Average Discharge Current Research Articles

Perturbations induced by electrostatic probe in the discharge of Hall thrusters.

Emissive and Langmuir probes are two widely used plasma diagnostic techniques that, when used properly, give access to a wide range of information on the plasma's ions and electrons. We show here that their use in small and medium power Hall thrusters produces large perturbations in the discharge characteristics. Potential measurements performed by both probes and non-invasive Laser Induced Fluorescence (LIF) spectroscopy highlight significant discrepancies in the discharge profile. This phenomenon is observed both in the 200 W and the 1.5 kW-class thrusters. In order to have a better understanding of these perturbations, ion velocity distribution functions are acquired by LIF spectroscopy at different positions in the smaller thruster, with and without the probes. Emissive probes are shown to produce the biggest perturbation, shifting the acceleration region upstream. The probe insertion is also shown to have significant effect on both the average discharge current, increasing it by as much as 30%, and its harmonic content in both amplitude and spectrum. These perturbations appear as the probe tip passes a threshold located between 0 and 5 mm downstream of the thruster exit plane.

Characteristics of a quasi-stationary non-self-sustained pulsed gas discharge with a hot cathode

The paper presents the electrode system and main characteristics of a non-selfsustained pulsed discharge with a hot cathode operating in argon at a pulse repetition frequency of 5 kHz and pulse duration of 50 ps. It is found that during the operation at a pressure of 0.3 Pa, the discharge current can reach amplitudes of up to 300 A and more with a rather high average discharge current of ∼ 100 A and low average discharge operating voltage of less than 20 V. The ion current from the discharge plasma to a Langmuir probe oscillates at frequency of about 100 kHz.

Role of a Conducting Vacuum Chamber in the Hall Effect Thruster Electrical Circuit

The role of the electrically conductive vacuum chamber wall in the completion of the discharge circuit of a Hall effect thruster is experimentally investigated. The Aerojet Rocketdyne T-140 laboratory-model Hall effect thruster, operating at a discharge voltage of 300 V, a discharge current of 5.16 A, and an anode flow rate of , serves as a representative Hall effect thruster testbed. The nominal facility operating pressure during thruster firings is torr, corrected for xenon. Two , square aluminum plates are placed adjacent to, but electrically isolated from, the walls of the stainless-steel vacuum chamber at two locations with respect to the center of the thruster exit plane: 4.3 m axially downstream along the thruster centerline and 2.3 m radially outward centered on the exit plane. The plates are configured in three distinct electrical configurations with corresponding measurements: 1) electrically grounded plates with measurements of currents to ground, 2) electrically isolated plates with measurements of floating voltages, and 3) isolated but electrically connected plates with measurements of the current conducted between them. The measurements are all taken simultaneously, with the discharge current oscillations of the thruster at a sampling frequency of 100 MHz. Measurements of the current conducted to ground in the electrically grounded configuration reveal that the axial and radial plates collect ion currents that are 13.6 and 10.7% of the discharge current, respectively; the collected current is coupled to the discharge current oscillations but is smaller in magnitude and phase delayed. In the electrically connected-plate configuration, 5.5% of the average discharge current is observed to flow from the axial plate to the radial plate, driven by a floating voltage difference of 7.6 V; this current is uncorrelated in time with the discharge current oscillations. These results indicate that the vacuum chamber conducts current and is a recombination site for a significant number of plume ions during Hall effect thruster operation.

Determination of trace heavy metals in environmental and biological samples by solution cathode glow discharge-atomic emission spectrometry and addition of ionic surfactants for improved sensitivity

Solution cathode glow discharge-atomic emission spectrometry (SCGD-AES) was evaluated for its ability to determine toxic heavy metals, including cadmium (Cd), mercury (Hg), lead (Pb), and chromium (Cr), in environmental and biological samples. A significant enhancement in heavy metal signal was observed by addition of a small amount of cetyltrimethylammoniumchloride (CTAC, C16H33 (CH3)3NCl) to the samples. The net intensity of atomic emission lines of Cd, Hg, Pb, and Cr increased by 2.1-, 4.8-, 6.6-, and 2.6-fold, respectively, after addition of 0.15% CTAC to the test solutions. The effects of ionic surfactants (CTAC) compared with non-ionic surfactants, e.g., Triton x-45 and Triton x-100, on the sensitivity of Cd, Hg, Pb, and Cr were also investigated in the present study. The enhancement effect is in the order Triton x-45<Triton x-100<CTAC for Hg, Pb and Cr and Triton x-45<CTAC<Triton x-100 for Cd. Addition of CTAC to the electrolyte solutions decreased the background intensity and fluctuation of atomic emission lines of studied metals. It also changed the surface tension and the viscosity, and increased average discharge current of electrolyte solution. SCGD sensitivity to the heavy metals greatly improved by addition of the surfactant. The improved detection limits of Cd, Hg, Pb, and Cr were 1.0, 7.0, 2.0, and 42 ng mL(-1), respectively. The proposed method was validated by quantifying Cd, Hg, Pb, and Cr in certified reference materials, including human hair (GBW 09101b) and stream sediment (GBW 07310 and GBW07311). Measurement results obtained for the determination of Cd, Hg, Pb, and Cr in the reference materials agreed well with reference values. This study improves the application of SCGD-AES in quantifying very low levels of Cd, Hg, Pb, and Cr from biological and environmental materials.

Enhancing the Surface Quality by Iso Pulse Generator in EDM Process

In automobile and aeronautical industries, complex moulds and dies is produced by Electrical Discharge Machining process. The surface finish is determined by the crater volume in EDM process. The amount of crater volume is influenced by the amount and distribution of discharge energy. The discharge energy is directly proportional to the average discharge current. This amount of current is determined by the duration of discharging effect. This study deals about evaluating the performance of iso current pulse generator on machining characteristics in EDM. Due to its ability of reducing stochastic nature in EDM process, iso pulse generator could produce better surface finish than conventional transistor pulse train generator with higher material removal rate.

Low-Current “Gliding Arc” in an Air Flow

This paper describes the result of the investigation of a gas discharge in a flow of air with electrode geometry typical for the so-called gliding arc. The feature characteristic of the experimental conditions is a rather low level of an average discharge current (of about 0.2 A). The discharge is initiated due to a spark breakdown in the narrow part of the gap. After that, the spark discharge is transformed into a kind of a glow discharge. At the subsequent stage, the plasma column travels in the gap under the effect of a gas flow, and the current is sustained in the regime of a normal glow discharge. The typical cathode-voltage-drop value in this regime is estimated to be about 300 V, an average electric field in the positive column plasma is (700-800) V/cm, and the neutral-particle temperature in the negative glow region is T ≈ 1100 K.

Exploration of aqueous zinc-hydrogen peroxide batteries

SUMMARY In zinc–hydrogen peroxide batteries, an active metal − zinc piece is used as the anode and ammonium chloride is used as the electrolyte in the anode zone. A soluble oxidant, hydrogen peroxide, is used as the active cathode substance, and sulfuric acid as an electrolyte in the cathode zone. Carbon felt, the sum of two apparent areas of which is 24 cm2, is used as an inert cathode with a PE-01 homogeneous membrane between the anode and cathode zones and 50 mL of solution in both the anode and cathode zones. The discharge characteristics of the batteries at 5 Ω were investigated for various concentrations of hydrogen peroxide, sulfuric acid and ammonium chloride solutions. When a 5-M ammonium chloride solution was used in the anode zone with a 3.2-M sulfuric acid and 3.52-M hydrogen peroxide mixed solution in the cathode zone, an average discharge current of 190 mA, an average output voltage of approximately 0.95 V and an actual gravimetric energy density of 42.73 W h kg−1 were obtained, and the discharge time of the batteries was more than 30 h. Copyright © 2011 John Wiley & Sons, Ltd.

High-speed dual Langmuir probe

In an effort to temporally resolve the electron density, electron temperature, and plasma potential for turbulent plasma discharges, a unique high-speed dual Langmuir probe (HDLP) has been developed. A traditional single Langmuir probe of cylindrical geometry (exposed to the plasma) is swept simultaneously with a nearby capacitance and noise compensating null probe (fully insulated from the plasma) to enable bias sweep rates on a microsecond timescale. Traditional thin-sheath Langmuir probe theory is applied for interpretation of the collected probe data. Data at a sweep rate of 100 kHz are presented; however the developed system is capable of running at 1 MHz-near the upper limit of the applied electrostatic Langmuir probe theory for the investigated plasma conditions. Large sets (100,000 sweeps at each of 352 spatial locations) of contiguous turbulent plasma properties are collected using simple electronics for probe bias driving and current measurement attaining 80 dB signal-to-noise measurements with dc to 1 MHz bandwidth. Near- and far-field plume measurements with the HDLP system are performed downstream from a modern Hall effect thruster where the time-averaged plasma properties exhibit the approximate ranges: electron density n(e) from (1x10(15))-(5x10(16)) m(-3), electron temperature T(e) from 1 to 3.5 eV, and plasma potential V(p) from 5 to 15 V. The thruster discharge of 200 V (constant anode potential) and 2 A (average discharge current) displays strong, 2.2 A peak-to-peak, current oscillations at 19 kHz, characteristic of the thruster "breathing mode" ionization instability. Large amplitude discharge current fluctuations are typical for most Hall thrusters, yet the HDLP system reveals the presence of the same 19 kHz fluctuations in n(e)(t), T(e)(t), and V(p)(t) throughout the entire plume with peak-to-peak divided by mean plasma properties that average 94%. The propagation delays between the discharge current fluctuations and the corresponding plasma density fluctuations agree well with expected ion transit-times observed with distinct plasma waves traveling away from the thruster at velocities >10 km/s.

White Layer Composition, Heat Treatment, and Crack Formation in Electric Discharge Machining Process

Characteristics of electric discharge machined (EDM) surfaces of normalized, quenched, and quenched and tempered-treated steels in kerosene and deionized-water dielectric liquids are investigated. Optical microscopy, scanning electron microscopy (SEM) and X-ray diffractometry are employed to analyze the machined surface. Surface cracks are examined in terms of white layer composition, heat treatment of the workpiece material, and operational parameters used, such as average discharge current and pulse-on duration. The present results reveal that base material properties and white layer composition have a distinctive function on crack formation that results in different crack network layouts on the surface and penetration depths in the substrate. Surface cracks, which initiate at the surface, travel down perpendicularly toward the interferential zone, and terminate at this interference, are mainly formed due to an increase in nonhomogeneities of metallurgical phases within the white layer. Such cracks are usually encountered on the surfaces when machining is performed in a hydrocarbon-based dielectric liquid using high pulse-on duration and low average discharge current. On the other hand, penetrating cracks, which penetrate the entire white layer thickness to an extent into the parent material, are mainly formed due to contraction of the recast structure joined to the circumferential edge of a crater rim during solidification. This type of crack is common when machining is performed in deionized water and the work material is brittle. Crack penetration depth is found to be proportional to the used pulse energy, and its path has a tendency to form parallel cracks to the machined surface at decreased pulse-on duration.

Time-resolved probe diagnostics of pulsed DC magnetron discharge during deposition of TiOx layers

The paper presents an experimental study of a pulsed DC unbalanced magnetron discharge with Ti target. This system is being used for deposition of nanocrystalline TiOx thin films. The unbalanced magnetron was operated in pulsed regime with the low repetition frequency of modulation cycle νp=250 Hz and relatively short active part of modulation cycle Ta=150 μs. Low average discharge current was used with maximum value about 1 A. Maximum instant discharge current in the active part of modulation cycle was 20 A. Relatively high plasma density up to value ne≈1018 m−3 was achieved at these conditions but due the low average power absorbed in the plasma the heat load applied on the target and the substrate was low. Time-resolved and spatially resolved Langmuir probe diagnostics was used for the systematic and detailed experimental study of the system. Experimental conditions as the instant value of discharge current, gas pressure in the reactor, gas mixture ratio Ar/O2, etc. were varied in a broad range. The time evolutions of plasma parameters such as electron density, electron temperature, plasma potential and EEPF were determined during the modulation cycle of pulse discharge excitation.

Battery-powered digital CMOS design

In this paper, we consider the problem of maximizing the battery life (or duration of service) in battery-powered CMOS circuits. We first show that the battery efficiency (or utilization factor) decreases as the average discharge current from the battery increases. The implication is that the battery life is a superlinear function of the average discharge current. Next we show that even if the average discharge current remains the same, different discharge current profiles (distributions) may result in very different battery lifetimes. In particular, the maximum battery life is achieved when the variance of the discharge current distribution is minimized. Analytical derivations and experimental results underline the importance of the correct modeling of the battery-hardware system as a whole and provide a more accurate basis (i.e., the battery discharge times delay product) for comparing various low-power optimization methodologies and techniques targeted toward battery-powered electronics. Finally, we calculate the optimal value of V/sub dd/ for a battery-powered VLSI circuit so as to minimize the product of the battery discharge times circuit delay.

Effect of cathode surface state on the characteristics of pulsed hollow-cathode glow discharges

A study is made of the effect of pulse repetition rate (0.1−103 s−1) and average discharge current (0–1 A) on the breakdown delay time and burning voltage of low-pressure glow discharges (p<0.1 Pa) in an electrode system of the reverse magnetron type with a large cathode surface area (≈103 cm2). It is shown that increasing the repetition rate leads to a many-fold reduction in the statistical spread in the delay time and in the discharge formation time, while the average discharge current has a significant effect on the burning voltage. The mechanism for the observed phenomena is interpreted qualitatively in terms of the presence of thin dielectric films on the cathode surface.

Ion energy and momentum flux dependence of diamond-like carbon film synthesis in radio frequency discharges

Abstract An investigation of the ion energy and ion momentum flux dependence of the properties of plasma-deposited diamond-like carbon (DLC) films is presented. The DLC films are deposited in low pressure radio frequency (RF) discharges operating at 13.3 Pa with methane as the source gas. The discharge is characterized in situ for the negative direct current (d.c.) self-bias voltage at the powered electrode, the average discharge current, and the total power, from which the average ion energy and average incident momentum flux can be determined. Ion energies explored in this study range from 50 to 250 eV. Films are deposited on [100] n-type silicon wafers and are characterized by a variety of analytical methods to determine film hardness, density, and stress. It is shown that the maximum film density (1.7 g/cm 3 ) and hardness (18 GPa) correspond to conditions of maximum deposition rate (5×10 −6 g/s) and maximum film stress (1.8 GPa) all at ion energies of approximately 160 eV. These values are comparable with those previously reported in the literature. The relationship between film properties and impinging ion energy and ion momentum flux is discussed within the framework of existing models of diamond-like carbon formation.

The Oscillations of the Positive Glow Corona

The paper treats experiments, computer simulations, and the physical explanation of the oscillatory behaviour of positive glow coronas at or below atmospheric density. Experiments show that most positivc glows have ionization regions that oscillate with frequencies depending on the gas, geometry, and average discharge current. Our circuit simulating program CIRCSIM with the general gas discharge elemcnt GPCSI reproduces quantitatively Beattie's sphere-to-plane coronas in air, and also our cylindrical coronas in air and in argon with additives. The oscillations demand a fast secondary ionization (feedback) mechanism, and are damp or inhibited by slower feedback processes like positive ion or metastable feedback, and detachment from negative ions. Linear damp oscillations are due to ionization region capacitance and equivalent inductance. Nonlinear self-sustained oscillations are formed by periodic quenching by the generated positive ions.

Ion energy distribution measurements in glow discharges

The energy distribution of ions hitting the cathode of dc and pulsed dc discharges were investigated under conditions typically for PACVD and plasma diffusion treatment. Energy and mass analysis was performed, using a two-stage differentially pumped plasma monitor. Gas atmospheres containing nitrogen, hydrogen and argon were investigated in the pressure range of a few mbar. The observed amount of ions with energies exceeding the sputtering threshold is considerably higher than theoretically expected. The density of ions striking the cathode in a low frequency pulsed discharge has proved to be strongly dependent on the average discharge current, whereas the operating voltage mainly affected their energy distribution functions.

Enhanced production of negative ions in low-pressure hydrogen and deuterium discharges

A new approach to enhance the production of negative ions in low-pressure hydrogen and deuterium discharges is described. The procedure, which requires on-off modulation of the discharge current, is applied to a conventional magnetic multicusp or ``bucket'' volume negative-ion source. The extracted ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ current from a 15-A discharge at a filling pressure of 2.4 mTorr is increased from 0.5 to 2.0 mA/${\mathrm{cm}}^{2}$ by discharge-current modulation at 10 kHz and 23% duty cycle. The ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ beam current is not significantly modulated (5%). The extracted electron current density and average discharge current are reduced by over a factor of 4. The work is relevant to production of ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ beams for neutral-beam injectors.

Current pulses in SF6 glow discharges

Sharp current pulses, with amplitudes of the order of hundreds of mA and frequencies of the order of 100 kHz, occur in SF6 glow discharges in the pressure range 1 to 10 Torr for average discharge currents up to 20 mA. The pulses are shown to be a phenomenon associated with a constricted positive column and a negative voltage-current discharge characteristic. A series of experiments is reported in which the amplitudes and frequencies of the current pulses are measured as functions of average discharge current, SF6 pressure, electrode separation, tube diameter and interelectrode capacitance. The results are described in terms of the charge stored on the interelectrode capacitance, and the dependence of the discharge processes of ionisation and attachment on E/N (axial electric field/neutral density) in the positive column. Further support for this description is provided by an examination of current pulses in high-pressure glow discharges through argon and neon. In these discharges electron-ion recombination plays a similar role to that of attachment in SF6 discharges.

One Consideration on Short-Gap, Atmospheric Discharge

The discharge characteristics at 760 Torr of air of needle-sphere electrodes with narrow gaps of 1mm or less are discussed in order to gain an understanding of the structure and mechanism of discharges such as those used in DC discharge-type plasma display panels (PDP). At the initiation of the discharge, the streamer corona is more predominant in 0.1 mm-gap discharge than in 1.0 mm-gap discharge. Pulsed discharge is observed to shift to DC discharge by increasing the average discharge current. The threshold current is observed to be larger in 0.1 mm-gap discharge than in 1.0 mm-gap discharge. With the use of a needle cathode, DC discharge is held at a smaller current than with other configurations and polarities. It can be estimated that the thickness of the cathode dark space and of the negative glow is smaller than 10 µm.

A Study of Low Voltage Condenser Discharge

As a guide to the solution of various problems concerning the emission spectrochemical quantitative analysis of iron and steel, a somewhat fundamental study is made by the use of a low voltage condenser discharge on the relation of the spectral line intensity of iron to the peak discharge current, average discharge current, discharge energy and excitation temperature under various excitation conditions. The spectral line intensity and the discharge energy are mutually related, both cecoming minimum at the critically damped discharge.The excitation temperature is observed to fall generally when the moving electrode method is used.

Dynamics of Corona Discharge between Cylindrical Electrodes

The qualitative explanation by Loeb of the constancy of the amplification factor with increasing corona currents with coaxial cylindrical electrodes used, as in conventional Geiger counters, is placed on a quantitative basis. As anticipated, it is shown that it is the screening action of the space charge which is responsible for the constancy and the stability of the average discharge current. The fluctuations superposed on the constant current, previously reported by us, are explained as a response of the system having a definite resonance frequency to the statistical fluctuations of the photoelectric current. The theoretical predictions are confirmed by experimental data.