3.12 Electron Cyclotron Resonance Heating (ECRH)

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Garey Dickerson
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1 3.1 Elctron Cyclotron Rsonanc Hating (ECRH) FANG Yud Introduction Elctron cyclotron rsonanc hating (ECRH) is an attractiv mthod for both sulmntary lasma hating and tmratur/currnt rofil control in tokamaks, bcaus its uniqu rortis: quasi-otical roagation, localizd absortion, high ffct and dirct lctron hating. EAST is a surconductor tokamak with circl/non-circl changabl lasma cross sction. On of its main objctivs is to raliz stady stat oration and to find and rformd in som kinds of advancd tokamak configuration. ECRH is on of th suitabl and hoful mthods to raliz ths objctivs. In rcnt twnty yars xrimnts, ECRH has bn rovd an ffctiv mthod for bulk and localizd lctron hating, tmratur and currnt rofil control, MHD control, rionization and currnt start-u, and a hoful mthod for non-inductiv currnt driv in tokamaks. Th ECRH systm will b st u and th ECRH xrimnts will b rformd in EAST tokamak. This rogram could b carrid out in two sts: Th first st Lowr frquncy gyrotrons and th corrsonding ECRH systm will b movd from HT- 7 tokamak to EAST. This systm will injct RF owr from th outsid of EAST lasma tours for O-mod ECRH hating. This lowr frquncy gyrotron is: 60GHz, about 00KW outut RF owr and uls lngth scond. Now this ty of gyrotron is availabl Th scond st Adding to a long uls (svral sconds-svral hundrd sconds uls lngth), 110GHz or 118GHz, svral hundrds KW gyrotron systm to th 60GHz systm. This kind of gyrotron is mor suitabl than th lowr frquncy and shortr uls lngth on for th surconductor tokamaks and thir highr lasma dnsity and high toroidal fild such as EAST. EAST can suly an aroriat condition for ECRH xrimnts. That is: (a) EAST is a vry flxibl tokamak dvic. Its toroidal fild can b chosn from 1.0 to 3.5 T, and th lasma dnsity can b changd from m -3 ; (b) EAST will orat normally with MW Ohmic hating owr, so MW ECRH owr should b nough to roduc som dfinitiv rsults; 55

2 (c) Th oration configuration of EAST is vry flxibl: circl/ non circl cross sctions with diffrnt k and, limitr or divrtor oration, which maks EAST an idal dvic to study ECRH from diffrnt ascts; (d) Th strong lowr hybrid currnt driv (LHCD) and ion cyclotron hating (ICRF) in EAST allows th invstigation of various synrgistic ffcts with ECRH in th filds of currnt driv and lctron or ion hating. () Strong diagnostics ability will b st u in EAST tokamak. So it is nsurs that ECRH xrimnt s rsults can b undrstood convnintly Exrimntal Program of ECRH on EAST Th ECRH study will carry out in EAST with both th ordinary and xtraordinary mods, which will b injctd from outsid and insid of th tours rsctivly. Aftr comlt th stting u of ECRH systm, both 60GHz and 110GHz (or 118GHz) gyrotrons will b usd in ECRH xrimnt in EAST Objctivs of roosd ECRH xrimnts Physics of wav absortion and bulk lctron hating To undrstanding of th hysics of high-owr absortion is on imortant objctiv. In EAST, w will carry out th ECRH xrimnt with both O-mod and X-mod, in both th fundamntal rsonanc hating and th scond harmonic hating. In xrimnts, th lctron hating rsults, such as lctron tmratur and ion tmratur, thir rofils, th lasma intrnal nrgy, will b chckd. In rsarch st1, th injctd EC owr is about kW, which won t b a small valu comard with th Ohmic hating owr 1-1.5MW. So th bulk hating xrimnts will giv som dfinit rsults. But, in th first rsarch st, if w want ut th rsonanc layrs in th cntral ara of th lasma column, th toroidal magntic fild in axis must b rducd to about -.5 T. Bcaus in this st, only th 60GHz wav gnration is usd Localizd hating and rofil control, its MHD ffcts Localizd lctron hating can chang th lctron tmratur rofil, and so th lasma currnt rofil. It is ossibl that us this localizd ECRH, w can find som suitabl q- rofils which can stabiliz som taring mods or kink mods, and som tims both of thm can b stabilizd simultanity. In xrimntal, th toroidal magntic fild would b adjustd and th microwav owr would b injctd at an aroriat angl for som scial ositions. Th various diagnostics would b usd to track th MHD bhaviors Synrgtic ffcts with LHCD or ICRF in EAST To study th synrgtic ffcts of ECRH during LHCD or ICRF hass is imortant for th invstigations in EAST. W will chck th functions of ECRH to LHCD drivn fficincy and to ICRF hating rsults. 56

3 It is rdictd that using ECRH to hat lctron and so incras th lctron tmratur will b bnficial for incras of LHCD drivn fficincy, and in a targt lasma with rlativ high lctron tmratur th ICRF hating rat should also b highr. Localizd ECRH would bring som changs of th tmratur and currnt rofil, which also roduc som ffcts to LHCD or ICRF. In this xrimnt, diffrnt ECRH mods will b tstd and various diagnostics will b usd to giv th rliabl data Currnt driv Th ositiv rsults of lctron cyclotron currnt driv (ECCD) has bn rovd in som tokamak xrimnts, vn if th drivn fficincy is low bcaus th rlativly low lasma aramtrs for ECCD in nowadays tokamaks. At rsnt, ECCD has bn considrd a hoful currnt driv mthod to driv lasma currnt in th cntr ara for nxt ractor scal tokamaks. Its advantag is that thr is not any limit for EC wav ntration into lasma intrnal rgion, if th wav frquncy is chosn high nough. In EAST, w will chck ECCD ffcts us both O-mod and X-mod in diffrnt injctd angls and diffrnt rsonanc layr ositions. It is stimatd that, th fficincy of ECCD in EAST will b low, so it is ncssary to dvlo som cods for th calculation of th drivn currnt by ECCD from th total lasma currnt Study of initiation and start-u has of dischargs It has bn rovd that alication of ECRH during lasma initiation is a ractical way of ralizing start-u in rlativly lowr loo voltag, saving substantially th volt-sconds which wr consumd during lasma start-u riod and forming a rroducibl currnt channl at a wll-dfind location away from th wall, so that th lasma-wall intraction can b rducd in this has and th subsqunt nrgy confinmnt tim will b influncd. In EAST, w will chck th functions of ECRH in th riod of lasma initiation us diffrnt ECRH mods and mak diffrnt wav nrgy dosition rgions. Th ECRH ffcts to gas r-ionization, lctron hating and thn th assing tim through th ak radiation zon which is causd by th imurity ions, th lowring of th loo voltag, th saving of voltsconds and th influnc for lasma confinmnt will b studid Basic undrstanding of ECRH in tokamak ECRH is in th highst rang of frquncis usd for lasma hating. Th rquird frquncy is gnrally th fundamntal or scond harmonic of th lctron cyclotron frquncis, which for nowadays tyical tokamak magntic fild is in th rang GHz, and for th ractor scal tokamaks, such as ITER is in GHz. ECRH has a numbr of advantags in wav transmission and absortion. Bcaus th frquncy usd in ECRH is vry high, th wav can roagat in lasmas such as in vacuum 57

4 and no any scial couling schm b rquird to gt it into th lasma, and it can b launchd from a siml wavguid artur or a quasi-otical mirror grou. About th wav absortion, EC wavs can b absorbd in quit narrow local ara around th cyclotron rsonanc layrs. Such localizd hating may b of us in controlling th currnt rofil in a tokamak, with a viw to surssing magnto-hydro dynamic instabilitis and in som cass to imrov lasma confinmnt bhaviors Wav roagation and its accssibility condition to rsonanc layr Although th wav absortion is dndnt on hot lasma ffcts roagation of wavs u to th cyclotron rsonanc layr is ssntially dtrmind by th cold lasma disrsion rlation. In a homognous lasma, thr xist two diffrnt wav mods from th Altron- Hattr disrsion rlation [1] : n k c / ω = 1 [αω (1 α)]/[ω (1 α) Ω sin θ ± Ω Γ] (1) = whr: Γ = [ Ω sin 4 α = ω / ω θ + 4ω (1 α) cos is th angl btwn wav roagation dirction and stady magntic fild. θ ] Th + and - signs in quation (1) corrsond th ordinary mod (O-mod) and xtraordinary mod (X-mod) rsctivly. 1/ For θ= π /, quation(1) can b simlifid to: n n = 1 ω = [( ω / ω ω ) Ω ω ]/[ ω ( ω ω Ω )] () for O-mod (3) for X-mod So, for O-mod, both b injctd from outsid or insid of th tours, th wav can roagat until rachs th oint whr th dnsity satisfis th rlation ω =ω. It mans that at for th fundamntal rsonanc layr, th accssibility condition of th wavs is: ω < (4) Ω and at th scond harmonic rsonanc layr, this condition bcoms to: ω < Ω (5) 4 58

5 And for X-mod, at th fundamntal rsonanc layr and th ur hybrid rsonanc layr (whos osition is vry nar from th fundamntal rsonanc layr), th condition for wav accssibility is: ω < Ω (for wav injctd from insid) (6) (Ths two rsonanc layrs can t b accssibl, if X-wavs injctd from outsid.) and at th scond harmonic rsonanc layr, this condition bcoms to: ω < (for wav injctd from outsid) (7) and Ω ω < Ω (for wav injctd from insid) (8) 6 For X-mod, th ur hybrid rsonanc layr xists in th osition, whr: ω =ω + Ω (9) and thr xist a cutoff layr for X-mod, whr: ( ω ω ω ) = Ω (10) Wav absortion Th strngth of wav absortion is charactrizd by th lasma otical dth. W can writ th rlation: P A = P [1 x( )] (11) 0 τ Whr P A is th absorbd wav owr, and P 0 is th total wav owr. About th wav absortion, w can summariz that: (a) For O-mod, dirct wav absortion at th fundamntal cyclotron frquncy occurs in th vicinity of th ECR layr and th absortion zon is accssibl from both th insid and outsid of th tours; (b) For X-mods, dirct wav absortion at th ECR layr is wak, but th indirct absortion, via mod convrsion at th ur hybrid layr into Brnstin wavs, is vry strong. This mak th wav absortion almost comlt in ECR layr; (c) At th scond harmonic cyclotron rsonanc, th X-mod is strongly absorbd both for injctions from insid and outsid; (d) In ovr-dns condition ( ω >ω ), thr xists a narrow angl of ordinary wav roagation, such that an O-mod from outsid of tours may b convrtd into X-mod and hnc b absorbd at th ECR zon in th form of Brnstin wavs. In summary, th O-mod hating is rfrabl for bulk lasma hating and for th rofil control in th intrior rgion of th lasmas. Th X-mod hating is surior for surfac and low-dnsity ara hating, scially insid th scra-off lasma intrctd by divrtor and also for low-dnsity lasma start-u. 59

6 Rlativistic ffcts and th down-shift frquncy hating Rlativistic ffcts maks th rsonanc vlocity, in th rgion whr th cyclotron frquncy is abov th wav frquncy, considrably lss than that thy would b according to non-rlativistic thory. So th wavs whos frquncy is blow th lctron cyclotron frquncy vrywhr can b absorbd and th lctron hating can aar in th rgions whr th mor highr magntic fild xist. This rlativistic ffct ar vry obvious, scially in th cass whr som high-nrgy lctron comonnts xist, such as in som RF hating or currnt driv rocsss. So, in th occasions of LHCD or ICRF, as w lan to carry out in EAST, this rlativistic ffct will lay an imortant rol, if w carry ECRH in ths cass at th sam tim Choic of Oration Paramtrs in ECRH xrimnt in EAST In ECRH xrimnts, th choic of oration aramtrs should b dcidd according to th microwav frquncy, thir injction mannrs and th hating mods. In ECRH xrimnts, two conditions must b satisfid Dnsity limit Both for O-mod or X-mod, thr xist th dnsity limits th wavs can transmit only in th aras in which th lasma dnsity is lowr than on of ths critic dnsitis. From quations (4)-(8), w can summaris th critic dnsitis as follows: Frquncy Critic dnsity (GHz) Injctd from (10 13 /cm 3 ) O-mod X-mod O-mod X-mod Outsid or insid Outsid or insid Outsid or insid Insid only Insid only Insid only Rsonanc condition For diffrnt wav frquncy and diffrnt hating mods, th rsonanc conditions ar as follows: Frquncy (GHz) Fundamntal (Tsla) scond harmonic (Tsla)

7 If on or svral of ths rsonanc layrs xist in th lasma rgion, th dnsity conditions mntiond abov ar satisfid and th wav roagation mannrs ar also suitabl, th wav nrgy will dosit mainly in ths rsonanc aras and th lctrons will b hatd Program lan In th first st of th ECRH xrimnt, on or two 60GHz gyrotrons will b usd. Th total outut RF owr will rach about kW. This RF systm can sustain th uls duration btwn scond. In th scond st of th ECRH xrimnt, xct th 60GHz systm, anothr 110GHz (or 118GHz) systm will b dvotd. Th latr on can sustain its working uls duration from svral sconds to svral hundrd sconds. Th schdul of EAST ECRH rsarch rogram is as follows: Yar St 1 St Th Tchnical Imlmntation of EAST ECRH Program Rsarch st1 In st1, only 60GHz systm will b st-u and installd in EAST tokamak. This 60GHz ECRH systm is mainly b movd from EAST tokamak, xct its wav launchr (antnna). About this systm, many of its subsystms will b comltd arly in HT-7 invstigations, such as owr suly systm, gyrotron gnrator, wav transmission lin, som rotction comonnts, som instrumnts for wav aramtr masuring. But in this systm, th wav launchr must b rst-u, bcaus th windows usd for ECRH in EAST and th sha and scals of EAST lasmas ar all diffrnt from HT GHz Gyrotron As th wav gnrator, th 60GHz gyrotrons (Ty: VGE-8060A, Producd by CPI Cororation, USA) will b usd in this st. Th main oration aramtrs of th VGE-8060A gyrotron ar as shown in tab.1. Th main magnt for th wav-articl intraction rgion in gyrotron is a grou of surconductiv solnoids, which can suly th ndd magntic fild around 3kG. Th magnt rquirs u to 30A DC currnt rgulation lss than Th additional sur-conducting solnoid magnt is in th cathod rgion, which rquirs about 3A DC currnt. 61

8 Tabl 1 Th Main Paramtrs of VGE-8060A Gyrotron Paramtrs Normal rformanc rang Working Frquncy (GHz) 60 Outut Mod TE 0 Bam Voltag (kv) -80 Bam Currnt (A) 8.1 Collctor Dissiation (kw) 650 Gun Anod Voltag (kv) 17 (with rsct to cathod) Hatr Voltag (V) 6.7 Hatr Currnt (A) 3 Body Currnt (ma) < 10 Gun Anod Currnt (ma) <3 Efficincy (%) ~ 30 Puls Duration (ms) Outut Powr (kw) 00 Cooling Tub Body Tub Collctor Outut window Watr Load Dionization Watr 18 gs, 100 si 150 gs, 115 si 10 gs, 30 si 100 gs, 70 si Transmission Lin Th 60GHz microwav outut from th gyrotron will b transmittd to th rgion nar ECRH window in EAST, with its lngth around 40 mitrs, by a transmission lin. Th transmission lin is basd on a circl ovr-mod cor wav-guid lin with its innr diamtr.7cm. Using of th ovr-mod transmission lin can rduc owr loss and gt nough owr sustain and assing ability. In th transmission lin, som 90 o bnds mad of corrugatd wav-guid with th sam diamtr ar ndd. Som othr imortant comonnts ar connctd in it too, such as th arc dtctor nar gyrotron s outut window, a rsistiv sction assmbly, a down-tar btwn gyrotron s window (6cm dia.) and.7cm wav-guid, mod convrtrs from TE 0 to TE 01 and 6

9 TE 01 to TE 11 rsctivly, DC isolator, dirctional coulrs in sctions of TE 0 and TE 01, vacuum cramic window Antnna Th quasi-otical Gaussian bam antnna will b adotd as ECRH wav launchr (antnna) in EAST. This antnna is consistd of som lan mirrors and llitical mirrors. This kind of antnna can launch wavs from th dsirabl sid of th tours (insid or outsid of th tours), and can focus th wav to any ointd ara in lasmas too. Fig.1 shows th structurs of 60GHz ECRH transmission lin. Fig.1 Th Transmission Lin for ECRH in EAST Rsarch st In st, a nw ECRH systm with its working frquncy 110GHz or 118GHz, outut owr 500kW-1MW and much longr uls lngth, will b addd. This systm is mor dsirabl for EAST both in its highr frquncy and longr uls lngth Gyrotrons- th Wav Gnrator For EAST usual oration conditions- th toroidial fild in 3-3.5T, th frquncy of around 110GHz for ECRH is mor suitabl. At rsnt, th 110GHz gyrotrons can b sulid by CPI Cororation (USA), GYCOM Cororation (Russia) or JAERI Institut (Jaan). Ths 110GHz gyrotrons hav bn usd succssfully in DIII-D and JT-60U tokamaks. Th outut owr abilitis of this kind of gyrotrons ar almost th sam, that is th outut owr is 0.7-1MW, with th uls lngth rach to 5-10 sconds and th outut mod from th gyrotrons ar 63

10 all th quasi-otical HE 11 mod which can dirctly b could into th quasi-otical transmission lin, which is mad u of th high ovrmod corrugatd wavguid. Th 118GHz gyrotrons can b sulid by Thomson Cororation (Franc). This kind of gyrotron can works in quasi-continuous stat (its uls lngth can rach to svral hundrd sconds) with th outut owr KW, and it will b usd in Tour-Sur tokamak xrimnt. In gnral, for th surconducting tokamaks, such as Tour-Sur and EAST, th quasi-continuous gyrotrons ar mor dsirabl. Tabl givs th main aramtrs of th 118GHz gyrotron. Tabl. Th Main Paramtrs of TH1506 Gyrotron Paramtrs Normalrformancrang Frquncy(GHz) 118 PakOututPowr(kW) 500 PulsLngth,max.(Sc) 10 Efficincy,min. 30% OututMod HE 11 ModPurity,min. 95% BamVoltag,max.(kV) -85 BamCurrnt,max.(A) GunAnodVoltag(kV) 30(withrscttocathod) HatrVoltag,acordc(V) 0 HatrCurrnt(A) 0 GunAnodCurrnt(mA) < Transmission Lin and Antnna For th vry high frquncy (110-10GHz) wavs, th quasi-otical transmission lin basd on th high ovr-mod corrugatd wav-guid is suitabl. Th advantags of this kind transmission lin ar its low loss charactr and th asy controlld bhavior for th wavs in it. At rsnt, th main comonnts of this kind quasi-otical transmission lin ar availabl, such as th bnds, dirctional coulr, mod convrtr for rgulating of th olarization dirction of th wavs. Bcaus th outut mods of ths kinds of gyrotron ar all th 64

11 Guasian bam mods, th couling btwn gyrotron outut window and th corrugatd wavguid is vry convnint. Th antnna usd in this high frquncy systm is also th quasi-otical mirror grou, which will b installd in th ur osition of EAST vacuum vssl though a to window. Th focusd mirrors and lan mirrors will b locatd clos nar th wall and so far from th lasmas. This mirror grou can introduc th wavs into any ositions in lasmas from insid or outsid convnintly. Similar kind of antnna of quasi-otical mirror grou has bn usd succssfully in som tokamak s xrimnts. Fig. shows th brif structur of ECRH antnna will b usd in EAST. Fig. Th Gaussian Bam Antnna of ECRH in EAST 65

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