Theory Modeling of Second Harmonic Electron Cyclotron Plasma Breakdown in a Quasi-Helical Plasma

An analytic model is presented to describe the breakdown processes of a Stellarator plasma produced by second harmonic cyclotron waves. The important wave-particle process involves the nonlinear interaction of ECRH waves with cold electrons trapped near the resonance region. The nonlinear interaction causes the initially cold electrons to undergo excursions in energy which can ionize the plasma through collisions with neutrals when enough wave power is provided. To describe the plasma population growth, a three-group model (energetic trapped electrons, energetic passing electrons and cold untrapped electrons) is presented that accounts for the nonlinear wave-particle interactions, pitch angle scattering and coalitional slowing down on neutrals, ionization processes and particle loss rates due to transport. The density evolution equations for each group can be solved and the population growth rate is computed. In this work, we emphasize the role of the magnetic configuration on the plasma loss rates and detail its effect of the plasma breakdown rate. We apply this model to various magnetic configurations available to the HSX experiment.