Penning plasma based simultaneous light emission source of visible and VUV lights

In this paper, a laboratory-based penning plasma discharge source is reported which has been developed in two anode configurations and is able to produce visible and VUV lights simultaneously. The developed source has simultaneous diagnostics facility using Langmuir probe and optical emission spectroscopy. The two anode configurations, namely, double ring and rectangular configurations, have been studied and compared for optimum use of the geometry for efficient light emissions and recording. The plasma is produced using helium gas and admixture of three noble gases including helium, neon, and argon. The source is capable to produce eight spectral lines for pure helium in the VUV range from 20 to 60 nm and total 24 spectral lines covering the wavelength range 20–106 nm for the admixture of gases. The large range of VUV lines is generated from gaseous admixture rather from the sputtered materials. The recorded spectrum shows that the plasma light radiations in both visible and VUV range are larger in double ring configuration than that of the rectangular configurations at the same discharge operating conditions. To clearly understand the difference, the imaging of the discharge using ICCD camera and particle-in-cell simulation using VORPAL have also been carried out. The effect of ion diffusion, metastable collision with the anode wall and the nonlinear effects are correlated to explain the results.     

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.     

Dynamics of the spatial structure of pulsed discharges in dense gases in point cathode−plane anode gaps and their erosion effect on the plane electrode surface

The dynamics of the spatial structure of the plasma of pulsed discharges in air and nitrogen in a nonuniform electric field and their erosion effect on the plane anode surface were studied experimentally. It is established that, at a nanosecond front of the voltage pulse, a diffuse discharge forms in the point cathode–plane anode gap due to the ionization wave propagating from the cathode. As the gap length decreases, the diffuse discharge transforms into a spark. A bright spot on the anode appears during the diffuse discharge, while the spark channel forms in the later discharge stage. The microstructure of autographs of anode spots and spark channels in discharges with durations of several nanoseconds is revealed. The autographs consist of up to 100 and more microcraters 5–100 μm in diameter. It is shown that, due to the short duration of the voltage pulse, a diffuse discharge can be implemented, several pulses of which do not produce appreciable erosion on the plane anode or the soot coating deposited on it.     

Investigation of the electron energy distribution function in hollow-cathode glow discharges in nitrogen and oxygen

The mechanism for the formation of the inverse electron distribution function is proposed and realized experimentally in a nitrogen plasma of a hollow-cathode glow discharge. It is shown theoretically and experimentally that, for a broad range of the parameters of an N₂ discharge, it is possible to form a significant dip in the profile of the electron distribution function in the energy range ε=2–4 eV and, accordingly, to produce the inverse distribution with df(ε)/d?>0. The formation of a dip is associated with both the vibrational excitation of N₂ molecules and the characteristic features of a hollow-cathode glow discharge. In such a discharge, the applied voltage drops preferentially across a narrow cathode sheath. In the main discharge region, the electric field E is weak (E<0.1 V/cm at a pressure of about p～0.1 torr) and does not heat the discharge plasma. The gas is ionized and the ionization-produced electrons are heated by a beam of fast electrons (with an energy of about 400 eV) emitted from the cathode. A high-energy electron beam plays an important role in the formation of a dip in the profile of the electron distribution function in the energy range in which the cross section for the vibrational excitation of nitrogen molecules is maximum. A plasma with an inverted electron distribution function can be used to create a population inversion in which more impurity molecules and atoms will exist in electronically excited states.     

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.     

Breakdown in helium in high-voltage open discharge with subnanosecond current front rise

Investigations of high-voltage open discharge in helium have shown a possibility of generation of current pulses with subnanosecond front rise, due to ultra-fast breakdown development. The open discharge is ignited between two planar cathodes with mesh anode in the middle between them. For gas pressure 6 Torr and 20 kV applied voltage, the rate of current rise reaches 500 A/(cm² ns) for current density 200 A/cm² and more. The time of breakdown development was measured for different helium pressures and a kinetic model of breakdown in open discharge is presented, based on elementary reactions for electrons, ions and fast atoms. The model also includes various cathode emission processes due to cathode bombardment by ions, fast atoms, electrons and photons of resonant radiation with Doppler shift of frequency. It is shown, that the dominating emission processes depend on the evolution of the discharge voltage during the breakdown. In the simulations, two cases of voltage behavior were considered: (i) the voltage is kept constant during the breakdown; (ii) the voltage is reduced with the growth of current. For the first case, the exponentially growing current is maintained due to photoemission by the resonant photons with Doppler-shifted frequency. For the second case, the dominating factor of current growth is the secondary electron emission. In both cases, the subnanosecond rise of discharge current was obtained. Also the effect of gas pressure on breakdown development was considered. It was found that for 20 Torr gas pressure the time of current rise decreases to 0.1 ns, which is in agreement with experimental data.     

Prebreakdown phase of an interelectrode discharge in wate

Results are presented from experimental studies of the prebreakdown phase of an electric discharge between point (anode) and plane (cathode) electrodes immersed in water with different initial conductivity. When a high-voltage pulse is applied, the induced conductivity is detected in the discharge gap. Its value is one order of magnitude higher than the initial conductivity. It is shown that the induced conductivity increases almost linearly with the initial conductivity. The induced conductivity correlates with the UV emission from the cathode surface. A qualitative analysis of the experimental results is performed.     

Experimental Study of Heating of a Liquid Cathode and Transfer of Its Components into the Gas Phase under the Action of a DC Discharge

An atmospheric-pressure dc discharge in air (i = 10–50 mA) with metal and liquid electrolyte electrodes was studied experimentally. An aqueous solution of sodium chloride (0.5 mol/L) was used as the cathode or anode. The electric field strength in the plasma and the cathode (anode) voltage drops were obtained from the measured dependences of the discharge voltage on the electrode gap length. The gas temperature was deduced from the spectral distribution of nitrogen emission in the band N₂(C³Π _u → B³Π _g , 0–2). The time dependences of the temperatures of the liquid electrolyte electrodes during the discharge and in its afterglow, as well as the evaporation rate of the solution, were determined experimentally. The contributions of ion bombardment and heat flux from the plasma to the heating of the liquid electrode and transfer of solvent (water) into the gas phase are discussed using the experimental data obtained.     

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.     

Excitation of RF Oscillations in a Discharge with Negative Differential Conductivity

The excitation of oscillations in a discharge with negative differential conductivity is studied experimentally. The possibility is demonstrated of amplifying oscillations in the cathode dark space at frequencies close to the electron plasma frequency of the positive-column plasma. The phase velocities of waves at these frequencies are determined. When the waves pass from the cathode dark space to the discharge positive column, their phase velocities decrease; the closer the frequency is to the electron plasma frequency, the more pronounced the decrease in the phase velocity. As the intensity of oscillations increases, the discharge becomes non-steady-state. This is confirmed by the time evolution of the current-voltage characteristic. The shape of the current-voltage characteristic, its splitting, and the rate at which it varies depend on the input RF power. The decrease in the cathode dark space indicates that the ionization processes in the discharge are strongly influenced by electron plasma oscillations excited due to the collective interaction of the electron beam formed at the cathode with the discharge plasma. It is these processes that determine the maximum values of both the frequency of the excited oscillations and the power that can be withdrawn from the discharge.     

Simulations of a surface glow discharge in a supersonic gas flow in the presence of external ionization

Results of two-dimensional hydrodynamic simulations of a surface glow discharge operating at pressures of 0.2–0.5 Torr in a nitrogen flow propagating with a velocity of 1000 m/s in the presence of external ionization are presented. The effect of the external ionization rate on discharge operation is analyzed. The current-voltage characteristics of the discharge are calculated for different intensities of external ionization in both the presence and absence of secondary electron emission from the cathode. The discharge structure and plasma parameters in the vicinity of the loaded electrode are considered. It is shown that, when the discharge operates at the expense of secondary emission from the cathode, the discharge current and cathode sheath configuration are insensitive to external ionization. It is also demonstrated that, even at a high rate of external ionization, the discharge operates due to secondary emission from the cathode.     

Double layer in a cylindrical hollow-cathode discharge

A dc cylindrical coaxial glow discharge with an inner grid anode has been studied. The region between the two electrodes is seen dark, while a brightly glowing region forms inside the grid anode up to the center. The current-voltage characteristic of a dc cylindrical glow discharge in nitrogen is similar to that of a normal glow discharge, while the normal glow discharge voltage decreases with increasing pressure. The minimum plasma potentials are observed in the hollow cathode region due to the accumulation of electrons at the back of the grid anode. At the center, some of the passed electrons are converged, so their potential is decreased. These electrons have a sufficient time to be redistributed to form one group with a Maxwellian electron energy distribution function. The electron temperature measured by electric probes varies from 1.6 to 3.6 eV, while the plasma density varies from 3.9 × 10¹⁶ to 7 × 10¹³ m⁻³, depending on the discharge current and probe position. The plasma density increases as the electrons move radially from the grid toward the central region, while their temperature decreases.     

Diel methane emission patterns from Scirpus lacustris and Phragmites australis

In mature Phragmites australis and Scirpus lacustris vegetated sediment methane was emitted almost exclusively by plant-mediated transport, whereas in unvegetated, but otherwise identical sediment, methane was emitted almost exclusively by ebullition. Diel variations in methane emission, with highest emission rates at daytime and emission peaks following sunrise, were demonstrated for Phragmites and Scirpus. The diel difference and magnitude of the emission peaks were much smaller for Scirpus than for Phragmites. In contrast to Phragmites, methane concentrations within Scirpus stems did not change significantly over the diel period. These patterns are consistent with a two-way transport mechanism for Phragmites (convective at daytime and diffusive at night-time) and an all day diffusive mechanism for Scirpus. The patterns could not be accounted for by diel variation in air and sediment temperature, plant transpiration, or photosynthetically coupled methane production. Comparison of the experimentally derived ratio of methane emission in helium and nitrogen under light and dark conditions with the theoretical derived ratio (calculated according to the kinetic theory of gases) confirmed the exploitation of the different transport mechanism for Phragmites and Scirpus. Methane emission from Phragmites correlated significantly with incident light, which probably drove the pressure differential associated with thermally induced convection. Decrease of the radial resistance of Scirpus stems for methane transport under light compared to dark conditions, in combination with morphological characteristics of the plant species, suggested that stomatal aperture, regulated by light, controls methane emission from Scirpus. Diel variation in bubble emission from the non-vegetated sediment coincided with sediment temperature changes. The results have important implications for sampling and scaling strategies for estimating methane emission from wetlands.     

Determination of the cathode and anode voltage drops in high power low-pressure amalgam lamps

For the first time, cathode and anode drops of powerful low-pressure amalgam lamps were measured. The lamp discharge current is 3.2 A, discharge current frequency is 43 kHz, linear electric power is 2.4 W/cm. The method of determination of a cathode drop is based on the change of a lamp operating voltage at variation of the electrode filament current at constant discharge current. The total (cathode plus anode) drop of voltage was measured by other, independent ways. The maximum cathode fall is 10.8 V; the anode fall corresponding to the maximal cathode fall is 2.4 V. It is shown that in powerful low pressure amalgam lamps the anode fall makes a considerable contribution (in certain cases, the basic one) to heating of electrodes. Therefore, the anode fall cannot be neglected, at design an electrode and ballast of amalgam lamps with operating discharge current frequency of tens of kHz.     

Some features of horizontally oriented low-current electric arc in air

The properties of an electric arc operating in open air at currents of lower than 1 A were studied experimentally. The rod cathode was oriented horizontally. Cylindrical rods and plane plates either installed strictly vertically in front of the cathode end or tilted at a certain angle served as the anode. It is shown that, with such an electrode configuration, it is possible to form a discharge channel much longer than the electrode gap length. Regimes of regular oscillations are revealed, and conditions for their appearance are established. The electric field strength in the arc column and the electron temperature near the anode are calculated.     

Influence of a transverse magnetic field on the characteristics of a dc gas discharge

The influence of a transverse magnetic field and the working-gas pressure on the rotation frequency of the current channel, as well as on the electric field in the positive column and the cathode voltage drop in a dc gas discharge, was studied experimentally. The working gases were pure hydrogen and hydrogen-methane, hydrogen-argon, and hydrogen-argon-methane mixtures. It is shown that a transverse (with respect to the discharge current) magnetic field stabilizes a normal glow discharge against a transition to an arc discharge at specific absorbed powers above 300 W/cm³. The cathode voltage drop and the electric field in the positive column are measured. It is shown that the electric field does not depend on the magnetic field strength, whereas the cathode voltage drop increases with increasing magnetic field. It is found that the rotation frequency of the current channel is a complicated function of the discharge parameters and attains 400 Hz.     

Amplitude−temporal characteristics of a supershort avalanche electron beam generated during subnanosecond breakdown in air and nitrogen

The amplitude?temporal characteristics of a supershort avalanche electron beam (SAEB) with an amplitude of up to 100 A, as well as of the breakdown voltage and discharge current, are studied experimentally with a picosecond time resolution. The waveforms of discharge and SAEB currents are synchronized with those of the voltage pulses. It is shown that the amplitude?temporal characteristics of the SAEB depend on the gap length and the designs of the gas diode and cathode. The mechanism for the generation of runaway electron beams in atmospheric-pressure gases is analyzed on the basis of the obtained experimental data.     

Properties of a spherical stratified gas discharge

Experimental data are obtained on the conditions for the stratification of spherical direct-current discharges and on the properties of the strata. The experiments were aimed at studying discharges in both molecular and inert gases at low pressures and were carried out with a steel chamber whose wall served as a cathode. An anode was placed at the center of the chamber. The discharge stratification was observed to be especially pronounced in media containing a small admixture of the vapor of a high-molecular substance (e.g., acetone). In discharges in pure inert gases, no strata were observed. The current-voltage characteristics of discharges at different pressures were obtained. The discharges were found to be unsteady: current pulses with a duration of about 1 microsecond and a characteristic repetition rate of about 1 kHz were detected against a steady current background and were found to correlate with the pulses of the integral emission from the discharge. The radius of each of the strata was determined as a function of its number and of the gas pressure and discharge current. The radial profiles of the time-averaged floating potential were measured in experiments with stratified discharges and with uniform discharges in argon.     

Dynamics of the optical emission from a high-voltage diffuse discharge in a rod-plane electrode system in atmospheric-pressure air

Results are presented from experimental investigations of the dynamics of optical emission from a nanosecond diffuse discharge in a rod-plane electrode system. A study was made of discharges in a 10-cm-long interelectrode gap in atmospheric-pressure air (the cathode being a 1-cm-diameter rod with a bullet-shaped end). The voltage across the discharge gap was 220 kV and the voltage pulse duration was 180 ns, the voltage rise time being 10 ns. In experiments, the discharges were observed to evolve through two stages: the bridging stage and the conduction stage. The bridging stage begins with intense optical emission from the cathode region, the onset of the emission being delayed with respect to the beginning of the voltage pulse. Simultaneously with the onset of optical emission, a displacement current corresponding to the motion of charged particles begins to be generated in the cathode region. The duration of this current corresponds to the time the emission front takes to bridge the gap. As the emission front reaches the anode region, the current increases abruptly, indicating the beginning of the conduction stage. It was found that the time delay of optical emission relative to the beginning of the voltage pulse largely governs the discharge parameters: as the time delay becomes longer, the emission front velocity in the bridging stage increases from 0.6 to 1.5 cm/ns, the probability of realizing a multichannel structure of the discharge becomes higher, and the discharge current and the intensity of X-ray emission from the discharge grow.     

Abnormal and subnormal regimes of a spherical glow discharge in the drift-diffusion approximation

A spherical glow discharge with a pointlike anode is considered in a self-consistent drift-diffusion approximation. The model includes the time-dependent continuity equations for ions and electrons in the drift-diffusion approximation and Poisson’s equation for the radial electric field. In finding steady-state distributions, Ohm’s law is used to relate the discharge voltage and discharge current. Steady-state distributions of the plasma parameters across the discharge gap, current-voltage characteristics, and cathode characteristics for an abnormal spherical discharge in molecular nitrogen are obtained. In a subnormal glow-discharge regime, oscillations in the conduction current, potential, and other discharge parameters are revealed. Similar regimes are also observed in conventional discharges in tubes.