Investigation of Nanosecond-Pulsed Plasma Initiation Phenomena in Liquids

Abstract

Nanosecond-pulsed plasma in liquids provide a high-energy environment within which several different non-negligible regimes of physics intersect, requiring new high-speed diagnostics and imaging techniques for better understanding and use in current and future applications. This dissertation highlights four experimental campaigns which investigate nanosecond-pulsed plasmas in liquids using the following imaging techniques: nanosecond optical imaging for time-resolved velocity measurements and shock imaging (using both backlight and shadowgraph), ultrafast phase-contrast X-ray imaging at the Advanced Photon Source (APS) for picosecond-exposure imaging of multiphase behavior during bright optical emission, and time-resolved Raman spectroscopy for investigation of possible plasma-induced phase transition. The first campaign presents the first-known diagnosis using X-ray phase contrast imaging (PCI) of these types of nanosecond-pulsed plasmas in liquids. X-ray PCI is insensitive to plasma optical emission, which facilitates imaging of previously-obscured phenomena and quantitative analysis using an X-ray diffraction model. Results herein indicate that primary plasma streamers are exclusively low-density phenomena, an important contribution to the ongoing debate over which phenomena governs plasma initiation. Additionally, this first set of PCI experiments demonstrates this plasma device as a relatively low-cost, portable, self-healing target for benchmarking next-generation imaging systems. The second and third campaigns build on the progress of the first in pursuit of this application, using a spark discharge target in liquid heptane (first-known PCI imaging of plasma-induced shocks) and in a mineral oil suspension of microparticles (bismuth and tungsten explosion/atomization), respectively, with X-ray diffraction and thermodynamic analysis. The fourth experimental campaign investigates a known mode transition from spherical to branched plasma structures when plasma energy density is increased. We hypothesize that spherical-to-branched transition may coincide with local production of high-pressure solid phases of water such that plasma initiation and propagation may be governed by stress-induced fracture mechanics, since these plasma events have sufficient energy to isentropically reach pressures indicative of Ice VI and VII; this hypothesis is supported by time-resolved Raman spectroscopy results of the plasma region of interest, since the phase transition qualitatively affects vibrational structure.