BETA-LIMIT IN LONG-PULSE TOKAMAK DISCHARGES

The maximum normalized beta achieved in long-pulse tokamak discharges at low collisionality falls significantly below both that observed in short pulse discharges and that predicted by the ideal MHD theory. Rec ent long-pulse experiments, in particular those simulating the Interna tional Thermonuclear Experimental Reactor (ITER) [M. Rosenbluth et al. , Plasma Physics and Controlled Nuclear Fusion (International Atomic E nergy Agency, Vienna, 1995), Vol. 2, p. 517] scenarios with low collis ionality nu(e), are often limited by low-m/n nonideal magnetohydrodyn amic (MHD) modes. The effect of saturated MHD modes is a reduction of the confinement time by 10%-20%, depending on the island size and loca tion, and can lead to a disruption. Recent theories on neoclassical de stabilization of tearing modes, including the effects of a perturbed h elical bootstrap current, are successful in explaining the qualitative behavior of the resistive modes and recent results are consistent wit h the size of the saturated islands. Also, a strong correlation is obs erved between the onset of these low-m/n modes with sawteeth, edge loc alized modes (ELM), or fishbone events. consistent with the seed islan d required by the theory. We will focus on a quantitative comparison b etween both the conventional resistive and neoclassical theories, and the experimental results of several machines, which have all observed these low-min nonideal modes. This enables us to single out the key is sues in projecting the long-pulse beta limits of ITER-size tokamaks an d also to discuss possible plasma control methods that can increase th e soft beta limit, decrease the seed perturbations, and/or diminish th e effects on confinement. (C) 1997 American Institute of Physics.