Parametric instabilities during high power helicon wave injection in DIII-D.

Helicon (whistler) waves at 0.476 GHz have been launched on DIII-D with powers up to 0.6 MW for heating and driving currents in the plasma core. Previously we have shown [1] that under experimental conditions in L-mode plasmas parametric decay instability (PDI) is expected in the edge plasma region, in agreement with measurements with magnetic pickup loops that show evidence of PDI corresponding to the ion cyclotron frequency and its harmonics at the outboard edge of the plasma. In one experiment, an energetic ion tail associated with PDI was observed. PDI has also been observed in H-mode plasmas. The dominant driver of PDI is the EXB and/or polarization drift velocity associated with the helicon wave which can drive ion-cyclotron quasi-modes and electrostatic sideband waves unstable [1]. We extend numerical calculations of growth rates and frequencies to different values of the edge plasma parameters, including PDI associated with possible parasitic excitation of the slow lower hybrid wave and predict strong PDI owing to the slow wave's large E_||. Estimation of convective thresholds and PDI saturation levels is in progress.

[1] M. Porkolab et al., in 24th Top. Conf. RF Power in Plasmas, Annapolis, MD, 2022.

^*Work supported by US DOE under DE-FC02-04ER54698 and DE-SC0016154.