The intrusion of an equipotential poloidal limiter into the edge plasma of a circular tokamak discharge distorts the axisymmetry in several ways: (1) it (partially) shorts out the top-to-bottom Pfirsch-Schlüter potentials, (2) it creates zones of reversed equilibrium current flow into the limiter, and (3) it generates an electrostatic field opposing the loop current. The resulting boundary mismatch between the outer layers and the inner axisymmetric Pfirsch-Schlüter layer increases the free energy available to drive the edge plasma unstable. A number of special limiters are proposed to symmetrize the edge plasma and thereby reduce the electrical and MHD activity in the boundary layer.
Effects of poloidal variation of neutral density on Pfirsch–Schlüter transport near the tokamak edge