High beta flux-conserving tokamaks

Numerical and analytic studies have shown that plasmas subject to external heating will naturally seek high beta equilibria whose characteristics are determined by flux conservation. These equilibria can be characterized by broad pressure profiles and plasma currents peaked toward the plasma surface. They possess the same q profiles as the low beta equilibria from which the heating commences, and consequently have reasonable stability properties against local MHD modes. These equilibria are naturally noncircular and tend to approximate a D-shape with moderate elongation of roughly one and one-half. The technological requirements for sustaining such equilibria are not extreme. Their confinement characteristics are such that one can contemplate attaining ignition conditions in moderate-size plasmas. The one remaining factor which must be analyzed is the stability of these equilibria to MHD modes. In this regard, there is no reason to suspect difficulty at moderate beta. However, since these equilibria have the capability of attaining arbitrarily high beta as far as the equilibrium of the plasma is concerned, it is important to investigate the limitations imposed on maximum attainable beta by MHD stability theory.