Recent Progress in Runaway Electron Research at TCV

Runaway electrons (REs) are a concern for fusion reactors from the startup to the termination of tokamak discharges. The sudden localized loss of a multi-MA RE beam can inflict severe damage to the first wall. The Tokamak à Configuration Variable (TCV) has recently explored various aspects of RE physics, designing and testing multiple strategies to prevent the formation of REs, reduce their number, or mitigate their effect. A density threshold above which RE generation can be avoided is identified and a corresponding effective critical field is determined. Resonant interaction between RE cyclotron motion and magnetic ripple oscillations is found to limit RE energy and illustrates how pitch-angle scattering can impede RE growth. RE confinement in low density TCV plasmas is good enough for the RE population to grow until it drives most of the toroidal current. The application of central electron cyclotron resonance heating under such conditions if found to enhance RE losses strongly enough for the entire RE population to be expelled in a few hundred milliseconds. RE beam formation can take place following disruptions triggered by massive gas injection. Full conversion to RE-driven current is observed. Successfully tested strategies to mitigate a post-disruption RE beam include safe ramp-down, regeneration of a healthy hot plasma, and most recently benign termination following massive injection of low-Z material. Combined achievements in avoiding startup RE generation, reducing an established flattop RE population, and mitigating a post-disruption RE beam, have furthered the prospects in safe operation for future tokamak reactors.