Microhollow Cathode Discharge Plasma Research Articles

Dynamics of plasma bullets by nanosecond pulsed micro-hollow cathode discharge within air

In this paper, the air plasma jet produced by micro-hollow cathode discharge (MHCD) is investigated. The discharge is powered by a positive nanosecond pulse high voltage supply. The waveforms of the discharge, the images of the jet, the evolution of the plasma bullet and the reactive species are obtained to analyze the characteristics of the MHCD plasma jet. It is found that the length of the plasma jet is almost proportional to the air flow rate of 2–6 slm. Two plasma bullets appear one after another during a single period of the voltage waveform, and both of the two plasma bullets are formed during the positive pulse voltage off. The propagation velocity of the two plasma bullets is on the order of several hundred m/s, which is approximate to that of the air flow. These results indicate that the gas flow has an important influence on the formation of this MHCD plasma jet.

Effects of reactor geometry on dissociating CO2 and electrode degradation in a MHCD plasma reactor

AbstractThis paper reports an experimental study on the effects of reactor geometry for dissociating carbon dioxide using a microhollow cathode discharge (MHCD) reactor, and the associated electrode degradation. A MHCD reactor consists of two hollow metal electrodes that are separated by dielectric material. The geometric reactor parameters studied were the dielectric material thickness and the diameter of the reactor hole. Dielectric thicknesses of 150, 300 and 450 μm and discharge hole diameters of 200, 400 and 515 μm were studied parametrically. The results of each parameter combination were discussed in terms of specific energy input (SEI), CO2 conversion, electrical‐to‐chemical energy conversion efficiency, and degradation rate of the electrodes. Overall, the results showed that, at constant SEI, increasing the dielectric thickness increased CO2 conversion and energy efficiency but decreased the degradation rate. Moreover, at a constant SEI, varying the hole diameter did not affect CO2 conversion or efficiency but the electrode degradation rate decreased with increasing hole diameter. The maximum efficiency observed was about 18.5% for the dielectric thickness of 450 μm, hole diameter of 400 μm, and a SEI of 0.1 eV/mol. With this reactor geometry, the maximum CO2 conversion was 17.3% at a SEI of 3.7 eV/mol. Moreover, the results showed that the rate of electrode degradation increased linearly with time at a rate ranging from 130 to 207 μm2/s, with a reactor lifetime of 13 hours at a SEI of 3.7 eV/mol. © 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.

Dissociation of carbon dioxide using a microhollow cathode discharge plasma reactor: effects of applied voltage, flow rate and concentration

This paper reports an experimental study on dissociating carbon dioxide (CO2) using a microhollow cathode discharge (MHCD) plasma reactor operated at 1 atm. The MHCD plasma reactors can be a promising technology for dissociating gases, including CO2, as they do not require catalysts, they operate at around room temperature, and can be inexpensively built and operated. In this study, CO2 balanced with the carrier gas argon (Ar) was fed through the MHCD reactor, and parametric experiments were conducted to investigate the effects of applied voltage, flow rate, and CO2 mole fraction in the influent on the composition of the products, energy conversion efficiency, and yield. Within the investigated parameter ranges, the maximum energy conversion efficiency of 14% was achieved when the specific energy input was 1.1 eV mol−1, whereas the maximum CO yield of 10.5% was achieved when the specific energy input was 4 eV mol−1. The results also showed that diluting CO2 with Ar increased the yield at an expense of a decrease in energy conversion efficiency. The results of this study provide insights for operating MHCD reactors for efficient gas dissociation at atmospheric pressure.

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.

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.

Optical Diagnostics of Breakdown Phase of Pulsed N2/He Atmospheric-Pressure Microhollow Cathode Discharge Plasma

The breakdown phase of a pulsed N2/He atmospheric-pressure microhollow cathode discharge plasma is studied using the time-resolved N2 optical emission spectra as well as the discharge current and voltage. The N2 emission intensity exhibits a sharp peak in the breakdown phase and becomes constant in the glow discharge phase. The temporal variation of the N2 emission spectra indicates that the N2 rotational temperature remains below 500 K immediately after discharge ignition but increases to ≈1000 K within ≈20 µs after the ignition. In the breakdown phase, the plasma with high excitation and dissociation rates is probably generated at the low neutral temperature. The prompt increase in neutral temperature indicated by the N2 rotational temperature is explained using a simple, zero-dimension model including the neutral heating due to electron–neutral collisions and the neutral cooling caused by the removal and refilling of the gas in the discharge volume.

Research of Making Large Volume Atmospheric Pressure Plasma by Parallel MCS Discharge

We research parallel microhollow cathode sustained (MCS) discharge plasma that is generated by parallel operations of the Microhollow cathode discharge (MHCD) plasma to produce a large volume atmospheric pressure plasma. We propose the cylindrical parallel MCS discharge plasma expecting electron supply by MHCD plasma and electron trapping effects of logarithm potential. Several MHCD electrodes are placed on cylindrical surface of 19 mm in radius and a thin wire is placed at a cylinder center axis. MHCD electrodes are supplied repetitive pulse voltage and the central wire anode is supplied DC voltage. So far, 8 parallel MCS discharge plasmas could be generated at 50 kPa. In this paper, the relationship between axial distance of MHCD electrodes and number of parallel discharge electrodes, and the condition to increase power supplied to MCS discharge were studied. Axial distance of MHCD electrodes were arranged to 6 mm and 16 parallel cylindrical MCS discharge in atmospheric pressure was generated. Power supplied to MCS discharge could be increased without decreasing number of parallel discharge electrodes by reducing current limiting resistor and shortening MHCD pulse width.

Hydrogen generation in a microhollow cathode discharge in high-pressure ammonia–argon gas mixtures

We explored the feasibility of using a single flow-through microhollow cathode discharge (MHCD) as a non-thermal plasma source for hydrogen (H2) production for portable fuel cell applications. The MHCD device consisted of two thin metal electrodes separated by a mica spacer with a single-hole, roughly 100μm in diameter, through all three layers. The efficiency of the MHCD reactor for H2 generation from NH3 was analyzed by monitoring the products formed in the discharge in a mass spectrometer. Using a gas mixture of up to 10% NH3 in Ar at pressures up to one atmosphere, the MHCD reactor achieved a maximum ammonia conversion of slightly more than 20%. The overall power efficiency of the MHCD reactor reached a peak value of about 11%. The dependence of NH3 conversion and power efficiency on the residence time of the gas in the MHCD plasma was studied. Experiments using pulsed excitation of the MHCD plasma indicated that pulsing can increase the power efficiency. Design and operating criteria are proposed for a microplasma-based H2 generator that can achieve a power efficiency above the break-even point, i.e., a microplasma reactor that requires less electrical power to generate and maintain the plasma than the power that can be obtained from the conversion of the H2 generated in the microplasma reactor.

Neon excimer emission from pulsed high-pressure microhollow cathode discharge plasmas

Microhollow cathode discharge (MHCD) plasmas in high-pressure Ne (up to and exceeding atmospheric pressure) are known to be efficient sources of Ne2∗ excimer radiation in the vacuum ultraviolet spectral region between 75 and 90nm. By operating the MHCD plasma in a pulsed direct current (dc) mode, we were able to increase the Ne2∗ excimer emission by up to 1 order of magnitude compared to the emission intensity obtained from the same MHCD plasma excited by a constant dc current. Time-resolved emission spectroscopic studies of the Ne2∗ excimer emission following pulsed dc excitation of an MHCD plasma in high-pressure Ne were carried out to elucidate the microscopic details of the excimer formation and destruction processes. Our studies provide direct evidence that quenching of the Ne2∗(3Σu) excimer molecules and other loss processes of the excimer molecules are important processes in high-pressure MHCD plasmas and represent, in fact, the dominant destruction channel of the Ne2∗(3Σu) excimer molecules in the MHCD plasma under a wide range of operating conditions.

Excimer formation in high-pressure microhollow cathode discharge plasmas in helium initiated by low-energy electron collisions

We report the observation of intense continuous vacuum ultraviolet radiation in the range 60–100 nm from microhollow cathode discharge plasmas in high-pressure He (up to 600 Torr). Two prominent emissions, a narrow, sharply peaked feature in the region of 58–65 nm and a broad emission from 65 to 95 nm are attributed to the He ∗ 2 first- and second-excimer continuum emission, respectively. We also observed several narrow atomic emission lines in the 95–125-nm range that correspond to atomic O, N, and H emission lines. We attribute the presence of these atomic line emissions to near-resonant energy transfer processes involving the He ∗ 2 excimers and trace concentrations of the impurities O 2, N 2, and H 2 in the discharge feed gas. The processes leading to the atomic line emissions in the present case are similar to the near-resonant energy transfer process observed previously in high-pressure microhollow cathode discharge plasmas in Ne/H 2 gas mixtures, which resulted in the emission of intense, monochromatic atomic hydrogen Lyman-α radiation (P. Kurunczi et al. J. Phys. B: At. Mol. Opt. Phys. 32 (1999) L651). He ∗ 2 excimer formation in a microhollow cathode discharge plasma is initiated by low-energy electron collisions (excitation of the metastable He levels or ionization of the He atoms) followed by three-body collisions. The emission of He ∗ 2 excimer radiation from a microhollow cathode discharge plasma indicates that these discharges are very efficient sources of energetic electrons, as the formation of He ∗ 2 excimers requires a sufficiently large number of electrons with energies well above 20 eV.