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

摘要

1.Introduction Over the last decade a substantial number of observations accumulated in the ECRH experiments in toroidal devices appeared not fitting into a paradigm that they are adequately described in the linear approximation for the microwave propagation and absorption. The most eloquent one among them is the pump mm-wave anomalous backscattering studied in detail in the X2 ECRH experiments at TEXTOR [1]. The radiation temperature of the observed frequency downshifted backscattering signal was shown to be thousand times larger than the electron temperature, whereas the amplitude of the signal appeared to be modulated at the magnetic island rotation frequency. The most intensive anomalous backscattering was observed at the plasma density in the magnetic island coinciding with the upper hybrid density for the frequency equal to the half value of the pump frequency. In order to explain the latter observation the theoretical model [2] was proposed recently. It interprets the anomalous backscattering as a consequence of the two-upper-hybrid (UH)-plasmon parametric decay of the pump wave possessing very low threshold due to the daughter UH waves trapping in a direction of the plasma inhomogeneity in the vicinity of a local maximum of the density profile often observed in the magnetic island. The theoretical model also predicts substantial (up to 25%) anomalous absorption of the pump power. This makes important further investigation of the mechanism of excitation and the scenarios of the saturation of low-power-threshold two-plasmon parametric decay instability (PDI) of mm-wave. In order to study this phenomenon in detail the model experiment has been performed at the linear device. The decay occurs in a plasma filament extended in the direction of the magnetic field and produced by a high-frequency discharge in a long quartz tube with its inner diameter of d=22 mm filled with argon at about 1 Pa pressure. The quartz tube passes through the waveguide (72 x 34 mm2) in p