Observation of parametric decay instability during ion Bernstein wave heating experiments onHT-7

Abstract

The parametric decay processes with a power threshold of 70 kW have beenobserved on the HT-7 tokamak. Two types of parametric decay instabilities havebeen identified. The pump wave decays into an ion cyclotron quasi-mode and anion Bernstein wave, or decays into a low-frequency electron Landau dampedquasi-mode and an ion Bernstein wave. The experimental observation showsgood agreement with theoretical prediction. Other peaks at a frequency ofnΩD are also observed. The main features of the observed radiofrequency spectra is due tothe parametric decay instability of the launched slow electron plasma pumpwave into an ion cyclotron quasi-mode and an electron plasma wave, dominatedby plasma inhomogeneity convective loss. As the plasma displacement shifts towardsthe antenna, a higher ω/Ω value at the antenna position and a higher wavefrequency are used to reduce the possibility of parametric decay instability (PDI) production. Theexperimental evidence clearly shows that a higher wave frequency, edge densityand value of ω/Ω in the scrap-off layer are favourable to prevent the production of PDIs.The PDI was effectively reduced and successfully avoided witha power level up to 300 kW.