Development of An Ion Energy Analyzer for the Energy- and Pitch-Angle-Resolved Measurement in Beam-Driven Field-Reversed Configuration Plasmas

Understanding the confinement and transport of fast ions is critical for improving and optimizing the performance of beam-driven, field-reversed configuration (FRC) plasmas. The C-2W device, which is currently being operated at TAE Technologies [1], produces an FRC plasma driven by eight neutral beam injectors[2]. In a beam-driven FRC equilibrium, the fast ion and thermal pressure are comparable, and hence, the velocity distribution of fast ions is essential for understanding steady-state equilibrium. The distribution also plays critical roles in transient phenomena, such as fluctuations or instabilities. Therefore, as a new diagnostic tool, we employ fast-loss ion detectors, widely used in the tokamak and stellarator communities[3-5]. The ion energy analyzer is designed by calculating fast ion trajectories within the energy range of 1 - 15 keV and pitch angles between 30 and 90 degrees, under different C-2W configurations where the typical magnetic field is approximately 0.4-0.7 T at the measurement location. The designed diagnostic features a PCB detector plate enclosed inside the probe housing, with multiple segmented copper layers. The segmentation is optimized to resolve fast ion energies and pitch angles with numerical simulation of the C-2W ion velocity distribution. Potential sources of error in the energy-resolved measurement induced by finite aperture size and space-charge effects are also evaluated. The device will be located near the mirror section in C-2W to investigate the interdependence of fast ion distribution and the FRC equilibrium, as well as to study transient phenomena.