Dependence of ion-cyclotron range of frequencies instabilities on species mix and fast-ion distribution: III. Novel sub-cyclotron modes

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Abstract
Dedicated DIII-D experiments studied instabilities above and below the ion cyclotron frequency by injecting thirteen different neutral beam populations into L-mode plasmas with different thermal compositions of H, D, and 3He at five different values of toroidal field. Two companion papers describe the results of these experiments in detail. This paper focuses on data and analysis of two instabilities that occur at ∼ 0.6 of the deuterium and hydrogen cyclotron frequencies f cD and f cH in mixed hydrogen and deuterium plasmas. The modes below f cD are often driven unstable by deuterium beams but are rarely destabilized by hydrogen beams. The modes below f CH are reported here for the first time in a fusion plasma. These sub − f cH modes are only driven by deuterium beams but, unlike the sub − f cD modes, they are only unstable in plasmas with deuterium concentrations that are ≲ 20 % ; they also have a smaller amplitude than the sub-deuterium modes. Addition of fully-ionized 3He to the plasma has little effect on the sub-cyclotron deuterium modes but is stabilizing for the sub-cyclotron hydrogen modes. The modes are excited in the core and have toroidal mode numbers around 20. These trends are compared with calculations of the growth rate in a simplified model. The theory successfully predicts global Alfvén eigenmode instability with frequencies and mode numbers close to experimental values, the stronger amplitude of the sub-deuterium modes, and the species-mix dependence of the sub-hydrogen modes. Increased damping associated with the ion-ion hybrid, cutoff, and crossover frequencies plausibly accounts for the stabilization of the sub − f cH modes when 3He is puffed. However, the relative stability of sub − f cH modes during hydrogen injection remains unexplained.
Funding Information
  • Fusion Energy Sciences (DE-AC52-07NA27344)

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