Current diffusion in the RFX magnetizing winding

RFX is a fusion Reversed Field Pinch (RFP) experiment under construction in Padua, Italy. Its air-core magnetizing winding is part of the poloidal-field winding and is devoted to inducing the toroidal plasma current by transformer action. This is obtained by slowly charging the winding to a specified current which is then forced to deca quickly. During this fast decay, the current density distribution inside each conductor behaves according to diffusion equations and this causes the transient stray magnetic field in the plasma region to rise well above the static value. A numerical approach, based on the volume integral formulation, is developed to study these diffusion phenomena. After a brief description of the mathematical basis of the method, the numerical analysis performed is presented and the stray-field evolution of a number of technical solutions considered to comply with the RFX design specifications are compared.