Investigation of gyrotron-based collective Thomson scattering for fast ion diagnostics in a compact high-field tokamak

As a promising method for fast ion diagnostics, collective Thomson scattering (CTS) can measure the one-dimensional velocity distribution of fast ions with high spatial and temporal resolution. The feasibility of diagnosing fast ions in a compact high-field tokamak by CTS was studied in this work, and the results showed that a wide range of probing frequencies could be applied. A high-frequency case and a low-frequency case were mainly considered for fast ion diagnostics in a compact high-field tokamak. The use of a high probing frequency could effectively avoid the refraction effect of the beams, while the application of a low probing frequency allows greater flexibility in the selection of scattering angle which may help to improve the spatial resolution. Based on typical plasma conditions (B₀ = 12.2 T, n_e0 = 4.3 × 10²⁰ m⁻³, T_e0 = 22.2 keV, T_i0 = 19.8 keV) for a compact high-field tokamak, a 220 GHz CTS diagnostic that utilizes a small scattering angle of θ = 30° and a 160 GHz CTS diagnostic that utilizes an orthogonal geometry were proposed. Further study showed that the high-frequency case could operate in a wider range of plasma conditions and provide more information on fast ions while the low-frequency case could achieve higher spatial resolution of the poloidal direction.