Strong anomalous absorption of an X-mode pump microwave in a plasma filament due to the two-plasmon decay instability

The effect of strong anomalous absorption of the X-mode pump wave associated with two upper-hybrid (UH) plasmon parametric decay is observed in a specially performed model experiment in a laboratory plasma. Strong microwave absorption takes place at a maximal plasma density in the plasma filament higher than the UH resonance value for the frequency equal to the half value of the pump frequency. The anomalous absorption efficiency is determined at the level of 70% in the beginning stage of the parametric decay instability and 40% in the following steady state. A theoretical model of the two UH plasmon decay in strongly inhomogeneous plasma filaments is developed. The localization, threshold, and growth rate of the instability are determined and found to be in agreement with experimental observations.