First Velocity-space Resolved Measurements of Relativistic Electron Losses in a Magnetically Confined Plasma

This letter reports on the first direct velocity-space resolved measurements of runaway electrons (REs) losses in magnetically confined plasmas using a unique Fast Particle Loss Detector (FPLD) installed on the magnetic confinement fusion device Tokamak & agrave; Configuration Variable. Clear signatures of REs are identified via multiple distinct regions of emission appearing on the FPLD scintillator-coated plate in plasma discharges that contain no fast ions but are characterised by significant RE populations, as monitored through measurements of gamma-ray emission. Some of these regions of emission on the FPLD scintillator result from direct impacts of REs that have passed, virtually unimpeded, through one of the FPLD's steel collimators. These RE impacts correspond to a region of momentum space centred around a pitch, lambda=v parallel to/v similar to +/- 0.8 and energy, E similar to 3 MeV. Simultaneous measurements of both co- and counter-current RE losses provide evidence for the detection of both passing and trapped REs. Full-orbit and Monte Carlo simulations of the interaction of the REs with the FPLD components indicate that a fraction of the REs can pass through the FPLD's collimator or through its graphite shielding, leading to multiple emission locations on the scintillator plate, with both features observed experimentally.