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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

The object of this review is to summarize the achievements of research on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994) and Marmar, Fusion Sci. Technol. 51, 261 (2007)] and to place that research in the context of the quest for practical fusion energy. C-Mod is a compact, high-field tokamak, whose unique design and operating parameters have produced a wealth of new and important results since it began operation in 1993, contributing data that extends tests of critical physical models into new parameter ranges and into new regimes. Using only high-power radio frequency (RF) waves for heating and current drive with innovative launching structures, C-Mod operates routinely at reactor level power densities and achieves plasma pressures higher than any other toroidal confinement device. C-Mod spearheaded the development of the vertical-target divertor and has always operated with high-Z metal plasma facing components—approaches subsequently adopted for ITER. C-Mod has made ground-breaking discoveries in divertor physics and plasma-material interactions at reactor-like power and particle fluxes and elucidated the critical role of cross-field transport in divertor operation, edge flows and the tokamak density limit. C-Mod developed the I-mode and the Enhanced Dα H-mode regimes, which have high performance without large edge localized modes and with pedestal transport self-regulated by short-wavelength electromagnetic waves. C-Mod has carried out pioneering studies of intrinsic rotation and demonstrated that self-generated flow shear can be strong enough in some cases to significantly modify transport. C-Mod made the first quantitative link between the pedestal temperature and the H-mode's performance, showing that the observed self-similar temperature profiles were consistent with critical-gradient-length theories and followed up with quantitative tests of nonlinear gyrokinetic models. RF research highlights include direct experimental observation of ion cyclotron range of frequency (ICRF) mode-conversion, ICRF flow drive, demonstration of lower-hybrid current drive at ITER-like densities and fields and, using a set of novel diagnostics, extensive validation of advanced RF codes. Disruption studies on C-Mod provided the first observation of non-axisymmetric halo currents and non-axisymmetric radiation in mitigated disruptions. A summary of important achievements and discoveries are included., United States. Dept. of Energy (Cooperative Agreement DE-FC02-99ER54512), United States. Dept. of Energy (Cooperative Agreement DE-FG03-94ER-54241), United States. Dept. of Energy (Cooperative Agreement DE-AC02-78ET- 51013), United States. Dept. of Energy (Cooperative Agreement DE-AC02-09CH11466), United States. Dept. of Energy (Cooperative Agreement DE-FG02-95ER54309), United States. Dept. of Energy (Cooperative Agreement DE-AC02-05CH11231), United States. Dept. of Energy (Cooperative Agreement DE-AC52-07NA27344), United States. Dept. of Energy (Cooperative Agreement DE-FG02- 97ER54392), United States. Dept. of Energy (Cooperative Agreement DE-SC00-02060)