Laser Wavefront Analyzer for Gas Puff Loads on Decade Quad

Abstract : The Objective of this contract was to develop and implement a Laser Wavefront Analyzer (LWA) to measure z-pinch initial gas density profiles suitable for the Decade Quad Simulator (DQ) at ABDC, TN. A probe laser beam passes through a supersonic gas jet, which distorts the laser wavefronts. The LWA measures such wavefront distortions which are in the range of ~lamba/50. We have designed, constructed and validated our prototype LWA. Gas densities, produced by using DQ gas puff hardware, have been measured. This report summarizes our results. The advantage of the LWA is that the gas density profile can be obtained within one or few shots. However, in the present LWA configuration, the air density-length product (~10(exp 21) sq cm (exp -2) along the line of sight is much higher than that of gas puff(~10(exp 18) sq cm). Variations in the air column limit the present arrangement to a sensitivity of: 5 x 10(exp 17) cubic cm. We have observed in related Fiber Optic Interferometer (FOI) measurements that the time scale of the air turbulence is on the order of 1 ms. The effects of the air turbulence can be minimized to a factor of approximately equal 10 when the reference image is taken within =500 microsecond of the gas measurement image by using two laser probe beams and/or by putting the LWA in a vacuum to reduce the air density-length product. With these improvements, the position uncertainty of the microlens focal spots can be reduced from the present 0.1 pixel to 0.01 pixels.