Publications

Z-pinches created using the Z accelerator generate {approximately}220 TW, 1.7 MJ radiation pulses that heat large ({approximately}10 cm{sup 3}) hohlraums to 100-150 eV temperatures for times of order 10 nsec. We are performing experiments exploiting this intense radiation to drive shock waves for equation of state studies. The shock pressures are typically 1-10 Mbar with 10 nsec duration in 6-mm-diameter samples. In this paper we demonstrate the ability to perform optical spectroscopy measurements on shocked samples located in close proximity to the z-pinch. These experiments are particularly well suited to optical spectroscopy measurements because of the relatively large sample size and long duration. The optical emission is collected using fiber optics and recorded with a streaked spectrograph. Other diagnostics include VISAR and active shock breakout measurements of the shocked sample and a suite of diagnostics that characterize the radiation drive. Our near term goal is to use the spectral emission to obtain the temperature of the shocked material. Longer term objectives include the examination of deviations of the spectrum from blackbody, line emission from lower density regions, determination of kinetic processes in molecular systems, evaluation of phase transitions such as the onset of metalization in transparent materials, and characterization of the plasma formed when the shock exits the rear surface. An initial set of data illustrating both the potential and the challenge of these measurements is described.

We are improving the understanding of pulsed-power-driven ion diodes using measurements of the charged particle distributions in the diode anode-cathode (AK) gap. We measure the time - and space-resolved electric field in the AK gap using Stark-shifted Li I 2s-2p emission. The ion density in the gap is determined from the electric field profile and the ion current density. The electron density is inferred by subtracting the net charge density, obtained from the derivative of the electric field profile, from the ion density. The measured electric field and charged particle distributions are compared with results from QUICKSILVER, a 3D particle-in-cell computer code. The comparison validates the fundamental concept of electron build-up in the AK gap. However, the PBFA II diode exhibits considerably richer physics than presently contained in the simulation, suggesting improvements both to the experiments and to our understanding of ion diode physics.

The CR-39/range-filter technique measures ion energy by determining the maximum filter thickness which ions can penetrate. CR-39 located behind the filter records the ions. This method is used to measure peak voltage in pulsed power accelerators. We investigated range and straggling effects in this diagnostic by exposing it to 8- and 15-MeV protons for both Al and Ta filters. The range agreed with published values to better than ±6%. The range straggling decreased for higher incident ion energy and lower atomic number, as expected, although there were differences up to a factor of 1.7 between the experimental values and predictions. The dependence of the track diameter distribution on ion energy enabled us to establish a signature which is characteristic of ions which penetrate a filter, via straggling. These results can be used to evaluate the errors present when this diagnostic is used to measure accelerator voltage.