Toroidal Alfven eigenmodes observed in low power JET deuterium-tritium plasmas

The Joint European Torus (JET) recently carried out an experimental campaign using a plasma consisting of both deuterium (D) and tritium (T). We observed a high-frequency mode using a reflectometer and an interferometer in a D-T plasma heated with low power neutral beam injection, PNBI = 11.6 MW. This mode was observed at a frequency f = 156 kHz and was located deep in the plasma. The observed mode was identified as a toroidal Alfven eigenmode (TAE) using the linear MHD code, MISHKA. The stability of 21 modes that match experimental measurements was investigated. Beam ions and fusion-born alpha particles were modelled using the full orbit particle tracking code LOCUST, which produces smooth distribution functions suitable for stability calculations without analytical fits or the use of moments. We calculated the stability of the 21 candidate modes using the HALO code, which models the wave-particle interaction. These calculations revealed that beam ions can drive TAEs with toroidal mode numbers n ≥ 8 with linear growth rates γd/ω ∼ 1%, while TAEs with n < 8 are damped by the beam ion population. This finding was supported by a simple analytical model. Alpha particles drive modes with significantly smaller linear growth rates, γα/ω ≲ 0.1% due to the low alpha power generated almost exclusively by beam-thermal fusion reactions. Non-ideal effects were calculated using complex resistivity in the CASTOR code, leading to an assessment of radiative, collisional, and continuum damping for all 21 candidate modes. Ion Landau damping was modelled using Maxwellian distribution functions for bulk D and T ions in HALO. Radiative damping, the dominant damping mechanism, suppresses modes with high toroidal mode numbers. Comparing the drive from energetic particles with damping from thermal particles, we find all but one of the candidate modes are damped. The single net-driven n = 9 TAE with a net growth rate γ/ω = 0.02% matches experimental observations with a lab frequency f = 163kHz and location R = 3.31m. The TAE was driven by co-passing particles through the v∥ = vA/5 resonance, with additional sideband resonances contributing significant drive.