The weight changes of Be tiles which functioned as a rail limiter in ISX-B for more than 3500 beam-heated discharges have been determined. The net weight loss for the limiter was 2.0 g, with the central tiles losing a total of 3.2 g and inboard tiles gaining 1.2 g. The weight loss is attributed primarily to the release of Be droplets as a result of limiter surface melting. The weight gains resulted from an inward flow of molten material along the limiter surface. The results indicate high erosion (melt loss) with incomplete and nonuniform redeposition (melt flow) of limiter material during periods of limiter melting

Edmonds, P.H.

England, A.C.

Gabbard, A.

Roberto, J.B.

Zuhr, R.A.

UNT Digital Library

NOTE: This is a draft of a paper being submitted for publication. Contentsof the paper should not be quoted nor referred to without permission ofthe authors.CONF-851174—12DE86 002246Weight Chanye Measurements of Erosion/Depositionat Beryllium L in i te r Tiles in ISX-BJ . B. Roberto, P. H. Edmonds, A. C. England, A. Gabbard, and R. A. ZuhrBy acceptance of this article, thepublisher or recipient acknowledgesthe U.S. Government's right toretain a nonexclusive, royalty-freelicense in and to any copyrightcovering the article.SOLID STATE DIVISIONOAK RIDGE NATIONAL LABORATORYOperated byMARTIN MARIETTA ENERGY SYSTEMS, INC.for theUNITED STATES DEPARTMENT OF ENERGYOak Ridge, TennesseeJuly 1985wmnm * its mmm s wuimWeight Change Measurements of Erosion/Depositionat Beryllium Limiter Tiles in ISX-BJ. B. Roberto, P. H. Edmonds, A. C. England, A. Gabbard, and R. A* Zuhr*Oak Ridge National LaboratoryOak Ridge, Tennessee 37831AbstractThe weight changes of Be tiles which functioned as a rail limiter in ISX-Bfor more than 3500 bean-heated discharges have been determined. The net weightloss for the limiter was 2.0 g, with the central tiles losing a total of 3.2 gand the inboard tiles gaining 1.2 g. The weight loss is attributed primarily tothe release of Be droplets as a result of limiter surface melting. The weightgains resulted from an inward flow of molten material along the limiter surface.The results indicate high erosion (melt loss) with incomplete and nonuniformredeposition (melt flow) of limiter material during periods of limiter melting.*0perated by Martin Marietta Energy Systems, Inc. under contractDE-AC05-840R21400 with the U. S. Department of Energy.DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.NSMMMi OF WS HRMIff B- 2 -1. IntroductionThe erosion of liiniters and other components exposed to high particle andheat fluxes in the plasma edge of fusion devices is a major concern in com-ponent lifetime and plasma contamination. Erosion rates have been measured ina variety of fusion devices at test lirniters and divertor plates using surfaceprobes (1-5), laser-induced fluorescence (6-9), thin-layer activation (10), andisotopic markers (11). These measurements have provided localized informationon erosion rates and mechanisms, but have not addressed the macroscopic, long-term erosion/redeposition behavior of lirniter and divertor structures. Inthis paper, we report the use of weight change measurements to monitor theerosion and redistribution of limiter material at a rail limiter during threemonths of beam-heated operation in the ISX-B tokamak.The weight change measurements were part of the ISX-B beryllium limiterexperiment which is described in detail elsewhere (12-14). The intent of thisexperiment was to evaluate the performance of beryllium as a limiter materialunder conditions of hydrogen fluence, thermal cycling, and heat fluxesexpected in breakeven machines. In particular, this experiment served as aprecursor to the possible use of beryllium limiters in the European JETdevice. The weight change results provide quantitative information on theerosion/deposition behavior of the beryllium limiter.2. ExperimentalThe limiter consisted of 12 individual beryllium tiles arranged in a railassembly and mounted at the top of ISX-B. Each tile was approximately10 x 10 x 2.5 cm3 an(j 260 g with the front surface curved to maintain aconstant power loading for a power 1/e decay length of 2 cm. Half of thetiles were tessellated with a square array of 1 cm deep slots to limit surface- 3 -stresses. The tessellations did not appear to influence the weight -.hangeresults. Photographs of the installed lirniter are shown in Figure 1 and theposition of the limiter in relation to the plasma is indicated in the inset ofFigure 2.The beryllium limiter functioned as the main limiter of ISX-B for more than3500 beam-heated discharges with typical plasma parameters Ip=116kA, Bj=llkG,ne=4 x 1013/cm3, Pg=0.8MW (0.2s), and t=0.3s. Under these conditions, the maxi-mum power flux to the limiter was 2 KW/cm^. Early in this sequence, the plasmacurrent was raised to 155 kA for 300 discharges to test the limiter at higherpower loads. The 155 kA discharges resulted in surface melting of the limiter(Figure 1) which had not been observed at 116 kA. The general topographicfeatures developed on the surface during this limited 155 kA high power opera-tion persisted throughout the experiment. The total deuterium fluence to thecentral 20 ci.î  of the limiter was approximately 2 x 10^2 D/crn^.Prior to installation in ISX-B, the limiter tiles were cleaned ultrasoni-cally in methyl alcohol and acetone, dried in a rough vacuum, and weighed to anaccuracy of <1 mg using a large mechanical balance. Following completion of theexperiment, the limiter was carefully disassembled and the individual tiles wereplaced in plastic bags which had been previously weighed. The tiles were thenreweighed in the bags to insure that there was no additional loss of materialdue to abrasion or handling.3. ResultsThe weight change results are summarized in Figure 2 and Table 1.Overall, the litniter showed a net weight loss of 2.0 g with the central tileslosing 3.2 g and inside tiles gaining 1.2 g. Surface probe measurements(15, 16) indicate that approximately 0.2 g of beryllium was uniformly- 4 -distributed around the torus. The remainder of the weight loss is presumed tobe in the form of small droplets of molten material which were observed on theinside wall and at the bottom of the vacuum vessel in the vicinity of thelimiter. One of the central tiles (Tile 7) had been gamma activated prior toinstallation of the limiter (17). Activity representing approximately 0.7 g ofberyllium was found on the inboard side of the liiniter following the experiment.The measured weight loss of Tile 7 was 1.1 g implying that more than half of thelost material was redistributed inward along the liiniter. This is consistentwith the observed weight gain of the inner tiles. Examination of the photo-graphs of Figure 1 suggests an inward directionality to the flow of moltenmaterial on the limiter surface which could account for the weight gain. Sincethe liiiiiter was mounted horizontally, this flow was probably driven by electro-magnetic forces.4. DiscussionIt is evident from Figures 1 and 2 that the limiter experiencedsubstantial erosion and redistribution of material during its operation inISX-B. Extrapolated to steady-state, the material loss at the central tileswould require daily tile replacement. Of course, this extrapolation would beunreliable if the weight loss occurred primarily during the high currentdischarges which resulted in lirniter melting. The existence of molten flowpatterns on the limiter surface and beryl!iur.i droplets on the vacuum vesselwalls strongly suggests that limiter melting was responsible for much of theweight change. The observation (15, 16) that only 10% of the beryllium losswas deposited uniformly on the torus .-'alls suggests that sputtering and eva-poration were less important erosion mechanisms.- 5 -Sputtering estimates based on available beryllium sputtering data (13)suggest that a combination of deuterium sputtering and self-sputtering of thelimiter could account for approximately 10% of the observed weight loss.Beryllium evaporation would contribute less than 1% of the weight loss for aplane lirniter surface, but could be significantly enhanced at local hot spotscreated by protrusions in the melted region with reduced thermal contact tothe bulk. There is evidence from spectroscopy (12) that the beryllium contentin the plasma was Much higher after the initial surface melting than beforethe melting for similar discharges. The beryllium density in the plasmasteadily increased as the base limiter temperature increased, further indi-cating the importance of thermal processes (12) in impurity introduction.Implantation of deuterium and impurities (19) is insufficient to account forthe observed weight gain of the inner tiles, but could contribute to the smallweight gain of the outer tiles. It is apparent from the directionality andvolume of the inward flow of molten material on the limiter surface that lirnitermelting during the period of high current operation was responsible for most ofthe beryllium redistribution. This horizontal flow suggests a position instabi-lity for the melt layer due to electromagnetic forces which could contributesignificantly to local erosion during fault conditions (such as disruptions)which result in surface melting.5. ConclusionsThis paper summarizes the results of weight change measurements which wereperformed on the individual segments of a beryllium rail lirniter during exten-sive beam-heated operation in ISX-B. Substantial erosion and redistributionof limiter material was observed, with the central tiles losing a total of3.2 g and the inner tiles gaining 1.2 g. The weight loss is attributed- 6 -primarily to the release of beryllium droplets following surface melting duringhigh current operation. An inward flow of molten material along the limitersurface accounted for most of the weight gain. This suggests a position insta-bility for the melt layer. Overall, the results indicate high erosion(primarily melt loss) and incomplete and non-uniform redeposition (primarilymelt flow) of limiter material for conditions which involve periods of limitersurface melting. The results also establish the usefulness of weight changemeasurements for monitoring long-term erosion/deposition behavior of limitersand other plasma edge components.ACKNOWLEDGEMENTSThe authors gratefully acknowledge the ISX-B team for their cooperation inperforming these experiments. We are particularly indebted to R. C. Isler forhelpful discussions and to ORNL analytical services for careful weighing of theBe tiles.- 7 -References1. G. Staudenmaier, P. Staib, and G. Venus, J. Nucl. Mater. 76-77, 445(1978).2. J. B. Roberto, R. A. Zuhr, and S. P. Withrow, J. Nucl. Mater. 93-94,146 (1980).3. R. £. Clausing, L. Heatherly, L. C. Emerson, and R. J. Coichin,J. Nucl. Mater. 111-112, 180 (1982).4. G. Mezey, J. W. Partridge, and G. M. McCracken, Proc. 9th Intern. VacuumCongress and 5th Intern. Conf. on Solid Surfaces (Madrid 1983)Extended Abstracts, p. 188.5. G. Staudenmaier, J. Vac. Sci. Technol. A3, 1091 (1985).6. B. Schweer, P. Bogen, E. Hintz, D. Rusbuldt, S. Gato, and K.-H. Steuer,J. Nucl. Mater. 111-112, 71 (1982).7t. E. Dullni, E. Hintz, J. B. Roberto, R. J. Colchin, and R. K. Richards,J. Nucl. Mater. 111-112, 61 (1982).8. H. L. Bay and B. Schweer, J. Nucl. Mater. 128-129, 257 (1984).9. T. B. Cook, P. W. King, J. B. Roberto, K. A. Stewart, and K. E. Yokoyama,J. Nucl. Mater. 128-129, 253 (1984).10. D. H. J. Goodall, T. W. Conlon, C. Sofield, and G. M. McCracken,J. Nucl. Mater. 76-77, 492 (1978).11. J. B. Roberto, J. Roth, E. Taglauer, and 0. W. Holland, J. Nucl. Mater.128-129, 244 (1984).12. R. C. Isler et al., submitted to Nuclear Fusion.13. P. K. Mioduszewski et al. (to be published).14. P. H. Edmonds et al., J. Vac. Sci. Techno!-,, _A3, 1100 (1985).15. R. A. Zuhr, Private Communication.16. R. A. Langley, M. B. Lewis, and R. A. Zuhr, submitted to J. Vac. Sci.Techno!. A.17. A. C. England, U. L. Hiliis, and P. H. Edmonds, ORNL report (in press)18. J. Bohdansky and J. Roth, J. Nucl. Mater. 122-123, 1417 (1984).19. R. A. Zuhr, submitted to J. Vac. Sci. Techno!. A.- 9 -Table 1. Summary of Weight Change ResultsTile Number Installed Weight (g) Weight Change (g)+0.0957+0.0400+0.8826+0.1462-0.8961-0.7425-1.1108-0.3235-0.0895-0.0092+0.0026+0.00111 (inside)23456739101112 (outside)260.8242266.2631260.5752267.1097261.0954266.5720260.6792260.4349260.5964266.3416259.5123266.9151- 10 -Figure CaptionsFigure 1 Photographs of the beryllium limiter before operation in ISX-B(top), following a period of high current operation whichresulted in surface melting (center), and at the end of theexperiment (bottom). Note that topographic features developedduring the high current operation continue to be recognizableafter More than 3500 beam-heated discharges.Figure 2 Weight change of the individual beryllium tiles as a function oftheir radial position. The location of the liraiter with respectto the plasma is indicated in the inset.ORNL-PHOTO 0004-85 FEDBEFORE FLUENCE 7/2/84 SHOT #64431Figure 1ORNL-DWG 85-133271.20.8IUJ-0.4-0.8 --1.2ISX-B Be LIMITERINSIDE OUTSIDENET DEPOSITIONo» o» -̂»CVJ§o1 2 3 4(TILE NUMBERS)88 ab o cj9 J. £ ^10 11 12110 15 20INSIDE25 30OUTSIDEDISTANCE ALONG THE LIMITER (cm)Figure 2

Weight change measurements of erosion/deposition at beryllium limiter tiles in ISX-B

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Weight change measurements of erosion/deposition at beryllium limiter tiles in ISX-B

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