Absolute calibration of Phase Contrast Imaging on HL-2A tokamak.*

Phase contrast imaging (PCI) has recently been developed on HL-2A tokamak. In this article we present the calibration of this diagnostic. This system is to diagnose chord integral density fluctuations by measuring the phase shift of a CO$_{\mathrm{2}}$ laser beam with a wavelength of 10.6 $\mu $m when the laser beam passes through plasma. Sound waves are used to calibrate PCI diagnostic. The signal series in different PCI channels show a pronounced modulation of incident laser beam by the sound wave. Frequency-wavenumber spectrum is achieved. Calibrations by sound waves with different frequencies exhibit a maximal wavenumber response of 12 cm$^{\mathrm{-1}}$. The conversion relationship between the chord integral plasma density fluctuation and the signal intensity is 2.3×10$^{\mathrm{13\thinspace }}$m$^{\mathrm{-2}}$/mV, indicating a high sensitivity.