Parametric instabilities associated with 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 current drive. Previously we have shown that under typical experimental conditions parametric decay instability (PDI) is expected in the edge plasma region. Measurements with magnetic pickup loops at the outboard edge of the plasma showed evidence of PDI corresponding to the local ion cyclotron frequency and its harmonics (ion cyclotron quasi-modes) and corresponding lower hybrid and/or ion Bernstein wave sidebands. Here we present the latest experimental observations of PDI under different plasma discharge conditions, and extend our numerical calculations of growth rates and frequencies to different values of the edge plasma parameters, including PDI associated with the E_|| of parasitically excited slow waves. For the first time we have also calculated excitation of PDI into ion sound quasi-modes and lower hybrid waves. The resulting broadening of the pump wave frequency spectrum due to this process may be indistinguishable from scattering by low frequency edge turbulence. Convective thresholds are also calculated to assess the possibility of pump wave depletion.



^*Work supported by US DOE under DE-FC02-04ER54698, DE-SC0016154, DE-SC0020337

and DE-SC0014264.