Effect of the electron drift in a transverse electric field on the width of spark channels in a multichannel sliding discharge

The effect of electron drift on the transverse size of the spark channels in a multichannel sliding discharge on a dielectric surface is considered in a semianalytic approximation. The strength of an electric field transverse to the channel axis is estimated by the mirror image method. The estimate obtained is used to analyze the differential equation for the density of electrons with allowance for their drift from the channel into the surrounding layers. It is shown that the channel expands to a certain steady-state radius at which an increase in the local electron density due to the ionization of atoms is balanced by its decrease due to the electron drift from the surface channel layer into the surrounding layers. Numerical estimates are carried out for the conditions of earlier experiments with discharges in He, Ne, Ar, and Xe. The analysis applies to the initial nanosecond stage of the spark development, when the hydrodynamic expansion of the channels is still insignificant.     

Suprathermal electrons in a vacuum spark discharge

Results of experiments on the detection of suprathermal electron beams in the plasma of a highcurrent low-inductance vacuum spark by means of space-resolved spectral X-ray polarimetry are presented. It is shown that the observed polarization of bremsstrahlung may be caused by an ~100-keV electron beam propagating along the discharge axis from the pinching region toward the anode. The influence of the discharge initiation conditions on the parameters of the generated electron beams is analyzed.     

Pulsed submicrosecond multichannel sliding discharges of opposite polarities: Filling of the discharge gap with spark channels

Results are presented from measurements of the discharge current and the factor of the discharge gap filling with spark channels during pulsed sliding discharges of opposite polarities in Ne, Ar, and Xe on an aluminum oxide ceramic surface. The measurements were performed in the regime of single pulses of submi-crosecond duration at discharge voltages of 0?C12 kV with two discharge chambers with different thicknesses of the ceramic plate (0.4 and 0.17 cm) and different electrode gap lengths (4 and 10.3 cm) at gas pressures of 30 and 100 kPa. The results obtained for discharges of opposite polarities are compared with one another, and common features of discharges in three gases are revealed. It is shown that the filling of the discharge gap with spark channels in the gases under study is more efficient in the case of the positive polarity of the discharge voltage, except Xe at a pressure of 100 kPa in the electrode gap of length 10.3 cm. The quasi-homogeneous regime of discharge in each of the three gases is attained easier at lower gas pressures. Comparison of the data on the filling factors of the discharge gap and the peak currents of opposite-polarity discharges for each gas at a given pressure indicates that the higher the discharge current, the more densely the discharge gap is filled with spark channels.     

X-ray spatial distribution of a low-inductance vacuum spark discharge

The paper presents the results of X-ray source spatial distribution measurements of a low-inductance vacuum spark discharge based on the analysis of helium-like ions spectral lines structure. The intensities of spectral lines of FeXXV ions and dielectronic satellites with spatial resolution were measured. The polarization of satellite line 1s2p ^{2 2} D _5/2-1s ²2p ² P _3/2 was found. The obtained dependence of maximum plasma parameters during micropinch events from initial plasma line density in discharge corresponds to the radiative collapse model.     

Quantification of monocarboxylic acids from a spark discharge synthesis

Summary A suite of sixteen monocarboxylic acids having carbon numbers 2 to 7, formed by the Miller-Urey spark discharge process, were identified and quantified by gas chromatography and mass fragmentography using a deuterium spiking technique. The molar concentration and isomeric distribution of these laboratory synthesized monocarboxylic acids are compared to those previously reported for the Murchison meteorite. They show similar trends, namely, decreasing molar concentration with increasing molecular weight, and, the ratio of normal/branched isomers tend toward smaller with increasing carbon numbers.     

Gasdynamic form of the sliding discharge

The development of a high-voltage discharge above the water surface is considered. The peculiarity of the observed type of discharge is explained by the process of water evaporation.     

Peculiarities of the spark discharge formation at a limiting ballast resistor

The results of the experimental studies of a spark discharge in the point-to-plane geometry in argon at a discharge current limited by a large ballast resistor are presented. A microstructure of a low-current spark discharge representing a formation consisting of a set of micron-size current filaments inside a spark current channel has been revealed.     

Specific features of an electric discharge operating between an electrolytic anode and a metal cathode

Results are presented from experimental studies of a high-current electric discharge operating between an St45 steel cathode and a service water anode in a wide range of air pressures. Peculiarities of discharge ignition and specific features of cathode and anode spots were revealed. The behavior of the current density on a service water anode was investigated for the first time. Comparison of the current densities j on the steel cathode and service water anode shows that, in the parameter range under study, Hehl’s law is not satisfied on the water anode. The two-dimensional distribution of the potential inside and on the surface of the service water anode was measured.     

Diffuse and constricted modes of a dc discharge in neon: Simulation of the hysteresis transition

Results are presented from theoretical studies of high-pressure (～100 Torr) dc discharges in neon. The diffuse and constricted discharge modes are studied using a model including the equation of balance for charged and excited particles, heat conduction equations for the neutral gas and plasma electrons, and Poisson’s equation for the radial electric field at a fixed total discharge current. A specific feature of the constricted mode in the investigated range of low fields and high degrees of ionization is that the excitation and ionization rates in the center of the discharge tube and at the periphery differ by several orders of magnitude. This implies that, in the constricted mode, the region where the electron energy distribution function is Maxwellian due to electron-electron collisions may adjoin the region (beyond the constriction zone) where the high-energy part of the distribution function is depleted. The hysteresis transition between the diffuse and constricted modes is analyzed. A transition from the constricted to the diffuse mode can be regarded as a manifestation of the nonlocal character of the formation of the electron distribution function, specifically, the diffusion of high-energy electrons capable of producing gas ionization from the central (constricted) region toward the periphery. The nonlocal formation of the distribution function is described by a nonlocal kinetic equation accounting for electron-electron collisions and electron transport along the radius of the discharge tube. Since only high-energy electrons produce gas ionization, the effect of the nonlocal formation of the electron distribution function is taken into account by introducing the effective temperature of the high-energy part of the distribution function and solving the equation for the radial profile of the high-energy part of the distribution function. This approach allows one to approximately take into account the nonlocal character of the electron distribution function without substantial expenditure of computer resources. The nonlocal model makes it possible to numerically simulate the hysteresis transition between the diffuse and constricted modes, which is impossible in the local approximation.     

Specific features of the radial distributions of plasma parameters in the initial segment of a supersonic jet generated by a pulsed capillary discharge

Results are presented from spectroscopic studies of the initial segment of a supersonic plasma jet generated by a pulsed capillary discharge with an ablative carbon-containing polymer wall. Specific features of the spatial distributions of the electron density and intensities of spectral components caused, in particular, by the high electron temperature in the central zone, much exceeding the normal temperature, as well as by the high nonisobaricity of the initial segment of the supersonic jet, are revealed. Measurements of the radiative properties of the hot jet core (the intensity and profile of the H_α and H_β Balmer lines and the relative intensities of C II lines) with high temporal (1–50 μs) and spatial (30–50 μm) resolutions made it possible to determine general features of the pressure and temperature distributions near the central shock. The presence of molecular components exhibiting their emission properties at the periphery of the plasma jet allowed the authors to estimate the parameters of the plasma in the jet region where “detached” shock waves form.     

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.     

On the relation between the index of the interelectrode gap filling with spark channels and the current of a pulsed multichannel sliding discharge in Ne,Ar, and Xe

Experimental results and model concepts concerning the relation between the index K of the interelectrode gap filling with spark channels and the peak current I _peak of a single-pulse submicrosecond multichannel complete sliding discharge on an alumina ceramic surface are discussed. The spatial structure of an incomplete discharge at the threshold for the surface spark breakdown of gas is considered. The experiments were performed with three gases, Ne, Ar, and Xe, at pressures of 30 and 100 kPa and opposite polarities of the discharge voltage, with two discharge chambers differing in the geometry of the discharge gap and the thickness of the ceramic plate. It is shown that, although the structure of the incomplete discharge at the threshold for spark breakdown varies from diffuse homogeneous to pronounced filamentary, the dependence \(K\left( {\sqrt[6]{{I_{peak} }}} \right)\) for a complete discharge is close to linear and can be qualitatively explained by the earlier proposed semiempirical model of the time evolution of the structure of a multichannel discharge. In particular, the estimated steepness of the dependence \(K\left( {\sqrt[6]{{I_{peak} }}} \right)\) agrees best with the experimental results when the local density of free electrons at the threshold for spark breakdown is 10¹⁶ cm^?3 or higher.     

Positive column of a glow discharge in neon with charged dust grains (a review)

The effect of charged micron-size dust grains (microparticles) on the electric parameters of the positive column of a low-pressure dc glow discharge in neon has been studied experimentally and numerically. Numerical analysis is carried out in the diffusion-drift approximation with allowance for the interaction of dust grains with metastable neon atoms. In a discharge with a dust grain cloud, the longitudinal electric field increases. As the number density of dust grains in an axisymmetric cylindrical dust cloud rises, the growth of the electric field saturates. It is shown that the contribution of metastable atoms to ionization is higher in a discharge with dust grains, in spite of the quenching of metastable atoms on dust grains. The processes of charging of dust grains and the dust cloud are considered. As the number density of dust grains rises, their charge decreases, while the space charge of the dust cloud increases. The results obtained can be used in plasma technologies involving microparticles.     

Effect of natural convection on the current-voltage characteristic of a DC discharge in neon at intermediate pressures

The parameters of the positive column of a glow discharge in neon are calculated with allowance for the induced hydrodynamic motion. It is shown that natural convection in the pressure range of ∼0.1 atm significantly affects the profiles of the parameters of the positive column and its current-voltage characteristic. The convection arising at large deposited energies improves heat removal, due to which the temperature in the central region of the discharge becomes lower than that calculated from the heat conduction equation. As a result, the current-voltage characteristic is shifted. With allowance for convection, the current-voltage characteristic changes at currents much lower than the critical current at which a transition into the constricted state is observed. This change is uniquely related to the Rayleigh number in the discharge. Thus, a simplified analysis of thermal conduction and diffusion, even with detailed account of kinetic processes occurring in the positive column, does not allow one to accurately calculate the current-voltage characteristic and other discharge parameters at intermediate gas pressures.     

Mechanism for electric breakdown in a chemically nonequilibrium system and the influence of the chain oxidation reaction in an H2-air mixture on the breakdown threshold

A mathematical model is developed and a numerical analysis is performed for an electric breakdown in a hydrogen-air mixture with a low concentration of H₂. It is shown that, at sufficiently low pressures p<10^?2 atm, a small molecular-hydrogen additive (η=5×10^?5–5×10^?3) decreases the reduced field of an electric breakdown in air by a factor of more than 2 because of the appearance of an additional detachment process associated with the chain hydrogen-oxidation reaction. Detailed calculations are performed for the mean number density of negative oxygen ions [O ₂ ^? ]=10³ cm^?3 and the hydrogen concentration in air [H₂]=0.5, 0.05, and 0.005%. It is found that, for [H₂]=0.005%, the breakdown can develop under the action of a geoelectric field of 1.3 V/cm at p?10^?4 atm.     

Emission from a transverse volume discharge in neon with small admixtures of water vapor and air

Emission from the plasma of a pulsed discharge in neon with small admixtures of water vapor and air in the wavelength range 210–620 nm is investigated. A transverse volume discharge with spark UV preionization is ignited in neon at a pressure of 100–200 kPa and charging voltage of U _ch≤20 kV. It is shown that the discharge acts as a source of pulsed UV radiation on OH(A-X) (λ=308–314 nm) and NO(B-X) (247.8 nm) transitions, which is of interest for the use in an ecologically safe lamp based on the mixtures of neon with water vapor and air. In the visible spectral region, plasma emission consists of the NeI (3s, 3s′-3p, 3p′) band and H_β 486.1-nm spectral line. On the short-wavelength side of the spectrum, a broadband emission (the third continuum of neon) is observed, whose intensity increases with increasing neon pressure and decreasing emission wavelength.     

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.     

Raman amplification of laser pulses near the threshold for plasma wave breaking

Equations are derived for the amplitudes of counter-propagating laser pulses near the threshold for plasma wave breaking, which allow one to describe laser pulses with durations on the order of the plasma oscillation period. In the quasi-monochromatic approximation, they take the form of conventional threewave equations with an additional nonlinearity for the plasma wave. The amplitudes of the amplified laser pulses estimated using these equations agree with results obtained by solving the complete equations. It is shown that Raman amplification of a weak quasi-monochromatic signal (plasma noise) in rarified plasma is significantly suppressed. At the same time, according to numerical simulations, the amplification of laser pulses with durations on the order of the plasma oscillation period is suppressed insignificantly. This result opens new prospects in the application of Raman compression of laser pulses without additional frequency modulation.     

Plasma convection near the threshold for MHD instability in nonparaxial magnetic confinement systems

Using a highly nonparaxial magnetic confinement system with an internal levitated ring as an example, it is shown that, in a plasma near the threshold for ideal MHD instability, the external heating and the original local dissipative processes may give rise to and maintain self-consistent nonlinear MHD convection, which leads to an essentially nonlocal, enhanced heat transport. A closed set of equations is derived that makes it possible to describe such convective processes in a weakly dissipative plasma with β～1. Numerical simulations carried out with a specially devised computer code demonstrate that the quasisteady regime of nonlinear convection actually exists and that the marginally stable profile of the plasma pressure is maintained. A large amount of data on the structure of the nascent convective flows is obtained and analyzed.     

STDP rule endowed with the BCM sliding threshold accounts for hippocampal heterosynaptic plasticity

We have combined the nearest neighbour additive spike-timing-dependent plasticity (STDP) rule with the Bienenstock, Cooper and Munro (BCM) sliding modification threshold in a computational model of heterosynaptic plasticity in the hippocampal dentate gyrus. As a result we can reproduce (1) homosynaptic long-term potentiation of the tetanized input, and (2) heterosynaptic long-term depression of the untetanized input, as observed in real experiments. Action Editor: Nicolas Brunel