OPTIMIZED PROFILES FOR IMPROVED CONFINEMENT AND STABILITY IN THE DIII-D TOKAMAK

Simultaneous achievement of high energy confinement, tau(E), and high plasma beta, beta, leads to an economically attractive compact tokamak fusion reactor. High confinement enhancement, H = tau(E)/tau(E)-ITER8 9P = 4, and high normalized beta beta(N) = beta/(I/aB) = 6%-m-T/MA. ha ve been obtained in DIII-D experimental discharges. These improved con finement and/or improved stability limits are observed in several DIII -D high performance operational regimes: VH-mode, high l(i) H-mode, se cond stable core, and high beta poloidal. We have identified several i mportant features of the improved performance in these discharges: det ails of the plasma shape, toroidal rotation or ExB flow profile, q pro file and current density profile, and pressure profile. From our impro ved physics understanding of these enhanced performance regimes, we ha ve developed operational scenarios which maintain the essential featur es of the improved confinement and which increase the stability limits using localized current profile control. The stability limit is incre ased by modifying the interior safety factor profile to be nonmonotoni c with high central q, while maintaining the edge current density cons istent with the improved transport regimes and the high edge bootstrap current. We have calculated high beta equilibria with beta(N) = 6.5, stable to ideal n=1 kinks and stable to ideal ballooning modes. The sa fety factor at the 95% flux surface is 6, the central q Value is 3.9 a nd 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 o f electron cyclotron current drive.