Parametric Decay Instabilities During Helicon Wave Injection on DIII-D and Projection to Future Fusion Reactors

Helicon (whistler) fast waves at 0.476 GHz have been launched on DIII-D with powers up to 0.8 MW for heating and current drive. RF probes and magnetic pickup loops located in the plasma edge region detected the presence of Parametric Decay Instabilities (PDIs), with frequencies corresponding to the local ion cyclotron frequency and its harmonics, as well as corresponding sidebands around the injected helicon pump wave frequency [1]. Here we present the latest experimental observations and the theoretical prediction of growth rates and frequencies of PDIs in DIII-D. We consider PDIs driven not only by the perpendicular electric fields of the fast helicon waves but also calculate growth rates driven by the E_|| of parasitically excited slow waves near the antenna. In agreement with past theoretical calculations [2] PDIs observed at the local ion cyclotron frequency and its harmonics are interpreted as ion cyclotron quasi-modes while the sidebands correspond to slow lower hybrid and/or ion Bernstein waves. Convective thresholds are also calculated to assess the possibility of pump wave depletion. It is found that in both high performance DIII-D plasmas and future reactor-grade plasmas PDIs driven by the dominant perpendicular electric field component of the helicon wave would not lead to pump wave depletion.

1. M. Porkolab, et al., AIP Conf. Proc. 2984, 070004 (2023).

2. M. Porkolab, Phys. Fluids 20, 2058 (1977) and Nuclear Fusion 18, 043009 (1978).