Identification of, and transition to, the second region of ideal MHD stability in tokamaks.

摘要

The second region of ideal MHD stability in tokamaks is studied by considering the behavior of the second region boundary for self- consistently calculated, marginally stable, second region equilibria and the characteristics of numerically computed transport sequences that achieve second stability. Equilibria with pressure profiles, p((psi)), that are marginally stable to the second region on each flux surface are generated numerically. This constraint eliminates p((psi)) as an independent variable, and reduces the predictor variables to the tokamak parameters and the q profile. The primary response functions considered are the plasma figures of merit, (beta) and (var epsilon)(beta)(sub p), and the normalized pressure gradient, (alpha). Variations of the radial wavenumber in the ballooning equation negligibly affect the second region boundary for these equilibria. The second region boundary is sensitive to variations in the q profile at small aspect ratio, A, and will stabilize or destabilize depending on the balance of higher order (var epsilon) = A(sup (minus)1) modifications of the normal field line curvature, (kappa)(sub n). These effects are a competition between the stabilizing geometric magnetic well of the toroidal field component of (kappa)(sub n) and the destabilizing poloidal field component of (kappa)(sub n). The latter term becomes competitive in high (var epsilon)(beta)(sub p) plasmas with large Shafranov shifts. Simple analytic models are presented that reproduce the scaling of the marginally stable second region values of (alpha) and (var epsilon)(beta)(sub p), and stability diagrams illustrating the behavior of the high-n unstable region for various parameters are shown. (ERA citation 15:036548)