Laser differential interferometer for diagnostics of gas-puff Z-pinch

Laser shearing interferometer (LSI) is suitable for measuring plasma density profile on condition that the spatial frequency shift of the interference fringes induced by the plasma is less than the spatial-carrier frequency of the fringes. When it is applied to diagnostics of plasmas with very large density gradient and divergence, such as current sheath produced by Z-pinch, the fringes pattern obtained in the experiment seldom meets the requirement. A new type of laser differential interferometer has been developed on the base of a cyclic LSI. By placing a ring interferometer between the two lenses of a four-focus imaging system, the interferometer has the ability to change the ratio of frequency shift to carrier frequency arbitrarily, and thus it is useful for the diagnostics of plasma with large density gradient and divergence. The principle of the four-focus differential interferometer was illustrated. Gas-puff experiments were conducted on the Yang accelerator. The interferograms of the imploding current sheath were obtained and the fringes in the sheath were distinguishable and countable. The electron density profiles and the current sheath shape were calculated from the interferograms.