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Microhollow electrode discharges (MHCD) [l] are gas discharges between closely spaced (submillimeter) electrodes containing openings with diameter, D, on the same order as the electrode gap. In previous experiments the reduction of the size of the cathode opening to 100 pm has allowed us to generate stable, direct current discharges in air up to atmospheric pressure [2]. The microhollow cathode discharges were operated at currents of up to 20 mA, corresponding to current densities of 250 A/cm2 and at average electric fields of 16 kV/cm. The gas temperature of the MHCD was determined by spectroscopic measurements of the excited vibrational states of the nitrogen molecules. First results indicate that the gas temperature at atmospheric pressure and currents of 20 mA is close to 2000 OK. Parallel operation of MHCDs can be achieved by ballasting each discharge resistively [3]. Without resistive ballast, the discharge itself needs to be resistive, that means the current voltage characteristic of the discharge must have a positive slope. Results of modeling [4] show an increase of the forward voltage with current at high current values. However, overheating of the electrodes prevents dc operation of parallel discharges in atmospheric air in this current range. In order to extend the range of operation into the high current mode, were the discharge becomes resistive, it needs to be pulsed. In pulsed operation, with pulse duration in the range from 1 to 100 microseconds, the current range could be extended to 80 mA. At higher current, glow-to-arc transition was observed. The results show, that pulsed operation at high current might allow to operate discharges in parallel without individual ballast.Further research on PIA (Brandenburg and Kline 1998) plasmas, glow discharge-type plasmas created in room air, is reported. These plasmas can be made in a modified 1 kW microwave oven (2.45 GHz) and can be sustained for an indefinite period by the microwaves. They have now been made in a much larger size using 915MHz industrial microwaves at 30-75kW of microwave power. These new plasmas range from roughly spherical and approximately 50cm in diameter to highly irregular and dynamic shapes of larger size. Thus it is demonstrated that the PIA phenomenon can be scaled upward in size and power. The properties of the 915MHz PIA plasmas appear similar to that made with 2.45GHz, except that visible spectra appear to display more bright bands. The plasma also appears to seek microwave sources, like a classic breakdown, rather than retreat from it, as is the behavior of the PIA at 2.45GHz. This suggests that the 915MHz plasmas may be more strongly driven and closer to a classical arc discharge in properties. Further measurements on PIA plasmas will be reported. We believe these plasmas are a new and unusual plasma state first reported by Manwaring and Powell and Finklestein (1 970) whom used 30kW at 75 MHz. Recent research results will be shown. Work supported by AFOSR.

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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...

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