduce MO into the plasma core by this mechanism. SS walls, on the other hand, pose no great hazard in this regard and are eroded primarily by sputtering. ACKNOWLEDGEMENTS Thanks are due to K. L. Wilson, H. I. Baskes, R. Bastasz, F. Greulich, and A. E. Pontau for helpful S.P.Orth, ._ _discussions,andtoW.L.Chrisman, _ .._ and B. ti. Brown, Jr., tor technical assistance.
REFERENCES [1] G. '9 [2] I. P. [3] I. P.
II. McCracken (1979) 889. Ali-Khan, K. Wienhold, J. Ali-Khan, K. Wienhold, J.
and P. E. Stott, Nucl. Fusion J. Dietz, F. Nucl. Mater. L. Dietz, F. Nucl. Mater.
Waelbroeck, and 74 (1978) 132. Waelbroeck, and 74 (1978) 138.
R.A. Kerst /Surface
[41 I. Ali-Khan,
modification
K. J. Dietz, F. Waelbroeck, and P. Wienhold, J. Nucl. Mater. 76 & 77 (1978) 263. [51 V. P. Pinchuk, A. N. Gorban', and V. G. Kornich, Sov. Phys.-Tech. Phys. 19 (1974) 808. L61 S. Okuda and S. Imoio, Jap: J. Appl. Phys. 19 (19801 971. [71 I. Ali-Khan, K. J. Dietz, F. G. Waelbroeck, and P. Wienhold, J. Nucl. Mater. 85 & 86 (1979) 1151. C81 G. Lisitano, M. Fontanesi, and E. Sindoni, Appl. Phys. Letters 16 (1970) 122. [91 S1.J. Zweben and R. 3. Taylor, UCLA Center for Plasma Physics and Fusion Enaineerina _I _a Report PPG-543 (1981). DO1 K. L. Wilson and M. I. Baskes, J. Nucl. Mater. 76 & 77 (1978) 291. Dl]J. R. Phillips and B. F. Dodge, A.I. Ch. E. Journal 14 (1968) 392. D2]M. I. Baskes, J. Nucl. Mater. 92 (1980) 318. n3lM. I. Baskes. Sandia National Laboratories _ _ Report SAND8&8201 (1980). 04]M. I. Baskes, J. Nucl. Mater. (to be published). n5]J. H. Evans, J. Nucl. Mater. 76 & 77 (1978) 228. n6]L. E. Murr, Interfacial Phenomena in Metals and Alloys (Addison-Wesley, 1975) p.123. [17]R. Behrisch, J. Bohdansky, G. H. Oetjen, J. Roth, G. Schilling, and H. Verbeek, J. Nucl. Mater. 60 (1976) 321. 08]R. E. Clausing, L. C. Emerson, and L. Heatherly, J. Nucl. Mater. 76 & 77 (1978) 199.
of 300 series stainless steel
443
Journal of Nuclear Materials 103 & 104 (1981) 439-444 ~rth-Ho~~d ~~~~g Company
SURFACE EDIFICATION
OF
300 SERIES STAINLESSSTEEL BY A Q~UTERIUM PLASMA* R. A. Kerst
Sandia National Laboratories Livermore, California 94550 USA
Samples of 300 series stainless steel (SS) and MO have been ex osed to 200 eV and 20 cm- s-l. The eV 03 ions from a deuterium plasma with fluxes up to 1.7 x 10 17 D exposed surfaces were examined by scanning and transmission electron microscopy. Although the highest fluxes produced evidence of erosion, no blisters were seen, The same exposure given to MO, however, produced extensive blistering, The experimental results are compared with theoretical computatjons of hydrogen transport in SS and interpreted in the context of a model for hydrogen blistering. l
1.
INTRODUCTION
Modification of the first-wall and limiters by the edge plasma of a fusion reactor has considerable impact on many areas of reactor operation. Erosion, whether by sputtering or by blistering and exfoliation, has a deleterious effect on wall lifetime and reactor Q-value [l]. Microstructural changes, such as bubble and microchannel formation, may affect helium ash removal as well as tritium inventory and tritium permeaIt is important to determine which of tion. numerous demonstrated and proposed mechanisms for surface modification are actually relevant to fusion reactor operation. Among the proposed mechanisms is the production of "metal snow," [Z-6] which may occur when a surface is exposed to an energetic flux of hydrogeni~ ions or atoms. The incident flux may, if the surface recombination rate is low, lead to a high gas pressure in preexisting If the pressure in the resultcracks or voids. ing blisters were to exceed some critical value, they would burst, releasing metal into Evidence of blister formathe vacuum chamber. tion on stainless steel surfaces exposed to an atomic hydrogen atmosphere has been reported previously [7f.
In the present work, samples of 300 series stainless steel (SS) were exposed to a deu~erium discharge in a piasma-materials interaction simulation device to determine what surface modifj~ation effects should be expected when SS is exposed to the edge plasma of a fusion In particular, the likelihood of proreactor. ducing metal snow is addressed. 2.
APPARATUS
The experimental apparatus consisted of a Lisitano coil discharge [S] produced in a SS 304 vacuum vessel, with an applied axial magnetic *This work was supported Department of Energy.
by the U. S.
field of 150 G for an RF frequency of 400 MHz; the plasma drifted along the magnetic field lines, forming an afterglow column outside the Lisitano coil. With an RF power input of 100 W, a plasma column of 6.0 cm diameter was produced in deuterium, with an electron density of 3.9 x 109 cm-3 and an electron temperature of 6.8 eY, as measured by a Langmuir probe located 30 cm from the Lisitano tail. The neutral gas pressure was controlled by a flow measurement system which operated a piezoelectric leak valve, the pressure being determined by the pumping speed of the system, In these experiments, the deuterium pressure ranged from 3 to 6 mforr. The plasma composition, as measured by a quadrupole mass spectrometer 41 cm from the Lisitano coil, was 11% Dt, 25% 03, and 62% Df, for a deuterium pressure of 4.0 mTorr. These numbers varied with pressure and should be taken as approximate, In most cases, ~70% of ionized D was in the form of D$. Energettc D was provided at the surface of a sample by a bias voltage, V . Samples were mounted on a movable holder whit $ had all parts except for the sample surface electrically insulated from the plasma by a glass-ceramic shroud. This holder was inserted into the afterglow at a point 10 cm from the Lisitano coil, where the plasma density was a factor 21.6 greater than at the 30 cm point. The energy of D striking a sample surface is taken as T
E = + e(V
where mined which 8.4 x 3.
P
- Y,),
f’f
Y is the plasma space potential, deterb$ the Langmuir probe. The vacuum chamber, was unbaked, achieved a base pressure of 10-8 Torr.
CALCULATIONS
The basis of the theory, described originally by Pinchuk et al. [5] and developed more recently bv Ali-Khan et al. [2-31. is that H which is implanted into the nearz&rface region of a metal results in a depth profile of the H concentration c(x,t) which depends upon the depth profile of the implantation T(x), the diffusion
coefficient D, the recombination constant K at the surface (x=0), and the implantation flux multiplied by the sticking probabiljty tx$(t). If a void exists at some depth x, then the H pressure in the void px(t) is determined by Sieverts' law: c(x,t) = co P,(t))* where co is the solubility. When pxft) reaches some critical value, determined by the size of the void, bubble growth may occur which leads to blistering and/or exfoliation. Thus the necessary conditions for the blistering and exfoliation are: (1) the material must contain preexisting voids,‘preferably of large size; (2) the gas must diffuse in the material rapidly enough to reach the bubble growth sites during implantation (but not so rapidly that c is significantly lowered at bubble growth sites by diffusion into the bulk); and (3) the effective flux a$ must be large enough to induce the In terms of physical paracritical pressure. meters, the requirements are high a+, low co, some optimum range of D, and large rfr=characteristic size of voids).
Considering typical values of plasma density and and scrape-off layer temperature, n and T thicknesses 6 ? or con f' emporary tokamaks, and assuming a 4.5, an upper bound for a$ at the wall is 5 x 1017 cm-2s-1 [9]. It is further assumed that the SS wall is at temperature T = 300 K and that D = 0.17 exp(-Ol$jl eV/kT) exp (-0.091 (cm2fs) [lo] an4 co =*7.7 x 10 ev/kT) (H * cm- atm-z)fll]. Baskes 1121 has calculated the recombination rate constant as
C
*2
where y is the surface free energy. Taking y ~250 N/m for SS [x6], the critical pressure for r = 1 nm is p ~5 x 104 atm, Using the solubility and temperature given above, the critical D concentration is c uik,;;;,4,,” is exceeded by c . able to search f??xblisters on IS surfaces which have been exposed to high fluxes of energetic 0. 4.
EXPERIMENTAL
RESULTS
Initially, an experimerit was done to test the accuracy of the DIFFUSE code in modeling lowenergy D implants. With the gas flow into the discharge chamber regulated, the plasma-wall interaction was manifested by fluctuations in the neutral D2 pressure Ap:
AP = b,,
(5)
where a+ is the net Cl flux into the wall (positive or-negative),
To be determined is whether this mechanism is valid for a fusion reactor wall made of 300 series SS under incident fluxes characteristic of the first wall environment.
l/2 c1 cl expC(Es
required to cause growth of a bubble having linear di~nsion r is [15]
and k is a constant
Inositivel.
Figure 1 shows a typical case where the discharge was operated in steady state for ~57 s, then turned off. The global influx of D to the wall was co stant during the discharge f@ ~6.7 x 1014 cm-%- P ), so that the changes in QI_+were due to recycling. Also shown in the figure is the prediction of ap from DIFFUSE. To realistically account for the finite time response of the pumping system, the output from DIFFUSE was used to program the gas source with no discharge running. The agreement is good, although the code slightly overestimates the recombination rate, as seen by the saturation level.
- ED)/kTl (3)
0
where cl = 2.6 x 10z5 molecules - K1j2 . AMUl/2/ atm s s * cm2, Es and ED are the activation energies for solubility and diffusion respectively, co* is the pre-exponential factor for the solubility, and M is the molecular weight of the gas molecules. The DIFFUSE code [13] was used to compute the evolution of c(x,t) for the above values of The effective D flux and materials parameters. time duration was to = 3600 s, although c(x,t) for the first 17 nm reaches a steady state after -1 s. Although it is important to insert the incident flux-beneath the surface [14], the detailed profile is not critical. In these computations, the depth of the implanted D was 0.1 rim. The peak concentration is found to be Cmax(X,t) 22.b X lo-' H/metal ratio at x zO.2 rim. The half-width of the profile is '40.2 pm. In the simplest approximation, the pressure
Samples of SS 304 and 316 were mechanically polished or electropolished and exposed to various deuterium discharge conditions. After plasma exposure, the samples were examined with optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) for evidence of blisters, bubbles, or exfoliation. Table 1 summarizes the parameters and results for several exposures. Df ions were accelerated to samples 3-7 and the c&rent was measured to determine the flux. Nuclear reaction analysis (NRA) of carbon paper samples exposed in this apparatus showed the assumption of Df as the main ion species to be valid. The NRA also showed that the current measurement underestimates the D flux by a factor ~2.2, due to eneraetic neutrals produced by recombination or charge exchange. Secondary electron emission was not significant. This correction factor was included in the flux values given in Table 1. Samples 1 and 2 were electrically floating in the plasma, so a current measurement was not possible. The flux to these samples was estimated as the same as that to the
R.A. Kerst /Surface modification of 300 series stainless steel
~~ . 6.0
0
too
50 TimeW
Figure
1.
Pressure changes induced in the discharge chamber by plasm-wall interaction vs. time. The solid curve is experimenta? data, and the dashed curve is computed from DIFFUSE [13].
wall (described above). This value may be low, as the samples were placed well into the plasma column.
In some cases, the same region of the sample was examined before and after the plasma exposure. . _ __ In all cases, a distinct exposure boundary existed, caused by a mask placed over each sample during exposure. The general result of post-exposure analysis was that SEm showed no morphological change in the surface due to plasma exposure. Figure 2 shows the surface of sample 3 both inside fa) and outside lb) the exDosed reaion. There ii no essential difference between ihe two
441
regions, all of the features in both cases being associated with the ~chanical polishing. The TEM of sample 5 also showed no bubbles down to a size of ~3 nm. Figure 3 shows the exposed region of sample 4 (similar to that of sample 6). The eroded or etched appearance of the overall surface is similar to that reported previously for H sputtered SS [17]. This effect was observed to be progressive in the case of a sample which was removed from the discharge and examined at A negative result with a regular intervals. lower flux of H ions on SS 304 was also reported previously by Clausing et al. [18]. A MO sample was exposed to the same plasma conditions as sample 4. The as-received material was foil, produced by powder metallur~ and zone In this case, the surface was extenrefined. sively covered by blisters, ranging in size from 22 to 30 pm, as shown in Figure 4. There was no exfoliation, however. An attempt was made to observe exfoliation from the walls of the plasma chamber. A grounded piece of In foil was placed in the bottom of the machine where it was not directly reached by the plasma, but had a vertical line of sight to the wall. After being in this position during ?,43 hr. of plasma discharges, the In surface was examined by Auger electron spectroscopy (AESj and SER. The AES showed no elements pertaining to any interior component of the machine, including The SEM, combined with X-ray analythe SS wall. sis, showed the surface to consist en irely of . The area of exoosed In was ~8 cm m!i In.
Figure 2.
5.
SEMs of sample 3, SS 304 03 energ 320 eV, flux = 2.6 x lo16 Dvcm-2sfluence = 1.9 x 1020 D-cm-2. (a) Un: exposed region. (b) Exposed region. All features seen are due to mechanical polishing. There is no eviden:e= of blistering.
Figure 3.
SEM of sample 4, SS 304 D< energy = 310 eV, flux = 1.7 x 1017 ti.cm-2s-l (max.), fluence = 3.2 x 1020 D.cme2. The enhanced erosion on the righthand side is due to crystallographic orientation.
Figure 4.
SEM of sample 7, MO, D' energy = 315 eV, flux = 1.6 x 1217 D*cm-2s-1 (max.), fluence = 3.0 x 1020 D.cm-2. The blisters seen range in size from s2 to 30 urn.
DISCUSSION AND CONCLUSIONS
In general, the SS samples exposed to D plasma in this experiment showed no evidence of blistering. In one instance only (sample 4), some features appeared on SS which could plausibly be identified as blister remnants. Even here, though, the surface was so eroded that such identification would be conjectural at best. Indeed, this fact is telling in itself. If hydrogen blistering does occur but only in conjunction with erosion from other sources, then it is not likely to be an important cause of high-Z impurity influx into fusion plasmas. Blisters did form on MO, however, as reported previously [5,6], which tends to substantiate the theory of Ali-Khan et al. [2-31. The difference between MO and SS with regard to blister formation may be due to the brittle character of the former. Since preexisting voids or cracks are required for exfoliation to occur, the relative ductility of SS tends to render it more immune. It is concluded from this work that brittle materials are susceptible to blistering in a hydrogen plasma, and possibly to exfoliation. Fio limiters,for example, which receive a high flux of ions, may introduce MO into the plasma core by this mechanism. SS walls, on the other hand, pose no great hazard in this regard and are eroded primarily by sputtering. ACKNOWLEDGEMENTS Thanks are due to K. L. Wilson, H. I. Baskes, R. Bastasz, F. Greulich, and A. E. Pontau for helpful S.P.Orth, ._ _discussions,andtoW.L.Chrisman, _ .._ and B. ti. Brown, Jr., tor technical assistance.
REFERENCES [1] G. '9 [2] I. P. [3] I. P.
II. McCracken (1979) 889. Ali-Khan, K. Wienhold, J. Ali-Khan, K. Wienhold, J.
and P. E. Stott, Nucl. Fusion J. Dietz, F. Nucl. Mater. L. Dietz, F. Nucl. Mater.
Waelbroeck, and 74 (1978) 132. Waelbroeck, and 74 (1978) 138.
R.A. Kerst /Surface
[41 I. Ali-Khan,
modification
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of 300 series stainless steel
443