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

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 3 He 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo>∼</mml:mo> </mml:mrow> <mml:mn>0.6</mml:mn> </mml:mrow> </mml:math> of the deuterium and hydrogen cyclotron frequencies <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mi>cD</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mi>cH</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> in mixed hydrogen and deuterium plasmas. The modes below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mi>cD</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> are often driven unstable by deuterium beams but are rarely destabilized by hydrogen beams. The modes below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mi>CH</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> are reported here for the first time in a fusion plasma. These <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mi>sub</mml:mi> <mml:mo>−</mml:mo> </mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mi>cH</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> modes are only driven by deuterium beams but, unlike the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mi>sub</mml:mi> <mml:mo>−</mml:mo> </mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mi>cD</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> modes, they are only unstable in plasmas with deuterium concentrations that are <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo>≲</mml:mo> </mml:mrow> <mml:mn>20</mml:mn> <mml:mi mathvariant="normal">%</mml:mi> </mml:mrow> </mml:math> ; they also have a smaller amplitude than the sub-deuterium modes. Addition of fully-ionized 3 He 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mi>sub</mml:mi> <mml:mo>−</mml:mo> </mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mi>cH</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> modes when 3 He is puffed. However, the relative stability of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mi>sub</mml:mi> <mml:mo>−</mml:mo> </mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mi>cH</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> modes during hydrogen injection remains unexplained.