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

Abstract

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 n 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/ 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/ 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.

Particle-in-cell simulations of parametric decay instability of radiofrequency wave in the ion cyclotron range of frequency in an inhomogeneous plasma

“…We consider an inhomogeneous plasma with both density and magnetic field varying with the radial position. The simulation parameters have been chosen to resemble those of the IBW experiments on the HT-7 tokamak [17]. In the simulations, we obtain resonant and nonresonant parametric decay channels similar to those observed in the experiments.…”

“…With more and more applications of rf waves for plasma heating and current drive on tokamak devices, the interactions between waves and plasmas have been investigated extensively [9][10][11][12][13][14][15][16][17][18]. When the input power of the incident wave reaches a megawatt, nonlinear phenomena occur and cannot be ignored.…”

“…When the input power of the incident wave reaches a megawatt, nonlinear phenomena occur and cannot be ignored. The observations of PDI during the ion cyclotron range of frequency (ICRF) heating experiments have been reported on different tokamak devices, such as ASDEX [10], JET [11], TEXTOR [12,13], JT-60 [14], DIII-D [9,15,16], HT-7 [17], Alcator C-Mod [18], etc. In these experiments, the observed decay processes involved (1) resonant decay into two ion Bernstein waves (IBWs), (2) nonresonant decay into an ion cyclotron quasimode (ICQM) plus an IBW, and (3) nonresonant decay into a low-frequency electron quasimode (EQM) and an IBW.…”

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Particle-in-cell simulations have been conducted to investigate the parametric decay processes of radiofrequency waves in the ion cyclotron range of frequency in inhomogeneous plasmas. By choosing the parameters close to those in the ion Bernstein wave heating experiments on the HT-7 tokamak (Li et al 2001 Plasma Phys. Control.

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Spectral Broadening of Ion Bernstein Wave Due to Parametric Decay Instabilities

“…To reveal these effects in relevant experiments, we choose the plasma parameters close to those of the ion Bernstein wave (IBW) heating experiments on HT-7 tokamak. [1,14] It is demonstrated that more decay channels appear and the frequency spectra broaden as the input power is increased. The characteristics of the spectral broadening and how this broadening comes out will be presented.…”