Low Threshold Parametric Decay Instabilities in ECRH experiments at toroidal devices

The experimental conditions leading to substantial reduction of backscattering decay instability threshold in ECRH experiments in toroidal devices are analyzed. It is shown that drastic decrease of threshold is provided by non monotonic behaviour of plasma density, which is often observed due to so-called density-pump-out effect or presence of magnetic islands, and by poloidal magnetic field inhomogeniety making possible localization of ion Bernstein decay waves. The corresponding ion Bernstein wave gain and the parametric decay instability pump power threshold is calculated. The possible experimental consequences of easy backscattering decay instability excitation are discussed.Comment: 12 pages, 8 figures, submitted to Nuclear Fusio

Enhanced microwave scattering with time-of-flight resolution

A RADAR-like modification of enhanced microwave scattering diagnostics is investigated both analytically and numerically. The method uses the fact that backscattering off the density fluctuation is localised to a narrow layer, whose position depends on the fluctuation wavenumber. Therefore, when a plasma is irradiated by a short microwave pulse, the time delay of the backscattered pulse contains the information on the shape of the fluctuation spectrum.This effect is strongly pronounced for scattering near the upper hybrid resonance where both incoming and outgoing waves are very slow. In laboratory and small tokamak plasmas, this technique permits observation of small-scale density fluctuations with a high spatial and wavenumber resolution. (orig.)Available from TIB Hannover: RO 9159(29)+a+b / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman