Optimum safety factor profile for an advanced tokamak with negative central shear

Abstract

The effect of the wall location on the stability of magnetohydrodynamic equilibria is considered for the case of an advanced tokamak with negative central magnetic shear (NCS). The equilibria have pressure and current profiles consistent with the proposed second stable core VH (SSC-VH) mode. In particular, the pressure probe has a finite edge pressure gradient that is consistent with high confinement as seen in experiments. The stability analysis shows that, for q0 = 3.9, all qmin in the range 1.8 ⩽ qmin ⩽ 3.3 are stable to n=1 and n=2 modes at βN = 5.0 with the DIII-D wall. However, there are two optimum minimum q (qmin) with respect to the wall distance from the plasma. For n=1 modes with q0 = 3.9, the optimum qmin is 2.1, with a secondary optimum at qmin = 2.85. The wall position for the n = 2 mode is more restrictive than that for the n = 1 mode but the optimum qmin values for both n = 1 and n = 2 are almost identical