Wall stabilization of high beta plasmas in DIII-D*

Detailed analysis of recent high beta discharges in the DIII-D [Plasma Physics Controiled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987, Vol. I, p. 1591 tokamak demonstrates that the resistive vacuum vessel can provide stabilization of low n magnetohydrodynamic (MHD) modes. The experimental beta values reaching up to &= 12.6% are more than 30% larger than the maximum stable beta calculated with no wall stabilization. Plasma rotation is essential for stabilization. When the plasma rotation slows sufficiently, unstable modes with the characteristics of the predicted "resistive wall" mode are observed. Through slowing of the plasma rotation between the 9 =2 and 4 =3 surfaces with the application of a nonaxisymmetric field, it has been determined that the rotation at the outer rational surfaces is most important, and that the critical rotation frequency is of the order of W27r= 1 kHz. 0 199.5 American Institute of Physics