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Neutral-beam-driven, sub-cyclotron compressional (CAE) and global (GAE)\Alfven eigenmodes are routinely excited in spherical tokamaks such as NSTX(-U)and MAST, have been observed on the conventional aspect ratio tokamak DIII-D,and may be unstable in ITER burning plasmas. Their presence has beenexperimentally linked to the anomalous flattening of electron temperatureprofiles at high beam power in NSTX, potentially limiting fusion performance. Adetailed understanding of CAE/GAE excitation, therefore, is vital to predicting(and ultimately controlling) their effects on plasma confinement. To this end,hybrid kinetic-MHD simulations are complemented with an analytic study of thelinear stability properties of CAEs and GAEs. Perturbative, local analytictheory has been used to derive new instability conditions for CAEs/GAEs drivenby realistic neutral beam distributions. A comprehensive set of simulations ofNSTX-like plasmas has been performed for a wide range of beam parameters,providing a wealth of information on CAE and GAE stability in sphericaltokamaks. Linear simulations show that the excitation of CAEs vs GAEs has acomplex dependence on the fast ion injection velocity and geometry,qualitatively described by the analytic theory developed in this thesis. Strongenergetic particle modifications of GAEs are found in simulations, indicatingthe existence of a new type of high frequency energetic particle mode. A crossvalidation between the theoretical stability bounds, simulation results, andexperimental measurements shows favorable agreement for both the unstable CAEand GAE spectra's dependence on fast ion parameters. The analytic resultsaccurately explain the recent experimental discovery of GAE stabilization withsmall amounts of off-axis beam injection on NSTX-U and suggest new techniquesfor control of these instabilities in future experiments.

Theory and Simulations of Compressional and Global Alfven Eigenmode Stability in Spherical Tokamaks