Magnetic field configuration and locked modes in RFX

The Reversed Field Pinch experiment RFX (R0 = 2 m, a = 0.46 m) is presently operated at approximately 500 kA of plasma current to establish confinement properties at low levels of energy and power. The magnetic configuration is presently characterized by rather large distortions that, like in other RFP experiments, play an important role in determining energy losses. Active control of the poloidal field configuration has been one of the main experimental objectives so far, and plasma behaviour has been improved by centering the plasma column and by reducing the stray field at the poloidal gaps; nevertheless, unstable MHD modes grow during the pulse and lock to the first wall, producing highly localized heat fluxes on it. First, the paper deals with horizontal plasma equilibrium, that has been controlled by means of a bias vertical field, since the long time constant of the stabilizing shell prevents fast control during the pulse. Then the field perturbations are analyzed and the characteristics of mode locking are described. Finally, the origin of mode locking and the effect of field errors on plasma confinement are discussed. (author) 14 refs., 6 figs