Calibration of a novel near-infrared phase contrast imaging on the HL-3 Tokamak

An original near-infrared phase contrast imaging (NI-PCI) diagnostic has recently been developed on the HL-3 tokamak. This diagnostic obtains the frequency–wavenumber spectrum of plasma density perturbations by measuring the phase shift of a near-infrared laser beam at a wavelength of 1550 nm as it traverses through the plasma, enabling broad spectrum turbulence measurements in HL-3 tokamak experiments. A prototype system of the NI-PCI diagnostic was constructed and calibrated using acoustic waves. Calibration results indicate that the system’s wavenumber measurement range is from $5.24{\mathrm{cm}}^{-1}$ to $35.32{\mathrm{cm}}^{-1}$, with a wavenumber resolution reaching $1.51{\text{cm}}^{-1}$ and a temporal resolution better than $1\mu s$. Compared to conventional PCI diagnostics, the upper limit of the wavenumber measurement for NI-PCI has been increased by 2.6 times under the same device conditions.