A simple method of equilibrium determination and its application to a RFP plasma with thin shell

A simple method is proposed to determine the equilibrium quantities (such as the plasma position and Shafranov's parameter Lambda ) in Tokamaks and RFPs. This method is available in the transient phase of a discharge, in which the plasma equilibrium is maintained by time-varying vertical fields, e.g. in the case where the plasma is surrounded by a thin shell. In this method, the external fields measured are matched to the approximate solution of the Grad-Shafranov equation in a vacuum by a fitting procedure. In the case of a circular plasma boundary, only a small number (>or=6) of the field measurements can provide a stable and rapid determination of equilibrium. A successful application to the REPUTE-1 RFP plasma with thin shell reveals that in most cases the plasma starts to move outward and downward before the plasma current begins to fall.