X-ray yield from pinch target implosions

Abstract

Summary form only given. The wire array z-pinch is an efficient x-ray source which has been proposed for use in indirect drive inertial confinement fusion schemes. Extensive research has focused on methods to enhance and manipulate the x-ray yield in a z-pinch. In recent experiments performed at the Nevada Terawatt Facility, it was observed that a center wire added as a target for conical array implosions resulted in an increase in x-ray yield when the diameter of the center wire was smaller than a threshold value which depended on the wire material. Investigation of this behavior was performed on Zebra, a 2 TW z-pinch generator which delivers a 1 MA current pulse to a load, with a 90 ns rise time. Aluminum cylindrical and conical wire arrays with similar implosion times were used to investigate the role of the center wire in the implosion. For each configuration the array wires diameter remained unchanged for all experiments, while Al, Ti, Cu, SS and W targets were used with diameters ranging from 10 μm - 1 mm. Comparing the soft x-ray yield (20 eV - 5 keV) without a center wire, the cylindrical arrays produced more x-rays than the conical wire arrays, which was expected since a portion of the kinetic energy of the conical implosion goes to producing a plasma jet. With the addition of a center wire, the conical wire array showed a positive correlation between the soft x-ray yield and the diameter of the target. This increase was significant enough to surpass the cylindrical wire array in soft x-ray yield. In conical wire array implosion the narrow end of the cone has an increased J×B force causing the narrow region to implode faster than the rest of the array, similar to an x-pinch. During the initial implosion, time-gated pinhole images recorded a bright x-ray burst at the narrow region of the cone. In addition there was also observed a bright, narrow, hard x-ray source along the length of the pinch. This talk will present the differences in soft (20 eV - 5 keV) and hard (1 keV - 5 keV) x-ray yield form different arrays and targets and will discuss the possible sources for the x-ray yield increase.