Global particle balance and wall recycling properties of long duration discharges on TRIAM-1M

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1 Global articl balanc and wall rcycling rortis of long duration dischargs on TRAM-M M. Sakamoto ), M. Yuno ), S. toh ), K. anada ), K. Nakamura ),. Zushi ), E. Jotaki ), M. asgawa ), S.V. Kulkarni ), A. yomasa ), S. Kawasaki ),. Nakashima ) ) Advancd Fusion Rsarch Cntr, Rsarch nstitut for Alid Mchanics, Kyushu Univrsity, Kasuga, Fukuoka, , Jaan ) nstitut for Plasma Rsarch, ndia -mail: sakamoto@triam.kyushu-u.ac.j Abstract. Th longst tokamak discharg with th duration of 6 s ( h min) was achivd. Th global articl balanc has bn invstigatd. n th longst discharg, th global balanc btwn th articl absortion and rlas of th wall was achivd around t~ min and thn th fuling was automatically stod. Aftr that th lasma dnsity was maintaind by th rcycling flux alon until th nd of th discharg. Th maximum wall invntory is about.6 x at t ~ min but it is finally rlasd from th wall at th nd of th discharg. Th global balanc sms to b causd by th incras in th hydrogn rlas from th main chambr rsulting from its tmratur ris. Morovr, it has bn obsrvd a larg diffrnc btwn rortis of wall rcycling in th continuous gas fd cas (i.. static condition) and th additional gas uff cas (i.. dynamic condition). n th static condition, th ffctiv articl confinmnt tim incrass almost linarly to about s during th on-minut discharg. n th dynamic condition, th dcay tim of th lctron dnsity just aftr th gas uff, i.. th ffctiv articl confinmnt tim, is. to. s during th on-minut discharg. Th larg diffrnc was also rroducd in th longst discharg. t is considrd that th nhancd wall uming is causd by th incras in fluxs of th diffusd ions and charg xchang nutrals du to th additional gas uff.. ntroduction Th achivmnt of stabl long uls oration is on of th rquirmnts for th futur fusion ractor. Th global articl balanc and its control ar critical in achiving th stady stat oration (SSO). Many studis on th global articl balanc hav bn carrid out, for xaml in th rfrncs [-6]. Th wall rcycling is a ky to undrstand th global articl balanc. Rcntly, th imortanc of th main chambr rcycling in divrtd lasmas has bn ointd out in Alcator C-Mod [7,8]. Th wall rcycling changs with a long tim constant dnding on th wall condition, which continus to chang during th discharg. Th ffct of th wall conditioning don bfor th discharg cannot b xctd in th SSO. t is ncssary to obtain th databas from th long uls xrimnts. Th wall rcycling dnds on not only th wall condition but also th articl flux out of th lasma. n transint hnomna such as dg localizd mods (ELMs) or th llt injction, th wall would contribut considrably to th uming of th transint articl flux out of th lasma. Such dynamic rsonss of th wall rcycling is considrd to lay an imortant rol in th oration of ELMy -mod, which is an ssntial oration in TER [9]. Th undrstanding of th dynamic rsons of th wall rcycling is imortant for th SSO as wll as th static rsons. n th surconducting tokamak TRAM-M, ultra-long dischargs with th duration of longr than hours hav bn obtaind. n this ar, th global articl balanc in th longst duration discharg (τ D ~ 6 s) and th static and dynamic rsonss of th wall rcycling will b rsntd.. Exrimntal Rsults TRAM-M (R major ~.8 m, a x b =. m x.8 m) is a surconducting tokamak. All of th lasma facing comonnts ar mad of high Z matrials: th oloidal limitrs and th

2 divrtor lats ar mad of molybdnum, and th main chambr is mad of stainlss stl. A low Z matrial coating has nvr bn don. All rsults rortd in this ar wr obtaind with hydrogn dischargs with a limitr configuration. Th fuling is fdback controlld to match th α lin intnsity at th cntr chord with th rfrnc signal. Th wavforms of th discharg with th duration of 6 s ar shown in Fig.. Th lasma currnt was almost constant of ~ ka during th discharg and was sustaind by.5 Gz LCD. Th lctron dnsity is dducd to b ~ 8 m -. Th RF owr dcrasd from 7.5 kw to 6.9 kw at t ~ 6.6 min to rduc th hat load to th limitr. Th rfrnc lvl for th fuling was dcrasd about 6.5 % at t ~ 6.5 min to xamin th static rsons of th wall rcycling. Th additional gas uff was carrid out twic as shown in Fig. (f) to xamin th dynamic rsons of th wall rcycling. Th α lin intnsity sontanously incrasd around t ~ min and th izolctric valv for th fuling was automatically closd. At t~ min, th global balanc btwn th hydrogn absortion and rlas of th wall was achivd, and thn th lasma dnsity was maintaind by th rcycling flux alon until th nd of th discharg. Th OV (78 nm) and Mo (86 nm) lin intnsitis wr almost constant during th discharg as shown in Fig. (c) and (d). No imurity accumulation was obsrvd. Th tim volution of th nutral gas rssur at th um duct (Fig. ()) suggsts that th nutral rssur in th main chambr incrasd aftr th global balanc rsulting from incras in th hydrogn rlas from th wall.. Global Particl Balanc A siml articl balanc in th main chambr can b writtn by dn whr = / dt N ful + dn / dt um wall () is th total numbr of hydrogn nutral (ka) α OV Mo P G ( -6 Torr) V izo (V) Wall invntory ( ) (c) (d) () (f) (g) Fdback Control uff (x.5) # on clos Fig. Tim volution of lasma currnt, α lin intnsity, (c) OV lin intnsity, (d) Mo lin intnsity, () nutral rssur at th um duct, (f) voltag of th izolctric valv and (g) wall invntory in th longst discharg. atoms in th chambr, N th total numbr of hydrogn ions in th lasma, ful th fuling rat, um th uming rat by th xtrnal um-unit and wall th nt wall uming rat, which mans th balanc btwn absortion rat and rlas rat of th wall. Namly, wall is ositiv whn th total amount of hydrogn absorbd by th wall is largr than that of hydrogn rlasd from th wall. This mans that th wall lays a rol of th articl sink. Whn wall is ngativ, th wall lays a rol of th articl sourc. Th first trm of th lft

3 hand sid of qn. () is dducd from th data of th ionization gaug at th um duct. N is assumd to b th sam as N, namly, Z ff = is assumd. ful is stimatd from th voltag alid to th izolctric valv. um is th roduct of th nutral rssur and th um sd of th xtrnal um-unit. Th unknown aramtr wall can b obtaind from th abov quation. n th stady stat condition, th nt wall uming rat is writtn by = () wall ful um sinc th lft hand sid of qn. () is ngligibly small. Th avragd wall uming rat from t ~ to min is stimatd to b. x 6 atoms m - s -. n this riod, th wall layd a rol of th articl sink. Aftr t ~ min, wall = um ~ 8 x 5 atoms m - s -, sinc th fuling was stod ( ful = ). n this riod, th wall layd a rol of th articl sourc and th ffctiv articl confinmnt tim, τ, is infinit. Th whol surfac ara (S~5m ) of th wall is usd for calculation of th wall uming rat although unvnnss du to th limitrs, th divrtor lats, th cooling is and so on is not takn into account. Th tim volution of th wall invntory is shown in Fig. (g). Th maximum wall invntory is ~.6 x hydrogn atoms. Th total amount of wall-umd hydrogn was finally rlasd from th wall until th nd of th discharg. Th α lin intnsity was masurd at four diffrnt toroidal ositions as shown in Fig.. Th signal at th ort D is about twic highr than th othrs as shown in Fig. du to th strong intraction btwn th lasma and th #9 limitr. All of th signals incrasd at th sam rat aftr th global balanc. This suggsts that th incras in th α lin intnsity (i.. influx to th lasma) was causd by th contribution from th whol toroidal ara. Tim volution of th tmratur of th lasma facing comonnts is shown in Fig.. Th tmratur is masurd with thrmocouls. Th tmratur of th main chambr incrasd artly u to ~ o C and it dnds on th distanc of th masuring oint from th cooling i. Th rang of th limitr surfac tmratur masurd with an infrard camra is about 6 to 8 o C rovidd that th # Limitr missivity of molybdnum is unity. Th Main chambr charactristic tim of th tmratur ris of A th limitr is ~ 5 min and that of th main B chambr including th divrtor lat is about C to 5 min. Th rsults of Fig. and Fig. # Limitr suggst that on candidat of th causs of th global balanc is th incras in D #9 Limitr hydrogn rlas from th main chambr du to its tmratur incras. D α Fig. To viw of th main chambr of TRAM-M. Thr sts of th oloidal limitrs (#, #, #9) ar installd. Th α lin intnsity is vrtically masurd at th orts A, B, C and D. Th tim volution of th α lin intnsity at th orts A, B, C and D. A C B Tmratur ( o C) # Limitr Divrtor lat Main chambr Fig. Tim volution of th tmratur of th # oloidal limitr, th divrtor lat, th main chambr.

4 . Static and Dynamic Prortis of Wall Rcycling To invstigat th dynamic rorty of wall rcycling, w carrid out th additional gas uffs in th.5 Gz LCD lasma. Th dcay tim, τ D, of th lctron dnsity just aftr th gas uff, i.. th ffctiv articl confinmnt tim, is ~.7 s and τ D of th α lin intnsity is ~. s as shown in Fig.. t is found that th dcay tim agrs wll with ach othr. Th tim volution of τ in th dynamic condition (i.. additional gas uff cas) is shown in Fig.5. Th data wr obtaind by that many gas uffs wr carrid out in thr on-minut dischargs. τ is almost constant with a valu of. s to. s during th on-minut discharg. τ in th static condition is valuatd from th following quation: τ = τ /(-R) () whr τ is a global articl confinmnt tim and R is a rcycling cofficint. t is known that R incrass with tim in a static condition in which th lasma dnsity is kt constant by continuous fuling []. This mans that th wall uming rat gradually dcrass and τ incrass with tim. n th.5 Gz LCD discharg ( n ~. x 9 m - ), τ incrass almost linarly to about s during th on-minut discharg as shown in Fig.5. Th dcay tim of n aftr th fuling is stod at t ~ s during th discharg is also shown in Fig.5 and it is consistnt with th valu of τ valuatd from qn. (). t is found that τ in th static condition is on ordr of magnitud highr than that of th dynamic condition. n th longst discharg, morovr, th static and dynamic rortis of wall rcycling wr also invstigatd. Additional gas uffs wr carrid out at t ~ 9 s and 5 s as shown in Fig. (f). Th valu of τ D of th α lin intnsity ar. s at t ~ 9 s and. s at t ~ 5 s as shown in Fig.6 and. W do not hav n data for this discharg but τ is xctd to b similar to τ D of th α lin intnsity from th rsult of Fig.. t is found that τ dducd from th rsult of Fig.6 and is th sam as that of th initial has of th discharg shown in Fig.5 vn though th wall invntory incrass with tim as shown in Fig. (g). On th othr hand, τ in th static condition is dducd to b on th ordr of s at t ~ 6 s from Fig.6 (c). t changs with tim (i.. th wall invntory) in contrast to th dynamic rorty. n ( 9 m - ) α τ (s) Tim (s) Fig. Tim volution of lin avragd lctron dnsity and α lin intnsity at th additional gas uff (hatch). Th fuling is automatically stod just aftr th gas uff and rstarts at th tim indicatd by th dashd lin Tim (s) Fig.5 Comarison of th ffctiv articl confinmnt tims btwn th static (on circl) and dynamic (closd triangl) conditions. Th closd circl is th dcay tim of n aftr th fuling is stod.

5 5 α Tim (sc) α Tim (sc) α.... (c) Fuling rstart. Fuling sto Tim (s) Fig.6, α lin intnsity at th additional gas uff with th duration of. s and (c) α lin intnsity whn th fuling is stod in th longst duration shown in Fig.. This larg diffrnc btwn τ in th static and dynamic conditions suggsts that th rcycling cofficint dcrass just aftr th gas uff, i.. th wall uming is nhancd by th additional gas uff, sinc th rduction of τ du to th dnsity incras by th gas uff is valuatd to b about %. t is considrd that th nhancd wall uming is causd by th incras in fluxs of th diffusd ions and charg xchang nutrals from th lasma to th wall du to th additional gas uff.. Summary n TRAM-M, wall rcycling xrimnts hav bn carrid out intnsivly. n this ar, th global articl balanc of th longst discharg with th duration of 6 s ( h min) and th wall rcycling rortis of th static and dynamic conditions ar rsntd. During th longst discharg, th nt wall uming rat bcam zro (i.. th global balanc btwn th articl absortion and rlas of th wall was achivd) and thn ngativ. Th wall layd a rol of a articl sink until t ~min and aftrwards it layd a rol of a articl sourc. Th total amount of wall-umd hydrogn was finally rlasd from th wall until th nd of th discharg. Th maximum wall invntory is about.6 x hydrogn atoms. Th global balanc sms to b causd by th incras in th hydrogn rlas from th main chambr rsulting from its tmratur ris. n th static condition, th ffctiv articl confinmnt tim incrass almost linarly to about s during th on-minut discharg. On th othr hand, th dcay tim of th lctron dnsity just aftr th gas uff, i.. th ffctiv articl confinmnt tim, is. to. s during th on-minut discharg in th dynamic condition. This larg diffrnc of th wall rcycling rortis btwn th static and dynamic conditions is also rroducd in th longst discharg. t is considrd that th nhancd wall uming is causd by th incras in fluxs of th diffusd ions and charg xchang nutrals from th lasma to th wall du to th additional gas uff. Rfrnc [] Sakamoto, M., t al., Nucl. Fusion () [] Tsitron, E., t al., Plasma Phys. Control. Fusion () [] Nakamura, Y., t al., J. Nucl. Matr. 9-9 () -. [] irooka, Y., t al., J. Nucl. Matr. 9-9 () -7. [5] Banno, T., t al., J. Nucl. Matr. - (995) [6] Grisolia, C., t al., J. Nucl. Matr. 96 (99) 8-8. [7] LaBombard, B., t al., Nucl. Fusion () -6. [8] Lischultz, B., Plasma Phys. Control. Fusion () [9] Shimomura, Y., t al., TER-FEAT oration, Nucl. Fusion () 9-6.