Observation of the strong anomalous absorption of the X-mode pump in a plasma filament due to the two-plasmon decay

The effect of strong anomalous absorption of the X-mode pump wave associated with the two-upper-hybrid-plasmon parametric decay is observed in a specially performed model experiment in a laboratory plasma. The strong microwave absorption takes place at the maximal plasma density higher than the upper-hybrid resonance value for the frequency equal to half of the pump frequency value. Its efficiency is determined at the level of 80% in the initial stage of the parametric decay instability saturation and 45% in the following steady state. The theoretical model of the two-upper-hybrid-plasmon decay in strongly inhomogeneous plasma is developed. The localization, threshold and growth rate of the instability are determined in agreement with experimental observations.