NBI-driven ion cyclotron instabilities at the W7-AS stellarator

Abstract

The lower hybrid (LH) plasma turbulence triggered by injection of quasi-transverse hydrogen beams at the W7-AS stellarator is investigated by the collective Thomson scattering (CTS) technique. Simultaneously, ion cyclotron emission (ICE) is measured with a broadband loop antenna. Good correlation of the ICE and CTS data is found at the LH frequency; the activity at lower ICE harmonics is also detected with injection of heating neutral beams. Theoretical interpretation of the observations is based on numerical simulations of fast ion distribution functions originating from different NBIs using a time-dependent bounce-averaged collisional Fokker–Planck code. With these distributions, the instabilities of electrostatic modes in the vicinity of ion cyclotron harmonics following from the solution of the local dispersion relation are investigated. Two cases are analysed: the hydrodynamic instability under the double-resonance condition when the LH frequency coincides with a high ion cyclotron harmonic (interpretation of the CTS results), and the kinetic instability of ion Bernstein waves at harmonics lower than the LH frequency (qualitative understanding of ICE data).