Plasma cathode sustained filamentary glow discharges in atmospheric air

Abstract

High-energy electron beams represent the most energyefficient way of gcncrating noncquilibrium plasmas, with power budget 2-3 orders of magnitude better than that of electric ficldsustained discharges. In this paper, we analyzc dynamics of plasmmas generated in dense gases by beams of energetic electrons.J w o distinct regimes are found, differing in a way that the excess negative charge brought in by the ionizing electron beam is removed. In the first regime, called a "fountain", the charge is removed by the back current of plasma electrons towards the inject!ion foil. In the second, called a "thunderstorm", a substantial cloud of negative chargc accumulates, and the increased electric field near the cloud causes a streamer to strike between thc cloud and a positive or grounded electrode, or between two diRerent clouds created by two different beams. A quantitative analysis, including electron beam propagation, clectrodynamics, charge particle kinetics, and a simplificd hcat balance, is performed in 1D approximation. Kinetics of high-energy primary and secondary electrons is described in the ' forward-back' approximation, while the di-ift-diffusion approximation is used for low-energy electrons.Electronegative gases, e.g., air, were found to favor the "thunderstorm" regime. Electron attachment to oxygen not only rcduccs the electron density, but also sharply slows down the mobility of negative charge carriers. This results in accumulation of electric charge at the "tip" of plasma column and an increase in electric field strength. However, as the beam heats the gas, at temperatures of about 2000 K electron detachment becomes strong, and the computed behavior of plasmas in air is close to that in nitrogen.Computations show interesting dynamic phenomenon when the beam acts like a hot blade in butter. Gas heating by the electron beam results in density decrease, allowing ihe beam to penetrate deeper. New portions of the gas are then heated, and thc beam can penetrate still farther. At longer timcs, this could lead to complex gas flow coupled with e-beam effects. Portions ofthe gas left behind will begin to cool, so that their density increases. This would cause a radial flow of cold gas from peripheral to the ccnt.ral regions. All this could eventually result in turbulence generation, oscillations of the beam penetration depth and of the local ionization fraction.The paper describes dynamics of plasmas produced by repetitively pulsed beams. This regime is shown to reduce the power budget for sustaining a given electron density compared with continuous beam case.Glow-to-arc transitions in filamentary glow discharges in atmospheric air can b: largely avoided by use of a plasma cathode, as has been demonstrated in short filamentary discharges in, air [I]. In these experiments a dc-driven microhollow cathode discharge (MHCD) was used as a plasma cathode to sustain a stable, direct current discharge: between the plasma cathode and a third positively biased electrode. We have, using the same concept, extended the gap dist...

Self-organization in cathode boundary layer microdischarges

“…Applications of these high pressure discharges range from plasma cathodes [7] and plasma reactors [8] to light sources. Particularly, research on 'MHCDs' has been stimulated by their application as excimer light sources [9].…”