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The microwave anomalous absorption in plasma due to the two-upper-hybrid-plasmon parametric decay instability

In the recent decade a substantial number of observations in the electron cyclotron (EC) plasma heating experiments in tokamaks and stellarators had been made that do not fit into a simple linear picture. The most distinct among them is the anomalous backscattering phenomena studied in detail in the second harmonic X-mode heating experiments at the Textor tokamak [1]. The theoretical model [2] proposed recently explains the anomalous backscattering as a result of the two upper-hybrid (UH) plasmon parametric decay instability (PDI) possessing very low threshold due to trapping of nonlinearly excited plasmons in the vicinity of the density maximum that accompanies the magnetic island. The theory [2] also predicts substantial (up to 25%) anomalous absorption due to this process. In the present paper the low-threshold two-UH-plasmon PDI is experimentally modelled. The decay occurs in a plasma filament produced by high-frequency discharge (27 MHz, 100 W) in long quartz tube with an inner diameter of 22 mm filled with argon (pressure about 1 Pa) oriented in the direction of the magnetic field. The tube passes through the waveguide (72 x 34 mm) in parallel to the wide wall. Using the waveguide the X-mode microwave pulses (up to 200 W) at frequency of 2.35 GHz, substantially higher than the EC and UH values, are incident onto the plasma. By means of optical and microwave diagnostics the strong anomalous absorption of the microwave power is observed in the plasma at density higher than the UH resonance value for the frequency equal to half value of the pump frequency. The effect localisation and its dependence on magnetic field, plasma density and microwave power are investigated. The threshold and growth rate of the anomalous phenomena are determined and compared to the specially developed theory predictions. The ion acoustic turbulence excited as a result of the PDI saturation via the cascade of decays is investigated theoretically and using the UHR backscattering diagnostics.