Time-resolved optical spectroscopy measurements of pre-pulse plasma formation on Z

The compression of annular wire arrays in z-pinch experiments produces large radiation pulses which are useful for high energy density research. The initial transformation of the wire array into a plasma shell impacts the subsequent implosion of the shell because it affects the current distribution, possibly seeding instabilities. On the Z accelerator the initial plasma is created when a ∼300 kA prepulse current flows through the 240--300 wire array during a ∼50 nsec period prior to the arrival of the main power pulse, approximately 100 nsec prior to the pinch. Ideally, the authors would like to measure the pre-pulse plasma density and temperature with nsec time resolution and sub ∼100 micron two-dimensional spatial resolution. Such measurements are difficult at large accelerators, such as Z, because of limited line of sight access and low experiment rates. Nevertheless, the unique conditions produced on Z discharges imply that it is important to measure the plasma formation characteristics on Z. The strategy is to perform an initial set of limited measurements at Z and interpret the results with the aid of models based on more detailed bench top experiments performed elsewhere. Subsequent more-detailed measurements at Z can then be pursued. The principle diagnostic for the pre-pulse plasma is time-resolved optical spectroscopy