G. Laity

System for time resolved spectral studies of pulsed atmospheric discharges in the visible to vacuum ultraviolet range

Vacuum ultraviolet (VUV) emission is believed to play a major role in the development of plasma streamers in pulsed atmospheric discharges, but detection of VUV light is difficult in pulsed experiments at atmospheric pressures. Since VUV light is absorbed in most standard optical materials as well, careful attention must be given to the selection of the lens and mirror optics used in these studies. Of highest interest is the VUV emission during the initial stage of pulsed atmospheric discharges, which has a typical duration in the nanosecond regime. An experiment was designed to study this fast initial stage of VUV emission coupled with fast optical imaging of streamer propagation, both with temporal resolution on the order of nanoseconds. A repetitive solid-state high voltage pulser was constructed which produces triggered flashover discharges with low jitter and consistent pulse amplitude. VUV emission is captured utilizing both photomultiplier and intensified charge-coupled device detectors during the fast stage of streamer propagation. These results are discussed in context with the streamer formation photographed in the visible wavelength regime with 3 ns exposure time.

VUV Emission and Streamer Formation in Pulsed Dielectric Surface Flashover at Atmospheric Pressure

There is a growing interest in the physics of surface flashover between the interface of atmosphere and vacuum in some high-power systems. More specifically, the quantitative role of vacuum ultraviolet (VUV) radiation for the photoionization leading to a streamer development during the initial stages of a breakdown is unknown. This paper describes an experimental setup used to measure the VUV radiation emitted from atmospheric flashover as well as time-resolved imaging of the flashover event. A pulser providing the voltage to the gap was designed with special considerations in mind, including long lifetime, low noise, and high reproducibility. This enabled the study of the flashover in various background gases with an emphasis on spectroscopic measurements. The calculated spectra are compared with the measured spectra, and it is found that atomic oxygen and nitrogen are responsible for most of the VUV production in an air breakdown at atmospheric pressure in the wavelength range of 115-180 nm. Time-resolved spectroscopy reveals that the VUV radiation is emitted during the initial stages while the streamers are developing.

Phenomenology of streamer propagation during pulsed dielectric surface flashover

There is a growing demand for understanding the physics of surface flashover, as it relates to the breakdown of electric fields on high power systems in the aerospace community. Specifically, the quantitative role of vacuum ultraviolet (VUV) radiation which is self-produced during the initial nanoseconds of surface flashover is virtually unknown. An experiment was constructed which allows detailed electrical and optical measurements of VUV emission during the timescales in which streamers are propagating before the transition into spark discharge. Repeated surface flashover events are generated using a solid-state high voltage pulser, with breakdown recorded in a number of gases at atmospheric pressure. Streamers are photographed using fast optical imaging with 3 ns resolution. Fast voltage and current diagnostics revealed a number of distinct stages of streamer development ranging from the onset of cathode directed streamers to the sharp current rise during final voltage collapse. The emission of VUV radiation is discussed in context to the observed streamer and electrical characteristics.

Simultaneous measurement of nitrogen and hydrogen dissociation from vacuum ultraviolet self-absorption spectroscopy in a developing low temperature plasma at atmospheric pressure

We demonstrate a method for determining the dissociation density of N and H atoms present in a developing low temperature plasma, based on the emission and self-absorption of vacuum ultraviolet radiation produced from the plasma. Spark plasmas are produced via pulsed discharge in N2/H2 mixtures at atmospheric pressure, where information on the dissociated densities of the constituent gas molecules is desired without employing invasive diagnostic techniques. By analyzing the self-absorption line profile of 121.5 nm Lyman-α H radiation emitted within the first ∼1.0 mm of plasma near the anode tip, a peak dissociated H atom concentration of 5.6 × 1017 cm−3 was observed ∼100 ns into spark formation, with an estimated electron density of 2.65 × 1018 cm−3 determined from Stark broadening. Similarly, simultaneous line fitting of the N 120.0/124.3 nm emission profiles revealed a peak dissociated N atom concentration of 3.8 × 1017 cm−3 during the same discharge period.

A passive measurement of dissociated atom densities in atmospheric pressure air discharge plasmas using vacuum ultraviolet self-absorption spectroscopy

We demonstrate a method for determining the dissociation degree of atmospheric pressure air discharges by measuring the self-absorption characteristics of vacuum ultraviolet radiation from O and N atoms in the plasma. The atom densities are determined by modeling the amount of radiation trapping present in the discharge, without the use of typical optical absorption diagnostic techniques which require external sources of probing radiation into the experiment. For an 8.0 mm spark discharge between needle electrodes at atmospheric pressure, typical peak O atom densities of 8.5 × 1017 cm−3 and peak N atom densities of 9.9 × 1017 cm−3 are observed within the first ∼1.0 mm of plasma near the anode tip by analyzing the OI and NI transitions in the 130.0–132.0 nm band of the vacuum ultraviolet spectrum.

Spatially Resolved VUV Spectral Imaging of Pulsed Atmospheric Flashover

The quantitative role of self-produced vacuum-ultraviolet (VUV) light on photoionization-dominated gas discharges is currently an area of interest in the aerospace community. In this paper, we present the images of the VUV spectroscopic analysis of a pulsed atmospheric flashover, where the spatial content of emission relative to electrode geometry has been preserved. The observed spatial profile of emission is dependent on radiating species in the range of 120-125 nm and is discussed in relation to the physics of nanosecond discharges.

Exploration of self-produced vacuum ultraviolet radiation from dielectric surface flashover at atmospheric pressure

This paper describes recent experiments to study selfproduced vacuum ultraviolet (VUV) emission from pulsed atmospheric plasma structures. While it has been classically believed that photo-ionization plays a significant role on plasma generation during fast timescales (i.e. streamers), the exact role of VUV radiation (energy greater than 7 eV) has only recently been explored and is currently an area of interest for the development of high power devices in the aerospace community. Since VUV emission is heavily absorbed by molecular oxygen and most optical materials, the direct observation of VUV radiation produced by atmospheric pressure plasmas is challenging. Experiments at Texas Tech University were performed with multiple vacuum monochromators, custom designed VUV transparent optical instruments, VUV sensitive intensified CCD and photomultiplier time-resolved diagnostics, and nanosecondtimescale electrical probes of the plasma. Previous studies were limited due to the non-linearity of the focusing optics used for VUV transmission, and thus the current experiment was designed to minimize chromatic abberation of recorded emission in the VUV regime of interest (115 – 135 nm). Quantitative observation of VUV emission from surface flashover in air revealed that the majority of emission is due to radiation from atomic oxygen and nitrogen in the wavelength range 130 – 135 nm, which has been confirmed by spectral calculation for an estimated Boltzmann temperature of 10 eV. High resolution spectral measurements in the range 115 – 130 nm also led to observation of various impurities along the surface, which were only observable due to the upgraded focusing system. Finally, time resolved measurements showed that the earliest VUV emission occurs during the streamer phase, where the recorded signal-to-noise ratio of the observed emission has been significantly increased due to more efficient optical diagnostics.

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.

Investigation of vacuum UV absorption during low-temperature plasma formation in N 2 /H 2 mixtures at atmospheric pressure

This paper describes recent advances in the study of self-generated emission of vacuum ultraviolet (VUV) radiation which is produced during the early time period leading to high voltage breakdown at atmospheric pressure. Previous studies of air breakdown showed the presence of 121.5 nm radiation which is spontaneously emitted by excited hydrogen atoms, HI. Since this Lyman-α line is self-absorbed, it enabled inferring various plasma parameters from recording emission spectra from 115-135 nm for species of HI and NI. For instance, measurements in H2/N2 mixtures have revealed that the highest amount of absorption via HI atoms occurs in the high field region near the anode, implying that significant H2 dissociation for radiation-trapping is occurring in this zone. Selective spatial measurements further showed that the apparent VUV emission centers (i.e. streamer heads) move away from the anode and the mechanisms leading to line broadening (i.e. Stark effect from space charge) are a function of streamer position. The presented self-absorption mechanisms are essential in quantitatively understanding the role of VUV radiation transport via absorption for photo-ionization during streamer breakdown, in which re-absorption of high energy photons is inherently a requirement.

Advanced imaging of pulsed atmospheric surface flashover

Vacuum Ultraviolet (VUV) radiation is commonly thought to enhance streamer formation, as it is energetic enough to cause photoionization in the gaseous volume. Light with wavelengths below 180 nm, i.e. VUV, is highly absorbed in the atmosphere which increases the difficulty of measuring any VUV emission from gaseous breakdown at atmospheric pressure. Nevertheless, VUV emission from pulsed surface flashover at atmospheric conditions was previously recorded at Texas Tech. A second generation system was designed to image VUV and visible emission directly while also preserving the spatial profile. The visible emission is imaged through an air-side focused ICCD, while VUV emission is imaged through a vacuum spectrograph. The variable length gap was excited with a pulser designed for a 100 ns rise time and 50 kV peak output. Captured images of visible light emission from streamers produced in oxygen are diffuse whereas nitrogen produces streamers that are segmented. VUV spatial images taken in oxygen reveal stronger emission closer to the cathode region, while nitrogen produces a more distributed intensity profile across the gap. While MgF2 enabled transmission and measurement of VUV, streamer characteristics recorded in the visible light spectrum of surface flashover on BK7 dielectric windows were also investigated. In this paper, the observed streamer images in both visible and VUV wavelength range will be discussed as it relates to surface flashover at atmospheric pressure.