Yury Z. Ionikh

Experimental and theoretical study of the transition between diffuse and contracted forms of the glow discharge in argon

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.

On NOx production and volatile organic compound removal in a pulsed microwave discharge in air

The production of NO and NO2 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 BaTiO3 pellets inside the microwave excitator. The concentrations of NO and NO2 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.

Production of molecules on a surface under plasma exposure: example of NO on pyrex

We propose a new experimental approach to the study of surface-catalysed nitric oxide production under plasma exposure. Stable nitrogen species are grafted to the surface of a pyrex discharge tube during N2 plasma pretreatment. These species are trapped by surface active sites and on being exposed to O2 plasma, they initiate the production of NO molecules, which are detected using tunable diode laser absorption spectroscopy. Supposing that nitrogen species are adsorbed N atoms, we estimate the initial surface coverage as [Nads] = 3 × 1013 cm−2. This gives an assessment of the lower boundary of the density of surface active sites.

Gas temperature measurements in weakly ionized glow discharges with filtered Rayleigh scattering

We report the first gas temperature measurements in plasmas to our knowledge obtained by filtered Rayleigh scattering (FRS). A narrow-linewidth Ti:sapphire laser is used as the illumination source, and a mercury filter provides strong suppression of elastic background. We perform measurements in weakly ionized glow discharges in pure argon and in an argon-plus-1%-nitrogen mixture. Where possible, we verify the FRS technique by comparing filtered measurements with unfiltered measurements. We present point measurements of axial temperature with uncertainties of less than 5%. We use a planar scheme to obtain radial temperature profiles with uncertainties of 10%.

NO formation mechanisms studied by infrared laser absorption in a single low-pressure plasma pulse

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 (N2(X, v > 12) + O) does not impact the NO concentration. Numerical computation of a simplified kinetics taking into account excited metastable state N2(A) for the NO formation shows good agreement.

Partial constriction in a glow discharge in argon with nitrogen admixture

The constriction of the positive column of a glow discharge in argon with nitrogen admixture (0.02–1%) was studied. The discharge was maintained in a tube of 2.8 cm inner diameter and 75 cm length at intermediate pressures (several tens of Torrs), at which the discharge constriction goes by a jump and the hysteresis effect is well pronounced. It was observed that the constriction begins near one of the electrodes and then the constricted region boundary propagates toward the other electrode. The reverse transition occurs in a similar way. The transition time in Ar : N2 mixtures appears to be essentially longer (up to 1 s) than that in pure argon. By varying the power supply voltage in the course of the transition, the boundary between the diffuse and constricted forms of the discharge could be stopped at some position between the electrodes. Such a partially constricted discharge (PCD) is stable and can exist for a long time. A PCD at various locations of the boundary can be formed, different locations being realized at different discharge voltages but at the same discharge current. This corresponds to a vertical segment in the voltage–current characteristic curve. It was found that this segment lies inside the hysteresis loop and connects two branches of the conventional I–V characteristic measured without affecting the discharge during the diffuse-to-constricted or reverse transitions. Plasma parameters in the diffuse and constricted positive columns are estimated and ionization mechanisms are analyzed. The possible reasons for the low velocity of the constriction front in Ar : N2 mixtures and the mechanisms of the stabilization of the PCD are also discussed.

Surface deactivation of vibrationally excited N2 studied using infrared titration combined with quantum cascade laser absorption spectroscopy

The wall de-excitation probability of vibrationally excited nitrogen molecules was determined using infrared (IR) titration with CO, CO2 and N2O. Gas mixtures of N2 with 0.05–0.5% of CO (CO2 or N2O) were excited by a pulsed dc discharge at p = 133 Pa in a cylindrical discharge tube. During the afterglow, the vibrational relaxation of titrating molecules was monitored in situ with quantum cascade laser absorption spectroscopy. The value of was deduced from measured vibrational relaxation times using a model of vibrational kinetics in N2. It was found that adsorption of IR tracers on the surface may increase the value of by a factor up to two, depending on the molecule and the surface material. It was demonstrated that N2O is the most inert and reliable tracer and it was used for the determination of on silica, Pyrex, TiO2, Al2O3 and anodized aluminum. Pretreatment of the silica surface by low-pressure plasma was found to have a strong effect on the vibrational de-excitation. Values of measured after O2, Ar and N2 plasma pretreatment of the same silica discharge tube were 5.7 × 10−4, 8.2 × 10−4 and 11 × 10−4, respectively. This study clearly demonstrates that the presence of adsorbed atoms and molecules on the surface may significantly alter the value of .

NO and NO2 production in pulsed low pressure dc discharge

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.

Experimental and theoretical studies of the electron temperature in nitrogen afterglow

In this paper, results of joint experimental and theoretical studies of the electron temperature in nitrogen afterglow at pulse-periodical excitation are presented. Electron energy distribution function (EEDF) in an afterglow of a pulsed direct current discharge has been measured by means of a time-resolved Langmuir probe technique. Electron concentration, vibrational temperature, and population of lower metastable electronic state of N/sub 2/ molecules have also been experimentally estimated at different delays after the discharge pulse. The results show that electron temperature in afterglow decreases with time, while the vibrational temperature remains almost constant. The EEDF has been calculated numerically from a steady-state Boltzmann equation, taking into account electron-electron collisions as well as superelastic collisions with vibrationally and electronically excited molecules. The vibrational distribution function was found numerically by solving a system of kinetic equations. Calculations show that the vibrational distribution function weakly varies within a cycle and is controlled by an average discharge power. Electron temperature in nitrogen afterglow for given populations of vibrational levels and of lower electronic level essentially depends on the electron concentration. Finally, a comparison of the theoretical and experimental results is performed.

Jumps And Bi‐stability Effects In Low Temperature Plasmas

In present paper the two kinds of bi-stability in low temperature plasmas are considered. The first kind is the bi-stability of the electron energy distribution function (EEDF) and related plasma parameters. Qualitative explanation of the possibility of the effect is given using Maxwellian EEDF approach. The review of physical conditions is done under which EEDF bi-stability was predicted using Boltzmann equation analysis and known experimental data interpreted in terms of bi-stability are described. The possible experiments are also discussed. The second kind is the bi-stability of the form of the positive column of glow discharge. The possibility of the existence of diffuse and constricted forms of positive column under the same discharge current is considered. It is found that under certain experimental conditions constricted and diffuse modes can simultaneously exist in the same discharge tube, i.e. steady state partially-constricted discharge can be realized. The problems of modeling of such kind of discharge are finally discussed.