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High power helicon waves (whistler or very high harmonic fast waves) at a frequency of 0.476 GHz have been launched on DIII-D with the goal of demonstrating efficient off-axis current generation in DIII-D AT plasmas. We have shown in recent work that under typical experimental conditions strong parametric decay instability (PDI) is expected in the pedestal region at injected RF power levels of 0.1-1.0 MW with corresponding electric fields of 10-30 kV/m [1]. The dominant driver of the PDI is the ExB and polarization drift velocity which can drive ion cyclotron quasi-modes and lower hybrid sideband waves unstable. Initial experimental results have been obtained with powers up to 0.3 MW showing evidence of strong PDI measured with high-frequency one-turn magnetic probes at frequencies set by the usual selection rules and the toroidal field strength at the outboard edge of the plasma; weak PDI has been observed at power levels as low as 10 kW. Here we present analytic and numerical assessment of growth rates, frequencies and convective thresholds for the PDIs, and compare them with experimental observations.

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