Study of Alfven eigenmode stability in Quasi-Poloidal Stellarator (QPS) plasma using a Landau closure model

The aim of this study is to analyze the linear stability of Alfven eigenmodes (AE) in the QPS device heated by a tangential neutral beam injector (NBI). The analysis is performed using the gyro-fluid code FAR3d, that solves the reduced MHD equations for the thermal plasma coupled with moments of the kinetic equation for the energetic particles (EP). The AE stability is calculated in several operational regimes of the tangential NBI: EP beta between 0.001 and 0.1, EP energy between 12 and 180 keV and different radial locations of the beam. The analysis is performed for vacuum and finite beta equilibria as well as QPS configurations with two and three periods. The EP beta threshold in the vacuum case is 0.001 and the AE frequency is lower as the energy of the EP population decreases. Toroidal Alfven eigenmodes with f = 80-120 kHz and elliptical AE between f = 120-350 kHz are triggered between the middle-outer plasma region (r/a> 0.5). The AE stability improves in the simulations with finite beta equilibria and three period configurations with respect to the vacuum case with two periods because the continuum gaps are slender, leading to a higher threshold of the EP beta, above 0.03 for the AEs triggered by the helical mode families. Helical effects are not strong enough to destabilize Helical Alfven eigenmodes, the AEs with the largest growth rates are triggered by the n= 1 and n= 2 toroidal families.

Automated summary

The aim of this study is to analyze the linear stability of Alfven eigenmodes (AE) in the QPS device heated by a tangential neutral beam injector (NBI). The analysis is performed for different operational regimes of the NBI, including EP beta, EP energy, and radial location of the beam. The stability of AE is improved in simulations with finite beta equilibria and three period configurations.