Toward improved modeling of parametric decay instabilities in the lower hybrid frequency range

Parametric decay instabilities (PDIs) are frequently observed during lower hybrid current drive

(LHCD) in fusion plasmas. PDIs modify the launched spectrum in the edge region and are believed to be responsible for the anomalous degradation of core current drive at high densities, commonly referred to as the lower hybrid density limit. While the nonlinear dispersion relation (NLDR) governing the decay modes in homogeneous plasmas has been extensively refined, the saturation mechanisms remain comparatively less understood. We assess the potential of commonly used saturation models to explain the PDI spectra observed in Alcator C-Mod experiments [1-2]. We also discuss ongoing efforts to extend existing models to account for turbulent density fluctuations and background gradients, with the goal of improving their applicability to both current experiments and future reactor scenarios.

[1] Baek, S. G., et al. 2014. “Characterization of the Onset of Ion Cyclotron Parametric Decay Instability of Lower Hybrid Waves in a Diverted Tokamak.” Physics of Plasmas 21 (6): 061511.

[2] Baek, S.G., et al. 2015. “High Density LHRF Experiments in Alcator C-Mod and Implications for Reactor Scale Devices.” Nuclear Fusion 55 (4): 043009.