Transition to the second region of ideal MHD stability

Transition to the second region of MHD stability has been examined in circular cross-section boundary tokamak geometry. Values of <β> and εβp, corresponding to equilibria which are marginally stable in the second region on each flux surface, are presented. The aspect ratio scaling of these functions is shown to be sensitive to the value of the on-axis safety factor q0. Equilibria evolving from the first to the second region of stability have been generated by a transport code which calculates the two-dimensional evolution of the magnetic configuration self-consistently with the thermal and particle diffusion. The transport model enhances the diffusion coefficients on flux surfaces which are unstable to high-n modes, thereby altering the pressure profile locally. The q-profile, modified by neutral beam driven current so that q0 > 1, reduces the size of the unstable region sufficiently to allow transition to the second region. Auxiliary power requirements for this transition are estimated. Stability of the transition equilibria to low-n external and internal modes is also examined. All ideal modes restabilize as εβp approaches and exceeds unity. A close fitting conducting wall is needed to stabilize low-n external kink modes. (author). 55 refs, 20 figs