LOWER-HYBRID TURBULENCE EXCITED BY A FAST TRANSVERSE ION-BEAM IN A MAGNETIZED PLASMA

Experimental and theoretical investigations of lower hybrid (LH) turbu lence in the W7-AS stellarator are presented. The turbulence is excite d by an ion beam, which is generated by a weak neutral hydrogen beam i njected transversely to the confining magnetic field. The instability is detected by collective Thomson scattering of powerful gyrotron radi ation. From the measured density dependence of the frequency it was id entified as an LH type of instability. The spectrum is characterized b y a narrow bandwidth in spite of the inherently poor radial resolution of the backscattering geometry. The theoretical model of an LH instab ility driven by a transverse fast ion component under the double reson ance condition (coincidence of the LH frequency with a high cyclotron harmonic of the fast ions) is developed. The instability growth rate i s derived. The stabilizing effect of high bulk ion temperatures was ob served experimentally, in accordance with theoretical modelling. An in stability saturation mechanism similar to the well known stochastic io n heating is proposed.