Latest results on quiescent and post-disruption runaway electron mitigation experiments at Frascati Tokamak Upgrade

TL;DR

This paper reports on recent experiments at Frascati Tokamak Upgrade demonstrating pellet-induced runaway electron suppression, avalanche multiplication phenomena, and a new strategy for RE energy suppression via density control using modulated ECRH.

Contribution

It introduces a novel approach to suppress runaway electrons by controlling large fan-like instabilities through density drops induced by modulated ECRH.

Findings

Pellet injections can suppress REs via burst MHD activity.

Avalanche multiplication of REs observed after pellet injection.

Density drops induced by modulated ECRH can suppress RE energy.

Abstract

Results from the last FTU campaigns on the deuterium large (wrt FTU volume) pellet REs suppression capability, mainly due to the induced burst MHD activity expelling REs seed are presented for discharges with 0.5 MA and 5.3T. Clear indications of avalanche multiplication of REs following single pellet injection on 0.36 MA flat-top discharges is shown together with quantitative indications of dissipative effects in terms of critical electrical field increase due to fan-like instabilities. Analysis of large fan-like instabilities on post-disruption RE beams, that seem to be correlated with low electrical field and background density drops, reveal their strong RE energy suppression capability suggesting a new strategy for RE energy suppression controlling large fan instabilities. We demonstrate how such density drops can be induced using modulated ECRH power on post-disruption beams.