RF Plasma Production and Heating Below Ion-Cyclotron Frequencies in Uragan Torsatrons

Transcription

1 EXW/P- RF Plasma Production and Hating Blow Ion-Cyclotron Frquncis in Uragan Torsatrons V.. Moisnko ), V.L. Brzhnyj ), V.N. Bondarnko ), P.Ya. Burchnko ), V.V. Chchkin ), M.B. Drval ), I.E. Garkusha ), L.I. Grigor va ), D. Hartmann ), R. Koch ), V.G. Konovalov ), V.D. Kotsubanov ), Y.D. Kramskoi ), A.E. Kulaga ), A.V. Lozin ), A.I. Lyssoivan ), S.M. Maznichnko ), V.K. Mironov ), I.N. Mysiura ) R.O. Pavlichnko ), V.S. Romanov ), A.N. Shapoval ), A.I. Skibnko ), A.S. Slavnyi ), Yu.S. Stadnik ), V.I. Trshin ), V.S. Voitsnya ) ) Institut of Plasma Physics NSC KIPT, Kharkiv, Ukrain ) Max-Planck-Institut für Plasmaphysik, Grifswald, Grmany ) Laboratory for Plasma Physics - ERM/KMS, Brussls, Blgium contact of main author: moisnk@ipp.kharkov.ua Ky words: radio frquncy xprimnts, impurity control, wall conditioning, torsatron. Abstract. In IPP-Kharkiv thr ar two torsatrons (stllarators) in opration. Th Alfvn rsonanc hating in a high ky-paralll rgim is usd on both machins. This mthod of hating is advantagous for small siz dvics sinc th hating can b accomplishd at lowr plasma dnsitis than th minority and scond harmonic hating. A sris of xprimnts is prformd aimd at study th faturs of th discharg with th THT (thr-half-turn) antnna. Elctron tmpraturs in th rang T >~..5kV ar achivd at th plasma dnsitis > ~.5.5 cm <. Plasma nrgy contnt is incrasd up to 5 tims with rspct to th plasma producd by th fram antnna prior to th THT antnna puls. A nw -strap shildd antnna is manufacturd and installd in Uragan-M. A high frquncy discharg for wall conditioning is introducd in Uragan-M torsatron. Th discharg is sustaind by th spcially dsignd small fram antnna. Rathr fficint hydrogn dissociation is achivd in such conditions. Slf-consistnt modl is dvlopd for simulations of plasma production with arbitrary ICRF antnnas. It includs th systm of th particl and nrgy balanc quations for th lctrons, ions and nutrals and th boundary problm for th Maxwll s quations. Th Maxwll s quations ar solvd at ach tim momnt for th currnt plasma dnsity and tmpratur distributions. Th Maxwll s quations solution allows dtrmining a local valu of th lctron RF hating powr, which influncs on th ionization rat and, in this way, on th volution of plasma dnsity. First rsults of calculations of RF plasma production in th Uragan-M stllarator with th fram-typ antnna ar prsntd.. RF hating blow ion-cyclotron frquncis in Uragan torsatrons Uragan-M is a small siz torsatron with l =, m = 9, R = m major radius, a. m avrag plasma radius and toroidal magntic fild B T. Th whol magntic systm is nclosd into a larg 5 m diamtr vacuum chambr. Th Alfvén rsonanc hating in a high k rgim is usd on this machin. This mthod of hating is advantagous for small siz dvics sinc th hating can b accomplishd at lowr plasma dnsitis than th minority and scond harmonic hating. Uragan-M is quippd with two antnnas. On is a fram-typ antnna for low dnsity plasma production. Anothr antnna in Uragan-M is an unshildd THT (thr-half-turn) antnna [] that consists of straps orintd in poloidal dirction (s FIG. ).

2 EXW/P- Th stratgy of usag of th Alfvén rsonanc hating with compact strap antnna (s.g. []) is a rsult of balanc of th tchnological rstriction to hav a compact antnna which occupis on dvic port and ncssity to supprss th xcitation of low k Alfvén rsonancs that rsid at th plasma priphry and caus plasma dg hating. Th xprimnts with th THT antnna wr attmptd arlir [, ]. Thy showd incras of plasma dnsity from n >.5 cm providd by a puls of th fram antnna to < > ~.5.5 cm. In that tim Uragan-M was not quippd with a diagnostic for masuring th lctron tmpratur. Sinc th xpctd hating in Alfvén rsonanc conditions is th lctron hating, th rsult of thos xprimnts rmains incomplt. Scond xprimnt also suffrd from hug radiation losss that rsultd in low plasma tmpratur what is indicatd by th light mission from low chargd stats of light impuritis. In currnt sris of xprimnts, th lctron cyclotron mission (ECE) diagnostics is mployd. On-axis magntic fild valu is is > ~.5 cm B =.7 T. For this magntic fild th cut-off valu of plasma dnsity. This valu is limiting for ECE diagnostics. Taking into account this fact, th xprimntal sris is organizd so that this lvl would b not xcdd dspit th highr lvls of plasma dnsitis ar achivabl. 7 ANTENNA I III 7 II II III I ANTENNA 6 I II III 5, FIG.. THT antnna. - optic spctroscopy - microwav rflctomtry and intrfromtry - prob masurmnts - ECE 5 - lasr impurity injction 6 - SXR 7 - CX nutral nrgy analyzrs - magntic diagnostics FIG.. Schm of Uragan-M diagnostics. Th diagnostics involvd into ths xprimnts ar shown on Fig.. Th frquncy of hating is chosn so, that th paking k valu in th antnna spctrum satisfy th Alfvén rsonanc condition at th plasma cor. Sinc th dvic is small and k is high th frquncy should b chosn quit clos to th ion cyclotron. It is f =.6 MHz for th fram antnna and f =.9 MHz for th THT antnna. For ths frquncis th ion cyclotron zon is prsnt in plasma column. In rgular dischargs th fram antnna crats plasma with th dnsity > ~.5 cm and tmpratur < T >~ kv. In th xprimnts th puls of th fram antnna gos first. Immdiatly aftr th fram antnna puls th THT antnna is switchd on. Th tmporal volution of th plasma paramtrs in such a shot is shown in Fig.. Th fram antnna ionizs th nutral gas during - ms and hats low-dnsity plasma. Th powr dlivrd to th antnnas is about P ~ kw for th fram antnna and P ~ 5 kw for th THT antnna. Plasma dnsity is > ~ cm during th fram

3 EXW/P- antnna puls. Aftr it th plasma dnsity grows rapidly to is ramping up to > ~.5 cm > ~.5 cm and, furthr at th nd of th THT antnna puls. Carbon CV lin mission is highr during th THT antnna puls. CIII lin mission is small during th whol shot, but thr is a hug rcombination pak aftr th THT antnna puls indicating that carbon is in highly ionizd stats in th plasma. Elctron tmpratur is calculatd from th radiation tmpratur by T = T ( ) rad th optical thicknss of th plasma / by mans of tokamak approximation formula for 5.6 < n > T R B [5] (hr th plasma dnsity is in cm -, lctron tmpratur is in V, torus major radius R is in cm, and th magntic fild is in G). P, *[kw] # H α, a.u. ECE, a.u. SXR, a.u. n=* cm - C V, a.u. C III, a.u. 6 tim, [ms] FIG.. Evolution of RF powr, radiation and lctron tmpratur, CV and CIII lins mission, plasma dnsity and SXR intnsity in a shot with continuous gas puff. Nutral 6 gas prssur is 9. Torr. FIG.. Evolution of RF powr, Hα hydrogn lin intnsity, ECE signal, soft x-ray intnsity, plasma dnsity and CV and CIII lins mission in a shot with a strong pulsd gas puff. Nutral gas prssur is 6 9. Torr. FIG. 5. Evolution of RF powr, radiation and lctron tmpratur, CV and CIII lins mission, plasma dnsity and Hα hydrogn lin intnsity in a shot with a modrat pulsd gas puff. Nutral gas prssur is 6.9 Torr. During THT antnna puls lctron tmpratur is lowr, but th lctron nrgy contnt is almost th sam as compard with th fram antnna plasma. It is difficult to incras plasma dnsity with continuous gas puff. For this purpos a pulsd gas puff is usd. An xampl of a puls with gas puff is givn in Fig.. Th gas puff causs a splash on H α hydrogn spctral lin mission. Th plasma dnsity, soft x-ray and CV mission signals grow substantially. Th ECE signal also starts to grow at th bginning of th gas puff puls, but thn dcrass. This bhaviour could b xplaind by locking of ECE mission in dns plasma. Soft x-ray diagnostics and CV radiation intnsity indicat that plasma is not cold. Th plasma dnsity coms clos to th valu > ~ cm dmonstrating th THT antnna ability to incras plasma dnsity to such valus.

4 EXW/P- With mor powrful gas puff th plasma dnsity could b furthr incrasd, but th diagnostics indicat dcras of th plasma nrgy contnt. Tuning th gas puff intnsity, th maximum plasma dnsity is dcrasd to valus > ~ 5 7 cm that allows on to nabl lctron tmpratur masurmnts. On of such pulss is shown in Fig. 5. In this puls during THT opration th dnsity is > ~ 5 cm and th lctron tmpratur T =.5kV is achivd in th plasma column cntr. Th spcific fatur of plasma dnsity bhaviour is that aftr th gas puff th dnsity dos not rturn to th valu which was bfor gas puff. Th radial distribution of th CV intnsity is calculatd puls by puls chord masurmnts (Fig. 6). It is cntral at both stags of th discharg. Th C + ion tmpratur is masurd using Dopplr spctromtry of a CV lin. Th stimats for Coulomb ion-ion collisions givs fast nrgy xchang btwn protons and C + ions. Thus thir tmpraturs should b clos to ach othr. Th ion tmpratur is smallr than th lctron on. It ramps up continuously during th THT antnna puls and rachs th valus clos to T = V by th puls nd. i missivity, a.u CV, λ=7.9 t (K) t (K). 6 coordinat, cm P, *[kw] n=* cm - #6 9-- #9 9-- #6 9-- # T i (V) 9.. ECE, a.u. #6 9-- # tim,ms 6 tim, [ms] FIG. 6 (uppr). Radial distributions of C + spctral lin missivity at tim momnts t = 9 ms (black) and t = 5 ms (rd). FIG. 7 (lowr). Evolution of C + ion tmpratur in modrat pulsd gas puff shots. FIG.. RF powr, plasma dnsity and ECE intnsity in two shots with diffrnt powr dlivrd to th fram antnna. Th influnc of fram antnna discharg on th THT antnna puls is illustratd in comparison of two shots with diffrnt powr dlivrd to th fram antnna (Fig. ). Dcras of powr rsults in som incras of plasma dnsity throughout th shot and connctd to this small dcras of plasma lctron tmpratur.. Wall conditioning dischargs in Uragan-M At Uragan-M torsatron (stllarator) th studis of th RF dischargs for wall conditioning hav bn carrid out. Th machin has th major plasma radius R =.7 m, th avrag minor plasma radius a =.m and th toroidal magntic fild B. T B. T. Th goal of

5 5 EXW/P- th discharg wall conditioning is th rmoval of adsorbd spcis from th wall so that thy may thn b pumpd out of th vacuum chambr. Th adsorbd atoms or molculs may b rmovd by th ion or atom impact owing to th momntum transfr or chmical intraction. If during th wall conditioning plasma is magntically confind, as would happn in suprconducting stllarators, th outflow of ions is not intnsiv and thir flux to th wall of th vacuum vssl is not uniformly distributd. Undr such conditions, th wall conditioning with chmically activ nutral atoms or molculs [6] is advantagous. Such nutrals ar producd intnsivly from a molcular gas in partially ionizd plasma whn th dgr of ionization is low. Such a scnario for wall conditioning is studid for th dischargs in hydrogn. In this scnario th claning agnts ar th hydrogn atoms rsulting from th dissociation of th hydrogn molculs. Thy hav Franck-Condon nrgis, about V. If th lctron tmpratur in th discharg is lss than th ionization thrshold, - V, th dissociation rat is highr than th ionization, and on lctron producs a numbr of nutral atoms during its liftim. FIG. 9. Fram antnna insid vacuum vssl. FIG.. Small fram antnna for wall conditioning (lft) and its position with rspct of th last closd magntic surfac (right). Th claning dischargs could b continuous or pulsd. In continuous dischargs th plasma - dnsity is low = cm. Such a low dnsity discharg could b sustaind by n xcitation of th slow wav. This wav could b xcitd by any antnna having currnts paralll to th stady magntic fild [7]. Th fram antnna is th most simpl antnna having such ability. Dgr of dissociation of H, % 6 5.x -5.x -.5x -.x - Hydrogn prssur, Torr FIG.. Dissociation dgr of hydrogn in low ( f =.MHz, rd curv) and high ( f =5MHz, blu curv) frquncy dischargs. Th disprsion quation for th wav is th following k = ( k k ε ) ε = E B ε E B /( EB), ε, whr ε ε = B ε B / B ar th prpndicular and paralll dilctric

6 6 EXW/P- tnsor componnts, k = / c. If th frquncy of th wav is highr than ion cyclotron th ω wav propagats if ε >. This rquirmnt imposs an uppr limit for plasma dnsity. In th first xprimntal sris th continuous RF dischargs in Uragan-M torsatron ar sustaind by th kw RF oscillator in th frquncy rang.5-. MHz []. This powr is coupld to plasma by a fram antnna (Fig. 9). Bcaus of th abov mntiond plasma dnsity limitation, plasma with low dnsity up to n = 9 cm - is sustaind. Th dissociation dgr of hydrogn givn by th optical masurmnts [] for such a discharg is displayd in Fig.. Th dissociation dgr is not high. To incras it, highr plasma dnsity is ncssary. To achiv highr discharg prformanc, a highr frquncy RF hating at 5 MHz is chosn. A nw small fram antnna (Fig. ) is manufacturd and installd in Uragan-M. This antnna shows bttr rsults in dissociation of hydrogn (Fig. ).. Slf-consistnt modl of th RF plasma production in stllarator.. Numrical modl Th modl of th radio-frquncy (RF) plasma production includs th systm of th balanc quations and th boundary problm for th Maxwll s quations. It is assumd that th gas in atomic hydrogn. Th systm of th balanc quations of particls and nrgy rads: ( kbnt ) kbnt = PRF kb H v nna (Ca + ) kb H iv nna + ( nt ), t dn n = σ v n n + D n, () i a dt τ E n dv + navv = n VV = const, whr n is th plasma dnsity, tmpratur, E n a is th nutral gas dnsity, T is th lctron k is th P RF is th RF powr dnsity, which is dlivrd to lctrons, B ε H =. 6V is th ionization thrshold for hydrogn atom, χ is th Boltzman s constant, hat transport cofficint, D is th diffusion cofficint, τ E is th particl confinmnt tim, σ ar th xcitation and ionization rats and V V is th vacuum chambr volum, σ v, i v C Φ / T.5 is th ratio of th ambipolar potntial nrgy to th lctron tmpratur. a = a Th balanc of th lctron nrgy includs th RF hating, nrgy losss for th xcitation and ionization of atoms and losss causd by th hat transport. Th balanc of th chargd particls includs accounts for th ionization and diffusion losss of particls. Th last quation in systm () rflcts th global balanc of th particls. It is assumd, that th nutral gas is uniformly distributd in th vacuum chambr volum, including th plasma column. Apart from plasma insid th confinmnt volum, th RF fild can produc plasma outsid it. Th losss of th chargd particls in this rgion hav a convction charactr: th particls scap to th wall along lins of forc of th magntic fild. Such losss of particls outsid th confinmnt volum ar dscribd in τ -approximation.

7 7 EXW/P- Th problm is solvd in cylindrical gomtry. Th plasma is assumd to b azimuthally symmtrical and uniformly distributd along plasma column. Th lngth of plasma cylindr is L = πr and th nds ar assumd to b idntical. To mak th systm of th quations () closd it is ncssary to dtrmin th singl xtrnal quantity in it, P RF (RF powr dnsity). This quantity can b found from th solution of th boundary problm for th Maxwll s quations: ω E ε( r) E i j = ωµ xt, c ( ) whr E is th lctric fild, j xt is th xtrnal RF currnts, ε is th dilctric tnsor. Th Maxwll s quations ar solvd at ach tim momnt for currnt plasma dnsity and tmpratur distributions. Th Maxwll s quations solution allows dtrmining th valu of local RF hating powr of th lctron plasma componnt which influncs on th ionization rat and, in this way, on th incras of plasma dnsity. Th Crank-Nicholson mthod is usd for th solving of systm of th balanc quations (). Th Maxwll s quations () ar solvd in D using th Fourir sris in th azimuthal and th longitudinal coordinats. For th discrtization in radial coordinat, th uniform finit lmnts mthod is mployd that uss a spcial st of wight (tst) and basis (shap) functions []... Exampl of calculations Th following paramtrs of calculations for Uragan-M stllarator ar chosn: th major radius of th torus is R =.7 cm, th radius of th plasma column is r = cm, th radius of th mtallic wall is a = cm, th radial coordinat of th antnna is l r = cm, th toroidal magntic fild is B = 5kG, th antnna siz of th fram antnna of azimuthal angl is ϕ a =, th angular siz of th fram antnna in th toroidal dirction ϑ a =.. Th currnt in antnna is assumd not varying along th conductors. Th first rsults of calculations of RF plasma production in th Uragan-M stllarator ar prsntd. Figs., display th profils of th plasma dnsity and th lctron tmpratur at th tim momnt t =.s. Figs. -6 display th tim volution of th plasma dnsity, dnsity of nutral gas and th lctron tmpratur..e+ 5 n, cm- 6.E+.E+ T, V.E+.E+ r, cm FIG.. Profil of plasma dnsity in t =. s. r, cm FIG.. Profil of lctron tmpratur in t =. s.

8 EXW/P- A charactristic fatur of th calculations is sufficintly high plasma tmpratur outsid th confinmnt volum (Fig. ). Sinc th hating powr pr particl is high at lowr plasma dnsitis (Fig. ) th lctron tmpratur is incrasd at th dg of th plasma column (Fig. ) whr th particl losss ar fastr. For th rason that th slow wav is focusd at th cntr of th plasma column, th pak of lctron tmpratur is obsrvd at this rgion (Fig. )..E+ E+ 6.E+ E+ <n>, cm-.e+ <na>, cm - 6E+ E+ <T>, V.E+ E+.E t, s FIG.. Tim volution of avrag plasma dnsity t, s FIG. 5. Tim volution of avrag nutral atoms dnsity t, s FIG.6. Tim volution of avrag lctron tmpratur. In th chosn rgim th plasma dnsity is casd to incras and th plasma production procss stagnats bcaus th powr is insufficint to complt burnout of nutral atoms (Figs., 5). At th initial stag of th plasma production sharp paks in tmpratur ar obsrvd (Fig. 6). Ths paks ar associatd with a sharp incras of th antnna loading rsistanc. It occurs whn th wav global rsonanc conditions in a plasma column is mt. At th initial stag of plasma production a slow wav damping is small, and th paks of th global rsonancs ar mor narrow and high. Acknowldgmnt This work is supportd in part by STCU projct 6. Authors ar thankful to th tchnical staff of stllarators dpartmnt for assistanc in th xprimnts. Rfrncs [] Moisnko, V.E. in IAEA Tchnical Committ Mting (Proc. th Int.Workshop on Stllarators, Kharkov 99), IAEA, Vinna (99) 7. [] Moisnko, V.E., t al., Plasma Physics Rports 5(9). [] Lysoivan, A.I., t al., Fusion Enginring and Dsign 6 (995) 5. [] Burchnko, P.Ya., t al., Visnyk KhNU, Sr. Phys. (), 559 () 5 (in Russian). [5] Bornatici, M., t al., Nucl. Fusion (9) 5. [6] Wintr, J., Plasma Phys. Control. Fusion (996) 5. [7] Moisnko, V.E. Transactions of Fusion Tchnology 9 () 65. [] Moisnko, V.E., t al., Wall conditioning RF dischargs in Uragan-M torsatron, 6th EPS Confrnc on Plasma Phys. Sofia, Jun 9 - July, 9 ECA Vol.E, P (9). [9] Lysojvan, A.I., t al., Nuclar Fusion (99) 6. [] Moisnko, V.E., t al., Problms of Atomic Scinc and Tchnology, Sris Plasma Physics (7), ().