Implosion dynamics and radiation characteristics of wire-array Z pinches on the Cornell Beam Research Accelerator

Abstract

Experimental results are presented that characterize the implosion dynamics and radiation output of wire-array Z pinches on the 1-MA, 100-ns rise-time Cornell Beam Research Accelerator ͑COBRA͒ ͓J. B. Greenly et al., Rev. Sci. Instrum. 79, 073501 ͑2008͔͒. The load geometries investigated include 20-mm-tall cylindrical arrays ranging from 4 to 16 mm in diameter, and consisting of 8, 16, or 32 wires of either tungsten, aluminum, or Invar ͑64% iron, 36% nickel͒. Diagnostics fielded include an optical streak camera, a time-gated extreme-ultraviolet framing camera, a laser shadowgraph system, time-integrated pinhole cameras, an x-ray wide-band focusing spectrograph with spatial resolution, an x-ray streak camera, a load voltage monitor, a Faraday cup, a bolometer, silicon diodes, and diamond photoconducting detectors. The data produced by the entire suite of diagnostics are analyzed and presented to provide a detailed picture of the overall implosion process and resulting radiation output on COBRA. The highest x-ray peak powers ͑300-500 GW͒ and total energy yields ͑6-10 kJ͒ were obtained using 4-mm-diameter arrays that stagnated before peak current. Additional findings include a decrease in soft x-ray radiation prior to stagnation as the initial wire spacing was changed from 1.6 mm to 785 m, and a timing correlation between the onset of energetic electrons, hard x-ray generation, and the arrival of trailing current on axis-a correlation that is likely due to the formation of micropinches. The details of these and other findings are presented and discussed.

Study of micro-pinches in wire-array Z pinches

“…Formation of axial necks in the pinch due to the implosion inhomogeneity was observed in previous papers. 16,17,19,28,34,35 Figure 2 shows the dynamics of radiation from necks in an Al 4-ray 24-wire star array and a 10-wire planar load taken with the Au photocathode without filters, E > 50 eV, over a 50 ns range. The necks are seen as bright spots in the streak images.…”

“…X-ray streak cameras have been used for investigation of the dynamics of micro-pinches (bright spots) in a single wire at a current of 160 kA, 5 in a fiber Z pinch at the early stage, 6 and in experiments at the COBRA generator with cylindrical wire arrays. 16 Bright spots with a lifetime <1 ns and size of 250-400 lm, in the keV range, were seen in the single wire. 5 Bifurcations of bright spots were observed in the fiber pinch.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] In this introduction, we will use words bright spots, micro-pinches, hot spots, and plasma points, as they are mentioned in the original cited articles. Apparently, plasma instabilities generate areas in the pinch with a high plasma density and temperature.…”

“…6 The formation of localized micro-pinches in wire arrays corresponded with bursts of x-ray radiation that were detected by photoconductive diodes in the keV range. 16 These x-ray bursts arose spontaneously on the pinch and lasted from sub-nanoseconds to a few nanoseconds. Electron temperatures of micropinches of 0.3-3 keV were measured in different wire-array Z pinches.…”

“…4,[18][19][20] Continuum spectra of hot spots in the keV range may be a signature of a high plasma density. 16,17 Recently, axially resolved spectroscopy and K-shell power measurements were applied to stagnated Al, Cu, and Ti Z pinches on the Z machine. 22 Segments of the pinch containing bright spots were compared to segments without bright spots.…”