Simultaneous measurements of unstable and stable Alfv\'en Eigenmodes in JET

TL;DR

This study reports the first simultaneous experimental observation of both unstable and stable Alfvén Eigenmodes in a JET tokamak, supported by kinetic-MHD modeling, enhancing understanding of mode stability for fusion devices.

Contribution

It provides the first simultaneous measurement of unstable and stable TAEs in JET, validated by advanced modeling, advancing the understanding of mode stability in fusion plasmas.

Findings

Unstable TAEs with n=3-5 observed and stabilized by fast ion drive.

Stable n=6 TAE resonantly excited with damping rate 1-4%.

Modeling shows good agreement with experimental mode frequencies and damping rates.

Abstract

In this paper, we report the novel experimental observation of both unstable and stable Toroidicity-induced Alfv\'en Eigenmodes (TAEs) measured simultaneously in a JET tokamak plasma. The three-ion-heating scheme (D-DNBI-3He) is employed to accelerate deuterons to MeV energies, thereby destabilizing TAEs with toroidal mode numbers n = 3-5, each decreasing in mode amplitude. At the same time, the Alfv\'en Eigenmode Active Diagnostic resonantly excites a stable n = 6 TAE with total normalized damping rate $-\gamma/\omega_0 \approx$ 1%-4%. Hybrid kinetic-MHD modeling with codes NOVA-K and MEGA both find eigenmodes with similar frequencies, mode structures, and radial locations as in experiment. NOVA-K demonstrates good agreement with the n = 3, 4, and 6 TAEs, matching the damping rate of the n = 6 mode within uncertainties and identifying radiative damping as the dominant contribution. Improved agreement is found with MEGA for all modes: the unstable n = 3-5 and stable n = 2, 6 modes, with the latter two stabilized by higher intrinsic damping and lower fast ion drive, respectively. While some discrepancies remain to be resolved, this unique validation effort gives us confidence in TAE stability predictions for future fusion devices.