Jared Miles

Laser REMPI Triggering of an Air Spark Gap with Nanosecond Jitter

A laser-triggering scheme for air spark gap switches was conceived and investigated for its potential to reduce shot-to-short time jitter. The scheme utilizes a pulsed ultraviolet laser of relatively low energy to generate resonant enhanced multi-photon ionization (REMPI) within the atmospheric air medium of the spark gap switch. With an applied voltage below the self-breakdown level, the laser-induced pre-ionization initiated avalanche breakdown within the gap and the subsequent triggering of the switch. This laser induced pre-ionization process relied solely on gas phase ionization and not surface effects, since the laser does not strike either electrode. This triggering scheme produced sub-nanosecond jitter with low enough laser power that it could be transmitted through fiber optics, which would be advantageous for multi-switch triggering of a high current pulse. The laser pre-ionization effects of space charge, electric field distribution, and active species within the gap were analyzed for their role in driving electron multiplication leading to avalanche breakdown below the self-breakdown voltage. Experimental results will be presented, including arc timing and statistical jitter measurements, as well as optical images and spectral analysis of the arc emission.

Resonant Laser Induced Breakdown for Fuel-Air Ignition

A resonant laser-induced breakdown scheme to provide precision spatial guidance and timing of fuel-air ignition is presented. This scheme could potentially provide a precision trigger leading to breakdown of a fuel-air flow within a high-voltage gap using a compact low power laser source. The laser scheme employs resonant enhanced multiphoton ionization to generate a pre-ionized path between the electrodes and thus spatially guiding the eventual spark, even when the laser path does not follow the electric field path directly. We present here experimental data that demonstrates precision laser induced breakdown on an air flow (no fuel) within a large scale high-voltage electrode system.

Non-Maxwellian electron energy distribution function in a pulsed plasma modeled with dual effective temperatures

A non-Maxwellian electron energy distribution function (EEDF) has been modeled within a pulsed rf inductively coupled plasma source with the aid of experimental emission spectra and Ar metastable measurements obtained by laser diode absorption. The lower energy portion of the EEDF up to the first excited state energy of 11.5 eV for argon was accurately measured with a Langmuir probe and satisfactorily modeled with a generalized two-parameter expression. Above 11.5 eV, though, inelastic collisions caused the EEDF to deviate from the lower energy generalized expression and soon after, the energy limit of accuracy of the Langmuir probe was approached. In this work, a unique EEDF model was applied for electron energies above 11.5 eV that accounts for spectral effects due to both direct excitation from the Ar ground state and step-wise excitation from the metastable state. Previously tabulated optical cross sections were used with experimental data to simulate the optical emission spectra using a theoretical n...

Plasma electron spectroscopy in microhollow discharge with integrated wallprobe

Summary form only given. The plasma characteristics of a novel microhollow electrode discharge with an integrated wall probe have been analyzed in noble gases. The microplasma device was fabricated with three layers of thin molybdenum metal sheets separated by two thin sheets of mica insulation. A 200 micrometer diameter hole formed a microhollow electrode cylindrical cavity that passed through the entire five layers. The outer layers of molybdenum acted as a hollow anode and hollow cathode, while the inner molybdenum layer formed an effective wall probe. The working gas could be introduced as a flow through the microcavity into open air or the entire microplasma device could be operated in a closed vacuum chamber with a selected gas composition and pressure. Under certain microdischarge conditions, wall probe I-V measurements showed peaks in the electron energy spectra revealing an excess of fast electrons at specific energies. Analysis of these preliminary experiments using plasma electron spectroscopy (PLES) techniques produced promising results, revealing evidence in the spectra of energy exchange with specific neutral and excited gas species. This demonstrates, for the first time, that a wall probe can be useful for interpretation of plasma properties and the plasma-boundary interaction in a microplasma with a nonlocal electron energy distribution at atmospheric pressure. The condition of nonlocality has been analyzed in relation to the dimension of this microhollow electrode discharge and the operating conditions. In noble gases with elastic collisions of electrons, the condition of nonlocality is typically met when p L <;10 Torr-cm, where p is the gas pressure and L is a characteristic plasma dimension. Further development of wall probes in microplasmas would enable a thorough understanding of non-Maxwellian electron energy distributions as well as the possible use of a device as an analytical sensor employing PLES analysis.

Suprathermal electron energy spectrum and nonlocally affected plasma-wall interaction in helium/air micro-plasma at atmospheric pressure

Details of ground-state and excited-state neutral atoms and molecules in an atmospheric-pressure micro-discharge plasma may be obtained by plasma electron spectroscopy (PLES), based on a wall probe. The presence and transport of energetic (suprathermal) electrons, having a nonlocal origin, are responsible for electrostatic charging of the plasma boundary surfaces to potentials many times that associated with the ambient electron kinetic energy. The energy-flux distribution function is shown to be controllable for applications involving analysis of composition and processes taking place in a multiphase (plasma-gas-solid), chemically reactive, interaction region.

N 2 rotational temperature analysis by laser induced fluorescence following resonant enhanced multiphotonion ization

It has been demonstrated that the rotational temperature of molecular nitrogen at atmospheric pressure can be determined by direct optical probing of the N2 (X, v=3D0) ground state with subsequent analysis of the rotational state distribution. A tunable probe laser was scanned over resonant enhanced multi-photon ionization transitions initiating from various N2 (X, v=3D0, J) states. At atmospheric pressure, the laser photo-ionization also induced nitrogen fluorescence bands. Analysis of the relative fluorescence as a function of laser wavelength produced a calculated N2 (X, v=3D0, J) rotation state distribution and the assignment of a rotational temperature.