Feasibility study of a local active correction system of magnetic field errors in RFX-mod

Highlights: • FE analyses performed to estimate error fields due to passive structures with cuts. • 3 poloidal gap configurations considered with and without local coil compensation. • 2D DFT calculated to compare the resulting error field harmonic spatial spectrum. • Butt-Joint Gap with local active correction system seems the most viable solution. - Abstract: In magnetic confinement fusion devices, close-fitting structures surrounding the plasma are needed to slow down the growth rate of MHD unstable modes to time scales accessible to real time feedback control systems. However, error fields are generated as the response of the passive structures to the dynamic equilibrium field configuration, due to the presence of the cuts needed to allow the penetration of electric and magnetic fields. Considerable improvements have been achieved in RFX-mod after starting operation of the active MHD control system, allowing the successful simultaneous feedback control of a wide spectrum of MHD instabilities. In this paper, a feasibility study of a new local active correction system of magnetic field errors is presented, together with a comparison of three different configurations of the passive structure cuts.