Finite pressure induced destruction of the plasma boundary in stellarators

Abstract

Free-boundary MHD equilibria in stellarators are investigated. Particular attention is placed on an analysis of the influence of the plasma induced magnetic fields on the shape and sizes of the plasma boundary. Only a weak sensitivity of the plasma boundary destruction on the pressure and current density distributions is found. The existing moderate pressure profile dependence increases on passing from a shearless system to a system with large magnetic shear. One way to reduce the damage is to use external axisymmetrical magnetic fields. The efficiency of such techniques is briefly discussed. The problem of plasma boundary determination from the results of external magnetic measurements is analysed, and the accuracy of the magnetic measurements available for a sufficiently accurate identification of the plasma boundary by the method of field line tracing is estimated. The limiting plasma pressure determined by the conditions for plasma equilibrium is also discussed