Feasibility study of fast-ion velocity-space tomography in KSTAR via phantom tests

The fast-ion velocity distribution function is crucial for understanding fast-ion behavior and transport in future burning plasmas. However, direct measurements of this distribution are difficult due to its high-dimensional nature, necessitating inference from diagnostic data. To infer fast-ion velocity distributions in KSTAR experimental conditions, we explored the feasibility of using measurements from fast-ion D_α (FIDA) diagnostics. We assessed the reconstruction quality for two phantoms, representing a possible fast-ion distribution scenario and local velocity-space structures. We calculated the phase-space weight function of FIDA measurements, required for tomographic inversion, by modeling the measurements, and also developed a tomography code with Phillips–Tikhonov regularization. The phantom test results revealed limitations in the reconstruction capability of current FIDA systems in KSTAR, particularly near low-pitch regions. We also identified the influence of spatial bias of the weight function of the current FIDA systems. Introducing a new FIDA system to tomographic inversion process provided wider coverage in velocity space and the weight function with reduced spatial bias, thereby improving reconstruction capability, especially in low-pitch regions. We also scanned noise levels in the phantom tests and observed the benefits of using prior information to mitigate degradation of the reconstruction quality caused by noise.