Negative Glow Plasma Research Articles

Electron Kinetics in the Hot-Cathode Negative Glow of a Helium Discharge

The electron kinetics in the negative glow of a thermionic-cathode discharge in helium is discussed based on the results of Langmuir and emissive probe measurements performed in the axial direction of a 5-cm-long discharge at 0.65 Torr helium pressure and low discharge current. The electron energy distribution function shows two groups of electrons: the primary ones, which are emitted by the cathode and accelerated into the cathode fall, and the secondary ones, which are produced by inelastic collisions between primary electrons and neutral atoms. The cathode fall voltage and primary-electron energy show a strong dependence on thermionic current intensity. Based on the experimental results obtained for a thermionic current corresponding to a cathode fall voltage slightly above the helium ionization potential, a simplified electron kinetic model using the Monte Carlo flux simulation is proposed to obtain the secondary-electron energy distribution. Since the axial profile of the measured plasma potential shows a potential well, which corresponds to a maximum in the negative glow plasma density, the simulated secondary-electron energy distribution can be obtained mainly as a result of the accumulation of low-energy electrons yielded from inelastic collisions.

Modeling of nonlocal slow-electron kinetics in a low-pressure negative-glow plasma

The kinetics of slow electrons is studied in a low-pressure negative-glow plasma (NGP). A method based on the nonlocal approach is proposed, which allows the nonlocal (nonequilibrium) nature of slow electrons to be accounted for in a physically transparent and numerically efficient manner. The slow electrons are divided into trapped (cold, Maxwellian) and free (superthermal, non-Maxwellian). It is shown that the superthermal (free) electrons are particularly important because they carry current and supply energy to the system of cold (trapped) electrons. A nonlocal energy-balance equation for the trapped electrons is derived, in which heating by superthermal electrons and diffusion cooling are found to be among the most important mechanisms. Simple expressions for the diffusion-cooling rate and the wall potential are determined. The proposed method is validated by the numerical solution of the full kinetic equation for a NGP in Ar. The energy and space distributions of electron fluxes are analyzed, and flux reversal (in energy space) is observed and explained. A comparison to experiment is carried out and close agreement is obtained. The proposed method can be useful in building fully kinetic, self-consistent models of various NGP-based discharge devices.

A theoretical and experimental study of a Penning recombination laser operating in a hollow cathode discharge

A theoretical and experimental study of a Penning recombination laser operating in the negative glow plasma of a pulsed high-voltage hollow cathode discharge at intermediate pressures is presented. A detailed kinetic model of the plasma - chemical reactions and radiative processes determining the inversion population on the transition is developed and the time-dependent behaviour of both the plasma and laser parameters (excited states populations, electron and ion densities, electron and gas temperatures, output power and gain) is numerically calculated. An extended experimental investigation of the main plasma and laser parameters is made and a comparison with theoretical results is presented.

Field reversal in the negative glow of a DC glow discharge

We present a simple analytical derivation of the position of the field reversal in the negative glow of a dc glow discharge. In the absence of charged particle losses, this position is shown to depend only on the relaxation length of the energetic electrons entering the glow, the sheath length and the gap length. The model shows that the position of the field reversal, if it exists, can be deep in the negative glow plasma; it is not necessarily at the sheath-negative glow boundary. A large part of the total number of ions created in the negative glow can therefore enter the sheath and return to the cathode. These model results are independent of the electron temperature in the glow.

Electric field in cathode layer of anomalous glow discharge

The cathode layer of anomalous glow discharge is calculated taking into account in a kinetic approximation the resonance charge transfer of ions emitted by the negative-glow plasma. The results of calculation of the electric-field distribution are in good agreement with experiment for a discharge potential in the kilovolt range.

Recombination lasers in a flowing negative glow discharge

A fast gas flow through a negative glow plasma creates an afterglow region where supercooled electrons are separated from energetic beam electronsto produce a population inversion by collisional recombination. A report on spatially resolved electron temperature measurements of the afterglow plasma showing electron temperatures in the range of 750-1100 K under steady-state conditions is presented. A 4-cm-long flowing hollow cathode discharge was used to obtain CW (continuous wave) recombination laser oscillation in ArI. Collisional recombination of singly and doubly ionized metal vapor species in the same type of plasma also produced pulsed laser action in infrared lines of PbI, ZnI, PbII, and SnII. The addition of H/sub 2/ is demonstrated to significantly increase the laser output intensity. >

The negative glow in a magnetic field in a helium discharge

The negative-glow plasma in a magnetic field of an inverse-brush cathode discharge has been studied by absolute emission spectroscopy. The absolute density of the levels was measured as a function of the distance from the cathode for different currents, pressures, and magnetic fields. A comparison of line emission from the positive column and the negative glow in helium has been made both with and without a magnetic field. A simple theoretical model is presented in order to explain the experimental results.

Quick Measurement and Digital Data Handling for Langmuir Probes

A quick measurement and data processing system for Langmuir probes was developed based on a digital technique. This system enabled us to avoid the effect of probe contamination and plasma-state variations on probe characteristics. For noise reduction, the system had an averaging function by digital accumulation of data from multiple scanning. A set of personal computer programs which automatically determine the electron temperature and density from probe-characteristic data was also developed. The system was applied to a negative glow plasma, and the plasma parameters were successfully determined.

Study of Intense Electron Beams Produced by High-Voltage Pulsed Glow Discharges

We report the generation of high-current-density (20 A/cm2) pulsed electron beams from high-voltage (48-100 kV) glow discharges using cathodes 7.5 cm in diameter. The pulse duration was determined by the energy of the pulse generator and varied between 0.2 μs and several microseconds, depending on the discharge current. The largest electron beam current (900 A) was obtained with an oxidized aluminum cathode in a helium-oxygen atmosphere. An oxidized magnesium cathode produced similar results, and a molybdenum cathode operated at considerably lower currents. A small-diameter (<1 mm) well-collimated beam of energetic electrons of very high current density (>1 kA/cm2) was also observed to develop in the center of the discharge. Electrostatic probe measurements show that the negative glow plasma density and the electron beam current have a similar spatial distribution. Electron temperatures of 1-1.5 eV were measured at 7 cm from the cathode. The plasma density (8.5 · 1011 cm-3 at 450 A) was found to depend linearly on the discharge current. In discharges at high currents a denser and higher temperature plasma region was observed to develop at approximately 20 cm from the cathode. We have modeled the process of electron beam generation and predicted the energy distribution of the electron beam. More than 95 percent of the electron beam energy is calculated to be within 10 percent of that corresponding to the discharge voltage.

The influence of N 2 partial pressure and substrate bias on electron density and temperature in a dc sputtering diode

Single probes and grid probes were used to study the density and the energy of the slow and secondary electron groups in the negative glow plasma of a dc diode sputtering system. The influence of a negatively biased electrode and of a reactive gas partial pressure were specially examined. When the bias voltage is varied from 0 to −200 V the densities of the slow and secondary electron groups clearly increased but their energies suffer little change. The electron density value can increase threefold. Introduction of a reactive gas (N 2) into the gas discharge (Ar) leads to a rapid decrease in the density and the energy of the slow electron group when a critical pressure p c is reached. The secondary group density decreases regularly when the nitrogen pressure increases but the energy remains constant.

Cw recombination laser in a flowing negative glow plasma

cw laser oscillation has been obtained in the 1.43-μm line of Cd I populated by electron-ion recombination in the flowing afterglow of a negative glow plasma. The metal vapor in the recombining plasma is produced at room temperature by cathode sputtering. Negative glow plasmas having an electron energy distribution in which energetic beam electrons and supercooled secondary electrons coexist under steady-state conditions are shown to be an attractive medium for the excitation of cw recombination lasers.

Electron energy distribution and inelastic collisions in a negative glow plasma

Electron energy distribution and inelastic collisions in a negative glow plasma

Untersuchungen zur Ladungsträgerbilanz einer Reflexentladung im axialen Magnetfeld / Investigations on the Particle Balance in a Reflex Discharge with an Axial Magnetic Field

A simple model for the particle balance in a reflex discharge with a superimposed axial magnetic field is presented based upon the results found for the negative glow of an anomalous glow discharge. The electron density and temperature of hydrogen and helium discharges are determined as functions of the discharge current, pressure and magnetic field; the ion current of a cylindrical Langmuir probe is evaluated according to the theory of Laframboise. Additional information on the ion composition of the plasma is obtained by comparison of the probe measurement with interferometric microwave measurements. The reflex discharge produces a negative glow plasma with Te = 2000 - 3000 °K and densities up to ne = 1013cm-3. For pressures below 0.01 Torr and strong magnetic fields the negative glow is transformed into a Penning discharge.

Reaction rate constants in steady-state hollow cathode discharges: N2 + H2O reactions

In a cylindrical hollow cathode discharge in N2 with traces of H2O, there is a distribution of ion types from the edge of the negative glow to the axis of the cylinder. This distribution was explored by using a small, radially movable probe with an entrance aperture to a mass spectrometer in the probe. Ions could then be extracted from the negative glow plasma from any desired radial location in the cylinder. The results show N2+ and N+ at the edge of the glow giving way to N2H+, H2O+, and H3O+ toward the axis. Clearly, ion-molecule reactions are proceeding in the field-free negative glow region. Quantitative rate constant measurements were obtained for formation and destruction of N2H+, H2+, N+, and the formation of H2O+ and H3O+. The measurements on N2+ and N+ formation by electron collisions give values for the density of high energy electrons in the negative glow. Adequate agreement is found with values obtained by other workers using other methods, where overlap occurs. The present new and different method thus lends support to older findings as well as providing new information.

The normal cold–cathode glow discharge

This paper is a critique of some aspects of the theory of norma cold–cathode glow discharges, with special reference to the development of the discharge in a direction normal to the cathode. It is pointed out that a theory of the normal cathode dark space is available on the basis of Poisson's equation and the Boltzmann transport equation, if conditions at both its cathode and negative glow boundaries are supposed known, whether or not the complexity of the Boltzmann computing is minimized by using as far as possible from the beginning the macroscopic ownsend coefficient a/p and mobilit–type relations, or other convenient approximations. Attention is called to the vital part played by the negative glow plasma, and the possibility of improving sharp–boundary cathode dark space theory by applying a Langmuir–Rohm criterion for positive space–charge to form is considered. A more fundamental approach is however to consider the whole discharge from the cathode to the middle of the negative glow or the anode as ...

Ion sampling from the negative glow plasma in a cylindrical hollow cathode

A new hollow-cathode design is described which is used for the continuous recording of radial density distributions of individual ion species in the negative glow region of a cylindrical hollow-cathode d.c. discharge. The ions effuse through a sampling probe P of 1 mm diameter, the orifice diameter being 20–50 μm. The radial density distributions of individual ion species in the negative glow region may be determined directly from the effusive ion currents i( r) if the probe potential is equal to the space potential. Due to elementary processes in the negative glow region, e.g. ion—molecule reactions, secondary and tertiary ions are formed whose density distributions over the radius differ appreciably from the distribution of primary ions produced by electron impact. The measurement method described in the present paper is the basis of the determination of reaction-rate constants from the radial ion-density distribution in the negative glow region of a cylindrical hollow-cathode d.c. discharge. A first evaluation of this method has already been given elsewhere. Further applications of the present measurement method include the recording of the radial field-strength distribution in the transition zone between negative glow and dark space near the glow edge.

Electron and ion momentum transfer from plasma electrical conductivity in a magnetic field

The electrical conductivity of a partially to strongly (Coulomb collision-dominated) ionized dc plasma in a dc magnetic field was investigated at low frequency by two experimental methods, one utilizing the Hall effect and the other a plasma interaction with an induced solenidal electric field. Electron-ion and electron-atom momentum transfer collision frequencies and ion mobility in the magnetic field (ion slip) were inferred. The helium abnormal negative glow plasma was used at pressures of 0.5 to 2.5 Torr, electron densities of 1 to 32 × 1017m−3 and temperature ≈ 600 °K; the magnetic field ranged up to 675 G. The data support Shkarofsky's theory of partial ionization. When ion motion becomes dominant, the resulting ion mobility is 75% larger that He+ mobility in helium taken from drift tube measurements. This disagreement suggests that further theoretical work on ion mobility in a plasma is needed.

A Simple Bakeable Hollow Cathode Device for the Direct Study of Plasma Constituents

The unique features of construction of a hollow cathode discharge apparatus are described. The device includes a highly important exit aperture that also serves as a Langmuir probe for exploring the negative glow plasma.

Experimentelle Bestimmung innerer und äußerer Parameter des negativen Glimmlichtplasmas einer zylindrischen Hohlkathodenentladung in Argon / Plasma Diagnostic Measurements in the Negative Glow of a Cylindrical Hollow Cathode Discharge

Abstract The negative glow plasma of a cylindrical hollow cathode dc discharge (0.05 - 1 Torr Argon, 1-30 mA) is investigated as to the position of the glow edge, the axial dependence of the cathode current density, the axial and radial potential distributions, the density and energy of electrons (from Langmuir probe measurements) as dependent on pressure, discharge current and position, etc. It is found that two groups of electrons exist whose energies are &lt; 0.5 eV ~ 3 eV and the ratio of their denisties being 3 up to 20. A phenomenological explanation is given for the relatively high denisty of the fast group, The results are compared with those of other auhors concerning hollow-cathode and linear discharges.

Velocity distribution of plasma electrons in the negative H2- and He-glow with superimposed longitudinal magnetic field

The negative glow plasma has been found nearly field free in axial direction. Therefore plasma electrons in the stationary glow can thermalize down to the temperature of the neutral gas, whenever their diffusion—and recombination—lifetime is high enough. Applying Boltzmann's equation to this problem, the conditions of thermalization of plasma electrons are derived as a function of the outer parameters of the plasma: vessel diameter 2R, neutral gas pressurep and longitudinal magnetic fieldB. — If plasma electrons have a too short diffusion—and recombination—lifetime to be in thermal equilibrium with the neutral gas, the electron energy increases. For this case the distribution function of plasma electrons is derived using Boltzmann's equation. Approximating the calculated energy distribution by a Maxwellian distribution function, the electron temperature in the glow is obtained as a function of the parameters:R, p, B.