Study of Alfvén Eigenmodes in the TJ-II Stellarator

Abstract

Energetic ion driven Alfvén Eigenmodes (AEs) in the NBI-heated plasma at the TJ-II heliac were studied by Heavy Ion Beam Probing (HIBP) in the core, and by Langmuir and Mirnov probes (LP and MP) at the edge. HIBP observed the locally (∼ 1 cm) resolved AE at radii -0.5 < ρ < 0.9. The set of AE branches with low poloidal numbers (m < 8) was detected by MP. The most plausible candidates are global, helical and toroidal AEs. AEs on the density, electric potential and poloidal magnetic field oscillations were detected by HIBP at frequencies 50 kHz < f_AE < 300 kHz with a high resolution (< 5 kHz). The amplitude of the AE potential oscillations δφ_AE ∼ 10 V was estimated. The MP and HIBP data have a high coherency at f_AE. When the density rises, AE frequency is decreasing, f_AE ∼ n_e^−1/2, but the cross-phase between the density and potential remains permanent. Poloidally resolved potential measurements by HIBP and LP shows high coherency and finite cross-phase at f_AE, resulting in finite electric field δE_pol. Depending on the cross-phase between δn_e and δE_pol, AEs may bring small or significant contribution to the turbulent particle flux Γ_E×B for the observed k_θ < 3 cm⁻¹.