Experimental Study of X-ray Production and Implosion Dynamics of Low-, Mid-, and High-Atomic-Number Materials on University-Scale Z-pinch Machines of Various Architecture
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Authors
Butcher, Christopher James
Issue Date
2022
Type
Dissertation
Language
Keywords
Linear Transformer Driver , Marx bank , Plasma , Pulsed Power , X-rays , Z-pinch
Alternative Title
Abstract
Earlier research with the novel Double Planar Wire Array (DPWA) and Double Planar Foil Liner (DPFL) loads imploded on the high-impedance UNR Zebra Marx bank generator showed them to be excellent radiators of x-rays. This work focuses heavily on implosions of DPWA loads of low- to mid- to high-atomic-number metals and low-atomic-number DPFL loads performed using the low-impedance UM MAIZE Linear Transformer Driver (LTD). The DPWAs consisted of two wire planes of micron-scale sized wires, while the DPFLs consisted of two planes of micron-scale thickness foils. Current from the machine causes the load planes to ablate, creating two sheets of plasma that pinch in the center of the arrays. As the load begins to implode, radiation in a broad range is emitted, and then detected using various diagnostics, such as an absolutely calibrated PCD, filtered Si-diodes, x-ray pinhole cameras, spectrometers, and a fast, visible light camera which captures plasma evolution. In contrast to the Marx bank (which has been in widespread use in pulsed power research for decades), the LTD is a relatively new pulsed power architecture with the theorized potential to be more efficient than the widely used Marx bank generators. However, up to this point, very little is known on how DPWAs and DPFLs implode on LTDs, so it is important to study. Also, by making comparisons to previous implosions of similar load types on the Zebra generator, we can better understand how the changes in machine architecture and current values affect the radiation emission and implosion dynamics. In addition, unlike the Zebra generator, the low-impedance of the MAIZE LTD makes the discharge current highly susceptible to changes in the load inductance. By studying the load inductance throughout the Z-pinching process of DPWAs and DPFLs on the MAIZE LTD, we can better optimize future loads for the potential of reaching higher peak currents, faster current risetimes, and greater x-ray emission. To perform experimental low-current produced plasma research on the UNR main campus, and to test x-ray diagnostics as well as train students, we have developed a hard x-ray source based on a vacuum diode with laser-plasma cathode triggering dubbed “Sparky-HXRS” (or Sparky Hard X-Ray Source). One of the main objectives of this research was to study the hard x-ray characteristic radiation which is believed to be caused by inner-shell ionization of neutral atoms by non-thermal electron beams propagating through the cold thermal plasma. Such hard x-ray characteristic radiation as well as its polarization properties has not yet been studied in detail in pulsed power plasmas. Sparky-HXRS was designed for producing monochromatic x-rays while keeping the production of bremsstrahlung low in comparison. Laser-driven vacuum x-ray diodes have been attractive for the generation of short-duration x-ray pulses in a compact set-up, which can be temporally synchronized with the laser pulse. Such a device can operate with any voltage, and, if operated with the optimal voltage, can provide better monochromatization of a particular radiation range. The device was designed such that any desired metal could be studied by using that material of interest to form the anode. A number of anode materials were studied with varying atomic numbers, including: brass (an alloy of copper and zinc), stainless steel (an alloy of iron and chromium), titanium, and tungsten. The development of the Sparky-HXRS device also included the development of an “open-air” spectropolarimeter, to study the polarization of characteristic x-rays.