Interpreting observations of ion cyclotron emission from large helical device plasmas with beam-injected ion populations

Abstract

Ion cyclotron emission (ICE) is detected from all large toroidal magnetically confined fusion (MCF) plasmas. It is a form of spontaneous suprathermal radiation, whose spectral peak frequencies correspond to sequential cyclotron harmonics of energetic ion species, evaluated at the emission location. In ICE phenomenology, an important parameter is the value of the ratio of energetic ion velocity nu(Energetic) to the local Alfven speed V-A. Here we focus on ICE measurements from heliotron-stellarator hydrogen plasmas, heated by energetic proton neutral beam injection (NBI) in the large helical device, for which nu(Energetic)/V-A takes values both larger (super-Alfvenic) and smaller (sub-Alfvenic) than unity. The collective relaxation of the NBI proton population, together with the thermal plasma, is studied using a particle-in-cell (PIC) code. This evolves the Maxwell-Lorentz system of equations for hundreds of millions of kinetic gyro-orbit-resolved ions and fluid electrons, self-consistently with the electric and magnetic fields. For LHD-relevant parameter sets, the spatiotemporal Fourier transforms of the fields yield, in the nonlinear saturated regime, good computational proxies for the observed ICE spectra in both the super-Alfvenic and sub-Alfvenic regimes for NBI protons. At early times in the PIC treatment, the computed growth rates correspond to analytical linear growth rates of the magnetoacoustic cyclotron instability (MCI), which was previously identified to underlie ICE from tokamak plasmas. The spatially localised PIC treatment does not include toroidal magnetic field geometry, nor background gradients in plasma parameters. Its success in simulating ICE spectra from both tokamak and, here, heliotron-stellarator plasmas suggests that the plasma parameters and ion energetic distribution at the emission location largely determine the ICE phenomenology. This is important for the future exploitation of ICE as a diagnostic for energetic ion populations in MCF plasmas. The capability to span the super-Alfvenic and sub-Alfvenic energetic ion regimes is a generic challenge in interpreting MCF plasma physics, and it is encouraging that this first principles computational treatment of ICE has now achieved this.