Microhollow Cathode Research Articles

Two-dimensional model of orificed micro-hollow cathode discharge for space application

In this paper, we describe results of self-consistent two-dimensional (x-z) particle-in-cell simulations, with a Monte Carlo collision model, of an orificed micro-hollow cathode operating in a planar diode geometry. The model includes thermionic electron emission with Schottky effect, secondary electron emission due to cathode bombardment by the plasma ions, several different collision processes, and a non-uniform xenon background gas density in the cathode-anode gap. Simulated results showing behavior of the plasma density, potential distribution, and energy flux towards the hollow cathode and orifice walls, are discussed. In addition, results of simulations showing the effect of different Xe gas pressures, orifice size, and cathode voltage, on operation of the micro-hollow cathode are presented.

Characterization of a Microhollow Cathode Discharge Plasma in Helium or Air with Water Vapor

Microhollow cathode discharge (MHCD) plasmas were generated in gas mixtures containing water vapor at pressures of up to 100 kPa of He or 20 kPa of air. The cathode diameter was 1.0 mm with a length of 2.0 mm. The electrical characteristics showed an abnormal glow mode. Spectroscopic measurements were carried out to examine the plasma and radicals. An analysis of the spectral profile of Hα at 656.3 nm enabled a derivation of the electron densities, namely 2×1014 cm−3 (at 10 kPa) and 6×1014 cm−3 (at 4 kPa) for the helium and air atmospheres, respectively, in the negative glow region. By comparing the observed OH radical spectra with those calculated by the simulation code LIFBASE, the gas temperature was deduced to be 900 K for 4 kPa of He at a discharge current of 50 mA.

A Gas Switch Triggered by a Microhollow Cathode Discharge (MHCD) Array With Lower Trigger Energy

A microhollow cathode discharge (MHCD) array was operated at atmospheric pressure with a pulse voltage of 2.5 kV supplied by a trigger circuit. Triggered by this MHCD array, a gas switch with an annular plate structure operated under a voltage of 27 kV with a fall time of 30 ns and a current of 400 A with a rise time of 15 ns was investigated. It is used for discharging the charge stored in a capacitor of 4 nF through a resistor of 50 Ω. The advantages of the switch are obvious: a compact trigger with lower trigger voltage. An electrical model including MHCD capacitance is built, and the simulation results agree well with the experimental results.

Simulation of N2 microhollow cathode discharge and cathode sputtering by using a PIC/MC model

The nitrogen microhollow cathode discharge and Ti cathode sputtering, bombarded by ions (N2+, N+), have been studied using a two-dimensional PIC/MCC model. The behavior of ions (N2+, N+) and sputtered atom (Ti), and the thermalization process of the sputtered atoms in a nitrogen microhollow cathode discharge are simulated. The results show that hollow cathode effect is due to electron oscillations in the overlapping negative glow under our simulation condition. The densities of ions (N2+, N+) in the microhollow cathode discharge are two orders in magnitude greater than that in the conventional hollow cathode discharge; but the distributions and sizes of the mean energy of the ions (N2+, N+) are almost the same. The density of N2+ is fivefold as much as that of N+ in the microdischarge space; however, the maximum of mean energy of the latter is twice larger than the former. For various parameters (P, T, V), the densities of ions(N2+, N+) bombarding the cathode internal surface are almost uniformly distributed, and their mean energy are almost the same. When these atoms are 0.15 mm away from the cathode. The sputtered atoms are almost thermalized completely.

Analysis of cathode geometry to minimize cathode erosion in direct current microplasma jet

Microplasma jets are now widely used for deposition, etching, and materials processing. The present study focuses on the investigation of the influence of cathode geometry on deposition quality, for microplasma jet deposition systems in low vacuum. The interest here is understanding the influence of hydrogen on sputtering and/or evaporation of the electrodes. Samples obtained with two cathode geometries with tapered and rectangular cross-sections have been investigated experimentally by scanning electron microscopy and energy dispersion X-ray spectroscopy. Samples obtained with a tapered-geometry cathode present heavy contamination, demonstrating cathode erosion, while samples obtained with a rectangular-cross-section cathode are free from contamination. These experimental characteristics were explained by modelling results showing a larger radial component of the electric field at the cathode inner wall of the tapered cathode. As a result, ion acceleration is larger, explaining the observed cathode erosion in this case. Results from the present investigation also show that the ratio of radial to axial field components is larger for the rectangular geometry case, thus, qualitatively explaining the presence of micro-hollow cathode discharge over a wide range of currents observed in this case. In the light of the above findings, the rectangular cathode geometry is considered to be more effective to achieve cleaner deposition.

Pressure Dependences of Gas Temperature and Electron Density in Microhollow Cathode Discharges in He and He+H2O Gases

Pressure Dependences of Gas Temperature and Electron Density in Microhollow Cathode Discharges in He and He+H₂O Gases

Study of the Discharge Mode in Micro-Hollow Cathode

In this study, micro-hollow cathode discharge (MHCD) is investigated by a fluid model with drift-diffusion approximation. The MHC device is a cathode/dielectric/anode sandwich structure with one hole of a diameter D=200 μm. The gas is a Ne/Xe mixture at a pressure p=50∼500 Torr. The evolutions of the discharge show that there are two different discharge modes. At larger pD the discharge plasma and high density excited species expand along the cathode surface and, a ringed discharge mode is formed. At smaller pD, the discharge plasma and the excited species expand along the axis of the cathode aperture to form a columnar discharge.

Pressure Dependences of Gas Temperature and Electron Density in Microhollow Cathode Discharges in He and He+H2O Gases

Microplasmas were generated using a cylindrical hollow cathode with a diameter of 0.5 mm and a length of 3 mm in He and He+H2O gases at pressures in the range of 20 to 765 Torr. Spectral line profiles of He I (667.8 nm) and Hα (656.3 nm) were measured to determine the plasma parameters. Gas temperature and electron density were evaluated by the analysis of spectral line broadening. These parameters linearly increased with gas pressure in the range of 300 to 765 Torr. The gas temperatures and electron densities at atmospheric pressure were obtained to be 610 K and 5.0×1014 cm-3 for He and 820 K and 3.5×1014 cm-3 for He+H2O microplasmas, respectively. A comparison of these values with those obtained in a previous work [M. Miclea et al.: J. Phys. D 38 (2005) 1709] demonstrated that our device could generate high-density plasmas with a large volume and a high degree of nonequilibrium. The influences of an admixture of H2O on the discharge properties and plasma parameters were also discussed.

Cold DC-Operated Air Plasma Jet for the Inactivation of Infectious Microorganisms

We evaluated a nonthermal plasma jet for a respective use to prevent infections from bacteria and yeasts. The plasma jet is generated from the flow of ambient air with 8 slm through a microhollow cathode discharge assembly that is operated with a direct current of 30 mA. With these parameters, the temperature in the jet reaches 43 °C at 10 mm from the discharge. Agar plates that were inoculated with Staphylococcus aureus, Pseudomonas aeruginosa, Acinetobacter baumannii, and Candida kefyr were treated at this distance, moving the plates through the jet in a meander that covered a 2 cm by 2 cm area. Different exposure times were realized by changing the speed of the movement and adjusting the distance between consecutive passes. S. aureus was most responsive to the exposure with a reduction in the number of colony forming units of 5.5 log steps in 40 s. All other microorganisms show a more gradual inactivation with exposure times. For all bacteria, a clearing of the treated area is achieved in about 2.5-3.5 min, corresponding to log-reduction factors of 5.5-6.5. Complete inactivation of the yeast requires about 7 min. Both S. aureus and C. kefyr show considerable inactivation also outside the immediate treatment area, while P. aeruginosa and A. baumannii do not. We conclude that differences in the morphologies of the membrane structures are responsible for the diverging results, together with a targeted response to different agents provided with the plasma jet. For the gram negative bacteria, we hold short-lived agents, acting across a short range, responsible, while for the other microorganisms, longer lived species seem more important. Our measurements show that neither heat, ultraviolet radiation, nor the generation of ozone can be responsible for the observed results. The most prominent long lived reaction product found is nitric oxide, which, by itself or through induced chemical reactions, might affect cell viability.

Exciplex emission induced by nanosecond-pulsed microdischarge arrays operating at high repetition rate frequency

Arrays of microhollow cathode discharges or cathode boundary layer discharges were operated in krypton–chlorine mixtures resulting in an intense ultraviolet emission at 222 nm. It was demonstrated that nanosecond duration pulses applied at high repetition frequency allowed one to simultaneously ignite and sustain discharges inside all the microcavities of the arrays without using any resistive ballast. Self-repelling properties of adjacent plasmas expanding onto the cathode surface for the negative polarity part of the high-voltage pulses were demonstrated, resulting in a self-limitation of the available cathode surface for an individual microdischarge. Spectroscopic investigations allowed us to determine the gas temperature and evolution of the 222 nm band intensity versus the partial pressures and deposited power. Optimum partial pressure ranges for chlorine and krypton were 0.2–0.8 mbar and 50–70 mbar, respectively. The 222 nm band intensity increased linearly with the deposited energy per pulse and the pulse repetition frequency provided that the mean deposited energy was lower than 7 W. Above this value a cooling of the device should be implemented.

Ultraviolet excimer radiation from nonequilibrium gas discharges and its application in photophysics, photochemistry and photobiology

Narrowband UV and VUV excimer radiation can be generated in a variety of nonequilibrium gas discharges: dielectric barrier discharges, microhollow cathode discharges, arrays of microplasmas, corona discharges. Excimer lamps (excilamps) are now available for a large number of wavelengths and in various geometrical shapes. The availability of nearly monochromatic photon fluxes ranging in energy up to 15 eV resulted in a number of innovative photo-induced processes in photophysics, photochemistry and photobiology. This report focuses on progress made in the last decade.

Hydrogen production by methanol reforming in a non-thermal atmospheric pressure microplasma reactor

On board reforming of hydrocarbons for fuel cell feed has become an attractive research topic due to the low energy densities of batteries. The implementation of a microplasma as a means for reforming the liquid fuel methanol is explored in this work. Hydrocarbon reforming is commonly accomplished through catalysis, but catalysts have a number of limitations such as poisoning, coking, coarsening, long start-up times and excessive costs. Published studies have shown the viability of plasma reforming but none have succeeded in achieving suitable system efficiencies for portable applications. Non-thermal microplasmas are particularly attractive for reforming due to their extremely high electron and power densities and the scale of microplasma devices make them well suited for portable applications. This study describes experimental microplasma reactors reforming methanol. The reactors are based on the microhollow cathode discharge (MHCD) structure fabricated with microelectromechanical systems (MEMS) fabrication techniques. Through modeling the reaction for all five experiments, conversions within the microchannel were found to be nearly 100%. Despite the variations in the five experiments due to input electrical power, flow rate and concentration, the model was validated in each test. The experiments discussed in this work show the promise of a portable, non-thermal microplasma reformer that generates hydrogen for fuel cells for portable power.

Effect of Cathode Length on Electrical Characteristics of a Microhollow Cathode Discharge in Helium

Effect of Cathode Length on Electrical Characteristics of a Microhollow Cathode Discharge in Helium

Effect of Cathode Length on Electrical Characteristics of a Microhollow Cathode Discharge in Helium

The electrical characteristics of a microhollow cathode discharge (MHCD) have been measured over a wide range of helium gas pressures from 10 to 900 Torr, by using cathodes of 0.5 mm diameter and 0.5–3.0 mm length. A relatively high voltage of the Paschen minimum at high pressure for the shortest cathode and a conventional hollow cathode mode for longer ones were observed. These are explained by a change of the plasma loss owing to the difference in cathode length. The radial extent of the negative glow inside the cathode hole increased with discharge current, and eventually spread along the outer surface. The threshold current for the extension of the plasma outside the cathode hole increased in proportion to the cathode length and to the square of the gas pressure. Thus, the longer cathode provides a favorable condition for the generation of reactive MHCD plasma at high-pressure and high-current operation. On the basis of these results and spectroscopic observation, the sustaining mechanism of an MHCD is briefly discussed.

Generation of microdischarges in diamond substrates

We report the generation of microdischarges in devices composed of microcrystalline diamond. Discharges were generated in device structures with microhollow cathode discharge geometries. One structure consisted of an insulating diamond wafer coated with boron-doped diamond layers on both sides. A second structure consisted of an insulating diamond wafer coated with metal layers on both sides. In each case, a single sub-millimetre hole was machined through the conductor–insulator–conductor structure. The discharges were generated in a helium atmosphere. Breakdown voltages were around 500 V and discharge currents in the range 0.1–2.5 mA were maintained by a sustaining dc voltage of 300 V.

A global model of the self-pulsing regime of micro-hollow cathode discharges

A global (volume-averaged) model of the self-pulsing regime of micro-hollow cathode discharges working in argon gas is proposed. The power balance is done using an equivalent circuit model of the discharge that allows the current and voltage dynamics to be calculated. The fraction of the total power dissipated in the discharge that contributes to electron heating is deduced from a sheath model previously described. The particle balance is first done in a very simplified reaction scheme involving only electrons, argon atomic ions, and argon molecular ions. In a second step, the excited states (the metastable state Ar*(3P2) and the resonant state Ar*(3P1)) are included in the particle balance equations. The models are compared to experiments and several conclusions are drawn. The model without excited states underestimates the electron density and does not capture well the trends in pressure. The model with the excited states is in better agreement which shows that multi-step ionization plays a significant role. The time-evolution of the electron density follows closely that of the discharge current but the excited states density presents two peaks: (i) the first at the early stage of the current peak due to direct excitation with high electron temperature, (ii) the second at the end of the current (and electron density) peak due to large production of excited states by electron-ion recombination at very low electron temperature.

Operating modes and power considerations of microhollow cathode discharge devices with elongated trenches

Microhollow cathode discharge (MHCD) devices generate microplasmas with high electron densities and reactive species, making them a prospective hydrocarbon reforming technology. Here we report on the electrical phenomena resulting from MHCD devices with elongated trenches using argon at atmospheric pressure and room temperature. Devices that were 100 μm wide, 100 μm deep, and 1 cm long exhibited self-pulsing current during constant voltage DC power application ranging from 0 mA to 32 mA. The capacitances for MHCDs with trenches 25, 100, and 250 μm wide were estimated to be 68, 70, and 33 μF respectively. A current-limited DC supply prevented self-pulsing, and resulted in abnormal, normal, or negative differential resistance (NDR), i.e. hollow cathode, operating modes. The NDR state manifested at lower current limits and occurred when the microplasma in the trench was discontinuous. Simulations from a corresponding, empirically determined circuit model showed larger total average power consumption (including the ballast resistance) during pulsed inputs (5.61–31.08 W) in comparison to constant voltage inputs (<1 W). These findings advance the development of these MHCDs for microplasma reforming applications, providing insights into operational modes and power consumption estimates critical to understanding the overall efficiency in the context of a future microplasma reforming system.

阵列微孔阴极放电触发的纳秒脉冲开关

阵列微孔阴极放电触发的纳秒脉冲开关

Development of a novel dielectric barrier microhollow cathode discharge for gaseous atomic emission spectroscopy

The present research demonstrates a novel microhollow cathode discharge based on the dielectric barrier discharge principle (DB-MHCD) for application as an analytical microplasma gas detector. The plasma is formed inside a microfabricated multilayer structure and is mainly confined in a bore with a diameter of 100–250 μm. The DB-MHCD operates with alternating rectangular voltages of 1–2 kVpp and a frequency of 50 kHz. The insulation of the electrodes by the dielectric layer prevents deterioration of the electrodes and eliminates contamination of the gaseous analyte with electrode material. This enables long-term operation of the DB-MHCD device over several days. The analytical performance of the DB-MHCD is demonstrated with halogenated hydrocarbons leading to an excellent detection limit of 27 ppb for gaseous Cl in He.

Exhaust Plume Characteristics of Microhollow Cathode Discharge Plasma Thruster

A microscale plasma thruster in a microhollow cathode discharge configuration is tested in dual-stage operational mode. Images of exhaust plume in various operating conditions are presented. The first-stage-only mode exhibits very weak exhaust plume at an 80 SCCM argon gas flow, while in the second-stage-only mode, despite the same power input, the exhaust plume is significantly enlarged. Dual-stage operation without a gas flow produces the largest exhaust plume resulting from electrostatic acceleration.