Response of a magnetically diverted tokamak plasma to a resonant magnetic perturbation

Abstract

The safety-factor profile of a magnetically diverted tokamak plasma diverges logarithmically as the magnetic separatrix (a.k.a. the last closed magnetic flux-surface) is approached. At first sight, this suggests that, when determining the response of such a plasma to a static, externally generated, resonant magnetic perturbation (RMP), it is necessary to include an infinite number of rational magnetic flux-surfaces in the calculation, the majority of which lie very close to the separatrix. In fact, when finite plasma resistivity is taken into account, this turns out not to be the case. Instead, it is only necessary to include rational surfaces that lie in the region 0<Psi<Psi_c, where Psi is the normalized poloidal magnetic flux, and Psi_c<1 can be calculated from the edge plasma parameters. It is estimated that Psi_c= 0.9985 for an n=1 RMP, and Psi_c=0.9952 for an n=4 RMP, in a typical JET H-mode plasma.