Results on quiescent and post-disruption runaway electrons mitigation experiments at Frascati Tokamak Upgrade

Analyses of experimental data collected in the last FTU campaign provide interesting results on Runaway Electrons (REs) suppression by means of large (wrt FTU plasma volume) deuterium pellets on RE quiescent discharges, mainly inducing bursts of MHD activity that expel the RE seeds. This phenomenon was found to be very reproducible on discharges at 0.5 MA and 5.3 T. Pellets injected during current ramp-ups are also shown to lead to a complete suppression of the RE seed; however, they triggered disruptions as well. These promising results on pellet cleaning effects for different RE quiescent scenarios are discussed. Avalanche multiplication of REs after single pellet injection on 0.36 MA RE quiescent discharges is reported. We provide quantitative indications of RE mitigation effects in terms of increased Connor-Hastie (critical) electrical field due to Anomalous Doppler Instabilities (ADI) for RE quiescent scenarios and post-disruption RE beams. Analysis of large fan-like instabilities on post-disruption RE beams, that seem correlated to the applied negative electrical field and the background density drops, revealed their strong capability to dissipate the RE energy increasing their pitch angle, expelling some of them, and transferring a large fraction of their magnetic energy to the background plasma. Then, ADI can lead to efficient RE energy dissipation (on FTU) as well as full conversion from runaway into thermal electrons (on TCV), indicating a new possible strategy for RE mitigation by controlling the onset of large fan-like instabilities. We also show that short pulses of ECRH, combined with D2 gas puffing, generate a density increase that gives way to a density decrease, when ECRH is switched off, triggering ADI and opening the path to a possible alternative RE mitigation strategy.