Andrew Fierro

Optical emission spectroscopy study in the VUV–VIS regimes of a developing low-temperature plasma in nitrogen gas

The mechanisms leading to the development of an atmospheric low temperature plasma along a surface under pulsed conditions is of current interest. In the early plasma phase, high energy photons are a contributing factor to the process of generating electron avalanches resulting in surface flashover. Since only photons in the vacuum ultraviolet (VUV) regime are energetic enough to cause step-ionization or direct ionization of atmospheric gases, an experiment has been set up to enable observations of photons with wavelengths shorter than 200 nm while still allowing observation up to 800 nm. A spectrum simulation software package has been developed to allow for temperature analysis on the developing plasma in the VUV region. Observations below 200 nm revealed a Boltzmann distributed excited state population corresponding to a temperature of 3.1 eV. Time-resolved emission spectroscopy measurements of the entire electrode region during the streamer phase of breakdown demonstrate the presence of molecular nitrogen emission lines from the second positive system. Further photomultiplier tube measurements of the spark phase showed a rapid decrease in intensity of the second positive system compared to that of a representative atomic emission line in the VUV regime. This emission dominates the ultraviolet–visible (UV–VIS) spectrum during the initial phases of breakdown with little detection of other sources of emission during this phase.

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.

Graphics processing unit accelerated three-dimensional model for the simulation of pulsed low-temperature plasmas

A 3-dimensional particle-in-cell/Monte Carlo collision simulation that is fully implemented on a graphics processing unit (GPU) is described and used to determine low-temperature plasma characteristics at high reduced electric field, E/n, in nitrogen gas. Details of implementation on the GPU using the NVIDIA Compute Unified Device Architecture framework are discussed with respect to efficient code execution. The software is capable of tracking around 10 × 106 particles with dynamic weighting and a total mesh size larger than 108 cells. Verification of the simulation is performed by comparing the electron energy distribution function and plasma transport parameters to known Boltzmann Equation (BE) solvers. Under the assumption of a uniform electric field and neglecting the build-up of positive ion space charge, the simulation agrees well with the BE solvers. The model is utilized to calculate plasma characteristics of a pulsed, parallel plate discharge. A photoionization model provides the simulation with ...

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.

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.

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.