Probe Measurements of Wave Fluctuations Excited in Glow Discharges

Fluctuations of the electric field, the charged particle density, the electron temperature and the plasma potential are simultaneously measured by a probe in the positive column of helium glow discharge at a few torr gas pressure by exciting a small amplitude ionization wave. It is proposed that these values can be determined by analyzing the probe current.     

Wall Recombination in Glow Discharges

We derived a model describing the radial distribution of the ion and electron densities and the radial electric field strength in the positive column of a glow discharge. The set of equations related to the plasma consists of the equations for particle and momentum conservation for the ions and electrons and the Poisson-equation. In a novel approach, the necessary boundary conditions in our model result from a system of balance equations for the charge carriers on the insulated wall surrounding the positive column.     

Negative Ion Kinetics in RF Glow Discharges

Using temporally and spatially resolved laser spectroscopy, we have determined the identities, approximate concentrations, effects on the local field, and kinetics of formation and loss of negative ions in RF discharges. Cl- and BCl3- are the dominant negative ions found in low-frequency discharges through Cl2 and BCl3, respectively. The electron affinity for Cl is measured to be 3.6118 ± 0.0005 eV. Negative ion kinetics are strongly affected by application of the RF field. Formation of negative ions by attachment of slow electrons in RF discharges is governed by the extent and duration of electron energy relaxation. Similarly, destruction of negative ions by collisional detachment and field extraction is dependent upon ion energy modulation. Thus, at low frequency, the anion density peaks at the beginning of the anodic and cathodic half-cycles after electrons have attached but before detachment and extraction have had time to occur. At higher frequencies, electrons have insufficient time to attach before they are reheated and the instantaneous anion density in the sheath is greatly reduced. When the negative ion density is comparable to the positive ion density, the plasma potential is observed to lie below the anode potential, double layers form between sheath and plasma, and anions and electrons are accelerated by large sheath fields to electrode surfaces.     

针-板和针-水电极大气压直流辉光放电的光谱特性

利用光谱学方法,对针-水电极和针-板电极直流辉光放电特性进行了比较研究。结果发现两种装置产生的放电都有明显的分区现象, 从阴极到阳极分别为负辉区、阴极暗区、正柱区和阳极辉区。针-板电极放电中可以清晰地观测到阳极暗区, 而针-水电极放电阳极暗区不明显。对比两种放电的伏安特性曲线,发现放电电压均随电流增大而减小,但相同电流下针-水电极间的电压大于针-板电极间的电压。由于伏安特性具有负斜率,且放电电流密度介于10-5～10-4 A·cm-2,说明两种装置中的放电均处于正常辉光放电阶段。在正常辉光放电的范围内比较两种放电的发射光谱, 发现发射光谱中都包含N₂的第二正带系(含波长为337.1 nm的谱线)和N+₂的第一负带系(含波长为391.4 nm的谱线),但相对强度不同。利用光谱学方法对放电发射谱的谱线强度比I_391.4／I_337.1和振动温度进行了空间分辨测量,发现相同位置处针-水电极放电的谱线强度比要比针-板电极放电的大,并且相同位置处针-水电极放电的振动温度高。     

Ionization Waves in Glow Discharges with Non-Uniform Magnetic Fields

The properties of ionization waves self-excited in glow discharges with a non-uniform axial magnetic fields are investigated experimentally. The fundamental frequency remains almost constant over the tube for the variable magnetic fields, while the amplitude decreases with increasing the field. The wavelength becomes longer at the cathode side of the magnetic coil and shorter at the anode side with increasing the local magnetic field. The dependence of the wavelength on the magnetic field is explained by the changes of the axial electric field in the non-uniform magnetic field.     

Effect of Silane Pressure on Silane-Hydrogen RF Glow Discharges

The intensity and time dependence of optical emission from silane and silane-hydrogen radio-frequency (RF) discharges have been measured as a function of silane pressure (0.05-1.0 torr). The rate of emission of H* and Si* resulting from dissociative excitation was found to decrease with increasing silane pressure in a manner consistent with a similar decrease in average electron energy. Results from a Monte Carlo plasma simulation code were used to compute the rate of optical emission. Comparison of theory and experiment shows good agreement for emission intensities and confirms for discharges operating at constant pressure and power density a decrease in electron density and average electron energy with increasing silane partial pressure in mixtures of silane and hydrogen. The time-dependent spatially averaged emission intensity of H* is experimentally nonsymmetric with a shape that is a systematic function of silane partial pressure. This systematic behavior is reproduced by the plasma simulation and is attributed to the change in the dc bias of the powered electrode, which becomes less negative with increasing silane partial pressure.     

Similarity Relations for Low Pressure Glow Discharges in Noble Gases

A simple three level atomic model that includes excitation, ionization, stepwise ionization, and diffusion is used to determine similarity relations for low pressure noble gas discharges. A two-temperature electron energy distribution is also included in the model. Similarity relations involving the reduced electric field, the electron main body temperature, the electron tail temperature, and the pressure-radius product are developed. It is found that the reduced electric field and the electron tail temperature depend on the discharge current as well as the pressure-radius product. Good agreement is obtained between the similarity relation predictions and experimental results in helium and neon discharges.     

Electrolyte Cathode Atmospheric Glow Discharges for Direct Solution Analysis

Abstract

The electrolyte cathode atmospheric glow discharge (ELCAD) invented in 1992 is a new optical emission source with upcoming application in the field of environmental protection as an outstanding instrument for monitoring the toxic heavy metal content of waters and wastewaters. The main operating parameters, mechanisms (secondary electron emission from the electrolyte cathode, self‐sustaining processes in the cathode dark space, dependence of the emitted line intensities on the discharge parameters, temperatures), and the analytical performance of this special discharge are presented through a critical review using the papers related to the ELCAD published from 1993 to 2006.     

Field Calculations for Permanent Magnets used for Glow Discharges

If a permanent magnet has both a homogeneous polarization inside the material and a linear demagnetization characteristic in external fields, its magnetic field can be expressed using a surface pole model. For magnets satisfying these conditions and, in addition, having a rectangular shape, the fields at any given point in space can be calculated analytically. An algorithm for this calculation is presented in a form that can easily be implemented into a computer program. In our experiments we used Nd₂Fe₁₄B magnets to support low pressure glow discharges by magnetic fields. The magnets can be seen as composed of elementary magnets with rectangular shape, for which the magnetic field distribution is calculable. We present results of field calculations for various configurations of permanent magnets that we used in hollow cathode and Penning discharges.     

Low Temperature Dry Etching of Graphite Anodes in Hydrogen Glow Discharges

To discribe the temperature dependence of the etching rate of graphite in a hydrogen glow discharge a model is discussed taking into accout the transition from CH₄ to CH as the main etching product with increasing temperature.     

Electrostatic Probe Measurements in the Glow Discharge Plasma of a D. C. Magnetron Sputtering System

The characteristics of the plasma surrounding the substrates in a planar magnetron sputtering system with a graphite target have been investigated by electrostatic probe measurements. The behaviour of the ion density n_i and the electron temperature T_e, determining the ion flux that can be extracted by the substrate, with the variation of the basic system parameters, has been studied.     

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 ?s 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.     

Study of Spectral Emission Behaviour in the Glow and Microwave Discharges of Oxygen

Because of the very important role of oxygen plasmas in various applications, both direct current and microwave discharges have been analysed from the point of view of emission spectra properties. In both cases silica discharge tubes with practically the same diameters were used. The following transitions of the oxygen discharges were studied: the atomic lines at 777.4 nm (⁵P ‐ ⁵S)and 747.7 nm (³D ‐ ³P⁰), and the head at 759.4 nm of the atmospheric system (b ¹Σ⁺_g, v = 0 ‐ X ³Σ^—_g, v = 0).     

Comparison of Spectra Excited in HF and DC Hollow Cathode Discharges

Spectra excited in hollow cathode discharges operating at high frequency (27.2 MHz) and direct currents have been compared. The cathodes made of copper, brass and steel and argon as a carrier gas were used. Line intensity ratios (I_hf/I_dc) for species excited at different pressures have been investigated and discussed.     

Discharge Operation Mode Manipulation of Radio Frequency Atmospheric Pressure Glow Discharges in Argon

Stable operation of radio frequency atmospheric glow discharges is widely desired by most processing applications for their conveniency and feasibility, many of which have been addressed with the noble gas of helium and low discharge current density in stable operation regime of α mode. Here, the operation mode manipulation of rf atmospheric pressure glow discharge in argon is investigated using a one‐dimensional self‐consistent modeling approach. Stable operation regime of α mode is found to expand with rinsing excitation frequency and reducing discharge gap distance. The sheath characteristics in terms of sheath voltage and sheath thickness are given to explain the of α ‐γ mode transition. Furthermore, stable operation of discharge in γ mode is proposed tobe achieved by introducing dielectric barriers, which is indicated by monotonously growing of applied voltage with discharge current density (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Computational Studies of Atmospheric-Pressure Methane–Hydrogen DC Micro Glow Discharges

Atmospheric-pressure methane–hydrogen micro glow discharges were computationally investigated using a 2-D hybrid model. The plasma model was solved simultaneously with a model for the external circuit. Simulations were conducted for a pin-to-plate electrode configuration with an interelectrode separation of 400 ${rm mu}hbox{m}$. The spatiotemporal evolutions of electrons, species densities, electric field, and electron and gas temperatures were studied. A total of 81 reactions were considered, which included electron–neutral, electron–ion, ion–neutral, and neutral–neutral reactions. An 84-step reaction mechanism consisting of 15 surface species and four deposited bulk species was considered. A time-stepping technique was employed to address the time scales of plasma transport (in microseconds) and neutral and fluid transport (in milliseconds) in 2-D simulations with detailed volume and surface chemistry. The simulations indicated $hbox{H}_{3}^{+}$ and $hbox{CH}_{5}^{+}$ ions to be the most prominent hydrogen and hydrocarbon ions. The gas temperature predictions suggested the discharge to be operating as a nonthermal glow discharge. The effect of discharge current on both plasma and deposition characteristics was studied. The simulations predicted a flat voltage–current characteristic, indicating the discharge to be operating in normal glow mode. The predicted voltage–current characteristic was found to be in favorable agreement with the experimental measurements. With an increase in discharge current, the deposition rate profile expanded in the lateral direction, suggesting that deposition occurred at the cathode spot.      

UV Radiation of Low Pressure XeCl* and KrCl* Glow Discharges

The positive column of low pressure DC glow discharges in Xe/Cl₂ and Kr/Cl₂ gas mixtures has been investigated with respect to its UV radiation power and radiative efficiency for a wide range of parameters (total gas pressure: 1–30 mbar, current 10–30 mA, partial pressure of Cl₂: 1–33%). Also the radial distribution of the particle number densities of XeCl* (B) and KrCl* (B) has been determined by absolutely calibrated emission spectroscopy. Optimum UV (190–350 nm) output and efficiency has been found in gas mixtures with a relative Cl₂ partial pressure of 2–3% and total gas pressure of 12–18 mbar. At these parameters, the UV radiation power per column length is greater than 40 Watt/m with a radiative efficiency of 15–18%. These discharges could be used for UV induced photochemical processes.     

Some Investigations of the Ignition and Development of Breakdown in Cylindrical Hollow Cathode Glow Discharges

The breakdown behaviour of a hollow cathode glow discharge is investigated in a cylindrical, hollow cathode structure having an internal diameter of 2 cm. The anode is a plane electrode across one end of the cathode cylinder. Pressures of argon between 20 and 107 Pa were used (0.15 to 0.80 torr), and applied voltages between 800 and 2500 V. It is shown that the statistical time lag for breakdown is in the range of ～ 1 ms and depends on the applied voltage, the gas pressure, and the history of operation of the discharge tube. The rise time of the discharge current ranges from about 10 ns at high pressure and voltage to about 200 ns at the lowest pressure and voltage used. The discharge propagates along the cathode axis at a speed of about 10⁸ cm s^?1. From the obtained data, a qualitative model of the first stage of the discharge is derived. Based on this model, a simple calculation gives values of Townsend modified first coefficient η at high values of E/N, 10⁴ < E/N < 8 · 10⁴ Td which fit well at the lowest E/N, where they approach the data of PENNING and KRUITHOF in argon. In contrast to the extremely short initial current-rise times, in the submicrosecond range, the discharge currents reach steady-state values only after about 300 μs.     

Open Resonators Excited by Wave Beams

We study open resonators on the basis of the scattering theory. In particular, we develop the theory of an open resonator with a dielectric plate. This theory makes it possible to find the eigenfrequencies of such a system and their dependences on various parameters. Dispersion due to the dielectric plate is determined. The theory developed can be used as a basis for the method of measuring extremely small dielectric losses in the short-wavelength part of the millimeter-wave band. We also analyze the properties of a two-mirror resonator as a reflective filter and formulate the quality requirements for the exciting beam.