Quasi-optical design for the cross-polarization scattering diagnostic on the HL-2M tokamak*

As the plasma beta (β) increases in high-performance tokamaks, electromagnetic turbulence becomes more significant, potentially constraining their operational range. To investigate this turbulence, a cross-polarization scattering (CPS) diagnostic system is being developed for the HL-2M tokamak, which is a new advanced tokamak with the capability of high-performance plasma. To optimize properties of the Q-band CPS diagnostic such as wavenumber resolution and signal-to-noise ratio, a quasi-optical system has been designed. The system includes a lens group for beam waist size optimization, a rotatable wire-grid polarizer for polarization adjustment, and a reflector group for matching the emission angle of the launched microwave. Laboratory tests demonstrated a beam spot size of around 2.5 cm at the target measurement location (near the plasma pedestal), cross-polarization isolation exceeding 50 dB, and poloidal and toroidal angle adjustment ranges of ±40 degrees and ±15 degrees, respectively. These results verify the system's feasibility through laboratory evaluations. The CPS diagnostic with the quasi-optical system has been installed on the HL-2M and is currently being tested in this experimental campaign.