Boivin, R.

Bombard, B.

Bombarda, F.

Bonoli, P.

Christensen, C.

Fairfax, S.

Fiore, C.

Goetz, J.

Golovato, S.

Graf, M.

Granetz, R.

Greenwald, M.

Horne, S.

Hsu, T.

Hubbard, A.

Hutchinson, I.

Irby, J.

Lipschultz, B.

Luke, T.

Marmar, E.

DSpace@MIT

PFC/JA-94-29Confinement of Ohmic and ICRFHeated Plasmas in Alcator C-ModM. Greenwald, R. Bovin, F. Bombarda1 , P. Bonoli,C. Christensen, S. Fairfax, C. Fiore, J. Goetz, S. Golovato,M. Graf, R. Granetz, S. Horne, T. Hsu, A. Hubbard,I. Hutchinson, J. Irby, B. LaBombard, B. Lipschultz, T. Luke,E. Marmar, M. May2 , G. McCracken, A. Niemczewski,P. O'Shea, M. Porkolab, J. Rice, J. Schachter, J. Snipes,P. Stek, Y. Takase, J. Terry, R. Watterson, B. Welch3 , S. WolfePlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139September 1994'ENEA-Frascati, Frascati, Italy.2 Johns Hopkins University, Baltimore, Md., USA3 University of Maryland, College Park, Md., USAThis work was supported by the U. S. Department of Energy Contract No. DE-AC02-78ET51013. Reproduction, translation, publication, use and disposal, in whole or in partby or for the United States government is permitted.INTERNATIONAL ATOMIC ENERGY AGENCYk FIFTEENTH INTERNATIONAL CONFERENCE ON PLASMA PHYSICSAND CONTROLLED NUCLEAR FUSION RESEARCHSeville, Spain, 26 September - 1 October 1994IAEA-CN-60/A2/4-P-2CONFINEMENT OF OHMIC AND ICRF HEATED PLASMAS INALCATOR C-MOD*M. Greenwald, R. Boivin, F. Bombardal, P. Bonoli, C. Christensen, S.Fairfax, C. Fiore, J. Goetz, S. Golovato, M. Graf, R. Granetz, S. Horne,T. Hsu, A. Hubbard, I. Hutchinson, J. Irby, B. Labombard, B. Lip-schultz, T. Luke, E. Marmar, M. May2, G. McCracken, A. Niemczewski,P. O'Shea, M. Porkolab, J. Rice, J. Schachter, J. Snipes, P. Stek, Y.Takase, J. Terry, R. Watterson, B. Welch3 , S. WolfeMIT - Plasma Fusion CenterCambridge, MA.United States of America* Supported by US DoE contract #DE-AC02-78ET510131 ENEA-Frascati, Frascati, Italy2 Johns Hopkins University, Baltimore, Md., USA3 University of Maryland., College Park, Md., USAThis is a preprint of a paper intended for presentation at a scientific meeting. Because of the provisional nature of itscontent and since changes of substance or detail may have to be made before publication, the preprint is made available on theunderstanding that it will not be cited in the literature or in any way be reproduced in its present form. The views expressed andthe statements made remain the responsibility of the named author(s); the views do not necessarily reflect those of the goiem-ment of the designating Member State(s) or of the designating organization(s). In particular, neither the IAEA nor any otherorganization or body sponsoring this meeting can be held responsible for any material reproduced in this preprint.IAEA CN-60/A214-P-2CONFINEMENT OF OHMIC AND ICRF HEATED PLASMAS INALCATOR C-MODAbstractA series of confinement experiments has been carried out in Alcator C-Mod.We have found that data from both ohmic and ICRF heated plasmas can be fittedwith an L-mode scaling law. The ohmic data show no scaling with density in anyregime, the tE of these discharges being as much as 2-3 times neo-Alcator. Forthe small size and relatively high current in this tokamak, L-mode confinement isoften larger than neo-Alcator. With ICRF heating, confinement degrades with in-creasing power approximately as P 0j' ~. Overall, the confinement properties ofthe ohmic and ICRF plasmas are apparently distinguished only by the level of in-put power. Ohmic and ICRF H-modes are obtained over a wide range of dis-charge parameters, extending the range in the international database for surfacepower density, P/S, by almost a factor of 10. The power threshold for elm-freedischarges was in rough agreement with the old ITER database scaling P/S =.044 nB. Elmy H-modes were obtained at powers at least a factor of two lowerthan the elm-free ones. These results were obtained with all molybdenum plasmafacing components and no special wall treatment or coating.1. IntroductionThe Alcator C-Mod tokamak is a compact high field device capable of pro-ducing shaped diverted plasmas. Confinement studies have been carried out inhydrogen and deuterium plasmas with .35 < Ip(MA) < 1.05; 3.5 <BT(T) <5.4;.20 < a(m) < .24; .65 <R(m) < .70; .7 < ne(10 2 0m-3) <2.4; 1.1 < K < 1.6.The Zeff is low, typically <1.5 for ne > 1.5x 1020. All of the plasma facing compo-nents are made of molybdenum with no special wall treatment or coating. Plasmastored energy is calculated by integration of the density and temperature profilesand by analysis of the MHD equilibrium. These generally agree to within 20%,2however for ICRF plasmas, Wmhd is systematically higher than Wkin. This sys-tematic difference is probably attributable to energy in the minority ion tail.When "no plasma" shots are available for subtraction of stray field effects, wecan also obtain the stored energy by analysis of the diamagetic loop signals.These are in good agreement with Wmhd. Shots for confinement analysis werechosen with "standard" criteria: quasi-steady state, sawtoothing, and non-disruptive.2. Ohmic L-Mode ConfinementEarly in our experimental campaign, clear differences between Alcator C-Modand its predecessor Alcator C emerged. In the ohmic plasmas, stored energy in-creased with Ip but not with ne. Figure 1 shows the measured TE plotted againstthe calculated neo-Alcator TE[I]. Note that for most discharges, the measured TEis greater than the neo-Alcator value. The increase in stored energy with Ip sug-gested an L-mode behavior; figure 2 shows the measured TE plotted againstTITERP-89[2 ]. The agreement in this case is clearly much better. Our conclusionis that our ohmic confinement is following an L-mode like scaling which for lowdensities and high currents can exceed neo-Alcator by as much as a factor of 3.T VS TeoAIctor - Ohmic Plasmos TE vs T - Ohmic Plosmos0.050 ' 0.0500.040 - 0.040 -0.030 9 0.0300.020 - 0.0200.010 - Hydrogen 0.010 y rogenF Deuterum Deutenumn0.000 L'.- _L_ 0.000 .0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.010 0.020 0.030 0.040 0.050re = .166 n.R"o0-A4ic" (s) TMW= .048 IpORloaxn.* Btuamu"P~ (s)Fig.1) Measured T E vs TE(neo-Alcator) Fig.2) Measured TE vs TTR89.P)scaling for ohmic data. The data do for ohmic data.not show agreement with the scaling.3A linear regression can be performed to calculate the best fit to a power lawscaling for the C-Mod data alone. The range of some parameters (a, R, BT) in ourdata base is too limited to include in the regression, however significant resultswere obtained by fitting against Ip, PRot, ne, K, and amu (effective ion mass). Itshould be noted that there are significant covariances in the database. In particu-lar Ip, ne, K, and amu are generally correlated. Of course for the ohmic data thereis an additional correlation between Ip and Poh. The results, which include datafor ICRF plasmas, are shown in figure 3. The coefficients are not distinguishablefrom ITER89-P within the error bars of the two fits.T KIN VS T qnu Ip z ne Ptot)Measured tE 0.06p.-*Fig.3) Measured 'TE Ao = 0.027vs power law regres- =.44 * 0.04sion, fitting to Ip, K -7i= 0.79 k 0.05ne, Ptot, and amu (ef- 7, 0.26 1 0.12fective mass). Both 0.04 -71, 0.30 ± 0.03ohmic and ICRF 7p = -0.60 * 0.02data are included. R 0.944The coefficients areessentially the same -as for ITER89-P 0.02-within the error bars.0 ICRFOhmic0.00.0.00 0.02 0.04 0.06*m (fit) = 0.027 Amu0"Ip-"xne0"Ptot-3. ICRF Heating and ConfinementICRF heating was performed using a dipole antenna (two current strapsspaced toroidally and driven out of phase) with up to 2 MW of power at 80 MHz.The heating scenario was hydrogen minority heating in deuterium at Bt = 5.3 T.Efficient heating was observed, roughly doubling electron and ion temperaturesat the higher RF powers. There were substantial variations in the relative tem-perature increase of the electrons and the ions. The minority concentration wastypically around 10% but was not well controlled or characterized in these ex-periments. The energy of the ion minority tail should decrease at higher densityand minority fraction, resulting in less slowing down on the electrons and more4(indirect) ion heating. Evidence so far suggests such a correlation but is not con-clusive at this point.Shot #940602019 Confinement Times - Shot#94060201940 TO(O) 0.6S0.050r~o) 0.0400.0300.0200.010 - -. Tujit- Tou..no2 0.000 . . . . . .1 0.0 0.2 0.4 0.6 0.8 1.02.1 F Time (sec)0 _0_ _ Fig.5) Measured TE vs time for the plasma0.50 0.60 0.70 0.80 shown in figure 4. tE neo-Aicator and TETime (sec) iTER89-P are included for comparison.The transients seen at the time of RF turn-Fig.4) Time histories of Te(0), Ti(0), on and turn-off are artifacts of the long av-ne, Zeff, and Picrf for shot 940602019 eraging period for the ECE data.Traces of Te(0) and Ti(0) vs time are shown in figure 4. The electron and iontemperatures are strongly modulated by sawteeth activity. The sawteeth them-selves are modified, their period increasing by factors of 2-4 over otherwise simi-lar ohmic plasmas. Despite a small increase in density and Zeff, Prad/Ptot is un-changed. Figure 5 shows traces of the calculated TE vs time - the data follow L-mode scaling right through the ohmic and ICRF phases. This point is reiterated infigure 6, where the stored energy is plotted vs total input power for severalranges of Ip. In figure 5, the larger value of TE determined from the MHD equi-librium calculation is likely due the effect of the ion tail energy.4. Local AnalysisInitial runs of the TRANSP[3] code have been performed. For the ohmicshots at Ip = .8 MA, ne = 1x10 2 0, we have found Xe(a/ 2 ) - 0.5 m2/sec. At lowdensities, where electron and ion transport can be separated, Xi ~ Xe and is5mildly anomalous with Xi/Xneo = 1-3 with large uncertainties. For a typicalICRF plasma, with Prf = 1.8x106, Ip = .8 MA, and ne = Ix I , the calculationsshow approximately 90% of the RF power is thermalized within r/a = .5 and anion tail with approximately 10-15 U out of a total stored energy of 60kJ. This isin good agreement with measurements of AW from kinetics, MHD, and diamag-netism. For the ICRF plasmas, Xe rises to about 1.3 with little change in Xi.Stored Energy vs Total Input PowerFig.6) Plasma ther-mal energy vs totalinput power forvarious ranges inIp. The solid linesshown are ITER89-P scaling.DL)40a'Li0C00 1 2Input Power (MW)5. H-modeA series of experiments was carried out to look for H-mode in ohmic plasmas.Because C-Mod operates at much higher toroidal field and density than other to-kamaks, it offers the opportunity to significantly expand the international data-base.With the ion grad-B drift toward the divertor, clear L-H transitions were ob-served. Further efforts to lower threshold power by ramping down BT to 3T andlowering the density, resulted in the achievement of elm-free H-modes[4]. Ac-companying the transition was a drop in Ha light, a strong rise in density andstored energy, a steepening of the edge profiles, and a decrease in edge turbu-lence. A plot of C-Mod data vs the "consensus" scaling P/S = .044 nB[5 is6000 0.8 MA - Ohmic 0 0.8 MA - ICRFO 0.6 MA - Ohmic M 0.6 MA - ICRFA 0.4 MA - Ohmic A 0.4 MA - ICRF... , . ... ,.... .. ..320shown in figure 7. While the elm-free data agree well with the empirical curve,elmy H-modes were obtained at substantially higher values of nB. No boroniza-tion or other coating on the all molybdenum first wall was necessary to obtainthese results. There are some indications that lithium coating, the remnants oflithium injection experiments, may lower the power threshold further. H-Modeswere also achieved with ICRF heating[8] with similar power thresholds as seenin the figure 7.ICRH and Ohmic H-Mode ThresholdFig.7) H-modethreshold data forohmic and ICRFdischarges. Thedashed line is theITER scaling P/S =.044nB and is agood fit to our elm-free data. Resultsfrom DIII-D[6] andASDEX-UG[7] areshown for compari-son.0.3CNEC,)1-1CL0.20.10.0ELM Free OHo ELMy OHo ELMy OH (Li Inj)X ELMy ICRH$ ELM Free ICRH.1-- ~-NE10)K)K ENE:0,-_-P/S = .0044nB11 3DIlD (Osborne et al. Nucl. Fus. 90)ASDEXU (Ryter et al. EPS 93)40nBT (10 9 m ,T)80 100The elm-free periods were short lived, lasting typically 50 msecs. These werelimited by an uncontrolled density rise and disruptions as q(a) dropped near 2(due to the BT ramp). While the energy confinement showed a large increase dur-ing the elm-free periods, it was not possible to make definitive measurements dueto the strong transients. TE in the L phase of these plasmas was around 30 msec.After the L-H transition, TE rose to 50-100 msec. The latter figure comes fromincluding the plasma dW/dt term which could be as large as 60% of the inputpower - thus the plasmas conditions were far from steady state. The ITER H-mode TE for these plasmas is around 55 msec [9]70 20References[1] PARKER, R., GREENWALD, M., LUCKHARDT, S., et al., Nucl. Fus. 25,9(1985)[2] YUSHMANOV, P.N., TAKIZUKA, T., RIEDEL, K.S., et al., Nucl. Fus. 30,10(1990)[3] HAWRYLUK, R., Physics of Plasmas Close to Thermonuclear Conditions,Proceedings of the course held in Varenna, Italy, Vol.1, pg.19 (1979)[4] SNIPES, J.A., GRANETZ R.S., GREENWALD, M. et al. to be published inNuclear Fusion (also available as PFC/JA-94-02)[5] RYTER, F., (for H-mode database working group), Cont. Fus. and PlasmaPhys. Vol 17C, 1-15[6] OSBORNE, T.H., BROOKS, N.N., BURRELL, K.H., et al., Nucl. Fus. 30,10 (1990)[7] RYTER, F., GRUBER, 0., BUCHL, K.. Cont Fus. and Plasma Phys. Vol17C, 1-23[8] SNIPES, J.A., GRANETZ R.S., GREENWALD, M. et al. to be published inProc. 21st EPS, Montpelier, 1994 (also available as PFC/JA-94-20)[9] KARDAUN, 0., et al. (for H-mode database working group), Proceedings of14th IAEA Conference on Plasma Physics and Controlled Nucl. Fusion Re-search.,. Wurzburg, Vol. 3, pg 251, 19928

Confinement of ohmic and ICRF heated plasmas in Alcator C-Mod

http://dspace.mit.edu/bitstream/1721.1/95219/1/94ja029_full.pdf

Confinement of ohmic and ICRF heated plasmas in Alcator C-Mod

Abstract

Similar works

Full text

Available Versions

DSpace@MIT