Fast-Ion Axial Bounce Resonance in a Linear Magnetic Fusion Device

We report observations of an energetic-particle mode (EPM) in a linear plasma device that corresponds to an axial bounce resonance of fast ions sourced from neutral-beam injection (NBI). In our experiment, novel dynamic shaping of a magnetic mirror field reduces the distance between the turning points of fast ions while introducing a minimum in the expected bounce frequency; this method decouples the plasma length from the fast-ion motion. We observe an axisymmetric magnetic mode with frequency evolution and axial extent that matches the predicted bounce motion. This experiment is repeated for two different NBI configurations with distinct bounce motions, thus confirming the mode's origin as an axial bounce resonance. We then show that the EPM persists during a transition from a magnetic mirror to a field-reversed configuration (FRC), making this the first observation of an axial bounce resonance in an FRC. Additionally, we use the spatial structure of the EPM to diagnose the change in magnetic topology from mirror to FRC. Finally, a 2D simulation of the FRC formation accurately models the experimental observations and confirms that the mode arises from the clustering of resonant fast ions. These results have insightful implications for the control and design of linear fusion devices with substantial populations of fast ions.