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.     

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.     

Low-pressure glow discharge in a Xenon/Chlorine mixture

The spatial, electrical, and optical characteristics of a transverse glow discharge and a volume discharge with a spherical anode and plane cathode in low-pressure Xe/Cl₂ mixtures are studied. It is shown that the transverse glow discharge in mixtures with a low chlorine content occupies most of the interelectrode gap and exists in the form of strata. As the total pressure (P≥300 Pa) and the partial chlorine pressure (P(Cl₂)≥80 Pa) increase, a solitary plasma domain with a volume of 1–2 cm³ forms in the discharge gap. It acts as a selective source of UV radiation in the XeCl(D-X) 236-nm, Cl₂ (D′-A′) 257-nm, and XeCl(B-X) 308-nm bands. In certain Xe/Cl₂ mixtures, plasma self-oscillations in the frequency range 1–100 kHz are observed. The current of a low-pressure volume discharge with a spherical anode and plane cathode and the emission from it have both a dc and an ac component. The pressure and composition of the working mixture, as well as the average current of the volume discharge are optimized to attain the maximum emission intensity of the XeCl(D,B-X) bands. Low-pressure volume discharges in xenon/chlorine mixtures can be used as active media in low-pressure large-aperture planar or cylindrical excimer-halogen lamps emitting modulated or repetitive pulsed UV radiation.     

Experimental study of a low-pressure glow discharge in air in large-diameter discharge tubes: I. conditions for the normal regime of a glow discharge

The initiation and characteristics of a low-pressure glow discharge in air in large-diameter discharge tubes are studied. A deviation from the Paschen law is observed: the breakdown curves U _dc(pL) shift toward the higher values of U _dc and pL as the interelectrode distance L increases. It is shown that the normal regime of a glow discharge is accompanied by gas ionization in the anode sheath. This takes place only for pL values lying to the right of the inflection point in the breakdown curve. The cathode-sheath characteristics in the normal and abnormal regimes of an air discharge for a duralumin cathode are determined. The axial profiles of the ion density, electron temperature, and plasma potential, as well as the anode voltage drop, are measured at various air pressures.     

H<Superscript>−</Superscript> ion density in the plasma of a low-voltage cesium-hydrogen discharge as a function of the cathode emission current

It was shown theoretically that the increase in the cathode emission current in a low-voltage cesium-hydrogen discharge to ≈10 A/cm² leads to an increase in the electron temperature in the anode plasma to T _e ≥ 1 eV. In this regime, the rate constant for the production of H^? ions via dissociative electron attachment to vibrationally excited H₂ molecules is close to its maximum value and the density of H^? ions is maximal (about 10¹³ cm^?3) in the anode plasma.     

Effect of the anode material on the X-ray spectrum of micropinch discharge plasma

The effect of the elemental composition of the anode material on the parameters and X-ray spectrum of micropinch discharge plasma have been studied using a low-inductance vacuum spark device. It is shown that the plasma electron temperature T _e and intensity of hard X-ray emission increase with increasing nuclear charge number Z of the anode material of the discharge system.     

Characteristic properties of the frame-antenna-produced RF discharge evolution in the Uragan-3M torsatron

In the ? = 3 Uragan-3M torsatron, hydrogen plasma is produced and heated by RF fields in the Alfvén range of frequencies (ω ? ω _ci ). To this end, a frame antenna with a broad spectrum of generated parallel wavenumbers is used. The RF discharge evolution is studied experimentally at different values of the RF power fed to the antenna (the anode voltage of the oscillator and the antenna current) and the initial pressure of the fueling gas. It is shown that, depending on the antenna current and hydrogen pressure, the discharge can operate in two regimes differing in the plasma density, temperature, and particle loss. The change in the discharge regime with increasing anode voltage is steplike in character. The particular values of the anode voltage and pressure at which the change occurs are affected by RF preionization or breakdown stabilization by a microwave discharge. The obtained results will be used in future experiments to choose the optimal regimes of the frame-antenna-produced RF discharge as a target for the production and heating of a denser plasma by another, shorter wavelength three-half-turn antenna.     

Self-sustained low pressure glow discharge with a hollow cathode at currents of tens of amperes

Self-sustained glow discharge with a hollow cathode was studied at high discharge currents (up to 30 A). Using a grid analyzer placed on the side flange of the hollow cathode, the ion and electron currents flowing in the cathode sheath were measured. At a discharge current of 30 A, pressure of 0.2?C2 Pa, and plasma density of 10¹¹ cm^?3, the coefficient of secondary ion-electron emission ?? calculated from the experimental data is found to be 0.1?C0.15. The dependences of the plasma parameters on the area of the small anode placed inside the larger hollow cathode are determined.     

Controlling the characteristics of a repetitive volume discharge in CFC-12 and its mixtures with argon

The parameters of a repetitive volume discharge in CF₂Cl₂ (CFC-12) and its mixtures with argon at pressures of P(CF₂Cl₂)≤0.4 kPa and P(Ar)≤1.2 kPa are studied. The discharge was ignited in an electrode system consisting of a spherical anode and a plane cathode by applying a dc voltage U_ch≤1 kV to the anode. The electrical and optical characteristics of a volume discharge (such as the current-voltage characteristics; the plasma emission spectra; and the waveforms of the discharge voltage, the discharge current, and the total intensity of plasma emission) are investigated. It is found that, by shunting the discharge gap with a pulsed capacitor with a capacitance of C₀≤3.5 nF, it is possible to control the amplitude and duration of the discharge current pulses, as well as the characteristics of the pulsed plasma emission. The increase in the capacitance C₀ from 20 to 3500 pF leads to a significant increase in the amplitude and duration of the discharge current pulses, whereas the pulse repetition rate decreases from 70 to 3 kHz. The glow discharge exists in the form of a domain with a height of up to 3 cm and diameter of 0.5–3.0 cm. The results obtained can be used to design an untriggered repetitive germicidal lamp emitting in the Cl₂(257/200 nm) and ArCl (175 nm) molecular bands and to develop plasmachemical methods for depositing amorphous fluorocarbon and chlorocarbon films.     

Spatiotemporal evolution of the current and the integral and spectral emission characteristics of a negative corona in nitrogen during its transformation into a spark

Results are presented from experimental studies of the conversion of a steady-state negative corona into a spark. It is found that a spark in a negative corona in nitrogen and air is formed in the absence of fast primary streamers. It is shown that, in atmospheric-pressure nitrogen, the conversion of a corona into a spark begins with the propagation of a plasma channel (secondary streamer) from the point electrode (cathode) to the plane electrode (anode). In contrast, the plasma channel in air originates near the plane electrode and then propagates towards the point electrode. The propagation velocity of the secondary streamer is very low, V=10³–10⁴ cm/s. Two possible scenarios of the formation of the spark channel in a negative corona in nitrogen are described on the basis of the concept of a contracted volume glow discharge. Results are presented from time-resolved spectral measurements of plasma emission from different regions of the corona during its transformation into a spark.     

Diagnostics of a nonequilibrium nitrogen plasma from the emission spectra of the second positive system of N2

A method is proposed for determining the electron density N _e and the electric field E in the non-equilibrium nitrogen plasma of a low-pressure discharge from the spectra of the second positive system of N₂. The method is based on measuring the specific energy deposition in the plasma and the distribution of nitrogen molecules over the vibrational levels of the C ³Π _u state, as well as on modeling this distribution for a given energy deposition. The fitting parameters of the model are the values of N _e and E. A kinetic model of the processes governing the steady-state density of the C ³Π _u nitrogen molecules is developed. The testing of this method showed it to be quite reliable. The method is of particular interest for diagnosing electrodeless discharges and provides detailed information on the processes occurring in the discharge plasma. Preliminary data are obtained on the plasma parameters in a cavity microwave discharge and an electrode microwave discharge. In particular, it is found that the electric field in an electrode microwave discharge in nitrogen is lower than that in a hydrogen discharge. This effect is shown to be produced by stepwise and associative processes with the participation of excited particles in nitrogen.     

Discharge dynamics and the production of active particles in a cathode-directed streamer

The emission spectroscopy technique is used to analyze a cathode-directed streamer discharge in air at atmospheric pressure in point-plane geometry at interelectrode distances of up to 100 mm and a high-voltage pulse amplitude of 18 kV. The densities of molecules in the N₂(C ³Π_u, v=0), N ₂ ⁺ (B ²Σ _u ⁺ , v=0) and NO(A ²Σ⁺, v=0) states are determined, and the reduced electric field in the streamer head is estimated. It is shown that the increase in the average electric field in the discharge gap substantially intensifies the production of active particles in the discharge plasma and makes the plasma more homogeneous. This effect is only related to the increase in the fraction of regions with a high electric field in the discharge gap and, as a result, the reduction of the discharge energy losses via rapidly thermalized degrees of freedom. The active particles are only produced in the streamer head, including the case in which the interelectrode gap is bridged by the streamer channel.     

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.     

Laser-optical measurements of the velocities of the plasma jets formed from different gases in a kilojoule-range plasma focus facility

The velocities of the plasma jets formed from Ne, N₂, Ar, and Xe gases in plasma focus facilities were determined by means of laser-optical shadowgraphy of the shock waves generated at the jet leading edge. In spite of the almost tenfold ratio between the atomic weights of these gases, the outflow velocities of the plasma jets formed in experiments with these gases differ by less than twice, in the range of (0.7–1.1) × 10⁷ cm/s under similar discharge conditions. The energies of the jet ions were found to vary from 0.7 keV for nitrogen to 4 keV for xenon.     

Measurements of the temperature of subsonic CO<Subscript>2</Subscript> induction plasma flows by analyzing their emission spectra

Results are presented from measurements of the temperature characteristics of subsonic CO² plasma flows generated by a 100-kW induction plasmatron at the Institute for Problems of Mechanics, Russian Academy of Sciences. The atomic excitation temperature T _a and the population temperature T _e of the electronic states of C2 molecules (both averaged over the jet diameter) were measured from the absolute intensities of the atomic spectral lines and the spectrum of C2 molecules in different generation regimes at gas pressures of 25–140 hPa and anode supply powers of 29–72 kW. The longitudinal and radial profiles of the temperatures were determined for some of these regimes and compared to those obtained from numerical calculations of equilibrium induction plasma flows in the discharge channel. For some generation regimes, the dependences of the averaged (over the line of sight) rotational and vibrational temperatures (T _r and T _v) on the discharge parameters, as well as the radial profiles of these temperatures, were determined from the best fit of the measured and calculated spectra of C₂ molecules (Swan bands). The self-absorption of molecular emission was observed at sufficiently high temperatures and gas pressures, and its influence on the measured values of the molecular temperatures T _e, T _v, and T _r was examined.     

Filling the vacuum chamber of a technological system with homogeneous plasma using a stationary glow discharge

Experimental study of a glow discharge with electrostatic confinement of electrons is carried out in the vacuum chamber volume V ≈ 0.12 m³ of a technological system “Bulat-6” in argon pressure range 0.005–5 Pa. The chamber is used as a hollow cathode of the discharge with the inner surface area S ≈ 1.5 m². It is equipped with two feedthroughs, which make it possible to immerse in the discharge plasma interchangeable anodes with surface area S _a ranging from ∼0.001 to ∼0.1 m², as well as floating electrodes isolated from both the chamber and the anode. Dependences of the cathode fall U _c = 0.4−3 kV on the pressure p at a constant discharge current in the range I = 0.2−2 A proved that aperture of the electron escape out of the electrostatic trap is equal to the sum S _o = S _a + S _f of the anode surface S _a and the floating electrode surface S _f . The sum S _o defines the lower limit p _o of the pressure range, in which U _c is independent of p. At p < p _o the cathode fall U _c grows up dramatically, when the pressure decreases, and the pressure p tends to the limit p ^ex, which is in fact the discharge extinction pressure. At p ≈ p ^ex electrons emitted by the cathode and the first generation of fast electrons produced in the cathode sheath spend almost all their energy up to 3 keV on heating the anode and the floating electrode up to 600–800°C and higher. In this case the gas in the chamber is being ionized by the next generations of electrons produced in the cathode sheath, their energy being one order of magnitude lower. When S _a < (2m/M)^1/2 S, where m is the electron mass and M is the ion mass, the anode may be additionally heated by plasma electrons accelerated by the anode fall of potential U _a up to 0.5 kV.     

Generalized Bohm’s criterion and negative anode voltage fall in electric discharges

The value of the voltage fall across the anode sheath is found as a function of the current density. Analytic solutions are obtained in a wide range of the ratio of the directed velocity of plasma electrons v ₀ to their thermal velocity v _T . It is shown that the voltage fall in a one-dimensional collisionless anode sheath is always negative. At the small values of v ₀/v _T , the obtained expression asymptotically transforms into the Langmuir formula. Generalized Bohm’s criterion for an electric discharge with allowance for the space charge density ρ(0), electric field E(0), ion velocity v _i (0), and ratio v ₀/v _T at the plasma-sheath interface is formulated. It is shown that the minimum value of the ion velocity v _i ^* (0) corresponds to the vanishing of the electric field at one point inside the sheath. The dependence of v _i ^* (0) on ρ(0), E(0), and v ₀/v _T determines the boundary of the existence domain of stationary solutions in the sheath. Using this criterion, the maximum possible degree of contraction of the electron current at the anode is determined for a short high-current vacuum arc discharge.     

Simulations of a longitudinal glow discharge in a hot air flow at atmospheric pressure

A one-dimensional axisymmetric time-dependent model of a discharge in a gas flow is developed. The model includes a fairly complete set of plasmochemical reactions and describes the heating and gasdynamic expansion of a plasma channel in air. The processes governing the distribution of nitrogen molecules in the N₂(C ³Π_u, v) state over vibrational levels are considered. The parameters of a longitudinal glow discharge in a hot (T ₀ = 1500?3000 K) air flow at atmospheric pressure are calculated. It is found that gas preheating considerably influences the parameters of the discharge channel. The results of calculations are compared with the experimental data.     

Two-dimensional simulation of the development of an inhomogeneous volume discharge in a Ne/Xe/HCl gas mixture

The kinetic processes accompanying plasma column formation in an inhomogeneous discharge in a Ne/Xe/HCl gas mixture at a pressure of 4 atm were investigated by using a two-dimensional model. Two cathode spots spaced by 0.7 cm were initiated by distorting the cathode surface at local points, which resulted in an increase in the field strength in the cathode region. Three regimes differing in the charging voltage, electric circuit inductance, and electric field strength at the local cathode points were considered. The spatiotemporal distributions of the discharge current; the electron density; and the densities of excited xenon atoms, HCl(v = 0) molecules in the ground state, and HCl(v > 0) molecules in vibrational levels were calculated. The development of the discharge with increasing the electron density from 10⁴ to 10¹⁶ cm^?3 was analyzed, and three characteristic stages in the evolution of the current distribution were demonstrated. The width of the plasma column was found to depend on the energy deposited in the discharge. The width of the plasma column was found to decrease in inverse proportion to the deposited energy due to spatiotemporal variations in the rates of electron production and loss. The calculated dependences of the cross-sectional area of the plasma column on the energy deposited in the discharge agree with the experimental results.     

Effect of an external RF field on the beam-plasma discharge in a magnetic field

The effect of an RF field on a steady-state beam-plasma discharge with a plane electrode placed parallel to a sheetlike electron beam is studied experimentally. The plasma parameters were measured by a single probe, and the electron distribution function was determined with the use of an electrostatic analyzer. The energy and current of the electron beam were E _B=2.5 keV and J _B=0.05–1.5 A, respectively. The working pressure was p=2×10^?5–10^?3 torr. The frequency of the external RF field was 13.56 MHz. Both the steady-state regimes in which the RF field had no effect on the plasma parameters and regimes with a pronounced effect of the RF field were observed. The experiments show that the regime of the discharge depends strongly on the plasma density and the magnetic field. The parametric instability is studied theoretically in the weak-turbulence approximation. It is shown that, due to the decay nature of the spectrum of plasma oscillations, the onset of instability is accompanied by the transfer of the energy of fluctuations over the spectrum, from the pump frequency toward its harmonics.