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Description
This work presents an electromagnetic model of the passive stabilizing shell (PSS) of RFX-mod2 experiment and a study on the related error fields.
RFX-mod2 is a major upgrade of the former RFX-mod machine. The PSS is now enclosed within a new wider vacuum vessel and is closer to the plasma thus improving its Magneto-Hydro-Dynamic (MHD) stability. Thanks to its engineering flexibility and active MHD control system, RFX-mod2 can operate in both low-current tokamak ($I_p \sim 50\div150\,kA$) and large current ($I_p$ up to $2\,MA$) Reversed Field Pinch (RFP) configurations. Moreover, the shaped tokamak operations will be capable of exploring positive and negative triangularity plasmas in ``exotic regimes'' such as the low-$q$ ($q_a<2$) and ultra-low-$q$ ($q_a<1$).
In preparation of integrated commissioning and first experiments, a three-dimensional electromagnetic model of the machine has been developed by using the CARIDDI code. The two poloidal cuts of the shell have been incorporated in the model, together with all the active coils (Magnetizing and Field Shaping Windings and the 192 saddle coils used for MHD control). In order to study the impact of these gaps, two models have been developed: the simplified butt-joint configuration and the realistic double overlap geometry.
A study on the error fields generated by the eddy currents induced on the PSS has been carried out for both the configurations (butt-joint and overlapped). In a first approach, a simplified filamentary plasma model has been used and the comparison revealed significant differences between the two PSS configurations. Then, using the MARS-F code, plasma response to different Error Fields has been calculated in toroidal geometry an RFP example case. The models and error field studies here presented pave the way to the development of controllers with error-field compensation, but will also be used during the commissioning phase for sensor identification.