Fast Ion Axial Bounce Resonance in C-2W

We report observations of an Energetic-Particle Mode (EPM) in the C-2W plasma device [1,2] that corresponds to an axial bounce resonance of fast ions sourced from Neutral-Beam Injection (NBI). In our experiment, a novel dynamic shaping of a magnetic mirror field monotonically reduces the distance between the turning points of fast ions while introducing a minimum in the expected bounce frequency [3]; this method decouples the plasma length from the motion of the fast ions. We observe an axisymmetric magnetic mode with frequency evolution and axial extent that matches the predicted bounce motion; this is repeated for two different NBI configurations, thus confirming its origin as an axial bounce resonance. Having established the mode's origin, we show that the EPM persists during a transition from a magnetic mirror to a Field-Reversed Configuration (FRC) and use the spatial structure to diagnose the change in magnetic topology [4]. Furthermore, a 2D simulation of the mirror and FRC formation accurately models the experimental observations and demonstrates that the mode arises from the clustering of resonant fast ions. These results may have implications for other and future linear fusion devices with substantial populations of fast ions.

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

[2] Gota, H. et al., Nucl. Fusion 64, 112014 (2024)

[3] Karbashewski S. et al., APS DPP Meeting Abstracts, TP12 00127, (2024)

[4] Roche, T. et al., Nat. Commun. 16, 3487 (2025)