David Z. Pai

Transitions between corona, glow, and spark regimes of nanosecond repetitively pulsed discharges in air at atmospheric pressure

In atmospheric pressure air preheated from 300 to 1000 K, the nanosecond repetitively pulsed (NRP) method has been used to generate corona, glow, and spark discharges. Experiments have been performed to determine the parameter space (applied voltage, pulse repetition frequency, ambient gas temperature, and interelectrode gap distance) of each discharge regime. In particular, the experimental conditions necessary for the glow regime of NRP discharges have been determined, with the notable result that there exists a minimum and maximum gap distance for its existence at a given ambient gas temperature. The minimum gap distance increases with decreasing gas temperature, whereas the maximum does not vary appreciably. To explain the experimental results, an analytical model is developed to explain the corona-to-glow (C-G) and glow-to-spark (G-S) transitions. The C-G transition is analyzed in terms of the avalanche-to-streamer transition and the breakdown field during the conduction phase following the establish...

Ionic wind generation by a wire-cylinder-plate corona discharge in air at atmospheric pressure

A wire-cylinder-plate electrode configuration is presented to generate ionic wind with a dc corona discharge in air at atmospheric pressure. The objective of the work is to maximize the power supplied to the flow in order to increase acceleration while avoiding breakdown. Thus, the proposed experimental setup addresses the problem of decoupling the mechanism of ion generation from that of ion acceleration. Using a wire-plate configuration as a reference, we have focused on improving the topography of the electric field to (1) separate the ionization and acceleration zones in space, and (2) guide the trajectory of charged particles as parallel to the median axis as possible. In the proposed wire-cylinder-plate setup, a dc corona discharge is generated in the space between a wire and two cylinders. The ions produced by the corona then drift past the cylinders and into a channel between two plates, where they undergo acceleration. To maximize the ionic wind it is found that the geometric configuration must be as compact as possible and that the voltage applied must be right below breakdown. Experimentally, the optimized wire-plate reference setup provides a maximum flow velocity of 8 m s−1, a flow rate per unit electrode length of 0.034 m2 s−1, and a thrust per unit electrode length of 0.24 N m−1. The wire-cylinder-plate configuration provides a maximum flow velocity of 10 m s−1, a flow rate per unit electrode length of 0.041 m2 s−1, and a thrust per unit electrode length of 0.35 N m−1. This 46% increase in thrust is obtained by increasing the electric power per unit electrode length by only 16% (from 175 to 210 W m−1), which confirms the gain in efficiency obtained with the decoupled system. In comparison with a simple wire-wire corona configuration, the wire-cylinder-plate configuration increases the ionic wind velocity by up to a factor of 3, and the thrust by an order of magnitude.

Nanomaterials synthesis at atmospheric pressure using nanosecond discharges

The application of nanosecond discharges towards nanomaterials synthesis at atmospheric pressure is explored in this perspective article. First, various plasma sources are evaluated in terms of the energy used to include one atom into the nanomaterial, which is shown to depend strongly on the electron temperature. Because of their high average electron temperature, nanosecond discharges could be used to achieve nanofabrication at a lower energy cost, and therefore with better efficiency, than with other plasma sources at atmospheric pressure. Transient spark discharges and nanosecond repetitively pulsed (NRP) discharges are suggested as particularly useful examples of nanosecond discharges generated at high repetition frequency. Nanosecond discharges also generate fast heating and cooling rates that could be exploited to produce metastable nanomaterials.

Images of Nanosecond Repetitively Pulsed Plasmas in Preheated Air at Atmospheric Pressure

Many potential applications require low-power nonthermal atmospheric-pressure air plasmas. We have generated such plasmas at 1000 K, using the nanosecond repetitively pulsed method. We present the images of the observed discharge regimes.

Plasma microreactor in supercritical xenon and its application to diamondoid synthesis

The generation of plasmas in a microreactor is demonstrated in xenon from atmospheric pressure up to supercritical conditions. Ac high voltage at a frequency of 15 kHz was applied across a 25-µm discharge gap between a tungsten wire and a fused silica micro-capillary tube in a coaxial configuration. Using this continuous flow supercritical fluid microreactor, it was possible to synthesize diamantane and other diamondoids up to nonamantane, using adamantane as a precursor and seed. It is anticipated that plasmas generated in supercritical fluid microreactors may not only allow faster fabrication of diamondoids, but also offer opportunities for the fabrication of other nanomaterials.

Images of a Nanosecond Repetitively Pulsed Glow Discharge Between Two Point Electrodes in Air at 300 K and at Atmospheric Pressure

For many applications of atmospheric pressure plasmas, a crucial issue is to obtain glow discharges at 300 K. We have generated such plasmas with a nanosecond repetitively pulsed method. We present experimental and simulated optical emission images of the dynamics of the formation of the glow regime at the early stages of its development.

Breakdown characteristics of a nanosecond-pulsed plasma discharge in supercritical air

We report on an experimental study of the behavior of a nanosecond-pulsed plasma discharge in air near the critical point. The plasma discharge was generated by a voltage pulse of 10?ns duration with amplitude up to 8?kV, applied between two pin electrodes separated by a gap distance of 25??m. The breakdown voltage and associated current were measured. The total electrical energy deposited per pulse was about 200??J. The results show a non-linearity of the plasma behavior at the critical point of air. An explanation is proposed based on strong density fluctuation characteristics of fluid behavior near the critical point.

Atmospheric-Pressure Discharges for the Fabrication of Surface-Based Metal Nanostructures

Various reactor configurations for generating atmospheric-pressure discharges were tested, and several types of nanostructures, including Mo nanoflakes, were successfully synthesized. Here, we present photographs of the discharges, as well as SEM images of representative nanostructures.

Field-emitting Townsend regime of surface dielectric barrier discharges emerging at high pressure up to supercritical conditions

Surface dielectric barrier discharges (DBDs) in CO2 from atmospheric pressure up to supercritical conditions generated using 10?kHz ac excitation are investigated experimentally. Using current?voltage and charge?voltage measurements, imaging, optical emission spectroscopy, and spontaneous Raman spectroscopy, we identify and characterize a field-emitting Townsend discharge regime that emerges above 0.7?MPa. An electrical model enables the calculation of the discharge-induced capacitances of the plasma and the dielectric, as well as the space-averaged values of the surface potential and the potential drop across the discharge. The space-averaged Laplacian field is accounted for in the circuit model by including the capacitance due to the fringe electric field from the electrode edge. The electrical characteristics are demonstrated to fit the description of atmospheric-pressure Townsend DBDs (Naud? et al 2005 J. Phys. D: Appl. Phys. 38 530?8), i.e. self-sustained DBDs with minimal space-charge effects. The purely continuum emission spectrum is due to electron?neutral bremsstrahlung corresponding to an average electron temperature of 2600?K. Raman spectra of CO2 near the critical point demonstrate that the average gas temperature increases by less than 1?K.

Uniform, Filamentary, and Striped Patterns in Helium Dielectric Barrier Discharge Cryoplasmas

Dielectric barrier discharge cryoplasmas were generated in helium at temperatures between 192 K and 7.6 K at pressures between 1.2 × 105 and 5.2 × 103 Pa. Different discharge structures were observed, such as uniform glow, filamentary, and striped patterns, whose formation depends on the neutral gas temperature, applied voltage, and frequency. At higher temperatures, in addition to He, optical emission spectra include bands due to impurities such as N2, while at temperatures below 20 K, only peaks due to He and He2 are observed.