The RFX magnet system

The reversed field pinch (RFP) magnetic confinement requires both toroidal and poloidal components for the magnetic field induction. As in tokamaks, the former is provided by the toroidal and poloidal components for the the ohmic heating (OH) winding and the equilibrium field (EF) winding. The two induction field components have similar amplitudes, so that the toroidal component required in a RFP is about one order of magnitude lower than that in a tokamak with equal plasma current and aspect ratio. Owing to the local stability properties, the TF coils have to be located as close as possible to the plasma but, at the same time, the magnetic field ripple from TF coils (as well as any kinds of stray fields) must be kept to a minimum. Another peculiarity of the TF winding is that it is required to operate with a time-varying current and at high voltage levels. From these points of view, the design of RFP TF windings presents much less technological problems than a tokamak of similar size. The RFP requires a high toroidal loop voltage during fast current rise and a relatively high toroidal loop voltage during the flat top. The main consequences for the RFX PF magnet systems are as follows: • - large flux and energy to be inductively stored; • - very high voltage across the OH and EF winding terminals, giving rise to substantial insulation problems; • - the very fast rate required for current rise may produce remarkable skin effects within the OH conductors, so that large cross-section conductors have to be avoided; • - the electrodynamic forces acting on the OH winding are large and comparable with those in tokamaks; • - in case of any fault, currents in the coils can rise beyond safety limits at a very high rate, leading to extremely critical conditions for the machine integrity . From a mechanical point of view, both the OH and the EF windings are subject to working conditions similar to those experienced in tokamaks. Thus, RFX windings were manufactured with a similar technology. To detect the TF and PF winding faults a very fast, hard-wired system has been developed, which is able to elaborate signals from specific probes and to decide on necessary protective actions. The present contribution deals with the whole magnet system of RFX, including all TF and PF magnets, mechanical structure, magnetic and electric measurement instrumentation, as well as the fast fault detection system. After a review of the basic concepts representing the theoretical background behind the main choices, all aspects and features concerning the magnet design are presented in detail and deeply discussed. The manufacturing technology is then presented together with the main problems met during manufacture, development and acceptance tests and the methods adopted in order to solve them are explained. On-site assembly procedures, testing and the first integrated RFX operation are finally described.