Active control for current dissipation of runaway electrons in TCV

A dedicated control algorithm for runaway suppression via ohmic coil has been implemented in TCV. A combination of the scenarios developed earlier in [1, 2] with Bt=1.45 T, inner wall limited circular plasmas, extremely low gas prefill and density target below 2E18 m-3 with Ne and Ar massive gas injection, at different pressure and valve opening times, allows further acceleration of the RE seeding population to form RE beams. The new software, developed on the basis of previous versions [1, 3, 4], is provided with a current quench detector that triggers a controlled current ramp-down with selectable decay rates. Sudden current losses, largely frequent in TCV discharges with high level of runaways and mainly attributable to MHD unstable modes, are detected and smooth drops in the current reference signal are triggered to avoid current loss recovery. The electric field measurement is used to limit the action of the ohmic coil on the RE beam current in order to maintain the loop voltage below predefined thresholds. Indeed, it has been noticed a correlation between the loop voltage level and RE beam destabilization by MHD activity. Hybrid controllers with time varying gains and switched feedforward terms have been introduced. Experimental results showing RE beam current dissipation with different ramp-down slopes with beams lasting from 50 to 400 ms are presented. Discussion on the possible strategies to further improve the tracking performances are provided.