IMPURITY BEHAVIOR IN HIGH-BETA PLASMAS IN PBX-M

In PBX, an elongated, indented tokamak, an accumulation of impurities leading to strongly peaked radiation profiles was observed, most frequently during the H-mode. The modified device, PBX-M, features a close-fitting stabilizing metallic shell that covers over 85% of the plasma surface and has the capability to produce highly indented plasmas. Two distinct modes of operation were used to generate high-beta equilibria in neutral beam heated discharges: 1) high current, highly indented high-beta(t) plasmas, where the plasma was in close proximity to the stabilizing plates and 2) low current, lesser indented high-beta(p) discharges where the plasma equilibrium shape was more oblate than the shell and the plasma was farther from the plates. In co-injected high-beta(t) plasmas, radiative losses were typically 15-25% of the input power and impurity accumulation was mild even in discharges lasting up to 650 ms with 250 ms H-modes. In high-beta(p) discharges, both co- and counter-injection was investigated. In discharges with tangential co-injection, total radiated power gradually peaked on axis, ultimately reaching 60% of the heating power. H-modes were frequently present and the high-Z impurity density profile was strongly peaked. In tangentially counter-injected discharges, the current profile modification led to the suppression of sawteeth and the near elimination of MHD activity outside the q = 1 surface. The improved particle confinement led to high electron density, high impurity radiation levels and a thermal collapse; however, the concentration and accumulation of metallic impurities was not as severe as the co-injected case.