Cathode Fall Thickness Research Articles

The Influence of Different Plasma Cell Discharges on the Performance Quality of Surgical Gown Samples.

An experimental study was performed on a low-density plasma discharge using two different configurations of the plasma cell cathode, namely, the one mesh system electrodes (OMSE) and the one mesh and three system electrodes (OMTSE), to determine the electrical characteristics of the plasma such as current–voltage characteristics, breakdown voltage (VB), Paschen curves, current density (J), cathode fall thickness (dc), and electron density of the treated sample. The influence of the electrical characteristics of the plasma fluid in the cathode fall region for different cathode configuration cells (OMSE and OMTSE) on the performance quality of a surgical gown was studied to determine surface modification, treatment efficiency, exposure time, wettability property, and mechanical properties. Over a very short exposure time, the treatment efficiency for the surgical gown surface of plasma over the mesh cathode at a distance equivalent to the cathode fall distance dc values of the OMTSE and for OMSE reached a maximum. The wettability property decreased from 90 to 40% for OMTSE over a 180 s exposure time and decreased from 90 to 10% for OMSE over a 160 s exposure time. The mechanisms of each stage of surgical gown treatment by plasma are described. In this study, the mechanical properties of the untreated and treated surgical gown samples such as the tensile strength and elongation percentage, ultimate tensile strength, yield strength, strain hardening, resilience, toughness, and fracture (breaking) point were studied. Plasma had a more positive effect on the mechanical properties of the OMSE reactor than those of the OMTSE reactor.

Investigation on the similarity law of low-pressure glow discharges based on the light intensity distributions in geometrically similar gaps

Experimental investigation of the light intensity distributions of a low-pressure glow discharge is carried out in several pairs of geometrically similar plane-parallel gaps, of which the aspect ratios and the products of the linear dimension and the gas pressure are the same. The discharge images are captured using a Charge Coupled Device camera, from which the corresponding axial light intensity distributions are presented. Based on the obtained light intensity distributions, the thicknesses of cathode fall layers were identified by measuring the distance between the peak glow position and the cathode boundary. The influence of the discharge current on the light intensity distributions on the geometrically similar gaps is also investigated. It was found that, for discharges in each pair of geometrically similar gaps, the reduced cathode fall thicknesses are observed to be identical when the discharge currents are the same. The similarity relation of the cathode fall thickness is validated for low-pressure glow discharges in gaps for different aspect ratios.

Discharge state transition and cathode fall thickness evolution during chromium HiPIMS discharge

The temporal evolutions of target voltage and current waveforms under different pulse voltage and working pressure conditions were studied during Cr high power impulse magnetron sputtering discharges. Target voltage and current characteristics demonstrated that when the pulse width was set as 200 μs, HiPIMS discharge went through a four-stage sequence during each pulse, Townsend discharge, glow discharge, afterglow, and pulse-off stages. A discharge state transition in the glow discharge stage happened at high pulse voltage and working pressure conditions. Furthermore, the dependence of reduced cathode fall thickness pdc on pulse voltage, working pressure, and normalized current density j/p2 was presented. It was found that gas rarefaction leads to a change of relationship between pdc and j/p2. A noticeable increase of the cathode fall thickness caused by gas rarefaction has been found.

Cathode fall thickness of abnormal glow discharges between parallel-plane electrodes in different radii at low pressure

In order to investigate the influence of electrode radius on the characteristics of cathode fall thickness, experiments of low-pressure (20 Pa ≤ p ≤ 30 Pa) abnormal glow discharge were carried out between parallel-plane electrodes in different radii keeping gap distance unchanged. Axial distributions of light intensity were obtained from the discharge images captured using a Charge Coupled Device camera. The assumption that the position of the negative glow peak coincides with the edge of cathode fall layer was verified based on a two-dimensional model, and the cathode fall thicknesses, dc, were calculated from the axial distributions of light intensity. It was observed that the position of peak emission shifts closer to the cathode as current or pressure grows. The dependence of cathode fall thickness on the gas pressure and normalized current J/p2 was presented, and it was found that for discharges between electrodes in large radius the curves of pdc against J/p2 were superimposed on each other, however, this phenomenon will not hold for discharges between the smaller electrodes. The reason for this phenomenon is that the transverse diffusions of charged particles are not the same in two gaps between electrodes with different radii.

The Magnetized Plasma Effect on Cathode Fall Thickness for Helium Gas Discharge

Our previous study showed that the thickness of the cathode fall region in magnetized DC argon plasma was about 3.3mm has been investigated using two different methods, namely: the axial potential distribution and the current density distribution along the glow discharge regions. The present study demonstrates the same measurements but carried out for Helium gas discharge at the edge and center of the cathode surface for the same characteristic of the DC (cold cathode) magnetron sputtering unit. The Ia–Va characteristic curves of the glow discharge and the axial potential distribution and the current density distribution have been investigated. Under the influence of magnetic field, the thickness of the cathode fall region for he discharge is about 2.5mm for the two methods in pressure (P) range of 1-4mbar. Apparently for helium gas discharge, a reduction of the cathode fall thickness (about 20%) has been found in the presence of a magnetic field at the center of the cathode and (about 37%) at the edge, furthermore stronger electric field at the edge of the cathode fall, and hence high rates of sputtering are expected. Keyword: DC glow discharge; cathode fall thickness; helium gas discharge; magnetic field PACS number; 52.25.Xz. Original Research Article Physical Science International Journal, 4(8): , 2014 1089

Disinfection of Microbes by Magnetized DC Plasma

A different combined effects for helium gas discharge such as: magnetic field strength, breakdown voltage, applied power, applied pressure, cathode fall thickness, edge effect, distribution of the electron temperature and density, and finally exposure time for Staphylococcus aureus substrate over slides at the cathode edge, are discussed under the influence of cold, nonthermal plasmas, ultra low pressure, and presence of the magnetic field for disinfection of bacteria for short exposure times, compatible to International Commission on Non-Ionizing Radiation Protection, Health Phys (ICNIRP) for healing applications. Furthermore, analyses of the experimental data of initial and final densities of cells alive, using survival curves, showed an impressive inhibitory effect of plasma discharge to the remaining survival of bacterial ratio under the influence of the magnetic field.

Determination of cathode fall thickness in magnetized dc plasma for argon gas discharge

The thickness of the cathode fall region (dc) in magnetized dc argon plasma has been investigated using two different methods, namely the axial potential distribution and the current density distribution along the glow discharge regions. The measurements have been carried out at the edge and center of the cathode surface. Dc (cold cathode) magnetron sputtering unit has been used. The radial and axial distributions of the magnetic field B, the Ia–Va characteristic curves of the glow discharge and the axial potential distribution and the current density distribution have been investigated. The thickness of the cathode fall region was between 2 and 3.3 mm for the two methods in pressure (P) range of 0.53–4 mbar. It is concluded that a noticeable reduction of the cathode fall thickness (about 33%) has been found in the presence of a magnetic field and at the center of the cathode and stronger electric field at the edge of the cathode fall, and hence high rates of sputtering are expected.

Cathode fall model and current-voltage characteristics of field emission driven direct current microplasmas

The post-breakdown characteristics of field emission driven microplasma are studied theoretically and numerically. A cathode fall model assuming a linearly varying electric field is used to obtain equations governing the operation of steady state field emission driven microplasmas. The results obtained from the model by solving these equations are compared with particle-in-cell with Monte Carlo collisions simulation results for parameters including the plasma potential, cathode fall thickness, ion number density in the cathode fall, and current density vs voltage curves. The model shows good overall agreement with the simulations but results in slightly overpredicted values for the plasma potential and the cathode fall thickness attributed to the assumed electric field profile. The current density vs voltage curves obtained show an arc region characterized by negative slope as well as an abnormal glow discharge characterized by a positive slope in gaps as small as 10 μm operating at atmospheric pressure. The model also retrieves the traditional macroscale current vs voltage theory in the absence of field emission.

Analytical model of a longitudinal hollow cathode discharge

This paper presents a simple analytical model of a longitudinal hollow cathode discharge used in metal vapour lasers. The model describes the principle relations between the voltage, current, plasma density and axial structure of the discharge. Contrary to standard dc discharges, this discharge does not require electron multiplication in the cathode fall (CF) to produce ions, but rather to satisfy the electron energy balance. A self-sustainment condition is obtained from the energy balance per electron–ion pair. From this, it follows that there is a maximum voltage at which the CF thickness tends to zero and the current density tends asymptotically to infinity. The discharge develops axial non-uniformity and an axial electric field in order to evacuate the created electrons to the anode, such that the characteristic time for transport losses is the same for electrons as for ions. The axial profiles of the current density, plasma density and potential are obtained from the electron continuity equation. It is shown that additional energy absorption from the axial field, similar to electron heating in dc positive columns, modifies the self-sustainment condition and thus leads to a shift in the voltage–current characteristic, depending on the cathode length.

Electrical characteristics of (N2-H2) gas mixture DC glow discharge

The electrical characteristics of DC Nitrogen-Hydrogen mixture glow discharge were investigated. The discharge current-voltage (I-V) characteristic curves of the discharge were measured at different gas pressures and gas mixture percentage. The axial distribution of both potential and electric field were studied at different H2percentage. The existence of the reversal electric field in the negative glow region was confirmed. Using the axial electric field distribution, the cathode fall thickness (Xc) was calculated. Second derivative of the single Langmuir probe electron current method was used to measure the electron energy distribution function (EEDF) in (N2-H2) gas mixture DC glow discharge at different H2 percentage. In the positive column region, a Maxwellian EEDF was found at different conditions, while in both cathode fall and negative glow regions, a non-Maxwellian EEDF was observed. Two groups of electrons were detected in these two regions. Key words: Glow discharge, axial potential and electric field distributions, Paschen's curves, Langmuir probes.

Influence of cathode temperature on the parameters of an atmospheric pressure dc glow discharge

It is shown that the cathode temperature has a significant influence on the properties of the cathode fall in a self-sustained, normal, dc, atmospheric pressure glow discharge in helium and, in particular, on the electric field distribution, the thickness of the cathode fall layer, the current density and the gas temperature. An increase in the cathode temperature leads to an increase in the gas temperature and cathode fall thickness, a decrease in the current density and a modification in the radial distribution of the electric field close to the cathode surface. A discussion based on a one-dimensional model of the cathode fall region including gas heating is presented.

Modeling of microdischarge devices: Pyramidal structures

Microdischarge (MD) devices are plasma sources typically operating at 100s Torr to atmospheric pressure with dimensions of 10s–100s μm. Their design in based on pd (pressure×characteristic dimension) scaling; smaller dimensions are enabled by higher operating pressures with typical devices operating with pd=1–10 Torr cm. MD devices have exhibited behavior that resemble both Townsend and hollow-cathode discharges, with bulk and beam electrons providing the dominant excitation, respectively. In this article, results from a two-dimensional computational study of MD devices operating in neon using a pyramidal cathode structure are discussed. Pressures of 400–1000 Torr and device dimensions of 15–40 μm are investigated. The onset of behavior resembling negative glow discharges with decreasing pressure correlates with an extension of cathode fall accelerated beam electrons into the bulk plasma. For constant applied voltage, peak electron densities increase with increasing pressure as the beam electrons are slowed in more confined regions. The MD devices typically require higher applied voltages to operate at lower pressures, and so resemble discharges obeying Paschen’s curve for breakdown. MD devices having similar magnitudes and spatial distributions of plasma and excited state densities can be obtained to dimensions of <15 μm by keeping pd and current density constant, and having a cathode fall thickness small compared to the characteristic dimension.

Cathode fall parameters of a self-sustained normal glow discharge in atmospheric-pressure helium

Results from comprehensive studies of a high-current self-sustained glow discharge in atmospheric-pressure helium are presented. The main parameters of the cathode fall, namely, the electric field pro-file, cathode fall thickness, current density, gas temperature, and heat flux to the cathode are determined. The results obtained are discussed using one-dimensional models of the cathode fall with allowance for volumetric heat release.

Application of the Child-Langmuir law to magnetron discharge plasmas

In order to investigate the influence of magnetic field on magnetron plasmas, the dependence of discharge-current density (J/sub d/) on magnetic flux densities (B) perpendicular to the electric field over the whole discharge space in a magnetron apparatus with flat multi-targets has been studied. The dependencies on working gas pressures (P) and voltages (V) have been also studied in the configuration. From the experimental results, it was found that J/sub d/ was approximately proportional to B/sup 2/, P/sup 2/ and V/sup 3/2/. The sheath structure in the apparatus has been investigated as a function of B by both a probe method and a spectroscopic one. The cathode-fall thickness was roughly proportional to B/sup -1/. From a numerical analysis on kinetics of /spl gamma/ electrons emitted from the cathode surface, it was found that the dependence of J/sub d/ on B could be explained by the Child-Langmuir law, in which the sheath thickness was replaced by the equivalent electron Larmor radius.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Emission spectroscopy of the plasma in the cathode region of N2-H2 abnormal glow discharges for steel surface nitriding

Plasma analysis using emission spectroscopy has been carried out in the cathode fall and the negative glow of N2-H2 abnormal glow discharges for steel surface nitriding. The spatial distributions of the selected excited states of N2, N, N2+, N+, NH, H2, H and Fe have been measured in terms of their dependence on the main process parameters, such as the cathode temperature (400-550 degrees C), H2 percentage in the gas mixture (10-70%) and total pressure in the nitriding reactor (400 and 800 Pa), corresponding to the given values of the discharge current (115-260 mA) and applied voltage (465-635 V). The experimental data presented yield reliable information on the occurrence of excited states in the cathode region and on the effect of the process parameters on the cathode fall thickness, which determines the energy distributions of ions bombarding the cathode and is an important input parameter in plasma process modelling.

Analysis of the cathode fall in high-voltage low-current gas discharges

An analytic investigation is made of the cathode-fall region in high-voltage low-current gas discharges. Explicit expressions for the variation of electric field, voltage drop, charge density, Joule heating, and cathode-fall thickness are obtained as functions of pressure, current density, gas properties, and distance from cathode. To first order, it is shown that the former two quantities depend on the two-thirds power of the ratio of the current density to pressure squared, that the electric field varies linearly in the cathode-fall region, and that the fall thickness is independent of the operating current and varies inversely with pressure. To illustrate the order of magnitudes involved, a numerical calculation is made using helium as the working gas.

High-Voltage Glow Discharges inD2Gas. II. Cathode Fall Theory

A one-dimensional theoretical treatment of the cathode fall region of 40-100 kv glow discharges in ${\mathrm{D}}_{2}$ gas is presented. The region is assumed to be fed with an external supply of slow ${\mathrm{D}}_{2}^{+}$ ions from an adjoining plasma at one boundary and an influx of secondary electrons from a cathode forming a second boundary. As a function of the current density of plasma ions the following quantities are calculated: the energy distributions and fluxes of ${\mathrm{D}}^{+}$, ${\mathrm{D}}_{2}^{+}$, and D particles incident on the cathode, the cathode electron current, the thickness of the cathode fall region, the ratio of cathode fall electron current to plasma ion current, the potential distribution including the effect of positive-ion space charge, and the voltage vs current characteristics for a fixed cathode fall thickness. The theoretical results are compared with the experimental results of Part I. Using the available cross-section data from the literature, account is taken of ${\mathrm{D}}_{2}^{+}$ charge exchange and dissociation, electron capture by ${\mathrm{D}}^{+}$, electron loss from D, ionization of the gas by fast ${\mathrm{D}}^{+}$, ${\mathrm{D}}_{2}^{+}$, and D, and ionization by cathode secondaries. Arbitrary approximations are made to a few of the unknown cross sections.