Interaction of the fast wave with a scrape-off layer plasma in tokamaks. The ion cyclotron parametric instabilities and the anomalous heating of ions

Abstract

The theory of the ion cyclotron (IC) electrostatic parametric instabilities of plasma which are driven by the elliptically polarized fast wave (FW) of the finite wavelength is developed. The growth rate of the IC quasimode decay instability, which was considered as a potential source of the generation of the high energy ions in scrape-off layer (SOL) during FW injection in tokamaks, is derived analytically for arbitrary values of the FW electric field and wavelengths of the unstable IC perturbations. The comprehensive numerical analysis of the dispersion equation for three wave system which contains the IC mode $φ_{i}\left(\mathbf{k}_{i}, ω\right)$ and its harmonics $φ_{i}\left(\mathbf{k}_{i}, ω-ω_{0}\right)$, $φ_{i} \left(\mathbf{k}_{i}, ω-2ω_{0}\right)$, where $ω_{0}$ is FW frequency, is performed. It reveals that the parametric IC instability for this wave system has the maximum growth rate for the IC waves with wavelength comparable with the thermal ion Larmor radius. It is found that the inverse electron Landau damping plays essential role in the development of this instability. The possible mechanism of the saturation of this instability is the scattering of ions by the ensemble of the IC waves with random phases, which limits the development of the instability on the high level. The anomalous heating rates of ions resulted from the interactions of ions with parametric IC turbulence is determined employing the developed quasilinear theory for the IC quasimode decay instability. The derived results reveals, that the experimentally observed anisotropic heating of cold SOL ions may be caused by the parametric IC turbulence in SOL. However, the IC parametric turbulence is unlikely to be responsible for the experimentally observed bursts of poorly confined suprathermal ions in the SOL of tokamak plasmas.