Analysis of parametric decay instabilities during helicon wave injection into tokamak plasma

The parametric decay instability (PDI) during helicon wave injection into tokamak plasmas is investigated based on its local dispersion relation under typical scrape-off layer conditions. The helicon wave can decay into a lower hybrid wave as the sideband mode and usually the ion cyclotron quasi-mode and its harmonics as low-frequency modes, which shows qualitative consistency with the experimental results of DIII-D tokamak [Pinsker et al., Nucl. Fusion 64, 126058 (2024)]. The parameter dependence of the PDI is also analyzed. The instability can be suppressed by a higher electron temperature or a stronger magnetic field. In contrast, increasing in the plasma density enhances the local growth rate, suggesting that helicon waves may also encounter a density limit similar to that observed for lower hybrid waves.