Absolute parametric decay instabilities of X2 microwave beams in reduced models and fully kinetic codes

Abstract

Nonlinear wave interactions known as parametric decay instabilities (PDIs) have been known to occur in fusion plasmas for many years. In the past, they have generally been considered of little importance in the context of high power microwave beams aimed for X2 heating due to high thresholds. Experimental evidence suggests that non-monotonic density structures, such as islands and ELMs, can lead to low thresholds absolute PDIs. The consequences include degraded electron cyclotron resonance heating performance, absorption into unintended modes and the production of strong scattering, which may damage microwave sensitive equipment. We present a reduced analytical 1D model for absolute PDI of an X-mode microwave beam into a cascade of electrostatic eigenmodes of a non-monotonic density structure. The model is solved numerically and compared with fully kinetic particle-in-cell (PIC) simulations which are computationally much more expensive. The primary decay, which initiates cascade, shows good agreement with the PIC simulations in terms of daughter frequencies, growth rates and saturation, in particular in a weakly nonlinear pump intensity regime. The secondary daughter waves of the reduced model, while visible, do not appear to be the dominant processes in the PIC simulations.