Stabilization of low-threshold two-plasmon parametric decay instability at oblique incidence of a microwave pump beam onto the plasma

It is shown that with an oblique incidence of a pump microwave in X2-mode electron cyclotron plasma heating (ECRH) experiments, the threshold power of a parametric decay instability (PDI) leading to excitation of two trapped upper hybrid waves increases and the anomalous absorption level decreases. It is proposed to use a small pump beam tilt along the magnetic field to suppress this most dangerous PDI, often observed in the X2-mode ECRH experiments.