Negative Glow Plasma Research Articles

On fundamental inconsistencies in a commonly used modification of a fluid model for glow discharge

This work considers the fundamental contradictions in the concept of one of the most well-known and widely used modifications of the fluid model for simulation of a glow discharge (GD), the ‘local mean energy approximation’ (LMEA). In this model, it is proposed to determine the kinetic coefficients in the electron particle and energy balance equations as functions of the electron mean energy (temperature) rather than local electric field, using a one-to-one correspondence between these parameters through the electron Boltzmann equation. It is shown that the scope of applicability of this model, like any other modification of the fluid model, is limited by the local mode of formation of the electron energy distribution function (EEDF). Therefore, as demonstrated by the examples of typical 1D and 2D problems for a GD in argon, its extension to the region of nonlocal EEDF is in no way justified and leads not only to serious errors in the results, but also to a logically intractable situation in attempts to apply the main postulate of the LMEA model to the region of a weak (or even reverse) electric field in a negative glow plasma. At the same time, the apparent reliability of calculations within the framework of the LMEA model for a number of parameters, in our opinion, only slows down progress in modeling of gas discharge plasma.

Numerical simulation and experimental diagnostics of fast electron kinetics and plasma parameters in a microhollow cathode discharges in helium

The paper presents the results of studies of plasma parameters and kinetics of fast electrons in the region of negative glow of a discharge with a microhollow cathode at high pressures based on a hybrid model that includes a kinetic description of electrons and a fluid description of the heavy plasma component. The results of numerical calculations are compared with the results of probe studies. It is shown that the hybrid model makes it possible to accurately describe the formation of electron energy distribution function peaks from fast electrons produced as a result of Penning ionization reactions and superelastic collisions. The results of numerical calculations make it possible to predict the parameters of the negative glow plasma in discharges with a microhollow cathode in pure helium and in helium with impurities of various gases, including organic ones.

Determination of organic impurities by plasma electron spectroscopy in nonlocal plasma at intermediate and high pressures

This study aims to improve impurity analysis by plasma electron spectroscopy for organic molecules. Various impurities can be registered simultaneously in one measurement, because the appearance energies of the characteristic Penning electrons vary for different chemical compounds. Herein, experimental studies were conducted on helium with alcohol vapor impurities in a nonlocal negative glow plasma of a short glow micro-discharge with an increase in pressure from 15 Torr to 150 Torr. As a result, plasma electron spectroscopy enables the detection of gas impurities in high-pressure (even at atmospheric) environments, which eliminates the need for expensive and cumbersome gas pumping systems and expands the scope of the method.

Operating modes in a low-pressure glow discharge with hollow cathode

The paper deals with investigations of the discharge in hydrogen in conditions close to those for the trigger units of pseudospark switches. The following modes of discharge sustaining are classified: the Townsend discharge, the suppressed glow discharge with hollow cathode, the intermediate mode of the gradual transition from the suppressed discharge to the ordinary glow discharge, and the ordinary discharge in which the negative glow plasma occupies the whole depth of the cathode cavity. As applied to the ordinary discharge, the theoretical model that offers a possibility to estimate the negative glow parameters is developed. In the current range 1.6–10 mA the electron densities in the negative glow region are 1.7–8.7 108 cm−3. The interpretation for the mechanism of the current passage to the anode is also proposed based on the model. In particular, the explanation is given for the fact that the region of the positive column plasma near the flat anode appears at a low discharge current.

Low-pressure discharge with hollow cathode and hollow anode in a trigger unit of pseudospark switch

This paper deals with the investigations of the auxiliary glow discharge in the trigger unit of the pseudospark switch. The trigger unit communicates with the grounded electrode of the main high-voltage gap due to the aperture in the flat part of one of the trigger electrodes. Then, the so-called parasitic current closes to the main electrode via this aperture. This paper is mainly concentrated on the problem of the formation of the parasitic current for different regimes of the auxiliary discharge burning and for different designs of the trigger system. At a low current, the auxiliary discharge burns in the suppressed regime, and an increase in the current results in the stepwise transition to the ordinary glow discharge. The conditions of sustaining the auxiliary discharge influence on the value of the parasitic current. In the experiments, the cavity of the trigger electrode can play a role of the hollow cathode or the hollow anode, so that the negative glow plasma or the positive column plasma is generated in this hollow electrode. It is demonstrated that for both cases, the parasitic current is determined by the ion flow from the plasma boundary.

Diagnostics and comparative analyzes of plasma parameters in micro hollow cathode discharges with an open and covered external surface of cathode in helium using an additional electrode

In this work, diagnostics and comparative analysis of microhollow cathode discharges with an open and covered external surface of the cathode in helium at atmospheric pressure were carried out. It is shown that by covering the external surface of the cathode, it is possible to achieve the effect when the current–voltage characteristics of the discharge grows in a wider range of discharge currents. Obviously, in this case, the negative glow (NG) plasma of the hollow cathode occupies a significant part of the discharge cell, as evidenced by the measurements carried out with the additional electrode. The electron temperature values determined from the I–U characteristic of the additional electrode are 0.3–0.4 eV for a microhollow cathode discharge with a covered external surface, which indicates that the plasma of such discharge is similar to NG plasma of glow discharge. The fast parts of the second derivatives of the I–U characteristic of the additional measuring electrode demonstrate the detection of the spectra of fast electrons produced as a result of Penning ionization reactions. Moreover, by covering the external surface of the cathode, conditions are created under which the entire discharge volume is occupied by a NG plasma, which makes it possible to achieve a better resolution for recording the spectra of fast electrons.

Diagnostics of large volume coaxial gridded hollow cathode DC discharge

Results of the experimental investigation of large volume DC glow discharge with coaxial grid electrodes in low-pressure helium are presented. Measurements of plasma parameters were carried out with a single-probe technique using the Impedans Langmuir Probe system. The distribution of the main plasma parameters including electron energy distribution functions was obtained. It is shown that the discharge volume is represented by negative-glow plasma with low electron temperatures of the order 0.3 eV. The second derivatives of probe I–V characteristics with respect to the potential evinced the presence of spectra of fast electrons born in reactions of Penning ionization of impurity particles with helium metastable atoms. We propose and implement a simple method for monitoring the presence of impurities and metastable atoms of the main gas and for estimation their densities.

Determining the spectrum of penning electrons by current to a wall probe in nonlocal negative glow plasma

It was shown that the spectrum of Penning electrons in a nonlocal equipotential plasma is associated with the second derivative of their current to a wall electrode (sensor). Analytical calculations are confirmed by experimental studies in a negative glow plasma of short DC glow discharge at moderate and high pressures (up to 150 Torr).

Cleaning Away the Oleic Acid Contaminant from Glass Surface by Negative Glow Plasma

Glass surfaces are contaminated by exposure to overheated oleic acid vapor and then cleaned by low-pressure plasma of a dc glow discharge in either air or argon. In this study, the oleic acid is used as model for hydrophobic organic contaminant and glass as a model for inorganic hydrophilic substrate. The processes of contamination and plasma cleaning are studied by atomic force microscopy (AFM), which revealed formation of oleic acid nanodroplets during contamination and, then, their removal during a few minutes of plasma cleaning. Water contact angle measurements show a much faster effect of plasma cleaning treatment, the contaminated surface changing from slightly hydrophobic to superhydrophilic in less than 30 s. This effect can be attributed to oxidation and hydroxylation of the oleic acid, as indicated by infrared absorption measurements. Also, the AFM indentation of the oleic acid nanodroplets after short plasma treatment indicates solidification of the remnant contaminant material.

Collisional electron spectroscopy method for gas analysis

Recently developed collisional electron spectroscopy (CES) method, based on identification of gas impurities by registration of groups of nonlocal fast electrons released by Penning ionization of the impurity particles by helium metastable atoms, is verified experimentally. Detection and identification of atoms and molecules of gas impurities in helium at pressures of 14 - 90 Torr with small admixtures of Ar, Kr, CO2, and N2 are carried out. The nonlocal negative glow plasma of short dc microdischarge is used as most suitable medium. Records of the energy spectra of penning electrons are performed by means of an additional electrode - sensor, located at the boundary of the discharge volume. Maxima appear in the electron energy spectra at the characteristic energies corresponding to Penning ionization of the impurity particles by helium metastable atoms.

Is the negative glow plasma of a direct current glow discharge negatively charged?

A classic problem in gas discharge physics is discussed: what is the sign of charge density in the negative glow region of a glow discharge? It is shown that traditional interpretations in text-books on gas discharge physics that states a negative charge of the negative glow plasma are based on analogies with a simple one-dimensional model of discharge. Because the real glow discharges with a positive column are always two-dimensional, the transversal (radial) term in divergence with the electric field can provide a non-monotonic axial profile of charge density in the plasma, while maintaining a positive sign. The numerical calculation of glow discharge is presented, showing a positive space charge in the negative glow under conditions, where a one-dimensional model of the discharge would predict a negative space charge.

Features of a near-cathode region in a gliding arc discharge in air flow

This paper describes the results of the investigation of a low-current atmospheric-pressure discharge in air flow with electrode geometry typical of the so-called gliding arc. At a current of fractions of an ampere the discharge burns in a regime of normal glow rather than in an arc regime and the area of the negative glow plasma smoothly moves over the cathode surface under the effect of gas flow. At the background of the glow discharge, occasional creation of a spark cathode spot can occur. Then the discharge becomes attached to the cathode surface and the site of current attachment does not move due to the gas flow. The site of current attachment continues its smooth displacement when the cathode spot is spontaneously extinguished. The events when the cathode spot abruptly jumps to a new place of attachment downstream are also observed.

Energy spectra of Penning electrons in non-local plasma at middle and high pressures

A recently-developed collisional electron spectroscopy (CES) method enabled us to measure the energy spectra of groups of fast non-local electrons in a collisional mode at high pressures, where no collisional energy relaxation of electrons in the different groups takes place in the volume, and the different groups of electrons behave independently of each other. We recorded the energy spectra of groups of fast electrons created via Penning ionization of Ar and N2 impurities by metastable He atoms at He pressures of 30 and 200 Torr. The experiments were conducted in the non-local negative glow plasma of a short dc microdischarge. The Penning electrons' energy spectra were recorded by means of an additional electrode – a sensor located at the boundary of the discharge volume, in contrast with the classical Langmuir probe. The spectra are characterized by the appearance of maxima at characteristic energies corresponding to the energy of the electrons released via Penning reactions. Using the Penning electrons' energy spectra, one can detect and identify the presence of different atomic and molecular admixtures in He at high pressures.

Registration of gas impurities in nonlocal plasma of helium microdischarge by an additional electrode — sensor

Identification of gas impurities in helium by selective registration of groups of fast electrons created in Penning ionization of impurities atoms or molecules by metastable helium atoms at pressures of 7–40 Torr is realized. The collisional electron spectroscopy (CES) method is applied and is experimentally verified. Identification of impurities atoms and molecules is accomplished in collisional regime of movement of the particles, where the different groups of electrons have no time to relax in energy by collisions in the volume and behave independently of each other. An original design of microplasma gas analyzer is proposed, containing only nonlocal negative glow plasma of a short dc microdischarge. Registration of the energy spectra of penning electrons by means of an additional electrode-sensor, located at the boundary of the discharge volume is performed. The sensor has large collecting area compared to classical Langmuir probes, contributing to significant enhancement in the measurements sensitivity. Maxima in the EEDF are recorded in helium with small admixtures of krypton, argon and air. The obtained maxima appear at low discharge currents and at characteristic energies corresponding exactly to the expected maxima for penning electrons of the known gas impurities used. The gas analyser is compact, simple in technical performance, has high sensitivity and its size is dramatically reduced compared to the existing devices for gas analysis. This work is an approach to the development of microdischarge gas analyzers for gas impurities detection like poison gases, gas pollutions in the atmosphere or in the industry etc.

Dynamics of plasma–electrode coupling in fluorescent lamp discharges

A time dependent model of a low pressure, mercury–rare gas discharge with thermionic electrode is presented. The model is applicable to ac-operated fluorescent lamps, which is the focus of this work. The model describes a one-dimensional negative glow plasma that is bounded on one side by a thermionic electrode and a sheath, and on the other by a positive column plasma. The electrode/sheath component of the model, together with the mutually interacting negative glow plasma, allows for self-consistent calculation of the electrode sheath potential. The model describes a smooth transition in the plasma parameters from electrode to positive column and thus reveals the spatial extent of the influence of the electrode and sheath processes. A detailed description of the model is presented, as well as results of calculations pertaining to a standard fluorescent lamp. Also shown are measurements from a 2 mm interferometer and an internal floating probe, both of which compare favourably with the calculations.

Enhanced IR hollow cathode laser in a 3He–Ne gas mixture

An experimental and theoretical study on 3He–Ne and 4He–Ne helical hollow cathode lasers is presented. Enhanced laser operation on the near IR NeI lines is observed when the natural isotope 4He is substituted by the lighter isotope 3He. A four-fold increase in the laser output power and a three-fold increase in the laser gain for the strongest NeI 1.1523 µm line is measured in the 3He–Ne gas mixture compared to the 4He–Ne gas mixture.On the basis of the theoretical analysis done by means of a non-stationary kinetic model for the negative glow plasma of 3He–Ne and 4He–Ne hollow cathode lasers, a study on the changes in the particle kinetics is carried out and an explanation of the experimental results is proposed. In the 3He–Ne mixture the electron temperature is lower than in the 4He–Ne mixture, while the gas temperature is higher. As a result the helium triplet metastable density and the rate constant for excitation transfer to neon atoms are higher in the 3He–Ne mixture. The lower laser level de-excitation due to intra-multiplet mixing of 2p1–10levels by 3He atoms is more efficient.

Dissociation of nitrogen in the plasma-cathode interface of glow discharges

Nitrogen dissociation in the cathode region of glow discharges was studied theoretically by Monte-Carlo simulation and experimentally by optical emission actionometry of lines emitted by atomic and molecular species of this region plasma. In the ranges 0.13–1.3 hPa and 0–0.5 mA/cm 2 of gas pressure and current density, dissociation rates of the negative glow plasma seem to be low at the level <1×10 −3. In the region close to the cathode these rates are two orders of magnitude higher than that observed in the negative glow. Fast particles (ions and neutrals) impact on the cathode surface seems to be a main source of free atoms here.

Self-organization in cathode boundary layer microdischarges

Direct current glow discharges in xenon between a flat, 100 µm thin cathode and a ring shaped anode, separated by a distance of 250 µm, were found to be stable up to atmospheric pressure. The glow discharge structure in this electrode configuration reduces to only the cathode fall and negative glow, with the negative glow plasma serving to conduct the current radially to the circular anode. Photographs taken in the visible range of the spectrum and at the wavelength of excimer emission for xenon (172 nm) indicate the transition from a homogeneous plasma to a structured plasma when the current is reduced beyond a critical value that is dependent on pressure. The plasma pattern consists of filamentary structures arranged in concentric circles. The structures are most pronounced at pressures below 200 Torr and become less regular when the pressure is increased. The self-organization of such plasmas indicates the existence of two branches of the voltage–current density (V–J) characteristic with positive slope. For conventional glow discharges in the current range of interest (milliampere), the only discharge mode with a positive slope of the V–J characteristic is the abnormal glow mode. At a critical current density, the discharge transfers from the abnormal glow into an arc. However, by cooling the cathode, it seems to be possible to stabilize the discharge, even in the glow-to-arc transition range. This second stable region in the V–J characteristic of such ‘cathode boundary layer discharges’ would explain the existence of a plasma pattern with two distinct values of current density at the same discharge voltage.

The characteristics of a confined discharge in a helium-chlorine gas mixture

The electrical and optical characteristics of a confined (cage) discharge in a He-Cl2 mixture at pressures within 0.1–1.5 kPa were studied. It was established that the negative glow plasma in this discharge is a source of wideband radiation in a wavelength range from 170 to 270 nm with the emission peaks at λmax=195 nm [Cl2(1Σ−1Π4)], 200 nm [C12**, and 258 nm [Cl2(D′-A′)]. Optimum conditions ensuring the maximum intensity of emission in the UV-VUV range due to transitions in the chlorine molecule have been established. The results are of interest from the standpoint of the development of a stationary shortwave low-pressure lamp filled with a He-Cl2 gas mixture.

Werner Lindinger

The scientific community lost a great friend and colleague with the tragic drowning of Werner Lindinger on Kauai, Hawaii, on 16 February 2001. He was a pioneer in the study of ion–molecule interactions.Werner was born in Brixlegg in the Austrian Tirol on 25 January 1944. In 1963, he enrolled at the University of Innsbruck, where he received his PhD in physics in 1972. His doctoral thesis was on the mass spectrometry of ions sampled from the negative glow plasma in a cylindrical hollow cathode. From 1967 to 1972, Werner was a part-time teaching assistant at the University of Innsbruck’s Institute of Atomic Physics. In 1972, he accepted a tenure track position at the institute.In 1973, Werner initiated his professional career as a Max Kade Foundation postdoctoral exchange fellow in the aeronomy laboratory of the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado, where he remained until 1975. His considerable talent and exceptional energy led to an extremely productive period there. He rigorously exploited the newly developed flowing afterglow technology for the measurement of thermal and low-energy ion–molecule interactions, ion mobilities, and ion reactions with mainly neutral molecules.Werner returned to the physics faculty at the University of Innsbruck in 1975. He was instrumental in developing an atomic and ion physics program that quickly achieved international recognition and led to a professorship in 1978 at the Institute of Experimental Physics in Innsbruck. Werner’s group made important contributions to ion–molecule reaction kinetics and a variety of ion–molecule interaction processes, and original contributions to thermochemistry. A notable example is their series of studies of molecular–ion vibrational quenching in neutral collisions. Those detailed studies led to greatly increased understanding of the mechanistic processes involved.In later years, Werner’s interests broadened from gaseous electronics. His group extended the application of ion flow systems to supersensitive detection of trace gases in an online, real-time manner in the 1980s by using ion–molecule reaction mass spectrometry (IMR-MS) based on charge exchange reactions and, in the 1990s, by using the proton transfer reaction mass spectrometry (PTR-MS) technique. Werner and his colleagues pioneered its use in a variety of applications in medicine and food analyses, and its use in highly time-resolved studies of the emissions from vegetation and biomass burning to the atmosphere. Many research groups around the world are now applying this technique for studies of biosphere–atmosphere interactions. At the time of his death, Werner was in Hawaii to install his instrument at the NOAA Clean Air Baseline Station on Mauna Loa.Werner lectured widely in Europe and the US for many years. He was a guest professor at the University of Trento in Italy and the University of Utah, Salt Lake City. His outstanding research on the measurement of thermal and low-energy ion–molecule interactions, ion mobilities, and ion reactions with primarily neutral molecules earned him, early in his career, the Fritz Kohlrausch Prize, the Austrian Physical Society’s most prestigious award, in 1976. His scientific achievement was recognized in 1997 with Austria’s highest science award, the Erwin Schrödinger Prize of the Austrian Academy of Science.Werner’s interests were broad and varied. He had a lively appreciation of art and music. He was a vigorous person physically. Like many Tiroleans, he was an accomplished skier. He also was an avid hiker and a regular tennis player. In later years, he became a serious equestrian, riding in two African safaris.Werner’s warm and outgoing personality led to the formation of many deep friendships in Boulder, many persisting actively throughout his life. He had friends not only at NOAA, but also at JILA and the University of Colorado’s chemistry department. Werner’s extraordinary joy of living made his friendship a rewarding and memorable experience. He will be sorely missed. Werner Lindinger PPT|High resolution© 2001 American Institute of Physics.