Optimized profiles for improved confinement and stability in the DIII-D tokamak

Abstract

Simultaneous achievement of high energy confinement, 'ZE, and high plasma beta, P. leads to an economically attractive compact tokamak fusion reactor. High confinement enhancement, H = 'CF/'ZE.ITER~~P = 4, and high normalized beta PN = P/(UaB) = 6%-m-T/MA. have been obtained in DIU-D experimental discharges.These improved confinement and/or improved stability limis are observed in several Dll-D high performance operational regimes: VH-mode, high 4 H-mode, second stable core, and high beta poloidal. We have identified several important features of the improved performance in these discharges: details of the plasma shape, toroidal rotation or ErB flow profile, q profile and current density profile, and pressure profile. From our improved physics understanding of these enhanced performance regimes, we have developed operational scenarios which maintain the essential features of the improved confinement and which increase the stability limits using localized current profile control. The stability limit is increased by modifying the interior safety factor profile to be nonmonotonic with high central q. while maintaining the edge current density consistent with the improved transport regimes and the high edge bootstrap current. We have calculated high beta equilibria with BN = 6.5, stable to ideal n=l kinks and stable to ideal ballooning modes.The safety factor at the 95% flux surface is 6. the central q value is 3.9 and the minimum in q is 2.6. The current density profile is maintained by the natural profile of the bootstrap current, and a modest amount of electron cyclotmn current drive. ' T E R ~R -S ~~)are observed in several operational regimes; VH-mode (Jackson 1991), high internal inductance (&) H-mode (Lao 1993a). and high poloidal @ (Politzer 1994). The performance in the high confinenent regimes observed in the JET, JT4OU, TFTR, and DIU-D tokamaks is no longer limited by transport or heating power, but instead are limited by stability limits at~high @T ( E T Team 1993. Mauel 1993, Zamstorff 1993. Taylor 1993. Strait 1993. Perfomance in present day tokamaks can be improved if the @-limit can be increased while maintaining the observed high confinement. Our ssategy for identifying 3 self-consistent high confinement high beta steady-state discharge scenario is to identify and maintain those features that are favorable for the high confinement and then modify the profiles to increase the stability limit, without adversely affecting the confinement. The features that we have identified that are favorable for high confinement are: (1) strong plasma shaping, high triangularity (6) and high elongation (K):(2) high plasma rotation. large shear in the ExB flow;(3) finite current density near the edge; (4) negative central shear; and (5) high q(0). We intend to show are compatible with steady-state high p.In the next section, we review a number of high performance regimes that have been identified in DlI-D experimental discharges, and discuss the features that we believe are important for achieving the high

Radial correlation length measurements on ASDEX Upgrade using correlation Doppler reflectometry

“…Plasma confinement improves with triangularity [23,24]. To test this relationship, the plasma shape was scanned from low to high triangularity in two similar L-mode upper single null discharges (#19146 and #19148) and the results are shown in figure 7.…”

Recent experimental studies of edge and internal transport barriers in the DIII-D tokamak

“…At high input powers, plasma performance of β N H 89 ∼ 7 for up to 10 τ E (∼1.6 s) has been achieved (c.f., β N H 89 ∼ 4 for standard ELMing H-mode). Here H 89 is the confinement enhancement factor relative to ITER89P scaling [37] and, β N the normalized beta, = β/(I/aB T ) [38]. This paper will review the recent advances in edge and internal transport barrier studies on DIII-D. Firstly, details of experimental results from pellet induced H-mode barriers will be presented.…”

Higher Fusion Power Gain with Current and Pressure Profile Control in Strongly Shaped DIII-D Tokamak Plasmas

“…Recent calculations [2][3][4] predict that a central region with negative shear in the safety factor will enhance plasma stability. The safety factor q is the number of toroidal transits of a field line for one poloidal transit, and the shear is ͑2V ͞q͒dq͞dV where V is the volume enclosed by the flux surface.…”