Imploding Z-pinch plasma diagnostics by using a laser shearing interferometer

Summary form only given. The density and current profiles of imploding Z-pinch plasmas are difficult parameters to measure, yet they are crucial to understand the implosion dynamics and stagnation physics of high current Z-pinches. The imploding plasmas have been simulated from their initial density distribution, implosion dynamics, to the final pinch radiation. But critical feedback information from the experiments to theory is still not available. Better measurements are needed of the motion of the plasma shell and the implosion current, which can be obtained using a laser shearing interferometer (LSI) in combination with a Laser Polarimetry measurement. This paper describes a LSI, for making spatially and temporally resolved measurements of the plasma density and current in imploding Z-pinches. Experiments were conducted on the /spl sim/700 kA Hawk, /spl sim/2.5 MA ACE4 and the 4.0 MA Double Eagle accelerators. The LSI passes collimated laser beams across the imploding z-pinch, which distorts the laser wavefront, with maximum distortion where the density gradient is highest. After passing through the pinch, the distorted wavefronts are split into two beams that are laterally displaced relative to one another. This shearing produces an interferogram, which is captured on film. In the experiments, a 150-ps laser pulse was split into 2 pulses with an inter-pulse delay of several tens ns. This pulse pair gave snap shots of the electron density profiles. Shearing interferograms at different times were obtained during the implosions. Electron density gradient up to 1020 cm/sup -2/ were measured. From the captured interferograms, electron density profiles and implosion velocities of the imploding plasmas were obtained; the growth rates and wavelengths of instabilities were derived. Knowing the initial gas distribution, the ionization states of the imploding plasma can be estimated, The results strongly motivate similar measurements on higher current drivers, such as Decade-Quad and Z with an upgraded 4-6 frame instrument.