m=0 perturbations of the magnetic surfaces in an RFP

An analysis of the m = 0 magnetic perturbation in the vacuum region of an RFP device is carried out using toroidal flux and toroidal field measurements. This perturbation is associated to the locked-mode phenomenon which is a coherent superimposition of modes, mainly m = 0, 1, locked in phase together. The m = 1 distortion of the magnetic surface is the dominant effect and was considered in a previous analysis. The purpose of this work is to study the m = 0 component using the same technique. The experimental data are elaborated with simple analytical expressions derived from the vacuum Maxwell equations and the general contravariant representation of the magnetic field in terms of the flux function. The m = 0 geometrical distortion of the magnetic surfaces is reconstructed. Its dependence on the plasma current and the impact on the plasma wall interaction are discussed.