Fiore, C.

Garnier, D.

Goetz, J.

Golovato, S.

Greenwald, M.

Hubbard, A.

Irby, J.

Marmar, E.

Martynov, A.

Ramos, J.

Rice, J.E.

Schachter, J.M.

Shea, P.

Stek, P.C.

Takase, Y.

Watterson, R.L.

Wolfe, S.M.

DSpace@MIT

PFC/JA-96-40Formation and Evolution of InternalTransport Barriers in Alcator C-ModD.T. Garnier, E.S. Marmar, C.L. Fiore, J.A. Goetz,S.N. Golovato,1 M.J. Greenwald, A.E. Hubbard, J.H. Irby,P.J. O'Shea, J.J. Ramos, J.E. Rice, J.M. Schachter, P.C. Stek,Y. Takase, R.L. Watterson,2 S.M. Wolfe, A. Martynov 3October 19961presently at Tokyo Electron America, Beverly, MA, USA.2presently at CPClare Corporation, Lexington, MA, USA.3 Keldysh Institute of Applied Mathematics, Moscow, Russia.To be published in the Proceedings of the 16th IAEA Fusion Energy Conference.This 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.F1-CN-64/AP2-15Formation and Evolution of Internal Transport Barriers in AlcatorC-Mod *AbstractCentral fueling of Alcator C-Mod plasmas with lithium and deuteriumpellets often leads to a strong reduction of core energy and particle transport.These transient modes, which typically persist for a few energy confinementtimes, are characterized by the development, during the post-pellet reheat, ofa very steep pressure gradient (scale length l, ; a/5) in the inner third ofthe plasma. Inside of the transport barrier, the ion thermal diffusivity dropsto values close to those predicted from neoclassical theory. The global energyconfinement time shows an increase of about 30% relative to L-mode scaling.Sawtooth suppression is typical, but not observed in all cases. The additionof up to 3 MW of ICRF auxiliary heating, shortly after the pellet injection,leads to high fusion reactivity, with D-D neutron rates enhanced by a factorof about 10 over L-mode discharges with similar input powers. The measuredcurrent density profile shows that a region of reversed magnetic shear exists atthe plasma core. The change in current profile is consistent with the calculatedbootstrap current created by the pressure gradient. MHD stability analysisindicates that these plasmas are near both the n = oc and the n = 1 marginalstability limits.1. INTRODUCTIONPellet-fueling induced confinement enhancement was first seen in ohmicdischarges on Alcator C [1] and was subsequently observed on other experi-ments. More recently, on JET, shear reversal in ICRF heated, pellet fueled,current-ramped discharges was reported [2,3], and the acronym, PEP, for PelletEnhanced Performance, was coined to describe them. With 80 MHz fixed-frequency sources, ICRF heated PEP modes have been obtained, without cur-rent ramping, in two regimes on Alcator C-Mod: at 5.3 tesla, hydrogen minoritydeuterium majority heating has been used, while at 7.9 tesla, heating with 3 Heminority in deuterium majority plasmas was used.2. EXPERIMENTAL OBSERVATIONSTypical plasma parameters for a 7.9 tesla case are illustrated in figure 1.Time histories of the key core parameters for this I, = 1 MA case clearly showthe peaking of density profile following the injection of a lithium pellet at 0.76seconds.Figure 2 compares the total plasma pressure profiles at 3 times in a dis-charge: during the pre-pellet ohmic phase; near the peak of the PEP modephase; and following the collapse of the core and return to L-mode. It is appar-ent that the transport improvement is restricted to the inner half of the profile.With the measured temperature and density profile time histories, along withcalculated ICRF power deposition and measured bolometric radiation profiles* Work supported by the U.S. Department of Energy, Contract No. DE-AC02-78ET51013.1as inputs, the TRANSP [4] code is used to calculate the evolution of transportcoefficients as well as current density profiles. According to these calculationsapproximately 30% of the current density at p = 0.2 is due to the pressure gra-dient driven bootstrap current, whose time history is shown in the second lastpanel of figure 1. The final panel of this same figure illustrates the time historiesof qO and q,,mi, consistent with the notion that the observed region of reversedshear is due to the increased bootstrap fraction.Line Averaged Electro Dnsity4-2 --Electron Density PeakednessCentral FPlasma Tem peratures--T20[Fusion Rate15 -_YI-100504-Cnrloam eprtrsT(L.r Bootstrap Current Density3- (p = 0.2)~2-01.4- q. and q' -.-.-. qOo1.0 -0.6-0.70 0.75 0.80 0.85 0.90Time (sec)Figure 1. Time histories of plasma parameters for an 8 tesla, 1 MA PEP-modeshot. The lithium pellet was injected at 0.76 seconds. The time histories ofbootstrap current density and q are calculated with TRANSP.24 IC RH Power2-H00.700.60- -----... Pre-PelletPEP Mode? 0.50- Post-PEPCL0.40-(DL../> 0.30-LL 0.200.10 ......0.00 -0.2 0.4 0.6 0.8Normalized Radius (r/a)Figure 2. Pressure profiles at 3 times: pre-pellet; shortly after the peak in theneutron rate; and after the collapse to L-mode.0.050.00- W ExperimentalA EFIT fit-0.05T)C-0.10 --0.15--0.200.65 0.70 0.75 0.80 0.85Major Radius (m)Figure 3. Comparison of measured magnetic field angle midplane profile withthe fitted profile from the EFIT equilibrium reconstruction. The measurementsare taken from a series of similar 5 tesla, 0.8 MA discharges, into which 2lithium pellets were injected: the first to induce PEP-mode; the second to makethe internal field measurements.Internal poloidal magnetic field profiles are measured in Alcator C-Mod,at a single time point in a discharge, using a lithium pellet ablation trail imag-ing technique [5]. Figure 3 compares the measured total magnetic field angle,3obtained in this fashion on a sequence of 4 similar discharges, to the result-ing EFIT [6] derived angles; the measured internal angles have been added asconstraints for the EFIT reconstruction of the equilibrium. Figure 4 illustratesthe corresponding current density and q profiles. The measurements were taken75 ms after the injection of the pellet which induced the PEP transition, 10 msafter the peak in the fusion neutron rate. The hollow q profile is apparent,with q0 = 2 and qmin just larger than 1. This was a 5.3 tesla, 0.8 MA case.TRANSP calculations indicate that the current density profile is roughly con-sistent with that expected when bootstrap effects are included, with 1/4 of thecurrent density at p = .3 driven by the bootstrap current.15 .8q 610E0 0-1.0 -0.5 0.0 0.5 1.0(R-R0)/aFigure 4. Current density and q profiles inferred from the midplane poloidalfield profile measurements shown in figure 3. The measurements are made 10ms after the peak neutron rate is reached.All PEP modes are observed to be transient. On C-Mod, they typicallylast from 50 to 100 ms. This is comparable to the resistive time scale for thecurrent profile to relax from reverse to normal shear, but cause and effect cannotbe established without measurements of the current profile time evolution. Sev-eral phenomena can apparently be ruled out as the cause of the transition outof PEP mode. As seen on ECE and soft x-ray tomography diagnostics, there isoften low m/n coherent MHD activity observed near the time of the transitionback to L-mode; however, in many cases this activity appears to saturate longbefore the collapse, and in some cases it is absent. The resumption of sawtooth-ing does not always correlate with the end of the PEP phase; often, the pressureprofile has relaxed back to those typical of L-mode, as much as 100 ms beforesawtoothing resumes.3. MHD STABILITYTo explore the ideal stability of C-Mod PEP mode discharges, the 7.9tesla shot already described was analyzed using the CAXE and KINX [7] set of42.0 -1.45UNSTABLE 1.05 3// 1.351.5- q..=.95 / 121.0-STABLEShot 950609013, 820 Ms.0.500 . . i0 5 10 15 20q*Figure 5. Ideal stability of an 8 tesla, 1 MA PEP-mode discharge (at 830 ms),and discharges with similar equilibria. Those equilibria falling to the left orabove the heavy lines are unstable to ideal modes.codes for equilibrium reconstruction and ideal MHD stability. Using q* and Opscalings, the stability of discharges over a range of these parameters, but withsimilar pressure and current density profiles, can be examined. Figure 5 showsthe results of such an investigation, which indicate that the basis equilibriumis stable, but is very close to the marginal stability limits both for the n = 1kink and n = oo ideal ballooning modes, as shown by the heavy lines. In allcases it is assumed that there is no conducting wall present. Thus a modestincrease in the plasma 3, or a slight drop in qmin could be expected to drive thedischarge unstable. These results further indicate that, for this current densityprofile, a 3 limit exists for the n = oo modes for /3N , 1.7; discharges with highernormalized 3 should be unstable to the high-n ballooning modes for values ofqmin up to 1.5. Discharges with qmin > 2 and small or negative central shearenter the region of "second stability" and are stable to high-n ballooning modeseven with large values of /.4. DISCUSSIONThere has recently been much attention focussed on reverse shear con-figurations and resultant improvements in confinement [8,9]. Typically, theseNCS/ERS discharges have many, if not all, of the macroscopic characteristics ofPEP mode discharges; NCS/ERS is accessed by tailoring the initial growth ofthe plasma to freeze in a hollow current profile, followed by strong neutral beamheating. The relative importance of the central fueling from the beams in ac-5- N 2 NON3n = 1..- =00I .cessing NCS/ERS is not known; however, this fueling cannot be separated fromthe central heating when NBI is the main auxiliary heating source. It is wellestablished that reverse shear alone is not sufficient to achieve the transition.Other modes of enhanced core confinement have also been identified, includingthe high 3p modes on JT-60U [10] and JET [11], and the CH-Mode on PBX-M [12]. In all of these cases, areas of commonality include enhanced particle andenergy confinement, with highly peaked core pressure profiles. In most, if notall cases, core fueling appears to play an important role, either through pelletor beam particle deposition. If it turns out that central particle fueling is oneof the necessary conditions to access these enhanced core confinement modes,then extrapolation to future devices, such as ITER, may be very difficult.[1] M. Greenwald, et al., Phys. Rev. Lett. 53(1984)352.[2] M. Hugon, et al., Nuclear Fusion 32(1992)33.[3] P. Smeulders, et al., Nuclear Fusion 35(1995)225.[4] R. Hawryluk, Physics of Plasmas Close to Thermonuclear Conditions, Pro-ceedings of the Course held in Varenna, Italy 1979, edited by B. Coppi, (Com-mission of the European Communities, Brussels, 1980) Vol. 1 , p.19[5] J.L. Terry, et al., Rev. Sci. Instrum. 63(1992)5191.[6] L.L. Lao, et al., Nuclear Fusion 30(1990)1035.[7] S. Medvedev, et al., in Proc. 20th EPS Conf. on Controlled Fusion andPlasma Phys., Lisbon, (1993)1279.[8] E.J. Strait, et al., Phys. Rev. Lett. 75(1995)4421.[9] F.M. Levinton, et al., Phys. Rev. Lett. 75(1995)4417.[10] T. Nishitani, et al., Nuclear Fusion 34(1994)1069.[11] B.W. Stallard, et al., Bull. Am. Phys. Soc. 40(1994).[12] PBX-M Group, Fusion Engineering Design 26(1995)83.6

Formation and evolution of internal transport barriers in Alcator C-Mod

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Formation and evolution of internal transport barriers in Alcator C-Mod

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