On the possibility of low-threshold absolute parametric decay instability in X2-mode ECRH experiments at the EAST tokamak

A comprehensive model of low-threshold two-plasmon parametric decay instability (PDI) of X2-mode is proposed, accounting for an arbitrary pump beam incidence angle and an inhomogeneous monotonic density profile. Based on this model, the possibility of absolute PDI excitation is demonstrated and its threshold for X2-mode ECRH experiments at the EAST tokamak is evaluated. The specific conditions and plasma parameters required for the onset of this instability are determined, demonstrating that it can be triggered under typical experimental scenarios.