Excitation of parametric instabilities during helicon wave injection into DIII-D^*

High power helicon (whistler) waves at 0.476 GHz have been launched on DIII-D with the goal of demonstrating efficient off-axis current generation. We have shown in past work [1,2] that under typical experimental conditions strong parametric decay instability (PDI) is expected in the pedestal region that may scatter the incident helicon wave and compromise the expected current drive efficiency. The dominant driver of the PDI is the ExB and/or polarization drift velocity which can drive ion cyclotron quasi-modes and lower hybrid (or high k ion-plasma or ion-Bernstein) sideband waves unstable. Using magnetic pickup loops, initial experimental results indicated evidence of PDI corresponding to the cyclotron frequency and its harmonics at the outboard edge of the plasma. Here we present numerical calculations of growth rates and frequencies and thereby estimate convective thresholds and PDI saturation levels and compare them with experimental observations [1,3].



[1] M. Porkolab et al, Bull. Am. Phys. Soc. (2021) https://meetings.aps.org/Meeting/DPP21/Session/UO07.7

[2] M. Porkolab and R.I. Pinsker, EPJ Web of Conferences 157, 03042 (2017).

[3] S.X. Tang, et al, this conference.

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