Design of machine upgrades for the RFX-mod experiment

After 10 years of operation since its major modification, an upgrade of the RFX-mod experiment is presently under design. The main objectives are the improvement of the control of magnetic confinement, plasma density and plasma wall interaction in both RFP and Tokamak configuration. The main design driver requirement for the improvement of the magnetic confinement control is the enhancement of the 'plasma-shell proximity', to reduce the deformation of the last close magnetic surface. This requirement calls for the removal of the present vacuum vessel, the fastening of a new first wall to the existing copper stabilising shell, including some poloidal and toroidal gap modifications, and the modification of the present toroidal support structure to provide the function of vacuum barrier. The critical aspects of the new torus assembly are the development of composite ceramic/metal joint solutions for the equatorial and poloidal joints of the toroidal support structure, in order to guarantee the vacuum boundary and the penetration of electro-magnetic fields within the plasma chamber. Moreover the new configuration requires the integration of a further active control system of local magnetic field at the poloidal gaps, in addition to the existing set of saddle coils distributed around the whole torus. For the enhancement of the control of plasma density and plasma wall interaction two main improvements are proposed: a new first wall with higher thermal conductivity material and optimized shape, to withstand the severe power load experienced in RFX, and a new distributed glow discharge cleaning system, to improve uniformity and frequency of wall conditioning. Finally an upgrade of the plasma diagnostic system, both in-vessel and ex-vessel, is also included in the proposal. The paper will present an overview of the engineering design of the new components of the RFX-mod and their integration with the existing machine assembly.