Low-threshold absolute parametric decay instability of microwave beam in X2 ECRH experiments with monotonic density profile

It is shown that the absolute parametric decay instability of a microwave beam of a finite width, resulting in the excitation of two electron Bernstein waves, can occur at monotonic parameters of the background plasma in X2-mode ECRH experiments at a modest pump power. The excitation of daughter waves, whose power deposition localization differs from that of the pump wave, can potentially explain the broadening of the power deposition profile and lack of the pump power absorbed in the electron cyclotron resonance layer that has been found in X2-mode ECRH experiments.