Analysis of Alfven eigenmode destabilization by energetic particles in Large Helical Device using a Landau-closure model

Abstract

Energetic particle populations in nuclear fusion experiments can destabilize the Alfven Eigenmodes through inverse Landau damping and couplings with gap modes in the shear Alfven continua. We use the reduced MHD equations to describe the linear evolution of the poloidal flux and the toroidal component of the vorticity in a full 3D system, coupled with equations of density and parallel velocity moments for the energetic particles. We add the Landau damping and resonant destabilization effects using a closure relation. We apply the model to study the Alfven mode stability in the inward-shifted configurations of the Large Helical Device (LHD), performing a parametric analysis of the energetic particle beta(beta(f)) in a range of realistic values, the ratios of the energetic particle thermal/Alfven velocities (V-th/V-A0), the magnetic Lundquist numbers (S) and the toroidal modes (n). The n = 1 and n = 2 TAEs are destabilized, although the n = 3 and n = 4 TAEs are weakly perturbed. The most unstable configurations are associated with the density gradients of energetic particles in the plasma core: the TAEs are destabilized, even for small energetic particle populations, if their thermal velocity is lower than 0.4 times the Alfven velocity. The frequency range of MHD bursts measured in the LHD are 50-70 kHz for the n = 1 and 60-80 kHz for the n = 2 TAE, which is consistent with the model predictions.