UC San Diego

Energy production is a continuing problem in the modern world, and nuclear fusion in tokamak reactors may be a viable solution. One remaining problem for tokamak is the generation of runaway electrons (RE) during shutdown of these reactors, the focus of this thesis. Energy and runaway electrons are both briefly reviewed, with emphasis on prior theses, prior theoretical developments, and prior experimental studies which establish context in the pre- existing body of knowledge. New experimental techniques tailored for studying RE are described. These techniques include plasma shaping optimized for RE generation via argon killer-pellet shutdown, which increased the probability of RE plateau in a shutdown from 30% to over 80%. A newly developed hard x-ray sensing scintillator array is described in detail, and this new diagnostic is used along with pre-existing diagnostics to explore the temporal and spatial character of hard x-ray emission resulting from RE. X-ray emission associated with RE impact at divertor strike points was observed after thermal quench (TQ) but before current quench (CQ). Instabilities of the RE current were observed during the plateau and at termination. Experiments probing RE in-situ by injecting polystyrene diagnostic pellets are also discussed. Pellets were observed disintegrating before reaching the last closed flux surface (LCFS), suggesting that substantial RE transport beyond the LCFS occurs, which is consistent with observed activation of the low field side midplane limiters. Inference of loop voltages during the pre-current quench (CQ) phase using inverse techniques and a discussion of limitations of this technique are also presented. Loop voltages exceeding 1kV are inferred peaking well before the beginning of CQ, and are capable of accelerating RE to energies of over 10MeV at the time of the first x-ray emission from RE impact with the wall. During the later CQ phase, this inferred voltage matches a simpler estimate for the loop voltage - LdI/dt