Progress of plasma density fluctuation measurements with phase contrast imaging on HL-2A tokamak

A CO₂ laser-based phase contrast imaging (PCI) diagnostic has been developed on HL-2A tokamak. It can detect line integrated plasma density fluctuations by measuring the phase shift of laser beam after being scattered by the bulk plasma. The diagnosed radial region ranges from $\rho \equiv r/a=0.625$ to 0.7. 32-channel HgCdTe detectors with alternative-current biased amplifiers are arranged in line at the imaging plane of the optical path. This PCI is able to diagnose density fluctuations with wavenumbers ranging from 2 cm^-1 to 15 cm^-1 and the time resolution is better than 2 μs. The first experimental data were achieved in 2018 spring campaign of HL-2A tokamak. High performance is confirmed in different discharging configurations and makes it a keen tool in broadband turbulence investigations.