Ionic and atomic absorption in stable spark discharges

Transient ionic and atomic emission-absorption measurements on a train of positionally-stable copper spark discharges are described. The measurements are made as a function of discharge current duration at constant amplitude and rise time. Spatial observation zones are isolated in the spark gap with a set of masks and a relay lens to estimate the movement of electrode vapor. Absorption measurements required custom circuitry for a pulsed hollow-cathode backlight and synchronously gated dual-channel boxcar integrator. Results indicated that electrode vapor moves primarily along the interelectrode axis in response to current duration, with substantial ionization remaining in the post-discharge period. Neutral atoms remain in the gap for long times after current cessation.     

High frequency vacuum spark discharge. Electrical and optical characteristics. Degree of ionization

We report on a study of the amplitude, shape and temporal features of the current, voltage and optical radiation of singly- and multiply-charged ions of the matrix in a high frequency vacuum spark discharge. The breakdown currents reach a few tens of amperes in a pulse of duration of a few tens of nanoseconds.Observations of the optical radiation of the discharge plasma show that multiply-charged ions are first formed rather than singly charged ones.The degree of ionization versus the high frequency voltage on the gap has been studied. For conventional mass-spectrometer conditions, the degree of ionization in the spark-discharge is shown not to exceed 1%.     

A High-Efficiency Reactor for the Pulsed Plasma Conversion of Methane

The authors recently developed a high-frequency pulsed plasma process for methane conversion to acetylene and hydrogen using a co-axial cylindrical (CAC) type of reactor. The energy efficiency represented by methane conversion rate per unit input energy has been improved so that such a pulsed plasma has potential for commercial acetylene production. A pulsed plasma consists of a pulsed corona discharge and a pulsed spark discharge. Most of energy is injected over the duration of the pulsed spark discharge. Methane conversion using this kind of pulsed plasma is a kind of pyrolysis enhanced by the pulsed spark discharge. In this study, a point-to-point (PTP) type of reactor that can produce a discharge channel over the duration of a pulse discharge was used for the pulsed plasma conversion of methane. The energy efficiency and carbon formation on electrodes have been improved. The influences of pulse frequency and pulse voltage on methane conversion rate and product selectivity were investigated. The features of methane conversion using PTP and CAC reactors were discussed.     

Acid-generating effect of plasma species and pulsed ultraviolet plasma radiation

The yield of acid residues generated in water at pH 3–13 by pulsed plasma radiation of spark discharge with a power of 0.59 W has been studied. The power per pulse of electrical discharge has been optimized to obtain the maximum chemical effect in water. The results are compared with similar data obtained by treatment with 900-W gliding discharge. It has been found that the energy efficiency of pulsed plasma radiation is 100 times that of gliding discharge plasma. The results make it possible to clarify the mechanism of the gliding discharge-induced formation of peroxynitrite.     

Einfluß des Gefüges und der Entladungsgase bei der Spektralanalyse von Al-Legierungen

Studies of the spectrochemical behaviour of aluminium alloys subjected to spark discharge at high or medium frequency demonstrate how particularly type and concentration of the alloying element, the discharge gas and structure of the alloy are bearing down on the shape of the analytical curve and influence the quantitative analysis. Changes in the microstructure of Al-alloys, discovered both after spark and general treatment of the samples, shift the analytical curve markedly whenever Cu, Mg or Zn is the alloying element. Spark generating in different ambient gases yields similar effects. Heat developing during spark discharge initiates surface melting on the electrodes with the consequence of metallurgical changes in the original alloy. Proof was given by microprobe analysis that a sample becomes homogeneous in the course of this secondary localized melting process. Depth and nature of the affected zone is solely given by the ambient gas during discharge, if all the other parameters during spark generation are kept constant. The study demonstrates the influence of (A) the thermo-chemical behaviour of the aluminium alloys and (B) of the physical properties of the alloying elements under investigation on the spectroanalytical results.     

Effects of 50 Hz sinusoidal magnetic fields and spark discharges on human lymphocytes in vitro

Human peripheral blood lymphocytes were exposed in vitro to 50 Hz sinusoidal magnetic fields of 30 μT, 300 μT, and 1 mT, and were monitored for genotoxic effects. No effects on chromosomal aberrations, micronuclei or proliferation indices were noticed. A weak effect on sister chromatid exchanges, noted in one series of experiments, could not be verified when the series was repeated. Exposure of whole blood to spark discharges of up to 3.65 kVcm⁻¹ and 2.6 μs duration (10 pulses) did not result In chromosomal aberrations. Cell destruction from spark discharges was extensive. The conductivity of blood during a spark discharge pulse was examined and was shown to decrease by a factor of 2–3 at the beginning of the pulse owing to the β-relaxation of cells. After 0.5 μs the conductivity approached a constant level.     

Study of Spark Discharge Assisted to Enhancement of Laser-Induced Breakdown Spectroscopic Detection for Metal Materials

In this work, laser triggered spark discharge was combined with laser-ablation under Air and Ar gases to investigate the characteristics of laser-ablated plasma emission. The experimental results show that the optical emission intensity is significantly enhanced by electric discharge compared to without discharge and the spectral emission time of plasma is much longer than that without discharge. The enhancement effect is more apparent in the presence of Ar ambient. In addition, the plasma temperature and electron density as well as limits of detections (LOD) have been determined. The better LOD can be attributed to the improvement of plasma. The higher plasma temperature and electron density indicate that the enhanced mechanism in emission intensity is predominated by the further excitation/ionization of the laser-ablated material by the spark discharge due to the energy deposition in the plasma.     

Atomic Oxygen Concentration in a Flowing Post-Discharge Reactor

Concentration of neutral oxygen atoms in the flowing post-discharge of a pure oxygen microwave discharge at different experimental conditions was determined with a nickel catalytic probe. The post-discharge reactor was setup for metal surface cleaning. It worked at the pressure between 20 and 100 Pa and at output power of the microwave plasma generator between 80 and 150 W. At those experimental conditions the O-atom density was found to be of the order of 10²¹ m^–3. It increased both with increasing pressure and microwave power. The degree of dissociation of oxygen molecules, on the other hand, decreased with increasing pressure.     

Influence of spark generator parameters in the analysis of graphite electrodes by spark source mass spectrometry

Spark source mass spectrometric analyses of doped carbon samples were performed at different pulse lengths, pulse frequencies and breakdown voltages. The pulse length and pulse frequencies had no influence on the analysis. The relation between breakdown voltage, spark gap and sample matrix was determined. With increasing breakdown voltage, the abundance of the multiply charged ions was found to decrease, whereas the abundance of the cluster ions increased. The breakdown voltage was also found to affect the absolute and relative intensities of different elements in different manners. Interpretation of these results led to some new considerations on the phenomena in a spark discharge.     

Evaporation of material and influence of auxiliary spark gap on the spectral excitation by means of laser emission spectroscopy for local analysis of graphite

For the determination of impurities in graphite, laser-micro-emission spectroscopy allows the analysis of much smaller sample areas than possible by spectroscopic analysis of arc or spark discharges.Because the maximum quantity of material which can be evaporated by a laser beam is only approximately 8μg, it was found necessary to introduce a spark gap above the evaporation pit to provide additional excitation of the plasma, thereby increasing the sensitivity of the analysis. In this way, the radiation intensity of the plasma was increased by two orders of magnitude.The effects of the spark gap parameters, voltage, capacitance and inductance, on the spectral excitation were investigated. Voltage and capacitance determine the energy supplied to the spark gap, whereas capacitance, together with inductance, controls the duration and characteristic of the discharge. To obtain the optimum additional excitation, the duration of the spark discharge had to be matched with the time taken for the material to evaporate.The catalytic effect of some impurity elements on the corrosion of graphite was identified by using the technique to analyse material in the corrosion pits of irradiated graphite fuel elements.     

Air microwave-induced plasma: Detection of NO in the post-discharge using an ArF laser

ArF laser-induced fluorescence (LIF) spectroscopy was applied to the investigation of the density of NO achieved in the post-discharge of an N₂-O₂ microwave-induced plasma (2450 MHz). The fluorescence spectrum provides a determination of NO vibrational temperature downstream the discharge. Absolute concentration profiles were determined in the low-temperature region of the post-discharge.     

Langmuir probe measurements of axial variation of plasma parameters in 27.1 MHz rf oxygen planar discharges

Langmuir probe studies have been performed on rf (27.1 MHz) discharges in O₂ under planar reactor conditions to determine the axial variation of the plasma parameters (positive ion density, electron temperature, and dc space potential) as a function of pressure (20–220 Pa) and power (10–150 W) or current (0.1–2 A). By monitoring the second derivative of the I–V probe characteristics, the suppression of the rf component in the probe circuit can be optimized. Referring to this problem, numerical studies provide relations for the determination of the residual rf component as well as of the dc component of the plasma potential at incomplete rf compensation. The positive ion density is obtained from the ion saturation currents. Here the effect of collisions between ions and neutral particles within the probe sheath (for p> 100 Pa) is considered. The electron energy distribution function is found to be of the Maxwellian type for all discharge conditions investigated here. If the pressure and the power exceed critical values, the axial charge carrier distribution is characterized by a valley formation in the bulk plasma center. This fact demonstrates that secondary electron emission due to ion impact on the electrode surfaces and following ionization by these electrons near the sheaths in front of the electrodes are significant processes for sustaining the discharge. At low pressures (p60 Pa) the dc plasma potential was found to be identical with the half-peak maintaining voltage of the discharge, in agreement with the model idea of a symmetric rf planar discharge where the rf voltage drop across the bulk plasma can be neglected. For growing pressure, however, the plasma system moves gradually toward a situation where the V-I characteristics of the discharge are significantly controlled by processes in the bulk plasma. This transition depends on the current density.     

Redox processes induced in methemoglobin solution by mercury lamp UV light and spark-discharge plasma radiation

The buildup of–SH groups in methemoglobin by irradiation with emission of spark discharge plasma in air and UV light from a low-pressure mercury lamp (λ = 253.7 nm) has been studied. The concentration of–SH groups increases the during spark discharge treatment, with the methionine amino acid residue in the methemoglobin molecule being reduced by nitrous and peroxynitrous acids; the bonds in the methemoglobin molecule are destroyed within 20 minutes of the spark treatment. By the action of UV radiation, methionine is reduced through chromophores (tryptophan, tyrosine) and methemoglobin is reduced by O₂^.- radical ions to oxy- and deoxyhemoglobin.     

Stabilization of the spark-discharge point on a sample surface by laser irradiation for steel analysis.

A combined technique with laser irradiation is suggested to control spark discharge for analytical use, having a unique feature that firing points of the spark discharge can be fixed by laser irradiation. Because the spark discharge easily initiates at particular surface sites, such as non-metallic inclusions, called selective discharge, the concentration of some elements sometimes deviates from their average one in spark discharge optical emission spectrometry. Therefore, stabilization of firing points on a sample surface could improve the analytical precision.     

Shrinkage effects in photopolymerizable resins containing filling agents: Application to stereophotolithography

The influence of porous additives with various structures on the shrinkage of resins used for stereolithographic applications was studied by measuring the dependence of the specific gravity of the charged polymer material as a function of the charge fraction. The investigations were conducted on difunctional monomers photopolymerized either by a radical (acrylate) or a cationic (epoxy) mechanism. The improvement resulting from the incorporation of charges in the formulation is accounted for in a semi-quantitative way by an adsorption-desorption equilibrium of monomer molecules between the pores and the bulk phase. Thus, the pores of the charges which are initially filled with monomer molecules play the role of buffers which discharge their content as the photopolymerization develops in the liquid phase. It is also worth mentioning that the viscosities of the photosensitive charged resins used in these experiments are quite suitable for use in stereophotolithography. © 1992 John Wiley & Sons, Inc.     

Features of the Current Sustainment in a Low-Current Discharge in Airflow

The paper relates to the investigations of a low-current discharge in a vortex airflow with the electrode configuration corresponding to classical coaxial plasmatron. The gas flow rate is varied from 0.1 to 0.3 g/s at an inner diameter of the plasmatron nozzle of 5 mm. The discharge is powered by dc voltage via a ballast resistor. Typical averaged current is changed from 0.06 to 0.15 A so that a maximum averaged power dissipated in the discharge amounts to 160 W. In these conditions, a luminous gas region at the plasmatron exit, which in most publications is associated with a plasma jet, is observed. The method for the jet diagnostics based on a usage of the additional electrodes at the plasmatron exit has been proposed. The main idea of the experiments is the elucidation of the problem whether the jet actually represents the plasma area or we have to apply the term “plasma” with care. In particular, in the case under discussion the main charged particles in the jet are electrons that are emitted from a plasma column located in the plasmatron nozzle. The model that describes the formation of electron flow in the jet has been proposed. Typical electron density in the jet estimated with a usage of the model is at a level of 10⁹ cm^?3.

     

Spark discharge assisted laser induced breakdown spectroscopy

Laser induced breakdown spectroscopy is combined with a spark discharge to operate in a laser triggered spark discharge mode. This spark discharge laser induced breakdown spectroscopy (SD-LIBS) is evaluated for Al and Cu targets in air under atmospheric pressure. Significant enhancement in the measured line intensities and the signal-to-background ratios, which depend on the spark discharge voltage and the laser fluence, is observed in spark discharge laser induced breakdown spectroscopy when compared to laser induced breakdown spectroscopy alone for similar laser conditions. The measured line intensities increase with the applied voltage for both targets, and the ratio of the measured line intensity using spark discharge laser induced breakdown spectroscopy to that using laser induced breakdown spectroscopy is found to increase as the laser fluence is decreased. For Al II 358.56, such intensity enhancement ratio increases from 50 to 400 as the laser fluence is decreased from 48 to 4 J/cm² at an applied voltage of 3.5 kV. Thus, spark discharge laser induced breakdown spectroscopy allows for using laser pulses with relatively low energy to ablate the studied material, causing less ablation, and hence less damage to its surface. Moreover, applying spark discharge laser induced breakdown spectroscopy gives up to 6-fold enhancement in the S / B ratio, compared to those obtained with laser induced breakdown spectroscopy for the investigated spectral emission lines.     

Simulation of Excimer Ultraviolet (EUV) Emission from a Coaxial Xenon Excimer Ultraviolet Lamp Driven by Distorted Bipolar Square Voltages

Simulation of excimer ultraviolet (EUV) emission from a coaxial xenon excimer ultraviolet lamp driven by distorted bipolar square voltages is presented in this study. A self-consistent radial one-dimensional fluid model, considering local mean energy approximation (LMEA), along with a set of simplified xenon plasma chemistry was employed to simulate the discharge physics and chemistry. Emitted powers of EUV light and deposited powers to the charged species were simulated by varying the values of four key parameters, which include the driving frequency, gas pressure, gap distance and number of dielectric layers. Results show that there are three distinct periods that include pre-discharge, discharge and post-discharge ones. It is found that intensive EUV (172 nm) emission occurs during the early part of the discharge period, which correlates very well in time with the power deposition through electrons. In addition, power deposition through \textXe ⁺ {\text{Xe}}^{ + } and \textXe₂ ⁺ {\text{Xe}}_{2}^{ + } occurs mainly in the discharge period and later part of discharge period, respectively. Surprisingly, the emission efficiency of 172 nm increases slightly with increasing driving frequency of power source, while it increases dramatically with increasing gap distance. In addition, the maximal emission efficiency is found to take place at gas pressure of 600 torr. The emission efficiency of one-dielectric case is found to be better than that of two-dielectric one. The underlying mechanisms in the above observations are discussed in detail in the paper.     

Improvement of Atmospheric Water Surface Discharge with Water Resistive Barrier

Atmospheric water surface discharge is a promising method for water treatment. The selection of discharge gap distance must take a pair of conflicting aspects into account: the chemical efficiency grows as the discharge gap distance decreases, while the spark breakdown voltage decreases as the gap distance decreases. To raise the spark breakdown voltage and the chemical efficiency of atmospheric pressure water surface discharge, resistive barrier discharge is introduced in this paper. Both the high voltage electrode and the ground electrode are suspended above water surface to form an electrode-water-electrode discharge system. The water layer plays the role of a resistive barrier which inhibits the growth rate of discharge current as voltage increases. Experiments conducted at different discharge gap distances and water conductivities indicate that both the spark breakdown voltage and the chemical efficiency are remarkably raised in comparison with traditional water surface discharge. After parameter optimization, the discharge reactor is scaled up with activated carbon fiber electrodes and advantages of water resistive barrier discharge are kept.     

Analysis and Review of Chemical Reactions and Transport Processes in Pulsed Electrical Discharge Plasma Formed Directly in Liquid Water

Electrical discharges formed directly in liquid water include three general cases where (a) streamer-like plasma channels form in, but do not span, the electrode gap, (b) spark discharges produce transient plasma channels that span the electrode gap, and (c) arc discharges form plasma channels with relatively longer life times. Other factors including the input energy (from <1?J/pulse to >1?kJ/pulse) as well as solution properties and the rates of energy delivery affect the nature of the discharge channels. An understanding of the formation of chemical species, including the highly reactive hydroxyl radical and more stable molecular species such as hydrogen and hydrogen peroxide, in such plasma requires determination of temporal and spatial variations of temperature, pressure, plasma volume, and electrical characteristics including current, voltage (electric field), and plasma conductivity. In spark and arc discharges analysis of the physical processes has focused on hydrodynamic and thermal characterization, while only a limited amount of work has connected these physical processes to chemical reactions. On the other hand, the most successful model of the chemical reactions in streamer-like discharges relies on simple assumptions concerning the temperature and pressure in the plasma channels, while analysis of the physical processes is more limited. This paper reviews the literature on the mathematical modeling of electrical discharges in liquid water spanning the range from streamer-like to spark and arc discharges, and compares the properties and processes in these electrical discharges to those in electron beam radiolysis and ultrasound.