Propagation of Cold Pulses and Heat Pulses in ASDEX Upgrade

Transcription

1 Propagation of Cold Pulss and Hat Pulss in ASDEX Upgrad R. Nu 1,F.Rytr 1,R.Dux 1, H.-U. Fahrbach 1, A. Jacchia,J.E.Kinsy 3,F.Lutrr 1, F. D Luca 4, G. Prvrzv 1, J. Stobr 1, W. Suttrop 1, ASDEX Upgrad Tam 1 1 Max-Planck-Institut für Plasmaphysik, EURATOM Association, Garching, Grmany Istituto di Fisica dl Plasma, Associazion EURATOM-ENEA-CNR, Milan, Italy 3 Gnral Atomics, San Digo, California, Unitd Stats of Amrica 4 INFM and Dipartimnto di Fisica, Univrsità dgli Studi di Milano, Milan, Italy -mail: Rudolf.Nu@ipp.mpg.d Abstract. Exprimnts on lctron hat transport wr prformd in th tokamak ASDEX Upgrad, mainly in ohmically hatd plasmas, applying ithr dg cooling by impurity injction or dg hat pulss with ECH. Rptitiv pulss within on plasma discharg wr mad allowing Fourir transformation of th tmpratur prturbation. This yilds a good signal to nois ratio up to high harmonics and allows a dtaild invstigation of th puls propagation. For dnsitis lowr than 1:8æ1 19 m,3, an incras of th cntral lctron tmpratur was found as th rspons to th dg cooling via impurity injction similar to obsrvations mad in othr tokamaks. Th invrsion dos not appar instantanously, but with a tim dlay roughly compatibl with diffusion. Modling of th propagation of th cold pulss in th framwork of th IFS-PPPL modl yilds qualitativ agrmnt. Howvr th prdictd incras of th ion tmpratur is not obsrvd xprimntally on th fast tim scal. Th rspons to ECH hat pulss is not prfctly symmtrical to cold puls xprimnts, but th similaritis suggst a common undrlying physical mchanism. No invrsion of th hat puls is found, instad th initial puls from th dg is associatd with a scond, much slowr hat puls in th cntr which is similar (and not symmtrical) to that of th cold pulss. It is found that th cntral incras is rlatd to th arrival of th puls clos to th invrsion radius and not to th initial puls. 1. Introduction Undrstanding nrgy transport in plasmas dvotd to fusion rsarch is th subjct of intnsiv studis. A particular obsrvation was mad in TEXT, aftr Lasr Blow Off impurity injction (LBO) [1]: th rspons to th dg cooling was a fast incras of th cntral lctron tmpratur, now known as non-local transport with amplitud rvrsal. Using hat pulss, cratd by a fast ramp of th plasma currnt cntral cooling was obsrvd in TEXT []. Furthr and xtnsiv studis followd ths initial obsrvations and mor information and rfrncs ar givn in th rviw papr [3]. Gnrally, th ffct appars at low dnsity, which suggsts that it is clarly visibl only whn lctrons and ions ar wakly coupld or whn T =T i is larg.. Exprimntal St-Up and Mthods Exprimnts wr carrid out in th tokamak ASDEX Upgrad (R = 1.65 m, a =.5 m, κ ç 1:6), applying ithr dg cooling or dg hat pulss. Th cold pulss wr producd by fast gas puffs and by impurity injction with a rptitiv lasr ablation systm, which allows puls-rats of up to Hz. Typical amounts of injctd particls ar a fw 1 18 in th cas of C and about 1 17 in th cas of Si and F. Th hat pulss wr launchd by Elctron Cyclotron Hating (ECH) providd by on 14 GHz gyrotron with about 35 kw of dpositd powr. Th X-mod scond harmonic was usd, nsuring a narrow (é 5 cm) singl pass absorption of almost 1% (optical dpth é 3) vn clos to th plasma dg. Off-axis dposition was achivd by adjusting th magntic fild or/and th launching mirror of th ECH. Th lctron tmpratur T is masurd by a 6 channl Elctron Cyclotron Emission (ECE) htrodyn radiomtr with a sampling rat of 31.5 khz. Th optical thicknss was sufficint for th masurmnt of T, at last for radii insid ρ = :9. Th lctron dnsity is gaind from a DCN intrfromtr, covring th main plasma and from a lithium bam diagnostic for th plasma dg. Using th Fourir mthod (FFT) to invstigat th propagation of rptitiv pulss, improvs th signal to nois ratio and allows to sparat th signal from th modulation causd by th

2 sawtth. Basd on th assumption of a diffusiv transport, th radial profils of th amplitud and phas of th hat pulss yild th hat diffusivity of th lctrons, χ HP [4,5]. If a convctiv ffct is prsnt, it will b obsrvd only on th amplitud profil as an asymmtry which dcrass with frquncy. Stp gradints of amplitud and phas indicat low transport and flat ons high transport. In our xprimnts th pulss had a low fundamntal frquncy (4 to Hz), but producd a larg numbr of harmonics, allowing th invstigation ovr a broad frquncy rang. 3. Propagation of Cold Pulss Cold pulss wr inducd, mostly in ohmically hatd dutrium plasmas, by injction of C, Si and F and also by 1 fast gas puffs with pur dutrium (5 ms duration). Figur F - injctions #15 1 shows tim tracs of th lctron tmpratur from a ρ =.15 discharg with I P = 6 ka and B t = :46 T at n ç 1 19 m,3 with a sris of cold pulss producd by F-LBO. An.3 nhancmnt of th cntral T is obsrvd as a raction to 3 th dg cooling. Th cntral hating dos not appar immdiatly,.4 but its onst is dlayd by at last 5 ms, and its maximum occurs 3 ms to 5 ms aftr th initial puls (s.5 also Fig. 4). Similar rsults ar obtaind by injcting C or Si and strongr dg cooling yilds a largr nhancmnt in.65 th cntr. Evn cold pulss initiatd by fast D puffs lad.78 1 to th sam bhaviour, indicating that th contribution of.86 th radiation from furthr insid th plasma is not rlvant..9 In fact, thr is a linar rlation btwn th dg cooling 1. and th cntral hat puls, indpndntly of th injctd lmnt Figur shows th rsult of th FFT of th ECE- tim (s) tmpratur data for th sam discharg. For all harmonics Figur 1: T at diffrnt radii a strong dcras in th amplitud occurs at ρ ç :5 corrlatd showing cold pulss from F in- with a clar phas jump at th sam radial position, jction. n is shown on th top. th so-calld invrsion radius. Sinc th FFT yilds only 8 Hz 3 Hz nois nois positiv valus for th amplitud, th phas jump rflcts th 4 Hz invrsion of th puls. Excpt for th rgion of th invrsion, 3 th amplitud of th prturbation shows a clar incras as th pulss propagat towards th cntr. This is in contradiction with purly diffusiv transport, vn apart from th 1 invrsion of th sign. Howvr, it cannot b attributd to #15 1 an inward convction, bcaus it dos not dcras with incrasing frquncy. Th ffct also cannot b producd on 5 8 Hz this fast tim scal by a rarrangmnt of th ohmic hating -5 Hz powr. Prforming cold pulss in dischargs with diffrnt -1 3 Hz B t and I P valus, it turnd out, that th invrsion radius of ρ th cold puls is always found outsid th sawtooth invrsion.8 1 radius. Its position is indpndnt of th strngth of th Figur : FFT data of discharg prturbation, of th lmnt injctd and also of th lctron 15 with F-LBO cold pulss. dnsity. Fig. 3 shows th sawtooth as wll as th cold puls invrsion radii vrsus th dg safty factor (q 95 ). Dnsity ramps rval that th invrsion of th cold pulss disappars at lin avragd dnsitis n max ç è1:5, 1:8è æ 1 19 m,3. Using th rptitiv injctions, it can b clarly dmonstratd in a singl discharg, that thr is no chang n (1 19 /m 3 ) _ ln( Amplitud (V) ) phas (dg) T (kv)

3 in th invrsion radius whn approaching n max. This is an xprimntal hint, that th absolut valu of T at th invrsion radius is not in dirct rlation to th invrsion mchanism. Looking at n èè=p T èè ç :35æ1 19 m,3 /V 1= as a critrion.8 for th obsrvation of th invrsion [6], th maximum valu of :4 æ 1 19 m,3 /V 1=, found in this work, complis vry wll..6 Injctions into H-plasmas and H-plasmas showd a similar ffct as in D-dischargs, as long as th cntral tmpratur was.4. high nough to fulfill th critrion. To intrprt th undrlying procsss, simulations using th IFS q 95 PPPL modl [7] th only modl yilding th invrsion [8] wr prformd. As input for th modl th xprimntal n profil and dg T, T i at ρ = :97 wr usd. n is kpt con- th sawtooth (æ) and th cold Figur 3: q-dpndnc of stant throughout th calculations. Th calculatd quilibrium T- puls invrsion radii (+: D, profils wr in good agrmnt with th xprimntal ons. Th æ: C,4: Si and æ: F),æ: amplitud of th prturbation at th dg was adjustd to match additional cntral ECH (3 th strngth of th cntral hat puls. A qualitativ agrmnt kw, cw), squar: invrsion with th xprimnt is obtaind and th invrsion radius is dscribd rathr wll, as shown in Fig. 4. It turns out howvr, that radii for th ECH pulss. 3 th dg cold puls has to b assumd much strongr in th modling and th cntral hat puls is too fast compard to th xpr imnt. Th ssntial physics ingrdint of th modl is a strong T i =T stabilising ffct in th critical gradint. This ffct propagats quickly ovr th radius and a fast nhancmnt of th ion 1.3 tmpratur ovr th whol radius follows, in raction to th dg -5 cold puls [8]. Th cntral incras of th lctron tmpratur is 5.4 a consqunc of th ion bhaviour. Th prdictd ris of th ion 1 tmpratur ( T i ç V) should b rflctd in an incrasd -5 nutron rat by a factor largr than two which is not sn in th 15 1 xprimnt. Th amplitud rvrsal and th invrsion radius on T rsult from th ovrlap of th initial cold puls on th lctrons.5-1 and th incras of T cratd by th ion bhaviour. This xplains th S-shapd tim volution of th modld T at ρ = :5 1 (solid lin). This bhaviour is not obsrvd xprimntally and.78 th wak S-shap of th masurd T is up-sid down compard to th modl. 4. Propagation of Hat Pulss Short hat pulss wr producd with ECH at th plasma dg, in dischargs with th sam paramtrs (I p, B T, n ) as for th cold pulss. T tim tracs for a dischargs with hat dg pulss ar shown in Fig. 5. Th ECH pulss (P ECH ç35 kw), rpatd vry 1 ms ( f = 1 Hz), had a duration of 5 ms and wr dpositd at ρ ç :85. Thir shap is invrs of th cold pulss from LBO with good approximation. Th channls furthr insid show that th pulss propagat with dcrasing amplitud and bcom smoothr, charactristic of a diffusion-lik procss. Th ρ ρ = tim [s] Figur 4: Tmporal volution of T ( ) and T i (- -) from th xprimnt and th IFS-PPPL-modl. hat pulss bcom almost invisibl at ρ ç :4 and th cntral channls do not xhibit th clar ngativ pulss xpctd from th symmtry with th LBO pulss. At most a vry wak dcras of T may b gussd at ρ ç :9 and insid. Clarly, th cntral channls xhibit

4 a smooth positiv hat puls with its maximum bing dlayd by about 3 to 5 ms. Th FFT data from discharg # 1116 ar givn in Fig. 6 for svral harmonics up to 5 Hz. Thy clarly dmonstrat th xistnc of an invrsion radius at ρ ç :4, whr th amplitud strongly dcrass and th phas quickly incrass. Pr analogy with th cold pulss, th trm invrsion radius is also usd, although th amplitud dos not chang sign. Figur 3 shows that th radial position of th invrsion radius for ECH pulss is closr to that of th sawtth than in th cold puls cass. 1.5 Th FFT provids additional information: Firstly, th radial n location of th ECH dposition at ρ = :83 is clarly indicatd at highr frquncis by th maximum of th amplitud and th minimum of th phas. Scondly, th propagation from th dposition towards th invrsion radius is diffusiv-lik yilding χ HP ç 3,5 m =s, indpndntly of th frquncy up to 6 Hz. This valu of χ HP is similar to th valu which could b xtractd from th phas only of th cold pulss. Not, howvr, th dcras of th amplitud whil th pulss propagat inwards, as xpctd from a diffusiv procss and in contrast to th LBO pulss. Nvrthlss th cntral rspons to ECH hat pulss rprsnts a non-local charactr: Th FFT data in th cntral rgion ar not th continuation of th puls propagation obsrvd up to ρ = :4, but th rsult of a scond dlayd tmpratur incras. This dlayd puls sms to hav th sam amplitud and roughly th sam phas ovr th whol cntral rgion and dos not show a clar propagating charactr. Locating th dposition at about mid-radius (ρ ç :6), th initial ECH puls is strong nough to b visibl on th T tim tracs in th cntral rgion and th initial and th dlayd puls can b clarly sparatd. Th jump in phas at th invrsion radius is almost th sam for th diffrnt positions of dposition. This dmonstrats that th raction of th plasma cntr is not dirctly rlatd to th initial prturbation but rathr dtrmind by th arrival of th puls in th rgion of th invrsion radius. Additionally, a clar asymmtry of th phas propagation is found. It indicats a lowr transport towards th cntr (χ HP ç 3, 5m /s) and a larg transport from th ECH dposition towards th dg (χ HP ç 7, 1 m /s), which is quit similar to th propagation of th sawtooth pulss. Dischargs with ECH hat pulss at n é 1:5 æ 1 19 m,3 do not xhibit an invrsion radius Lin avragd dnsity [1 m ] ECRH pulss ρ tim [s] Elctron tmpratur [kv] Figur 5: T at diffrnt radii showing ECH hat pulss. ln( Amplitud (V) ) phas (dg) Hz nois 5 Hz nois # Hz Hz 3 Hz 4 Hz 5 Hz ρ Figur 6: FFT data of discharg 1116 with ECH hat pulss. To quantify th transport changs in th cas of th ECH pulss, simulations with th ASTRA transport cod [9] wr prformd. Th most important fatur of th stady stat tmpratur profil is th chang of slop around ρ tor = :4, which rquirs a low χ valu in th cntr. A hat pinch, which would also b abl to caus th cntral paking, is xcludd by th xprimntal modulation rsults prsntd abov. Simulating th ECH pulss yilds a χ HP ç 3, 5m /s for puls propagation towards th cntr and also a strong rduction around ρ tor = :4. This stp provids th dcras of th amplitud and th variation of th phas as th pulss approach th invrsion radius, but it is not sufficint to rproduc th incras of

5 th cntral T following th dg hat puls with th tim dlay of 3 ms. For this purpos a tim dpndnc is ncssary and χ is modulatd in tim by only 15%, in th cntral rgion. Although this procdur dos not provid any physical xplanation, it dmonstrats that a small chang in χ is sufficint to xplain th obsrvations, making th undrlying physics difficult to idntify. It was also attmptd to simulat th ECH hat pulss using th IFS-PPPL modl. As xpctd, th xprimntal obsrvation cannot b rproducd by th modl, bcaus it provids an invrsion of th tmpratur prturbation. Thr is also no xprimntal indication of any dcras of th nutron mission rat as prdictd by th ITG modl. 5. Discussion Th cold pulss as wll as th hat pulss xhibit a comparabl tmpratur incras which occurs on th sam slow tim scal. This similarity suggsts a common undrlying physical mchanism which changs transport in th sam mannr around and insid th invrsion radius. Th diffrnt bhaviour outsid th invrsion radius may rsult from th following: Th ECH pulss do not crat any dnsity chang whras th cold pulss ar causd and accompanid by a particl sourc and nrgy sink. Thrfor cold pulss affct both, ions and lctrons whras ECH pulss hat xclusivly th lctrons. Espcially th diffrnc in th bhaviour of th amplitud of th prturbation may b du to th intraction with ions, or causd by th som combination of nrgy and particl transport. In th rgion insid th invrsion radius, th following common proprtis of cold and hat pulss ar vidnt: Th cntral tmpratur incras is rlatd to th arrival of th initial prturbation clos to th invrsion radius and th tim dlay of th cntral tmpratur incras is comparabl. Th cntral non-local raction occurs only blow th sam limit in dnsity and th amplitud in th cntr is proportional to that of th initial prturbation at th dg. Our study suggsts th xistnc of diffrnt transport mchanisms insid and outsid of th invrsion radius. Outsid, transport sms to b govrnd by critical gradint physics for th lctrons at last, as shown by th puls propagation and in particular by th phas asymmtry. Insid th invrsion radius, transport is low as indicatd by th rquird stp in χ. On may link th low transport in th innr rgion with th wak shar sustaind by th sawtooth activity. In such a situation with two radial rgions dominatd ach by diffrnt transport mchanisms, on may imagin that th arrival of cold or hat pulss at th transition zon is abl to slightly modify th transport at last in th innr rgion and causs th obsrvd ffct. Acknowldgmnt W ar glad to thank P. Mantica and A.G. Ptrs for stimulating discussions, and W. Dorland for having kindly givn us th routin of th IFS-PPPL modl. Th modling of th cold pulss with th IFS-PPPL modl by J. Kinsy was partly supportd by DOE contract Grant No. DE-FG3-95ER5439. Rfrncs ë1ë GENTLE, K.W., t al., Phys. Rv. Ltt. 74, (1995) 36. ëë GENTLE, K.W., t al., Phys. Plasmas 4, (1997) ë3ë CALLEN, J.D. and KISSICK M.W., Plasma Phys. Contr. Fusion 39, (1997) B173. ë4ë LOPES CARDOZO, N.J., Plasma Phys. Contr. Fusion 37, (1995) 799 and rf. thrin. ë5ë JACCHIA, A., t al., Phys. Fluids B 3, (1991) 333. ë6ë KISSICK M.W., t al., Nucl. Fusion 38, (1998) 81. ë7ë KOTSCHENREUTHER, M., t al., Phys. Plasmas, (1995) 381. ë8ë KINSEY, J.E., t al., Phys. Plasmas 5, (1998) ë9ë PEREVERZEV, G.V., t al., Rport 5/4 MPI für Plasmaphysik, Garching (1991).