Field-Aligned Plasma-Potential Structure Formed by Local Electron Cyclotron Resonance

Transcription

1 J. Plasma Fusion Rs. SERIES, YoL.4 (21) 69-'14 Fild-Alignd Plasma-Potntial Structur Formd by Local Elctron Cyclotron Rsonanc HATAKEYAMA Rikizo*, KANEKO Toshiro and SATO Noriyoshi Graduat School of Enginring, Tohoku [,lnivrsity, Sndai , Japan (Rcivd: 5 Dcmbr 2 / Accptd:27 August 21) Abstract Th significanc of basic xprimnts on fild-alignd plasma-potntial structur formd by local lctron cyclotron rsonanc (ECR) is claimd basd on th historical dvlopmnt of th invstigation on lctric doubl layr and lctrostatic potntial confinmnt of opn-ndd fusion-orintd plasmas. In th prsnc of a singl ECR point in simpl mirror-typ configurations of magntic fild, a potntial dip (thrmal barrir) appars around this point, bing followd by a subsqunt potntial hump (plug potntial) along a collisionlss pjasma flow. Th obsrvd phnomnon givs a clar-cut physics to th formation of fild-alignd plug potntial with thrmal barrir, which is closly rlatd to th doubl layr formation triggrd by a ngativ dip. Kywords: fild-alignd structur, local lctron cyclotron rsonanc, plug/barrir potntial, ffctiv confinmnt 1. Introduction Th formation and control of plasma potntial along magntic-fild lins has attractd gnral attntion in laboratory, spac and fusion-orintd plasmas bcaus th potntial structur holds th ky of wav-nonlinar dvlopmnt, chargd-particl acclration, plasma confinmnt and hat-transport rduction. In that connction, local lctron cyclotron rsonanc (ECR) as a typical principl of radio-frquncy hating in inhomognous magntic filds is ndowd with important attribut, bcaus it is supposd to induc intmrption of fild-alignd lctron flow, rsulting in th formation of local plasma-potntial structur undr th charg nutrality condition. This procss can b applid to hat-flux control in divrtd torus plasmas, whr th combination of VB along a divrtor channl and ECR hating (ECRH) could yild thrmal dik formation [1]. It can also b applid to improvd confinmnt in tandm-mirror plasmas. Historically spaking, th original tandm-mirror scnario for plug/ * Corr sponding author' s -mail : ci.tohoku.ac jp barrir potntial formation [2] simultanously rquird a nutral bam injction (NBI) producing sloshing ions and two ECR points in ach cll. This scnario was dmonstratd by th TMX-U [3] and GAMMA l [4] xprimnts in 1984 and 1985, rspctivly. In this stag, howvr, w pointd out that th undrlying physics of th abov-mntiond ida is rlatd to doubl-layr formation [5] triggrd by a small potntial dip inducing currnt limitation. Actually in a laboratory xprimnt in 198, w could alrady find th potntial dip and ris lik th plug/barrir in a DC-discharg positiv column [6]. Th sam was found in a particl simulation in 198 [7]. Thrfor, w claimd that only a singl ECR point is sufficint for th plug/barrir formation. Thraftr, our basic xprimnts hav dmonstratd this novl scnario [8]. On th othr hand, rcnt GAMMA l xprimnts hav shown in 1995 that th plug/barrir potntial is formd without NBI in th prsnc of th two by Th Japan Socity of Plasma Scinc and Nuclar Fusion Rsarch 69

2 Hatakyama R. t al., Fild-Alignd Plasma-Potntial Structur Formd by Local Elctron Cyclotron Rsonanc points in ach cll [9]. Furthrmor, only fundamntal ECRH, i.., th singl ECRH is now usd to produc th plug/barrir potntial in th GAMMA l xprimnts t11. Thus, in ordr to systmatically clarify ssntial faturs of th problm, it has bn rquird to invstigat dtails of potntial formation and control du to local ECR undr simplifid configurations. Hr, thr kinds of basic xprimnts ar carrid out using a singl-ndd Q machin. Scondly, a comparison btwn our rsults and GAMMA l rsults is mad. Thirdly, a particl-in-cll (PIC) computr simulation is prformd. 2. Exprimntal Apparatus A collisionlss plasma is producd by surfac ionization of potassium atoms on a hot tungstn plat undr an lctron-rich condition and is confind by a magntic fild B of a fw kg in th Q1-Upgrad machin, as shown schmatically in th uppr figur in Fig. l. A plasma-flow puls is injctd along th magntic fild (t = ) by applying a stp to a ngativly-biasd grid in front of th hot plat. A small Langmuir prob is usd to masur plasma paramtrs and thir axial profil. Th plasma dnsity, lctron and ion tmpraturs, and ion flow nrgy ar- ns= 1 x 1 cm-3, 2" =.2 Y, Tio a T.o' and E1s = 17", rspctivly. A background gas prssur is 4 x 1-7 Torr. A microwav with frquncy ol2n = 6 GHz and powr Pu = - I W is launchd into th plasma through a circular wavguid at th othr nd of th machin (z = 5 cm), propagating toward th hot plat (2 = lfg cm). Th subscript stands for th paramtrs in th cas of Pp = W. 3. Exprimntal Rsults and Discussion Th first xprimnt is concrnd with a singlpoint rsonanc in symmtric magntic-wll configurations as shown in th lowr figur in Fig. l. Th bottom of th magntic wll is locatd at th machin cntr. A mirror ratio R- is dfind as th ratio of B around z = +1 cm to that at z - cm. Th profil for R- = 1.68 is typically usd in th following xprimnt. Th microwav propagats toward th righthand sid in th rgion of ala"" < I and th ECR taks plac in th vicinity of lila". = (a""127t: lctron cyclotron frquncy) dnotd by th arrow at z = cm. Figur 2 shows typical profils of plasma and lctron dnsity n" in th cas ofr. = 1.68 and Pp =.5 W at t =.6 msc which corrsponds to th tim ncssary for th plasma flow to arriv at th ECR rgion. Th potntial structur consists of a potntial dip Ao (< ) formd around th ECR point and a subsqunt potntial hump A@o (> ) along th plasma flow. n" gradually dcrass toward th downstram Z-z ".r" --5T"s / f^t ^Qd t) El o l Fig. 1 1 Schmatic of xprimntal stup and symmtric magntic-wll configuration Fig. 2 Spatial profils of plasma and lctron dnsity n at t=.6 msc with B- = 1.68 for Pp =.5 w. 7

3 Hatakyama R. t al., Fild-Alignd Plasma-Potntial Structur Formd by Local Elctron Cyclotron Rsonanc rgion du to an ambipolar diffusion. Howvr, a stagnation of lctrons is gnratd in th ECR rgion and th stp dnsity drop is subsquntly formd. Namly, th plasma flow is obsrvd to b drastically pluggd by th potntial structur formd by th ECR. Th lctron tmpratur Z" incrass \p to T./T"s = 25-3 around th ECR rgion, and th I" diffrnc btwn th upstram and downstram rgions is maintaind. A@o is of th ordr of th ion flow nrgy and larg nough to plug ions, so calld, "plug potntial". -Ap6 is about fiv tims Z., prvnting cold lctrons supplid by th plasma sourc from mrging with hot lctrons in th ECR rgion, so calld, "thrmal barrir potntial". Such valus of A@o and A@6 ar plottd as functions of P, with R- kpt constant as givn in Fig. 3, and instad R. with P, kpt constant [s Fig. 4(b): cross marksl. In any cas, both A@o and -A@6 incras at first, gradually saturating for largr P, or R-. It is to b rmarkd that th saturation valu of A@o is always of th ordr of th ion flow nrgy d6, plugging most of th ions so as not to pass through th magntic wll rgion. This is bcaus th lctrons ar wll trappd and -4@6 is larg nough for most of th lctrons to b rflctd for larg P, or R- undr th charg nutrality condition. Lt us prsnt rsults in th scond cas of a singlpoint rsonanc in asymmtric magntic-wll configurations. Th aim of this xprimnt is to clarify which mirror ratio btwn upstram (R,) and downstram (R6) ons is mor ffctiv in th plug/ barrir potntial formation du to local ECR. Firstly, w chang R6 with Ru kpt constant 1.3 as shown in Fig. 4(a), whr R6 and Ru ar dfind as ratios ofb around z = -7 cm and z = 1 cm, rspctivly, to that at z = cm. Th valus of th plug and barrir potntials incras with an incras in th downstram mirror ratio, saturating for largr R6 for Pu =.8 W, as sn in Fig. a@). Nxt, w chang th upstram mirror ratio with Ro kpt constant In this cas, howvr, no changs of th plug and barrir potntials ar obsrvd for any Ro largr than unity. Thus, th downstram mirror ratio is prdominant in th plug/barrir potntial formation, and only th downstram mirror ratio is ndd to b incrasd for th confinmnt prformanc. Th third xprimnt is concmd with two-points rsonancs in symmtric magntic-wll configurations. Whn w dcras an avrag magntic-fild strngth of th symmtric configuration (a), instad of th singl ECR point, two ECR points appar in (b) and (c) configurations as shown in Fig. 5. In accordanc with Pp (W) Fig. 3 Potntial dip A@oand hump A,Qovs Prat t=.6 msc with F- = (a) 4 3 tt2 (b) L Rd Fig. 4 (a) Asymmtric magntic-wll configuration. (b) Potntial dip L,Qo and hump L.Qrvs Ro at t =.6 msc for Pp=.8 W, Crosss dnot rsults with R" = Ro. 7l

4 Hatakyama R. t al., Fild-Alignd Plasma-Potntial Structur Formd by Local Elctron Cyclotron Rsonanc Z (cm) Fig. 5 Symmtric magntic-wll configuration with twopoints rsonancs. _1 I Fig. 6 Potntial profils in th magntic configurations of Fig. 5 at f =.6 msc for Pr =.5 W. th configuration variation, th masurd potntial gratly varis in profil as dmonstratd in Fig. 6. Although not so apprciabl chang is noticd in th plug-potntial valu, a vry dp and widsprad potntial wll is formd as an intrval of th ECR points bcoms longr. Pay attntion to th ordinat-scal diffrnc (on ordr of magnitud diffrnc) btwn (a) and (c) configurations in Fig. 6. In th configuration (c) th plug and barrir potntial valus ar masurd with th microwav powr (P p = O - 1 W) as a paramtr. Th barrir potntial dpth gratly incrass without saturating, bing diffrnt from th plug potntial hight. Th sam tndncy is obtaind whn R* is incrasd with P, kpt constant in th prsnc of two-points rsonancs. Such a strongly-modifid potntial profil in th barrir rgion is xplaind by an additional xprimntal-rsult in divrging magntic-fild configurations [ 1]. Whn ECR taks plac at a position in a divrging-b-gradint rgion, th lctron magntic momnt kg mull2$ incrass, bing accompanid by -Lt"YllB acclration of lctrons. Thn th strong ionacclration potntial is formd so as to maintain th charg nutrality condition. In cas of closing this sction, it is to b mphasizd that th potntial-formation mchanism is basd only on th slctiv lctron trapping du to prpndicular Z. hating in th magntic wll and lctrostatic ion trapping rsultd slf-cosisitntly to satisfy th charg-nutrality condition. Such a formation of th potntial structur with a positiv potntial rgion and a local minimum is th sam phnomnon as a doubl layr formation du to th rflction of lctrons in a currnt-carrying plasma ll2,l3l. Thn th convntional scnario for th tandm mirror dvics is not ncssary, which nds two ECR points: th on for barrir formation and th othr for plug formation. A singl ECR is sufficint to provid th plug/barrir potntial structur. 4. Gomparison with GAMMA 1 Rsults Hr w compar our basic rsults with rsults of th GAMMA 1 xprimnts, whr only th fundamntal ECRH is applid to th B-gradint tgion of th plug/barrir mirror cll. Th rprsntativ paramtrs ar as follows. A@o = V for Q7-U,.3 kv (-.6 kv) for GAMMA lo. n" = I x 1 cm-3 for Q1-U and 5 x lrr cm-3 (5 x lrr cm-3; in th plug rgion for GAMMA 1. Eig - 2 V for Q1-U and th ion tmpratur paralll to th magntic fild 4rr

5 Hatakyama R. t al., Fild-Alignd Plasma-Potntial Structur Formd by Local Elctron Cyclotron Rsonanc V (35 V) for GAMMA 1. Th upstram-rgion lctron tmpratur 2"" =.35 V for Qr-U and cntral-cll lctron tmpratur Tu = 6-8 V (6-8 V) for GAMMA 1. In th GAMMA l cas both th two opration mods, i.., ECRH startup [14,15] and hot ion [1], ar rfrrd, and th valus writtn in th parnthss dscribd just abov ar obtaind from th xprimnt of th hot ion mod. In ordr to b in lin with th GAMMA l situation, on th othr hand, our magntic configuration is dtrmind as shown in Fig. 7(a), whr th ECR point is locatd in a B-gradint rgion. As xpctd, almost th sam potntial profil as dscribd so far is xprimntally confirmd. It is to b notd that thr is a diffrnc of ion vlocity distribution functions btwn th Q1-U and GAMMA 1 plasmas: a truncatd half- Maxwllian for Q1-U and a Maxwllian for GAMMA 1. Figur 7(b) givs th summarizd rsult, whr th plug(ecr)-rgion Z" at th abscissa is normalizd by 7.", and A@o at th ordinat is normalizd by d6 in th (a) 3 JI (b) 5? l-!v3 o UJ \t d2 g 2 I 4 1 L ECR -5 s. Qr-UPGRADE.? 9-9-s-- O GAMMA IO(ECRH) O GAMMA l(hotion) l 2 T / Tc Fig. 7 (a) Magntic configuration corrsponding to th GAMMA 1 situation. (b) Comparison of normalizd potntial hump A@o vs normalizd lctron tmpratur I in th ECR rgion. Q7-U cas and Z;11 in th GAMMA cas. Although th normalizd A@o incrass with an incras in th normalizd Z" and appars to saturat for largr T"/T."in a similar way in both dvics, th normalizd A@o in th GAMMA 1 attains to a valu btwn two and four tims as much as that in th Q7-U. This is du to th diffrnc btwn th ion vlocity distributions and th larg A,Q, is xpctd to b ncssary to plug th tailcomponnt ions in th GAMMA 1, th dtail of which is discussd latr. Whn th normalizd A@o is plottd as a function of P, normalizd by n"s (S: plasma cross sction), th normalizd P, in th Q7-U, which is rquird to mak A,Qo larg nough to plug most of th ions, is smallr than that in th GAMMA 1. This may b causd by th highr hating fficincy of th ECR in th collisionlss plasma of Q1-U whil a part of th microwav powr is usd for a plasma production in th GAMMA 1. It is to b rmarkd that A@o in our work plays th sam rol on ion confinmnt paralll to th magntic fild as in th GAMMA 1, although A@o and E16 in th Q7-U ar much smallr than A@o and Zilq in such a big fusionorintd dvic as th GAMMA PIC Computr Simulation In ordr to undrstand physical dtails of th potntial formation such as a A@o diffrnc btwn Q7-U and GAMMA 1 xprimnts, a PIC computr simulation with a two-and-a-half dimnsional lctrostatic cod is prformd basd on a Q-machin mittr modl [6]. Thr is a rsrvoir of th plasma particls at z = and x =.3L, -.7L,, from which lctrons and ions ar mittd toward a floating collctor nt z = 7, and ; = - L,. An xtrnally applid lctromagntic wav with right-hand circular polarization propagats along -z toward th rsrvoir, coming across a rsonanc point on th way. Typical simulation paramtrs ar as follows. Th charg to mass ratio:. (lctron),.25 (ion). Th systm lngth: l. = 5I21o.s, L,= l28lr*s (2.p"s: Dby lngth). Th stp lngth: o)r"slt =.2 (r4"5: lctron plasma frquncy). Th wav lctric-fild strngth: El(T"sl L.r; =.2. Th subscript S stands for th paramtrs at th rsrvoir. Figur 8 prsnts a typical rsult on th potntial, lctron- and ion-dnsity profils at arst = 24 undr th symmtric magntic-wll configuration of R^ = 2 with th singl-point rsonanc at zllo"s = 256 such as in Fig. 1. Th simulation convincingly confirms th xprimntal rsult on th plug (Af")/barrir (A@6) 73

6 Hatakyama R. t al., Fild-Alignd Plasma-Potntial Structur Formd by Local Elctron Cyclotron Rsonanc a o \ o o c q -n/ns ----ni / nis Ap Ao z?ts,,s Fig.8 Profils of potntial {. lctron n" and ion n, dnsitis in th PIC simulation = 24 with B-=2. (singl ECR point) for EllT"rld"*"1 =.2. potntial formation. In addition, th simulation for th first tim clarifis a spatial dviation of th lctron- and ion-dnsity profils, gnrating a charg sparation to form th plug/barrir potntial. This is bcaus lctrons ar trappd at th B-wll bottom and ions stagnat du to th potntial dclration. Th sparation distanc Az (= 41D"s) almost corrsponds to th stimatd dviation of th ion plasma oscillation around th trappd lctrons, i.., th ion Dby lngth (= 2lLo'1). In ordr to invstigat ffcts of th ion nrgy distribution function on th potntial formation, th distribution ahad of th rsrvoir xit is systmatically changd. Whn th ion-flow nrgy is incrasd with th ion tmpratur kpt almost constant and on th contrary th lattr is incrasd with th formr kpt almost constant, th plug potntial is corrspondingly incrasd and found to b always formd so as to rflct vn th high-nrgy tail componnt in th ion distribution. This rsult consistntly xplains th A@Ddiffrnc btwn th Qr-U and GAMMA Conclusions According to th basic xprimnts on th potntial formation and control du to local lctron cvclotron rsonanc, th following conclusions ar obtaind. Th plug/barrir potntial is wll formd and controlld with a singl ECR point in mirror-typ configurations of magntic fild. Th scal lngth of th potntial structur is of th ordr of th ion Dby lngth around th ECR point. Th barrir potntial formation is nhancd by lctron trapping and/or ion acclration du to 1tYB. Th plug potntial is dtrmind by th ion kintics (slf consistntly formd so as to rflct ions including a highr-tail componnt in th vlocity distribution function). Th GAMMA 1 rsults ar consistnt with our simpl scnario supportd by th basic xprimnts ( quantitativ agrmnt to som xtnt). Acknowldgmnts W thank M. Inutak and many scintists in th GAMMA 1 group for thir usful discussion and commnts. Th authors ar indbtd to S. Ishiguro for his coopration and H. Ishida for his assistanc. Rfrncs [1] T. Ohkawa, J. Plasma Fusion Rs. 64, 35 (199). [2] D.E. Baldwin and B.G. Logan, Phys. Rv. Ltt.43, l3r8 (1979). t3l D.P. Grubb r al., Phys. Rv. Lu. 53,783 (1984). t4l M.Inutak t al.,phys. Rv. Ltt.55,939 (1985). t5l N. Sato, Proc. (Invitd Paprs) Symp. Plasma Doubl Layrs, Roskild, 1982 (Risg National Laboratory, Roskild, 1982) p [6] J.S. Lvin and F.W. Crawford, J. Plasma Physics 24,3s9 (198). t1l T.Sato and H. Okuda, Phys. Rv. Ltt. 44,74O (198). [8] T. Kanko t al., Phys. Rv. Ltt. 8, 262 (1998) and J. Phys. Soc. Jpn. 69,26 (2). [9] T. Tamano, Phys. Plasmas 2,2321 (1995). t1l K. Yatsu / a/., Nucl. Fusion 39,177 (1999). I l] T. Kanko t al.,ieee Trans. Plasma Sci.2E, 1747 (2). [2] S. Torvdn t al., Plasma Phys. Control. Fusion 27, 143 (l985). tl3l A. Hasgawa and T. Sato, Phys. Fluids 25, 632 (1982). [4] T. Saito r al., Proc. Int. Conf. Opn Plasma Confinmnt Systms for Fusion, Nobosibirsk, 1993 (World Scintific, Singapor, 1993) p.l2l. [15] I. Katanuma t al., Phys. Plasmas 3,2218 (1996). [16] S. Ishiguro t al., Phys. Plasmas2,327l (1995). 74