Observation of an Axisymmetric Energetic Particle Mode driven by Axial Bounce Oscillations of Fast Ions in C-2W

The C-2W fusion device at TAE Technologies uses neutral beam injection (NBI) to sustain and heat either Field-Reversed Configurations (FRCs) or magnetic mirror plasmas [1]; in both configurations, fast ions delivered by NBI form a significant fraction of the confined particles and carry azimuthal current. This work reports on the observation of an axisymmetric energetic particle mode (EPM) in FRCs and mirrors. Dedicated experiments using magnetic mirrors to dynamically scan the turning points demonstrate that the fast ion axial bounce motion drives the mode and is likely due to an axial bunching of fast ions (see related theory and modeling poster by E. Granstedt). In mirrors and FRCs the mode has odd axial parity about the mid-plane and a characteristic parity swapping of higher-order harmonics; a linear array of magnetic diagnostics reveals the mode is axially standing and has an amplitude peak at the expected location of the fast ion turning points. Several experiments are analyzed to highlight the control of the mode using beam energy, axial injection angle, and gas species; notably, each of these parameters can be used to reduce the drive for the mode and suppress the oscillations.  An analysis of the mode across thousands of FRC discharges provides an empirical scaling of the fast ion bounce frequency based on easily measured macro parameters of the equilibrium, which is otherwise difficult to determine in C-2W FRCs due to the lack of internal field measurements.

[1] Gota et al., Nuclear Fusion 61, 106039 (2021)