Experimental Studies of Multi-Wire X Pinches on the Cobra Pulsed Power Generator

Abstract

Summary form only given. The rebuilt COBRA pulsed power generator, which provides a variable current pulse (e.g., 100-150 ns rise-times; 0.6-1 MA peak current), has extended the range of parameters that can be used to study X pinches at Cornell University. X pinches constructed with two to twelve wires of materials that range from aluminum to tungsten, including alloys such as Invar, have been tested. Selected wire diameters typically ranged from 12.5 to 75 mum. In all cases, attempts were made to minimize the axial length over which the wires touched at the cross point. The influence of various pulse shapes on the formation of X-pinch plasma, as well as on the general dynamics of the pinch implosion process, has been studied using a set of diagnostics with either spatial or temporal resolution. A set of XUV, soft X-ray, and hard X-ray sensitive photodiodes, such as XRD, PCD, and AXUV-HS, were used with various filters to study the X-pinch radiation over a wide energy range that spanned from XUV up to 50 keV X-rays. There were two general setups employed to study the dynamics of X pinches on COBRA. The first consisted of two X pinches placed in the electron return current circuit, along with a cylindrical wire array serving as the primary load. This setup enabled the study of two X pinches, as well as provided X-ray backlighting for wire array imaging. The X pinches replaced two of the four standard return current posts that connect to the anode hardware. Currents going through the two X pinches, and through the remaining two return current posts were all measured separately. The other general setup studied was that of a standard X pinch load on COBRA. For both setups, a spherically bent, mica crystal spectrograph was used in addition to a flat KAP crystal spectrograph to monitor the soft X-ray self-emission of the X pinches, the results of which are presented herein. Over the course of these experiments, observed current pulses were grouped into two categories, characterized by two peaks separated in time by approximately 50-75 ns. When the first peak was the larger of the two, the X pinch generated a smaller, hotter plasma than when the second peak was the larger. The influence of the two current shapes on the micro-pinch plasma parameters and on the soft X-ray energy yield is discussed in detail