Low-threshold two-plasmon decay of an extraordinary pump wave in a plasma edge blob
E. Z. Gusakov, A. Yu. PopovThe effect of low-threshold parametric decay instability within a coherent structure (blob) at the plasma edge is considered. The analysis focuses on the decay of the extraordinary pump wave into a pair of electron Bernstein (EB) waves with similar frequencies. The emission at frequencies corresponding to these daughter waves was observed recently in X2-mode electron cyclotron resonance heating (ECRH) experiments at ASDEX Upgrade and TCV tokamaks. The EB waves are found for the first time to be three-dimensionally trapped within a plasma edge blob, leading to a substantial decrease in the power threshold for their parametric excitation. To illustrate the findings, the model is applied to the X2-mode ECRH scenarios at the ASDEX Upgrade tokamak.