Optimization of RFX-mod2 gap configuration by estimating the magnetic error fields due to the passive structure currents

A major refurbishment of the toroidal complex of the RFX-mod device is in progress and will include the removal of the Inconel vacuum vessel and a modification of the stainless steel supporting structure to be made vacuum tight. The plasma facing graphite tiles will be mounted onto the inner surface of the copper shell so as to increase the plasma proximity factor. New operation regimes are expected to provide a significant reduction of the amplitude of RFP tearing modes with magnetic chaos mitigation and confinement improvement. On the other hand, due to the shorter distance from the passive structures, the plasma is expected to be even more sensitive to magnetic field errors at the plasma boundary, produced by the induced currents near the cuts of the same structures. In preparation to the new RFX-mod2 experiments, a thorough revision of the conditions triggering the error fields due to the eddy currents was undertaken. An analysis of the static and dynamic equilibrium magnetic field has been carried out in RFX-mod and RFX-mod2 to estimate the magnetic field driving the shell eddy currents. A lower equilibrium magnetic field should be required in RFX-mod2. Analyses of different configurations of the poloidal gap were also carried out by a specialized computational tool. A solution with a spaced poloidal gap and a more extended overlapping was envisaged capable of maintaining the same error fields at the closer plasma boundary with the same forcing field and meeting the more stringent insulation and assembly requirements of RFX-mod2.