Maddaluno, G.

Martinelli, A.

English

MPG.PuRe

786 RADIAL AND POLOIDAL DISTRIBUTION OF H1PURITlES AND DEUTERIUtl DEPOSITED ON THE FT LHIlTER DURING OHMIC DISCHARGES G. ~Iadda l uno Associaz i one EURATotl-ENEA su lla Fusione, Centro Ricerche Energia Frascati, C.P, 65 - 00044 Frascati, Rome, Italy A.P. tlartinelli tlax Planck lost. fur Plasmaphysik, EURATON A:; .::;ociation , 8046 Carching F.R.C. INTRODUC'flON In a machine with limiter operation, such 3S the Fra scati Toka mak (FT), the limiter is the main source of foreign atoms affecting the plasma purity and makes a large contribution to the total recycling of the I>.'orking gas [I J. In turn the surface composition and topography of the limiter are cha nged, erosion-redeposition phenomena taking place because of t he plasma flux. The knowledge of the radia l and poloida 1 distributions of impurities and deute r ium deposited on the limiter can be of great importance for un-derstandi ng the complex plasma-limiter interaction, particularly with re-gard to the observed asymrnetries o f the scrape-off pa rame ters [2}. In this work the sta inles s steel f ull poloidal limiter of FT [3] was equipped with six graphite t arge ts in order to study the spatial distribu-tions of i mpurities and deuterium within larger radial and polo idal ranges with respect to previous i nves t igat ions [4,S]. EXPERIMENT Three ISx I0x2 mm 3 graphi te (grade EK98, Ringsdorff) targe ts were mount ed on thf' ion sidf' and three on the e l ectron side of th e stainless steel limiter support and poloida lly distributed a ll around the limiter circumfe-rence, as shown i n Fig. 1. Fig. 1: Schematic view of the main poloidal FT limiter s howin g the p~ sition of graphite targets. The collecting surfaces of the targets were o riented perpe ndicular-ly to the toroidal magnetic field. IThe reliable measurement point clos-est to the plasma edge was at r-a = 1.3 cm , where a = 20 cm is the plasma r adius. All the targets but one were ex-posed to 623 ohmic discharges in the April/tlay 1986 experimental period; once , when the limiter wa s t emporari-ly extracted after 494 shots, the tar get mounted on the bottom ion side (ll i n Fig. 1) was removed . The main plasma paramf'ters were I = 300~600 kA, B = 6+8 T, n O.7+2~7xI014 cm- 3 . The toroidal fi~ld was parallel t o the plasma current. After removal from the limiter the targets were sur fa ce analysed by SEM, PlXE and NRA. -787 N 'g E • 3 Poloida l positon (deg rees) Fig . 2: Radially averaged poloidal dist r i-bution of meta l and deut erium depo s ited on the graphite targets. RESULTS Metal Deposit Analysis The metal cove~age (Fe+Cr+Ni), averaged over the seven experi men t al po i nts radial l y measur~ ed by PIXE on each of the six graphite targets, was fo und to be 3+sxI0 18 atoms cm- 2 (more t han a t housand mono layers) with no large poloidal 0[" di-rectional (electron vs ion side) diffe r ences (Fig. 2). The ch romium concentra -tion on all the targe t s was abo ut twice as large as i n the s t ainless steel u ~ed as l imit -er (AtS I 316) a nd wall CArS I 304) material. The target sub-j ected to fewe r disc harges t han the ot her ones showed the same amount of metal deposit . With respec t to the radi-a l distribution of metallic i mpurities, a concentration continuously de -creasing with the distance from the plasma edge was found in the inner part of t he limiter on the electron side (target 3E), with an e - foldiog length of - 1 cm, and, less clearly, on the ioo side (Fig. 3). In the other parts of the limiter the radia l d istribution was almost uniform, with a smooth maximum at intermediate positions between the top a nd bottom of the targets . De u terium Analysis - Higher radiallv averaged de u terium concentrations were measured on the ion side (8+9xI0 1g atoms cm- 2) than on t he e l ect ron side (- 3x l0 16 atoms cm- 2 ) by the D(3He, p)4He nuclear react ion on the targets wh i ch had been subjected to an t,"qual number of d ischarge s and showed simi-lar i mpurity d istributions (Fig. 2). Unlike meta lli c impurities , lower D concentration by a fa ctor 2+3 was fo und on the target 11 with respect to the other targets facing t he ion side. The rad i al distribution was ra t her uniform (Fig. 3), with sha llow maxima and minima occurri ng at different pos i t i ons, but the target 4E show-ed a 0 concentration c l ea rly increasing with t he distance from the plasma. DISCUSSION The general pattern observed in the meta l deposit (a s we l l as in lhe deuterium concentration) is t he result of several hundred both norma l and disruptive dis charges. The eva l uation of impurity fluxes in the scrape-off from the measured concentrations is to be carefully considered because of the different impurity amounts released i n such a large number of di s -c ha rges. Erosion action o f t he plasma and non linear dependence of the depo s it thickness on the number of discharges can also lead to misleading results. 788 I n effec t a nonlinear de-N , l o lO'9r----------------:O pendence is infer r ed by paring target II with othe r ones. , § ~ Moreover the metallic impurities co llected by de -position pro bes mounted on the limiter sides are large ly the res ult of loca l rede position close to the ero:-sion area. According t o the meas -• ,- 21 ured values of density and 21 31 scrape-off (6) (;t - lxl0 12 ~temperature in the FT .;:r- ---r..3E cm - 3 , T = 15 eVeat 1+2 cm tn 31,; e !5 11 .If" outside the plasma edge) ;;0 ~1E and the calcu lated ioniza -E 2E ...... -:::r _21'",'" .. /~...) 11 tion rate coefficients (i.e. "; 1Ee-~~~_~ .... ..t::r _..6-_ <av> ~ IXIO-7 cm3 5- 1 f or _ 3E ~--::g:=" --..f'r- "' "7 2E a Cr atom [7]), a par t i cle ~ 2. 1016 L _-'--_--'-_--'-_---" __ '-_L_-"-_-,J spu tte red from the Hmi t ee Cl 11 15 19 23 27 side with an ene rgy of a r - a (mm) few eV has a mean free pa th Fig. 3: ~Ieta l and deu te rium concentrat i on on the graphite targets as a fun ction of the radial distance from the plasma. See Fig. 1 for the poloidal pOSi t ion of the different for electron impact ioniza-tion as short as "" 1 cm . Therefore it has a high pro babili t y of being ionized and of re turning to the ta rgets. limiter under the combined action of the friction with the background plasma and the acceleration by a pre- sheath electric field . Atoms released with thermal energies by evapora-tion have an even higher probability of rede position owing to their shorter ionization mean free path. The refore the large Cr /(Fe+Ni+Cr) ratio observed on all the targets is probably due to preferent ial Cr evaporation from the neighbouring hot zones of the l i miter. The dec r ease of metal deposition wi t h increasing distance from the plasma as found on the inner targets (especially on t he electron side) could be related to a poloidally asymmetriC radial profile of energy flux to t he limiter. A minimum density (and energy) e-folding length has been found in t he inner part of the poloidal cross section of Alcator-C by Lang-muir probes [2]. Particle and energy £luxes concentrated on the upper part of the el-liptically shaped mushrooms (see Fig. 1) cause a release of metal atoms minly directed onto closed flux surfaces inside the limiter radius [81. On the other hand , l onger energy and particle e-fold i ng lengths cause t he release of metal impurities which are ionized in the scrape- off and di-rect ly collec t ed by the limiter and overlap the impurity out flux from the main plasma. The result could be a nearly constant deposit as obse rved on t he targets apa rt from those on the inside of t he poloida l cross section . With respect to the working gas concentrations, the deuteri um implant-ed in t he t argets within - 1 ~ from the su r face (the depth range of NRA ana 789 lysis) seems to correspond to the expected ratio of hydrogen isotopes to host atoms in stainless steel, i.e. , - 0.01 [9 ]. However the pattern of metal deposition, which is in form of droplets, especially farther from the plasma edge (as found by SEM) , as well as the high temperature the limiter surface reachs during the discharges, suggests that a large part of de ute -rium is not trapped in the metal layer I but is retained i n the graphite suhst r ate or in co-deposited carbon. Carbon is known to trap"'" 40 times as much hydrogen as stainl ess steel within the range of ion implantation [9]. The measured 0 concentration ( ..... 3';-9xI0 1S atoms cm- 2 ) corresponds ap-proximately to the saturation level of deuterium implanted in graphite with the energy expected in the FT scrape-off (E = 5 kT = 75 eV) [10]. The flatness of the deuterium radial profiles as observed ein five of tbe tar-gets could be ascribed to both saturation effects and high temperature ex -cursions close to the plasma edge . For the target 3E, the sharp radial de-crease of the metal depQsit could account for the increase of deute rium wit h the distance from the plasma. CONCLUSIONS Radial and poloidal profiles of impurities and deuterium deposited on the FT limi ter during about 600 ohmic discharges were investigated by sur-fa ce analyses (SEM, PIXE and NRA) of six graphi t e targets mounted on the SS limiter support. Netd (Fe+Cr+Ni) concentrations as high as 5XI0 18 atoms cm- 2 were found with no large poloidal or directional differences. The chromium con-centration was twice as large as in stainless steel, probably due to pre-ferential evaporation from the limiter. The radial profi le of impurities was rather flat apart from on the i nside of the poloidal cross section, especially on the electron side ('1-e+Ni+Cr ... 1 em), perhaps as a result of asymmetry in the density and energy scrape - off lengths. The overall deuterium concentrat i on was higher on the ion (8-:-9xI016 atoms cm- 2 ) than on the electron side (- 3XlO16 atoms cm- 2) and poloidally uniform. The radial deuterium distribution did not exhibit any decay with the distance from the plasma. Further analyses on a new set of targets mounted on a limiter with INCONEL mushrooms are in progress. [1 J [2l [3 [4J [5J [6J [7J [8J [9] [10 ] FOOTNOTE AND REFERENCES F. De 1'larco et al., Nucl. Fusion 26, 1193 (1986) B. La Bombard, 1'I.1.T. Plasma Fusion Center Report PFC/RR-86-6 (1986) C. Alessandrini, C, Ferro and t-l. Samuelli, 11th Symposium on Fusion Technology, Oxford (1980) p. 1067 C. Ferro et a!., 12th European Con£. on Controlled Fus i on and Pl asma Physics, Budapest (1985), Part 11, p. 555 E . Franconi, F. Brossa, J. Nucl. Hater. 145-1 47,691 (1987) v. Perieoli - Rido l fini, Plasma Physics Controlled Fusion 27 , 493 (1985) M.A. Lennon, K.L. Bell et al., CuI ham Report CLH - R-270 (1986) G.F. t-latthews et al., Plasma Physics Controlled Fusion 29, 189 (1987) R.A. Langley et al.) Nuel. Fusion, special Issue 1984 --S.A. Cohen, G.t!. McCracken, J. Nucl. Hater. 84, 157 ( 1979) 

Radial and poloidal distribution of impurities and deuterium deposited on the FT limiter during ohmic discharges

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Radial and poloidal distribution of impurities and deuterium deposited on the FT limiter during ohmic discharges

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