J. Stephens

Experimental and theoretical evaluation of surface coated exploding wires

This paper discusses the effect of surface coatings on exploding wire behavior. Three different surface coatings of different thicknesses and materials have been studied, each with a 99.99% pure silver conducting core. Experimentally, the wires are subjected to peak current densities in excess of 107 A/cm2 on a microsecond time scale. High Speed intensified Charge-Coupled Device (iCCD) images. A theoretical one-dimensional finite difference model has been developed to predict wire behavior and determine the mechanism(s) responsible for the deviations in behavior induced by the presence of a surface coating.

Charged electret deposition for the manipulation of high power microwave flashover delay times

A quasi-permanent charged electret is embedded into the radiation window of a high power microwave system. It was experimentally observed that the additional electrostatic field introduced by the electret alters the delay times associated with the development of plasma at the window surface, resulting from high power microwave excitation. The magnitudes of both the statistical and formative delay times are investigated in detail for different pressures. Experimental observations are related to calculated discharge parameters using known E/p dependent properties.

Nanosecond, repetitively pulsed microdischarge vacuum ultraviolet source

A microdischarge is driven by short pulses (80 ns FWHM) with peak current levels up to 80 A, with a repetition frequency of 1 MHz (1 pulse/μs) allowing for ∼550 W input power. Experiments in pure argon (Ar2*, 127 nm) and argon-hydrogen (Lyman-α, 121.6 nm) were conducted. Using short pulses, the argon excimer emission was not observed. Alternatively, Ar-H2 operated at both higher power and efficiency (0.63%) whenever pulsed. Using Ar-H2, the experiments result in an average generated vacuum ultraviolet power just above 3.4 W with a peak power of 42.8 W, entirely at Lyman-α.

The Impact of Wire Environment on Electro-Explosive Fuse Performance

The environment surrounding an exploding wire is known be a controlling factor in electro-explosive fuse performance. Recent experiments have shown that the application of an insulating surface coating to the fuse wire can significantly increase the rate of impedance transition and impedance magnitude of the exploding wire. This paper discusses the performance of surface coated fuses tested in commonly used solid and gaseous media. For comparison, these experiments are compared to bare wire fuse experiments in identical environments. Previously developed exploding wire models are utilized to aid in the interpretation of the experimental fuse behavior. Differential wire voltage, voltage pulse length, and degree of post vaporization conduction (i.e., restrike) are discussed for each experiment.

Optimizing wire parameters in exploding wire arrays

Exploding wire arrays as fast switches are of interest for explosive pulsed power applications utilizing magnetic flux compression. This type of opening switch has proven effective in producing pulses of several hundred kilovolts into vacuum diode loads. The research presented here discusses an exploding wire array capable of producing single digit kilo-joules, 100 nanosecond pulses when driven by a 45 kilo-Amp current with a waveform closely resembling typical magnetic flux compression output. For this specific parameter range, the optimal fuse design was developed based on the experimental behavior of the fuse under variation of parameters such as wire spacing, shielding, and quenching medium. Each fuse is composed of several silver wires arranged in a straight wire cylindrical array and is typically pressurized in a chamber filled with about 0.6 MPa of SF6. The tradeoff between wire spacing and voltage output was addressed by designing four different fuse termination pairs each with a diameter that increased wire spacing from 5 to 20 mm in 5 mm increments. A wire shield test was also conducted as an extension to the wire spacing experiment to uncover any mutual radiative effects between wires on fuse opening behavior. The optimum fuse design, including the optimum fuse wire diameter, will be discussed with a 20 Ohm resistive load as well as a vacuum diode load with similar impedance.

Discrete photon implementation for plasma simulations

The self-produced light emission from pulsed plasma discharges and its impact on plasma development are challenging to characterize through simulation and modeling, chiefly due to the large number of radiating species and limited computer memory. Often, photo-processes, such as photo-ionization or photo-emission of electrons, are implemented through over-simplifying approximations or neglected altogether. Here, a method applicable to plasma simulations is implemented in a Particle-in-Cell /Monte Carlo Collision model, which is capable of discretely tracking photons and their corresponding wavelengths. Combined with the appropriate cross sections or quantum yields, a wavelength dependent model for photo-ionization or photo-emission may be implemented. Additionally, by resolving the wavelengths of each photon, an emission spectrum for a region of interest may be generated. Simulations for a pure nitrogen environment reveal that the calculated emission profile of the second positive system agrees well with the experimental spectrum of a pulsed, nanosecond discharge in the same spectral region.

Influence of VUV illumination on breakdown mechanics: pre-ionization, direct photoionization, and discharge initiation

A microdischarge (MD) vacuum ultraviolet (VUV) light source is fired onto a N2–NO (99.92 : 0.08%) target gas. The minor gas constituent, NO, was chosen for its ionization potential (9.23 eV) and photoionization cross-section (1.4 × 10−18 cm2) at the wavelength of interest (121.6 nm, 10.2 eV). The result is a plasma generated entirely by volume photoionization in a N2–NO background. Using a very low electric field amplitude, charge carriers are drifted though the photoplasma at picoampere levels, serving as a non-invasive diagnostic. Using a simple one-dimensional fluid approximation for the low electric field condition, theoretical predictions of photoplasma current were found to be in meaningful agreement with experimental data. The impact of direct photoionization and pre-ionization on nanosecond timescale high voltage breakdown yielded two primary observations: (1) a significant reduction in the formative delay time necessary for spark formation, and (2) almost complete elimination of the statistical delay time. Again utilizing one-dimensional fluid approximations, reasonable agreement between experimental and simulated breakdown voltage was observed. Utilizing the same VUV source to illuminate a HV spark gap biased to about 95% self-breakdown voltage revealed that direct volume photoionization alone was insufficient to trigger breakdown of the high voltage gap. However, permitting electrode illumination, the same source was found to be capable of triggering breakdown in the undervoltaged gap, albeit with a large temporal jitter.

Electric field enhanced conductivity in strongly coupled dense metal plasma

Experimentation with dense metal plasma has shown that non-negligible increases in plasma conductivity are induced when a relatively low electric field (∼6 kV/cm) is applied. Existing conductivity models assume that atoms, electrons, and ions all exist in thermal equilibrium. This assumption is invalidated by the application of an appreciable electric field, where electrons are accelerated to energies comparable to the ionization potential of the surrounding atoms. Experimental data obtained from electrically exploded silver wire is compared with a finite difference hydrodynamic model that makes use of the SESAME equation-of-state database. Free electron generation through both thermal and electric field excitations, and their effect on plasma conductivity are applied and discussed.

Photoionization capable, extreme and vacuum ultraviolet emission in developing low temperature plasmas in air

Experimental observation of photoionization capable extreme ultraviolet and vacuum ultraviolet emission from nanosecond timescale, developing low temperature plasmas (i.e. streamer discharges) in atmospheric air is presented. Applying short high voltage pulses enabled the observation of the onset of plasma formation exclusively by removing the external excitation before spark development was achieved. Contrary to the common assumption that radiative transitions from the b1∏u (Birge-Hopfield I) and b′1∑+ u (Birge-Hopfield II) singlet states of N2 are the primary contributors to photoionization events, these results indicate that radiative transitions from the c′4 1∑+ u (Carroll-Yoshino) singlet state of N2 are dominant in developing low temperature plasmas in air. In addition to c′4 transitions, photoionization capable transitions from atomic and singly ionized atomic oxygen were also observed. The inclusion of c′4 1∑+ u transitions into a statistical photoionization model coupled with a fluid model enabled streamer growth in the simulation of positive streamers.

Time-discretized extreme and vacuum ultraviolet spectroscopy of spark discharges in air, N2 and O2

In this paper we present time-discretized spectra of spark discharges in air, N2 and O2. In previous work, a system for temporally resolved spectral analysis of extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) emission from spark discharges was presented, along with some initial results. As was noted in this paper, statistical variances and the lacking of an apparatus sensitivity profile limited the usability of the data obtained. We have investigated the cause of these variances and improved the setup to reduce their effect. We also investigated the apparatus sensitivity profile to correct the intensity of measured lines. Newly obtained spectra in dry air, N2 and O2 are presented. Air and N2 show high emission in the vicinity of 100 nm, where direct photoionization of molecular oxygen is possible, in the first 250 ns of the discharge. We conclude this emission originates from nitrogen, which has several intense molecular transitions in this region. This finding is confirmed by our experimental results which show the emission in this region is much lower in oxygen.