Beckmann, M.

Gruber, O.

Günter, S.

Lackner, K.

Leuterer, F.

Maraschek, M.

Mc Carthy, P.

Meister, H.

Pereverzev, G.

Salzmann, H.

Schweinzer, J.

Suttrop, W.

Wolf, R.

English

MPG.PuRe

26th EPS Conf. on Contr. Fusion and Plasma Physics, Maastricht, 14 - 18 June 1999      ECA Vol.23J (1999) 1821 - 18241821Simultaneous Attainment of High Electron and Ion Temperaturesin ITB-Discharges with ECCD on ASDEX UpgradeF. Leuterer, S. Günter, R. C. Wolf, M. Beckmann, O. Gruber,K. Lackner, M. Maraschek, P. J. Mc Carthy*, H. Meister, G. Pereverzev,H. Salzmann, J. Schweinzer, W. Suttrop, and the ASDEX Upgrade TeamMax-Planck-Institut für Plasmaphysik, EURATOM-Association, D-85748 Garching*University College Cork, Association EURATOM-DCU, Cork, IrlandIntroduction. Internal transport barriers have been observed in various experiments in whichcurrent rise and heating pulses were controlled to produce low or reversed central shear. Thishas previously been successful in two extreme regimes, in which ion temperature gradientdriven (ITG) modes should be either stable, or relatively easy to stabilize by rotational shear,namely in cases with pure electron heating (e.g. in [1]) and relatively cold ions, and indischarges with strong ion heating and Ti significantly larger than Te [2, 3]. Internal transportbarrier (ITB) discharges with have been obtained on JET transiently, but this featurewas lost, when strong ion heating was applied [4]. As the mode considered responsible foranomalous transport in discharges with strong ion heating - the ITG mode - is destabilizedby increasing the ratio , these observations have to be viewed as very critical for theprospects of a reactor based on advanced confinement modes, as -particle heating willpredominantly heat the electrons, resulting in .We report on first experiments on ASDEX Upgrade, in which central ECRH heating wasapplied during the ITB phase of NBI heated discharges with controlled current ramp. In thesedischarges an ECRH power of 1.25 MW was launched in the second harmonic X-mode. Thetoroidal injection angle of the focused ECRH beams was varied so as to produce, in additionto electron heating, also co- or counter current drive. The discharges presented here werelimiter discharges, leaning on the high-field side protection shield, with an L-mode edge. Atconstant applied ECRH power, marked differences were observed between the three cases ofco-, pure heating and counter-current drive, with the latter giving the best performance andthe longest lasting internal barriers for both ions and electrons. This was to be expected,as the counter-current drive was conceived to compensate for the tendency of the centralq-value to decrease, leading in turn to the disappearance of the reversed shear region. In fact,the co-injection case, in which this unfavorable current profile development should be evenaccelerated, shows values of , which for Ti are much lower compared to the casewithout ECRH power application. In this co-case, measurements of the q-profile based onthe motional Stark effect (MSE) diagnostic and the equilibrium code CLISTE [5] show thedisappearance of the reversed shear region about 500 ms after the application of the electroncyclotron current drive (ECCD).Negative central shear configuration. In the discharges presented in this paper, a centralreversed shear configuration is attained by a reduction of the current diffusion by applying5 MW of NBI during the current ramp-up phase at line averaged densities between[6]. The resulting shear is strongly reversed in the plasma center with inital q0values exceeding 6 and . However, due to the reduced but still continuous currentdiffusion q evolves towards a monotonic profile with q0 finally approaching one, if MHD26th EPS CCFPP 1999 ; F.Leuterer et al.: Simultaneous Attainment of High Electron and Ion Temperature...1822instabilities do not terminate the discharge earlier. Without ECRH an ITB is formed, whichis reflected in central Ti values in excess of 10 keV corresponding to a ion thermal diffusivityi at neoclassical level. While Te is a factor of two to three below Ti in the plasma core, thecentral e is also . At the time when qmin reaches 2 a (2,1) mode is observed whichat least transiently deteriorates the confinement properties, resulting in a sharp drop of Ti.Analysis with linear and non-linear resistive MHD codes predict (2,1) double tearing modesto become unstable for the q-profiles measured in these plasmas [7]. To investigate the effectof electron heating and current drive on stability and confinement, central ECCD was appliedat a time when qmin approaches 2, either to support the shear reversal or accelerate its decay.Central co- and counter-ECCD. Figure 1 shows the time traces of the central Ti (attor = 0.06) together with the ECE measurements at tor = 0.2 (the innermost ECE channelavailable during central ECRH power application) for three different cases: without ECRH,and with co- and counter current drive.#12229q02468qminq0#12224qminq0(a) (b)#12231q0qmin(c)051015T (keV)0.2 0.4 0.6 0.8 1.0 1.2Time (s)TiTeECCD0.2 0.4 0.6 0.8 1.0 1.2Time (s)TiTeTiTe0.2 0.4 0.6 0.8 1.0 1.2Time (s)ECCDFig. 1.: Comparison of three types of discharges with heating in the current ramp-up phase:(a) NBI and counter-ECCD, (b) reference case with NBI only, and (c) NBI and co-ECCD.Shown are the evolution of central and minimum q, the difference of which reflects theamount of shear reversal, and maximum values of ion and electron temperature from chargeexchange recombination spectroscopy and ECE, respectively. The ECE channels are locatedat because of the central ECRH deposition.For counter-current drive (figure 1(a)) the electron temperature, even 0.12 m from the center,reaches a value of 13 keV, which is confirmed also by Thomson scattering and is most ofthe time higher than the central value of the ions. The large drop of Ti before ECRH isswitched on is attributed to the (2,1) mode, which disappears when qmin has passed throughthe q = 2 surface. Raytracing calculations with the TORAY code show an ECR depositionwithin tor = 0.2. Neglecting current diffusion, the calculated driven current initially is 82 kAand rises to 134 kA due to the strong increase of the electron temperature. The corresponding26th EPS CCFPP 1999 ; F.Leuterer et al.: Simultaneous Attainment of High Electron and Ion Temperature...1823current density is of the order of the total current density measured with the MSE diagnosticin the plasma center. Depending on the exact location of the ECRH deposition, the diffusiontimes required for the build up of these currents vary between 50 and 200 – 300 ms. Thetemporal evolution of the central and minimum q exhibits a slowdown of the current profiledecay shortly after the application of counter-ECCD. At 1.06 s the discharge is terminated bya disruption triggered by the accidental tripping of two gyrotrons.Figure 1(b) shows the equivalent traces for a reference discharge without ECRH power, inwhich Ti reaches a similar high value, but the maximum Te remains at 5 keV. The behavior ofthe q-profile derived from CLISTE using the MSE measurements is comparable to the counter-ECCD case. However, the slope of the central MSE polarization angles which is a measureof q0 show a small but visible difference suggesting a larger q0 with counter-current drive.Probably the parametrization of the current profile in the present version of the equilibriumcode is not sufficiently flexible to bring out these small and very localized differences.With the ECR current drive in co-direction (figure 1(c)) the (2,1) mode, which in the othertwo cases has disappeared again, develops into a continuous mode with decreasing frequencyand exists throughout the ECCD phase which inhibits the recovery of the ITB. Ti being equalto Te remains at relatively low values also in the plasma center. The increasing modulationof the Te amplitude in figure 1(c) is caused by the (2,1) mode. 300 ms after the switch-onof co-ECCD, both, q0 and qmin start to drop finally reaching unity. Two main effects maycontribute to the faster decay of the reversed shear: On the one hand, the central currentdensity is increased by co-current drive and, on the other hand, the boot strap current due tothe deteriorated confinement is reduced.#122291.00.0 0.2 0.4 0.6 0.8tor051015T (keV)TiTe1.00.0 0.2 0.4 0.6 0.8torTiTe#122241.00.0 0.2 0.4 0.6 0.8torTiTe#12231(a) (b) (c)Fig. 2.: Comparison of the temperature profiles at 0.97 s of the three types of discharges withheating in the current ramp-up phase: (a) NBI and counter ECCD, (b) reference case withNBI only, and (c) NBI and co ECCD. The strong electron temperature gradient is only seenin case of counter-ECCD.The temperature profiles of the three cases are shown in figure 2, evidencing the strongelectron temperature gradients with counter-ECCD. Without ECRH the electrons are mainlyheated through the ions and therefore do not follow the increase of Ti in the plasma center.For co-ECCD due to an increased radial transport .The comparison between the toroidal rotational velocities of counter-ECCD and NBI onlydischarges does not exhibit marked differences (figure 3). Before the ECCD is switched onvtor is similar in both cases. During ECCD the rotational velocity of the plasma appears to26th EPS CCFPP 1999 ; F.Leuterer et al.: Simultaneous Attainment of High Electron and Ion Temperature...1824rise more slowly, but finally reaches the same value as in the reference discharge (0.981 s).At this time also the radial profiles are of comparable shape.0.0 0.2 0.4 0.6 0.8 1.00.659 s#12229#122240.981 stor0.50 0.60 0.70 0.80 0.90 1.00Time (s)counter ECCD(#12229)NBI only(#12224)1.2 MW ECCD01.0•1052.0•1053.0•1054.0•1055.0•105v    (m/s)torFig. 3.: Time evolution and radial profiles of toroidal rotational velocity (vtor), comparingthe counter-ECCD with the reference discharge (NBI only) before and after the applicationof ECCD.Summary. It has been demonstrated that with a combination of neutral beam injection andelectron cyclotron current drive into plasmas with negative central shear it is possible to attaininternal transport barriers for both electrons and ions simultaneously. In the cases presented,where during ECCD the minimum q-value has already dropped below two, the high centralelectron temperature in combination with the amount of shear reversal seems to play a criticalrole in the ability to avoid MHD instabilities which strongly deteriorate the improved coretransport properties. Only if the high value of q0 is sustained, supported by counter currentdrive, it was possible to reach high central electron temperatures. Here, the stabilizing effectof reduced resistivity on double tearing modes could serve as an explanation.These experiments have addressed one of the most critical elements for the applicability ofadvanced scenarios to a fusion reactor, namely their behavior under simultaneous strong ionand electron heating, at , with an outcome giving strong support to the case.References[1] P. Burati et al., Phys. Rev. Lett. 82 , 560 (1999).[2] F. M. Levinton et al., Phys. Rev. Lett. 75 , 4417 (1995).[3] B. W. Rice et al., Phys. Plasmas 3, 1983 (1996).[4] F. X. Söldner et al., Nucl. Fusion 39 , 407 (1999).[5] P. J. Mc Carthy and W. Schneider, IPP Report 5/85 (1999).[6] R. C. Wolf et al., Proc. 17th Int. Conf. on Plasma Physics and Controlled Fusion Research,IAEA-F1–CN-69/EXP1/12 (Yokohama 1998).[7] S. Günter et al., 26th Eur. Conf. on Controlled Fusion and Plasma Physics, submitted toPlasma Phys. Contr. Fusion (Maastricht 1999).

Simultaneous Attainment of High Electron and Ion Temperatures in ITB-Discharges with ECCD on ASDEX Upgrade

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Simultaneous Attainment of High Electron and Ion Temperatures in ITB-Discharges with ECCD on ASDEX Upgrade

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