Session UF2: High Pressure Discharges II

Photon Emission Dynamics during Low-Temperature Plasma Formation

This paper discusses the experimentally observed dynamics of photon emission in the vacuum ultraviolet (VUV) and ultraviolet (UV) spectral regions during the early formation phase of pulsed atmospheric plasma with nanosecond resolution. A 40 kV high voltage pulse with 100 ns risetime is utilized to breakdown a variable millimeter sized gap in various gases at atmospheric pressure. Spatially-resolved PMT measurements reveal that early photon emission originates near the anode and that the source of VUV emission travels from anode to cathode at a velocity on the order of $10^{7}$ cm/s, which is consistent with streamer velocities in volume breakdown reported elsewhere. It is also found, for instance, in pure nitrogen that the second positive system is the main contributor to the emitted light spectrum between 200 to 800 nm during the plasma formation phase while atomic nitrogen dominates the wavelength range between 115 to 200 nm. Under the investigated conditions, it is further elucidated that excited atomic nitrogen is formed in a two-step process rather than in a single electron collision with molecular nitrogen. Current observations in $H_{2}$ discharges demonstrate strong self-absorption for the Lyman-alpha transition coupled with appreciable Stark line broadening.   

Micro-Plasma Discharges From Charge Rollers in Print Engines

Conductive charge rollers (CR) are components in print engines of, for example, laser printers for charging of photoconductor (PC) surfaces. The charging results from an atmospheric plasma produced between the biased CR and the PC. During charging, the PC behaves like a perfect insulator with a conductivity $<$ 10$^{-15}$/$\Omega \cdot $cm. The charging process is essentially that of a dielectric-barrier-discharge. If operated with a dc or quasi-dc voltage, the discharge is terminated by surface charges on the PC. The charging process is continuous as the CR and PC surfaces move at speeds of tens to hundreds of cm-s$^{-1}$. The discharge is then reignited as the voltage drop between the CR and incoming uncharged surface of the PC rebounds. In this investigation, multi-dimensional computer modeling of the CR to PC charging process has been conducted. The computer model, \textit{nonPDPSIM}, solves transport equations for charged and neutral species, Poisson's equation, and the electron energy conservation equation for electron temperature. A Monte Carlo simulation is used to track sheath accelerated secondary electrons and the energy of ions incident onto surfaces. Radiation transport is included. We found that the applied voltage waveform and material properties of CR are important to operation. The uniformity of surface charges on the PC is sensitive to the material properties and speed of the moving surface. Parametric results for uniformity of charging of the PC will be discussed.   

Slow Lightning in Water Plasmoids

Water plasmoids are produced when a capacitor is discharged into a cathode at the surface of a weakly conducting water electrolyte. The resulting plasma jet forms a glowing spherical plasmoid which persists in air for up to 0.3 s and resembles ball lightning in some respects. This study shows that during the plasmoid's formation stage, surface discharges with unusual characteristics carry the large instantaneous discharge current. The liquid-surface discharges have some characteristics of both conventional solid-surface discharges (branching, fractal structure) and glow discharges (approximately constant current density from the discharge plasma to the water surface over a wide range of current). Dynamically, the surface discharge resembles a two-dimensional version of a lightning leader, but develops at much lower speeds: a maximum of about 0.3 m/s for the surface discharges in this study, compared to lightning leader speeds of 100 to 100,000 m/s. The low conductivity of the water used (about 20 mS/m) means that the surface discharges are interacting with a resistive barrier, which allows a significant tangential electric field on the surface. High-speed photography of the discharges is supplemented by spectroscopic and other experimental studies.   

High-Voltage Discharge in Air and Nitrogen, Guided by Femtosecond Laser

The ability of a low energy (El$<$2mJ), femtosecond laser pulse to modify a high energy (Ed$>$1J), high voltage discharge is examined in detail. The geometry and breakdown voltage of a long filamentary submicrosecond high-voltage pulse discharge are studied by noting the effect on initial streamer formation, the breakdown location and overall geometry, and through voltage and current waveforms. The laser pulse is focused in the inter-electrode gap, producing a weakly-ionized plasma filament and effectively decreasing the high voltage breakdown and the geometry of both the initial streamer formation and the high voltage filamentary breakdown. The electric pulse is characterized by rapid voltage rise, dU/dt$>$2$\times$10E11 V/s. High voltage breakdown results in a current pulse of 30-80ns with voltage amplitude of up to 120kV. The guiding effect is considered for delay times up to 100mcs. The initial stage of breakdown -- development a streamer tree -- was observed in detail. Tests were performed in air and nitrogen to better illuminate the physical mechanism of our observed laser-based guiding, particularly at small (less 2mcs) and large (up to 100mcs) delays. The largest decrease in the breakdown voltage occurs at early time, and the effect remains significant in pure nitrogen at delay times up to 5mcs. Both gas kinetic and gasdynamic analyses are used to determine the effects due to ionization and density drops in both air and nitrogen.   

Phase-resolved excitation dynamics of a pulsed 13.56 MHz asymmetric surface barrier discharge in atmospheric-pressure argon

Atmospheric-pressure discharges with non-thermal ions and electrons are an active topic of international research. This is because the excitation of reactive species, without extensive vacuum systems and significant heat dissipation in the sample, facilitates the surface modification of sensitive materials in a practical and cost-effective way. Asymmetric surface barrier discharges (ASBDs) are a variant of the parallel-plate barrier discharge, whereby the electrodes are offset and the ionisation region is between one electrode and the dielectric, i.e. Propagation occurs over the surface of the insulator with relatively easy access to the sample. Phase-resolved optical emission spectroscopy has been used to study the breakdown behavior of a pulsed 13.56 MHz ASBD in atmospheric-pressure argon. Two directions of observation and the calculation of the electron-impact excitation from the electronic ground state facilitate the study of highly repeatable streamer dynamics and the propagation of distinct ionisation fronts.   

COMSOL Modeling of Transport of Neutral Radicals to Substrate Surfaces Located Downstream from an Atmospheric Pressure Weakly Ionized Plasma Reactor

An Atmospheric Pressure Weakly Ionized Plasma (APWIP) Reactor generates a significant number of charged particles and neutral radicals. In our work the carrier gas is argon and the precursor molecule is acetylene. The APWIP is generated by corona discharges associated with an array of high voltage metal needles facing a grounded metal screen. Neutral radical transport downstream from the grounded screen to the substrate via diffusion and convection will be modeled with COMSOL, a finite element software package. Substrates will include objects with various shapes and characteristic dimensions that range from nanometers to centimeters. After the model is validated against canonical problems with known solutions, thin film deposition rates will be compared with experimentally measured results. Substrate geometries will include discs, spheres, fibers and highly porous surfaces such as those found on asphalt road surfaces. A single generic neutral radical will be used to represent the entire family of neutral radicals resulting from acetylene bond scission by free electron impact.   

Precursor ionization ahead of laser-supported detonation wave in air and argon

Laser-produced plasma in a gaseous form is considered, which has attracted great interest for use in many devices. After breakdown one of possible mechanisms of occurrence of this process is noted as laser-supported detonation wave. This wave consisting of the shock wave and the beam absorbing plasma travels at several kilometers per second along the laser beam channel in the direction opposite to the beam incidence. A Nd: Glass laser and a TEA CO$_{2}$ laser were utilized. According to shadowgraph and spectroscopic studies, the wave has a velocity of 1-10 km/s, an electron temperature of 2-5 eV and an electron density of 10$^{24}$ m$^{-3}$ after breakdown. For simplicity, the discussion is restricted to one-dimensional flows that considers the radiation from plasma and the collisional ionization by laser irradiation. Assuming that UV photons radiating from laser plasma induce photoionization ahead of ionization front, this ionization frequency $f_{p}$ at the distance $l_{p}$ (mean free path of photon) from the wave front corresponds to 10$^{10}$ s$^{-1}$. This is higher than the collisional ionization frequency (10$^{5-6}$ s$^{-1})$. Analytical velocities ($f_{p}l_{p})$ describing the avalanche ionization in the pre-ionization layer agree with the experimentally observed velocities. These results does not depend on background gas and laser-wavelength.   

Numerical studies of filamentary plasma formation in high power millimeter wave field

Filamentary structure characterizes millimeter-wave discharge in air and the ionization front propagates at supersonic speed in a high power millimeter-wave, generating a shock wave. In this study, the filamentary structure was studied experimentally and analytically using a 170GHz Gyrotron at the peak intensity range of 50 kW/cm$^2$ to 200kW/cm$^2$. On the propagation process of ionization front, it is important to investigate steady plasma formation process in a filamentary form through millimeter wave. Each filamentary element observed in the ionization front propagates not along or perpendicular to the electric field, but obliquely. To solve this mechanism, 2-dimensional numerical analysis was conducted assuming this phenomenon as a plasma fluid model. In dozens of times the size of plasma element scale, the steady plasma structure formation was simulated, and the calculation was compared with previous experimental results. The calculated formation patterns were in good qualitative agreement with experiments. The calculation model provides a physical interpretation of the pattern formation and dynamics. From the interpretation, it was found that accurate ionization model in low electric field is needed for good agreement with experiments. Moreover, for a quantitative agreement, not only the ionization model but also consideration of 3-dimensional effects are necessary, since 2-dimensional simulation cannot estimate accurate wave reflection and interaction by plasma.