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

The parametric decay processes with a power threshold of 70 kW have been observed on the HT-7 tokamak. Two types of parametric decay instabilities have been identified. The pump wave decays into an ion cyclotron quasi-mode and an ion Bernstein wave, or decays into a low-frequency electron Landau damped quasi-mode and an ion Bernstein wave. The experimental observation shows good agreement with theoretical prediction. Other peaks at a frequency of nΩ_D are also observed. The main features of the observed radiofrequency spectra is due to the parametric decay instability of the launched slow electron plasma pump wave into an ion cyclotron quasi-mode and an electron plasma wave, dominated by plasma inhomogeneity convective loss. As the plasma displacement shifts towards the antenna, a higher ω/Ω value at the antenna position and a higher wave frequency are used to reduce the possibility of parametric decay instability (PDI) production. The experimental evidence clearly shows that a higher wave frequency, edge density and 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 with a power level up to 300 kW.