Microwave-plasma interaction experiments with the versatile toroidal facility

Laboratory experiments with the Versatile Toroidal Facility (VTF) have been conducted to investigate the microwave propagation and attenuation in the ECRH generated plasmas. These experiments are aimed at studying the anomalous absorption of electromagnetic waves in turbulent plasmas. The location of the upper hybrid resonance layer as determined by Langmuir probe data agrees qualitatively with the location determined by the transmission diagnostic. Discrepancies between the two methods have been complicated by the small signal to noise ratio near the important regions due to turbulent scattering. Low frequency density fluctuations commonly observed in toroidal plasmas are believed to be produced by the drift wave instabilities. They can significantly scatter incident diagnostic waves when δn/n much-gt ρi/Ln where δn/n is the fractional density fluctuation, ρi is the ion Larmour radius, and L, is the scale length of the density gradients. Experiments will also be carried out on the Versatile Toroidal Facility (VTF) for the comparative studies of parametrically excited plasma waves by the transversely and obliquely injected electromagnetic waves. These experiments are motivated by the ionospheric plasma heating experiments performed at Arecibo, Puerto Rico using a ground-based HF heater. The cascading spectra of the PDI (parametric decay instabilities) produced Langmuir waves were observed in the vertical heating experiments as expected. In the oblique heating experiments the excitation of PDI was found; however, the cascading spectrum was not seen. Rather, a broad, highly structured spectrum separated by 15 KHz or so from the PDI produced spectrum was generated indicating other processes excited by the obliquely propagating heater waves can compete with the PDI. Preliminary results obtained from the VTF experiments will be presented and discussed