Analysis of Alfven Eigenmodes destabilization by energetic particles in TJ-II using a Landau-closure model

TL;DR

This paper models the destabilization of Alfven Eigenmodes by energetic particles in TJ-II using a Landau-closure approach, revealing mode couplings, stability conditions, and frequency behaviors relevant to plasma confinement.

Contribution

It introduces a comprehensive 3D reduced MHD model with Landau damping effects to analyze AE stability and mode interactions in TJ-II, including helical couplings and frequency sweeping phenomena.

Findings

Identification of destabilized helical Alfven Eigenmodes with frequencies 50-400 kHz.

Prediction of mode stabilization conditions based on iota profile adjustments.

Reproduction of observed AE frequency sweeping as a function of iota profile changes.

Abstract

Alfven Eigenmodes (AE) can be destabilized by energetic particles in neutral beam injection (NBI) heated plasmas through inverse Landau damping and couplings with gap modes in the shear Alfven continua. We describe the linear evolution of the poloidal flux and the toroidal component of the vorticity in a full 3D system using the reduced MHD equations, density and parallel velocity moments for the energetic particles as well as the geodesic acoustic wave dynamics. A closure relation adds the Landau damping and resonant destabilization effects in the model. We apply the model to study the Alfven modes stability in TJ-II, performing a parametric analysis in a range of realistic values of energetic particle $\beta$ (beta_f), ratios of thermal/Alfven velocities (Vth/VA0), energetic particle density profiles and toroidal modes (n) including toroidal and helical couplings. The study predicts a large helical coupling between different toroidal modes and the destabilization of helical Alfven Eigenmodes (HAE) with frequencies similar to the AE activity measured in TJ-II, between 50 - 400 kHz. The analysis has also revealed the destabilization of GAE (Global Alfven Eigenmodes), TAE (Toroidal Alfven Eigenmodes) and EPM (Energetic Particle Modes). For the modes considered here, optimized TJ-II operations require a iota profile in the range of [0.845, 0.979] to stabilize AEs in the inner and middle plasma. AEs in the plasma periphery cannot be fully stabilized, although for a configuration with iota = [0.945, 1.079], only n=7,11,15 AE are unstable with a growth rate 4 times smaller compared to the standard iota = [1.54,1.68] case and a frequency of 100 kHz. We reproduce the frequency sweeping evolution of the AE frequency observed in TJ-II as the iota profile is varied. The AE frequency sweeping is caused by consecutive changes of the instability dominant modes between different helical families.