The constriction of the positive column of a glow discharge in argon was studied both experimentally and theoretically. In experiments the direct current discharge was maintained in a cylindrical glass tube of 3 cm internal diameter and 75 cm length. The voltage–current U(I) characteristics of the discharge were measured at a gas pressure P from 1 to 120 Torr in a wide range of discharge currents. At P > 20 Torr the measured U(I) characteristics display the classical hysteresis effect: the transition from the diffuse to the contracted discharge form (with increasing current) occurs at a current higher than that for reverse transition (with decreasing current). It was also found that in some cases the so-called partially contracted form of the discharge is realized, when the diffuse and contracted forms coexist in the discharge tube.To calculate the plasma parameters under experimental conditions a 1D axial-symmetric discharge model for pure argon was developed. The details of the model are described and the results of simulations are presented. In particular, the electric field strength E in the positive column was calculated as a function of the discharge current. Theoretical E(I) characteristics are compared with those derived from the experiment. For the first time, the detailed kinetic model without the usage of fit parameters predicts the hysteresis effect in pure Ar with parameters of diffuse and constricted forms of the discharge in good agreement with the experiment.

The formation of NO molecules during a single plasma pulse in a low-pressure dc discharge is measured using time resolved tunable diode laser absorption spectroscopy in the infrared region. The pulse duration ranges from 280 µs to 16 ms and the pulse current ranges from 20 to 80 mA. The gas pressure is 133 Pa. Experimental results show that NO density is about proportional to the product of the pulse current times the pulse duration. NO formation mechanisms are discussed. We show that reaction of oxygen atoms with vibrationally excited nitrogen molecules (N 2 (X, v > 12) + O) does not impact the NO concentration. Numerical computation of a simplified kinetics taking into account excited metastable state N 2 (A) for the NO formation shows good agreement.

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Electrical breakdown resulting in the ignition of a low-pressure low-current glow discharge is investigated in long (length much larger than the diameter) tubes. New features characterizing the breakdown are found. Breakdown begins with synchronous sharp drop of the anode voltage and the peak in the anode current, which is not accompanied by the current at the grounded cathode. This proves the existence of the first (initial) breakdown occurring between the highvoltage electrode and the nearby section of the tube wall. Simultaneously, an ionization wave starts from the anode. The cathode current initiates noticeably later, at the moment when the ionization wave reaches the cathode. The distribution of the breakdown statistic delay time is governed by the Laue law. This study has revealed a profound effect on the breakdown of illumination of the tubes by visible-spectrum light. Illumination diminishes the average breakdown delay time; for the breakdown mode when breakdown occurs at the pulse leading edge this leads to a decrease in the average breakdown voltage. The long-wavelength threshold of the effect is 520 nm. Electron photodesorption from the wall surface is supposed to be the mechanism of the effect. Quantum efficiency for this process is 0.6×10 −9 . Unlike in most previous studies, all the measurements were carried out with unshielded tubes; screening of the tube by a grounded shield has a strong influence on the breakdown characteristics.

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The production of NO and NO 2 and the removal of 3-pentanone, as an example for volatile organic compounds (VOCs), in a pulsed microwave discharge in air, near atmospheric pressure has been studied. The influence of changing the pulse duration from 25 to 500 µs and of the pulse repetition rate from 10 to 500 Hz is reported. At a relatively high pressure of p = 800 mbar, plasma ignition is achieved by inserting BaTiO 3 pellets inside the microwave excitator. The concentrations of NO and NO 2 have been monitored by infrared tunable diode laser absorption spectroscopy. It was found that their concentrations increase monotonically with the average power injected into the plasma. Further, the efficiency of the pulsed microwave discharge for VOC oxidation, in this case of 1400 ppm of 3-pentanone in dry air, has been studied. The VOC removal efficiency has been determined using gas chromatography. The oxidative efficiency of the discharge was found to increase linearly with the pulse repetition rate as well as with the pulse duration, the power duty cycle ratio being the key parameter.

Spark-generated shock waves were studied in glow discharges in argon and argon–nitrogen mixtures. Ultraviolet filtered Rayleigh scattering was used to measure radial profiles of gas temperature, and the laser schlieren method was used to measure shock arrival times and axial density gradients. Time accurate, inviscid, axisymmetric fluid dynamics computations were run and results compared with the experiments. Our simulation show that changes in shock structure and velocity in weakly ionized gases are explained by classical gas dynamics, with the critical role of thermal and multi-dimensional effects (transverse gradients, shock curvature, etc.). A direct proof of the thermal mechanism was obtained by pulsing the discharge. With a sub-millisecond delay between starting the discharge and shock launch, plasma parameters reach their steady-state values, but the temperature is still low, laser schlieren signals are virtually identical to those without the discharge, differing dramatically from the signals in discharges with fully established temperature profiles.

Simultaneous measurements of both NO and NO2 are performed downstream a pulsed low pressure dc discharge in flowing air using tunable diode laser absorption spectroscopy in the infrared region. Pulse duration and repetition rate range from 20 μs to 5 ms and from 50 to 1000 Hz, respectively. The gas pressure is 4 mbar and the peak current is 80 mA. Experimental results show that NO and NO2 production depends only on the duty cycle ratio, that is, on the average power at a given current. A numerical computation of a simplified kinetics agrees well with experiment results.