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 to the local Alfvén speed . 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 takes values both larger (super-Alfvénic) and smaller (sub-Alfvénic) 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-Alfvénic and sub-Alfvénic 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-Alfvénic and sub-Alfvénic 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.

Origin of ion cyclotron emission at the proton cyclotron frequency from the core of deuterium plasmas in the ASDEX-Upgrade tokamak

“…Let us summarise the physical motivation for our choice of v ⊥ . First it is of the order v A , which previous work has shown maximises the drive of the MCI [36,37,38,44,56]. Second, it is consistent with the results of the Orb and Dimon full orbit tracking codes for this pulse.…”

“…The simulation is set up in slab geometry, corresponding to the local approximation; it does not incorporate any toroidal effects or eigenmode structure [32,33,34,48,49,50,51,52,53,54,55]. The omission of toroidal effects is expected to have only marginal consequences for modelling the ICE phenomenology addressed here, given the success of calculations and computations that use the local approximation in explaining results from JET and TFTR [1,2,4,3,7,38,36], as well as more recent ICE observations from KSTAR [40,41,42] and LHD [56,44].…”

“…The Alfvén velocity in the core ASDEX-Upgrade plasma was 5 to 6 × 10 6 ms −1 ; this value is lower than the proton birth velocity (2.4 × 10 7 ms −1 ), but is a factor of two above the NBI deuteron velocity (2.4 × 10 6 ms −1 ) at injection. Previous studies have shown [36,37,38,44,45] that the MCI is more readily excited for v ⊥ /v A 1, where v A is the local Alfvén speed. For this reason, it was concluded [8] that the most likely driver of this ICE is fusion-born protons.…”

First observation and interpretation of spontaneous collective radiation from fusion-born ions in a stellarator plasma

“…This PIC-hybrid approach was recently applied successfully to the interpretation of ICE from NBI protons in hydrogen and deuterium plasmas in LHD [14,15]. In related work, the applicability of PIC computations for interpreting ICE from a subset of fusionborn protons in deuterium plasmas was recently demonstrated for KSTAR observations [27,28], and also for ICE driven by NBI deuterons in KSTAR deuterium plasmas [36],…”

“…This is in contrast to ICE spectra from LHD where the spectral peaks are very close to successive ion cyclotron harmonics, driven by hydrogen (deuterium) NBI in hydrogen (deuterium) plasmas, see Refs. [11,12,13,14,15].…”