Study of the dark phase in the initial stage of the positive column formation in an argon glow discharge

The initial stage of the positive column formation in an argon glow discharge is investigated both experimentally and theoretically. A decrease in the plasma radiation intensity (the so-called “dark phase”) was observed experimentally over a time period of about 1 ms. A similar dip was also observed in the time dependence of the electric field strength. The time evolution of the population of the lowest metastable state of Ar was measured. A relevant theoretical model has been developed and used to perform calculations for the actual experimental conditions. A comparison between the numerical and experimental results shows that the model adequately describes the processes that occur during the formation of the positive column in an argon glow discharge. Experimental and theoretical study shows that the dark-phase effect is related to an excessive amount of metastable Ar atoms at the beginning of a discharge and, consequently, to high rates of stepwise ionization and chemionization.     

Long-lived Ar-Hg plasma in the afterglow of a high-current pulsed discharge

High-density (n > 10¹² cm^?3) argon-mercury plasma produced by a short (t ～ 20 μs) high-power pulsed discharge in argon with an admixture of mercury vapor at a discharge current of ～50 A, an argon pressure of ～4 mm Hg, and a mercury vapor pressure of ～10^?3 mm Hg was studied using optical spectroscopy and radio physics methods. It is found that the lifetime of this plasma after the end of the discharge pulse is up to 10^?2 s. It is shown that such an abnormally long lifetime of such an afterglow plasma, as compared to the plasma of an argon discharge without an admixture of mercury vapor, is related to the long residence time of atoms and ions of both argon and mercury in highly excited states due to chemi-ionization processes involving long-lived metastable argon ions. It is suggested that dissociative recombination of highly excited molecular ions of argon play an important role in the transfer of excitation to argon atoms and ions that are close to autoionization states.     

Dynamics of Breakdown in a Low-Pressure Argon–Mercury Mixture in a Long Discharge Tube

Breakdown dynamics in the course of glow discharge ignition in a long discharge tube (80 cm in length and 25 mm in diameter) filled with argon at a pressure of 3–4 Torr and mercury vapor at room temperature was studied experimentally. Rectangular voltage pulses with amplitudes from 1 to 2.5 kV were applied to the tube anode, the cathode being grounded. Complex electrical and optical measurements of the breakdown dynamics were carried out. Breakdown begins with a primary discharge between the anode and the tube wall. In this stage, a jump in the anode current and a sharp decrease in the anode voltage are observed and prebreakdown ionization wave arises near the anode. The cathode current appears only after the ionization wave reaches the cathode. The wave propagation velocity was measured at different points along the tube axis. The wave emission spectrum contains Hg, Ar, and Ar⁺ lines. The intensities of these lines measured at a fixed point exhibit very different time behaviors. The effect of the tube shielding on the breakdown characteristics was examined. It is found that, at a sufficiently narrow gap between the shield and the tube, this effect can be substantial.     

Influence of a Nitrogen Admixture on the Anomalous Memory Effect in the Breakdown of Low-Pressure Argon in a Long Discharge Tube

The memory effect (the dependence of the dynamic breakdown voltage U _b on the time interval τ between voltage pulses) in pulse-periodic discharges in pure argon and the Ar + 1%N₂ mixture was studied experimentally. The discharge was ignited in a 2.8-cm-diameter tube with an interelectrode distance of 75 cm. The measurements were performed at gas pressures of P = 1, 2, and 5 Torr and discharge currents in a steady stage of the discharge of I = 20 and 56 mA. Breakdown was produced by applying positive-polarity voltage pulses, the time interval between pulses being in the range of τ = 0.5–40 ms. In this range of τ values, a local maximum (the anomalous memory effect) was observed in the dependence U _b (τ). It is shown that addition of nitrogen to argon substantially narrows the range of τ values at which this effect takes place. To analyze the measurement results, the plasma parameters in a steady-state discharge (in both pure argon and the Ar + 1%N₂ mixture) and its afterglow were calculated for the given experimental conditions. Analysis of the experimental data shows that the influence of the nitrogen admixture on the shape of the dependence U _b (τ) is, to a large extent, caused by the change in the decay rate of the argon afterglow plasma in the presence of a nitrogen admixture.     

Influence of a Nitrogen Admixture on the Value and Radial Profile of the Metastable Argon Atom Density in a DC Glow Discharge in Argon

Plasma Physics Reports - Results of measurements of the value and radial profile of the density of Ar(3P2) metastables in a dc discharge in pure argon and Ar + 0.1%N2 and Ar + 1%N2 mixtures are...     

Experimental study of a multipoint cathode corona in an argon flow

Results are presented from experimental studies of a multipoint negative corona in an atmospheric-pressure argon flow. It is shown that a decrease in the interpoint distance, gas circulation through the discharge gap, and the adjustment of ballast resistances in the corona supply circuit allow one to stabilize the discharge and enlarge the operating range of discharge currents.     

Interaction of near-IR laser radiation with plasma of a continuous optical discharge

The interaction of 1.07-μm laser radiation with plasma of a continuous optical discharge (COD) in xenon and argon at a pressure of p = 3–25 bar and temperature of T = 15 kK has been studied. The threshold power required to sustain COD is found to decrease with increasing gas pressure to P _t < 30 W in xenon at p > 20 bar and to P _t < 350 W in argon at p > 15 bar. This effect is explained by an increase in the coefficient of laser radiation absorption to 20?25 cm^–1 in Xe and 1?2 cm^–1 in Ar due to electronic transitions between the broadened excited atomic levels. The COD characteristics also depend on the laser beam refraction in plasma. This effect can be partially compensated by a tighter focusing of the laser beam. COD is applied as a broadband light source with a high spectral brightness.     

Experimental and theoretical studies of the optical and electrical characteristics of a high-pressure pulsed discharge in argon

The optical and electrical characteristics of pulsed discharges in pure Ar at pressures of up to 7 atm, at which the discharge becomes unstable, are studied in a simple experimental device with automatic preionization. The gas temperature in the discharge is estimated from the width of the recorded emission spectrum. An analytical model of the vibrational relaxation of Ar ^*₂ (v) is used to better determine the constants of the vibrational-translational relaxation of Ar ^*₂ (v) molecules in their collisions with Ar atoms. The zerodimensional numerical model of a pulsed discharge in Ar is modified. The experimental and calculated results are compared in detail. Good agreement is achieved between the measured and calculated time dependences of the electrode voltage and the intensity of spontaneous emission in the pressure range of 1–6 atm, as well as between the measured and calculated values of the gas temperature at pressures of 3–6 atm. Preliminary results from numerical studies of the possibility of achieving generation are discussed.

     

Study of the dark phase in the initial stage of the positive column formation in a neon glow discharge

The initial stage of the positive column formation in a neon glow discharge is investigated both experimentally and theoretically. A decrease in the plasma radiation intensity (the so-called “dark phase”) was observed experimentally over a time period of about 1 ms. A similar dip was also observed in the time dependence of the electric field strength. The measured population of the lower metastable states of Ne was found to have a maximum at the beginning of the dark phase. A relevant theoretical model has been developed and used to perform calculations for the actual experimental conditions. A comparison between the numerical and experimental results shows that the model adequately describes the processes that occur during the formation of the positive column in a neon glow discharge. Experimental and theoretical studies show that the dark-phase effect is related to the excessive amount of metastable Ne atoms at the beginning of a discharge and, accordingly, to the high rates of stepwise ionization and chemionization.     

Production of excimer molecules and the excitation of nitrogen in a steady-state low-pressure electric discharge

Conditions for the simultaneous production of argon and xenon chlorides and excited nitrogen molecules in a longitudinal dc glow discharge in Ar/Cl₂/air, Xe/Cl₂/air, and Ar/Xe/Cl₂/air mixtures are studied. The electrical parameters of the plasma and its optical characteristics in the 130-to 350-nm wavelength range are investigated. It is shown that a small admixture of air added to argon or xenon leads to the production of excited nitrogen molecules, whose decay is accompanied by the molecular band emission in the range Δλ=176–271 nm. The conditions for simultaneous emission of the ArCl(B-X), XeCl(B-X), and nitrogen molecular bands are determined.     

Partially constricted glow discharge in an argon-nitrogen mixture

The characteristics of a glow discharge in Ar:N₂ mixtures with a low content of nitrogen (0.02–1%) are studied experimentally. Most studies were performed at pressures higher than 10 Torr, at which the discharge constriction goes by a jump and the hysteresis effect is well pronounced. It is found that the time during which the discharge switches from the diffuse to the constricted mode (and back) can reach ～1 s. The transition between these modes begins with the development of a constriction at one end of the positive column. Then, the constricted part of the discharge extends toward the other end until it occupies the entire column. The reverse transition occurs in a similar way. By varying the parameters of the electric circuit during the transition, the constriction front can be stopped to form a steady-state partially constricted discharge. It is shown that this type of discharge corresponds to points lying inside the hysteresis loop of the conventional I-V characteristic measured without affecting the discharge during a transition. A comparative analysis of the discharge characteristics in Ar:N₂ mixtures and in pure argon is performed.     

Relationship between linear energy transfer and behavioral toxicity in rats following exposure to protons and heavy particles.

Rats were exposed to protons (155 MeV) or to helium (165 MeV/amu), neon (522 MeV/amu) or argon (670 MeV/amu) particles to evaluate the behavioral toxicity of these types of radiations. Behavioral toxicity was assessed using the conditioned taste aversion paradigm. Exposure to all types of radiation produced dose-dependent increases in the intensity of the acquired taste aversion. However, the intensity of the aversions, measured as the dose that produced a 50% decrease in the intake of the sucrose-conditioned stimulus, did not show significant variation as a function of the linear energy transfer (LET) of the radiation. The results are discussed in terms of the relationship between LET and behavioral toxicity.     

Irradiation of mammalian cultured cells with a collimated heavy-ion microbeam

As the first step for the analysis of the biological effect of heavy charged-particle radiation, we established a method for the irradiation of individual cells with a heavy-ion microbeam apparatus at JAERI-Takasaki. CHO-K1 cells attached on a thin film of an ion track detector, CR-39, were automatically detected under a fluorescence microscope and irradiated individually with an 40Ar13+ ion (11.5 MeV/nucleon, LET 1260 keV/microm) microbeam. Without killing the irradiated cells, trajectories of irradiated ions were visualized as etch pits by treatment of the CR-39 with an alkaline-ethanol solution at 37 degrees C. The exact positions of ion hits were determined by overlaying images of both cells and etch pits. The cells that were irradiated with argon ions showed a reduced growth in postirradiation observations. Moreover, a single hit of an argon ion to the cell nucleus resulted in strong growth inhibition. These results tell us that our verified irradiation method enables us to start a precise study of the effects of high-LET radiation on cells.     

Dark phase effect in the evolution of the positive column of a glow discharge in nitrogen

The characteristics of the initial stage of the formation of the positive column of a glow discharge in nitrogen at reduced pressures are studied experimentally and numerically. A dip in the plasma emission intensity in the initial stage of the discharge (the so-called “dark phase”) is observed experimentally at the positive polarity of the high-voltage electrode (the cathode is grounded). The dark phase is preceded by an ionization wave (IW). When the anode is grounded, neither an IW nor a dip in the discharge emission intensity are observed. A theoretical model capable of describing the discharge development under the actual experimental conditions is constructed. It is shown that the dark phase effect may be caused by the high electron density (above the steady-state one) produced in the gas during the passage of the IW across the discharge gap. This mechanism of the dark phase formation differs from the mechanism proposed earlier to explain a similar effect in noble gases. Additional experiments carried out with pure argon, helium, and helium with a nitrogen admixture have shown that, in the case of a grounded cathode, gas breakdown is also accompanied by the passage of an IW, whereas in the case of a grounded anode, no IW is observed; however, the dark phase is present in both cases. It is shown using computer simulations that, in nitrogen (in contrast to noble gases), the mechanism resulting in the dark phase effect does not operate in the absence of an IW.     

Influence of the chlorine concentration on the radiation efficiency of a XeCl exciplex lamp

The influence of the chlorine concentration on the radiation efficiency of coaxial exciplex lamps (excilamps) excited by a dielectric barrier discharge (DBD) in binary Xe-Cl₂ mixtures at pressures of 240–250 Torr is investigated experimentally and theoretically. The experiments were carried out at Cl₂ concentrations in the range of 0.01–1%. The DBD characteristics were calculated in the framework of a one-dimensional hydrodynamic model at Cl₂ concentrations in the range of 0.1–5%. It is found that the radiation intensities of the emission bands of Xe*₂(172 nm) and XeCl* (308 nm) are comparable when the chlorine concentration in the mixture is in the range of 0.01–0.1%. In this case, in the mixture, the radiation intensity of the Xe*₂ molecule rapidly decreases with increasing Cl₂ concentration and, at a chlorine concentration of ≥0.2%, the radiation of the B → X band of XeCl* molecules with a peak at 308 nm dominates in the discharge radiation. The radiation efficiency of this band reaches its maximum value at chlorine concentrations in the range of 0.4–0.5%. The calculated efficiencies of DBD radiation exceed those obtained experimentally. This is due to limitations of the one-dimensional model, which assumes the discharge to be uniform in the transverse direction, whereas the actual excilamp discharge is highly inhomogeneous. The influence of the chlorine concentration on the properties of the DBD plasma in binary Xe-Cl₂ mixtures is studied numerically. It is shown that an increase in the Cl₂ concentration in the mixture leads to the attachment of electrons to chlorine atoms and a decrease in the electron density and discharge conductivity. As a result, the electric field and the voltage drop across the discharge gap increase, which, in turn, leads to an increase in the average electron energy and the probability of dissociation of Cl₂ molecules and ionization of Xe atoms and Cl₂ molecules. The total energy deposited in the discharge rises with increasing chlorine concentration due to an increase in the power spent on the heating of positive and negative ions. The power dissipated by electrons decreases with increasing chlorine concentration in the working mixture. Recommendations on the choice of the chlorine content in the mixture for reducing the intensity of VUV radiation of the second continuum of the Xe*₂ excimer without a substantial decrease in the excilamp efficiency are formulated.     

Surface Modification of Melamine-Formaldehyde (MF-R) Macroparticles in Complex Plasma

The surface modification of melamine-formaldehyde (MF-R) macroparticles (4.12 ± 0.09 μm in diameter) in dc glow discharges in neon, argon, and an argon–oxygen mixture (90% Ar, 10% O₂) was studied experimentally. The macroparticles were treated in the discharge plasma for 10, 20, 40, and 60 min. The macroparticles were placed in ordered plasma–dust structures and then extracted from them. The results of atomic force microscopy of the surface profile are presented. Quantitative data on destruction of the surface layer and aspects of its modification are discussed. The amount of substance removed from the particle surface for the exposure time was calculated using the fractal analysis method.     

Experimental study of the processes accompanying argon breakdown in a long discharge tube at a reduced pressure

Results are presented from experimental studies of the breakdown stage of a low-pressure discharge (1 and 5 Torr) in a glass tube the length of which (75 cm) is much larger than its diameter (2.8 cm). Breakdowns occurred under the action of positive voltage pulses with an amplitude of up to 9.4 kV and a characteristic rise time of 2–50 μs. The discharge current in the steady-state mode was 10–120 mA. The electrode voltage, discharge current, and radiation from the discharge gap were detected simultaneously. The dynamic breakdown voltage was measured, the prebreakdown ionization wave was recorded, and its velocity was determined. The dependence of the discharge parameters on the time interval between voltage pulses (the socalled “memory effect”) was analyzed. The memory effect manifests itself in a decrease or an increase in the breakdown voltage and a substantial decrease in its statistical scatter. The time interval between pulses in this case can reach 0.5 s. The effect of illumination of the discharge tube with a light source on the breakdown was studied. It is found that the irradiation of the anode region of the tube by radiation with wavelengths of ≤500 nm substantially reduces the dynamic breakdown voltage. Qualitative explanations of the obtained results are offered.     

Effect of a DC external electric field on the properties of a nonuniform microwave discharge in hydrogen at reduced pressures

The effect of a dc external electrical field on the properties of a highly nonuniform electrode microwave discharge in hydrogen at a pressure of 1 Torr was studied using optical emission spectroscopy and selfconsistent two-dimensional simulations. It is shown that the negative voltage applied to the antenna electrode with respect to the grounded chamber increases the discharge radiation intensity, while the positive voltage does not affect the discharge properties. The simulation results agree well with the experimental data.     

Simulation of the ignition of a methane-air mixture by a high-voltage nanosecond discharge

The ignition dynamics of a CH₄: O₂: N₂: Ar = 1: 4: 15: 80 mixture by a high-voltage nanosecond discharge is simulated numerically with allowance for experimental data on the dynamics of the discharge current and discharge electric field. The calculated induction time agrees well with experimental data. It is shown that active particles produced in the discharge at a relatively low deposited energy can reduce the induction time by two orders of magnitude. Comparison of simulation results for mixtures with and without nitrogen shows that addition of nitrogen to the mixture leads to a decrease in the average electron energy in the discharge and gives rise to new mechanisms for accumulation of oxygen atoms due to the excitation of nitrogen electronic states and their subsequent quenching in collisions with oxygen molecules. Acceleration of the discharge-initiated ignition is caused by a faster initiation of chain reactions due to the production of active particles, first of all oxygen atoms, in the discharge.