DENSITY-FLUCTUATIONS AND SPECTRAL BROADENING DURING THE LOWER HYBRID CURRENT DRIVE EXPERIMENT ON ASDEX

The broadening of the injected radiofrequency (RF) line in the frequency spectra detected during the lower hybrid (LH) experiment in ASDEX is studied. The parametric decay instabilities (PDI) and the scattering due to density fluctuations are considered as possible causes of the broadening. The analysis of the parametric dispersion relation shows that parametric instabilities driven by ion sound quasi-modes have the highest growth rates, but their RF power threshold is higher than the RF power available in the experiment. This has been confirmed by the negative results of the measurements performed to detect the presence of PDI. Also, measurements of the density fluctuation behaviour during RF power injection have been made. The results obtained from observations with a microwave reflectometer and a Langmuir probe show a correlation of the fluctuations in the scrape-off plasma with the line broadening. It is concluded that the line broadening is due to scattering of the launched waves by density fluctuations whose level is enhanced, via a non-linear process, by the injected RF power.