Microhollow Cathode Discharge Research Articles

Absolute atomic nitrogen density spatial mapping in three MHCD configurations

In this work, nanosecond two-photon absorption laser induced fluorescence (TALIF) is used to perform spatial mappings of the absolute density of nitrogen atoms generated in a micro-hollow cathode discharge (MHCD). The MHCD is operated in the normal regime, with a DC discharge current of 1.6 mA and the plasma is ignited in a 20% Ar/ 80% N2 gas mixture. A 1-inch diameter aluminum substrate, acting as a third electrode (second anode), is placed further away from the MHCD to emulate a deposition substrate. The spatial profile of the N atoms is measured in three MHCD configurations. First, we study a MHCD having the same pressure (50 mbar) on both sides of the anode/cathode electrodes and the N atoms simply diffuse in three dimensions from the MHCD. The recorded N atoms density profile in this case satisfies our expectations, i.e. the maximal density is found at the axis of the hole, close to the MHCD. However, when we introduce a pressure differential, thus creating a plasma jet, an unexpected N atoms distribution is measured with maximum densities away from the jet axis. This behavior cannot be simply explained by the TALIF measurements. Then, as a first simplified approach in this work, we turn our attention to the role of the gas flow pattern. Compressible gas flow simulations show a correlation between the jet width and the radial distribution of the N atoms at different axial distances from the gap. Finally, a DC positive voltage is applied to the third electrode (second anode), which ignites a micro cathode sustained discharge (MCSD). The presence of the pressure differential unveils two stable working regimes depending on the current repartition between the two anodes. The MCSD enables an homogenization of the density profile along the surface of the substrate, which is suitable for nitride deposition applications.

Influence of the pulse polarity on micro-hollow cathode helium plasma jet

Helium plasma jets generated by micro-hollow cathode discharge (MHCD) with the square-wave power source of different polarities are investigated in this work. The effects of positive and negative polarity pulses on the MHCD and plasma jet were compared, and the time-resolved optical emission spectra of excited species ( and O) were studied. The results confirm that the electric field is a key factor for the propagation of the jet during the rising edge of the positive current pulse, while the gas expansion is mainly important for the jet propagation during the current stable phase. The time-resolved spectra show that the generation of specie O in the jet-driven by the electric field is more efficient.

Experiment and simulation on the micro-hollow cathode sustained discharge in helium with different geometries of the second anode

The micro-discharge with a hollow cathode and a second anode with different geometries is experimentally and numerically studied. Effects of the second anode on the micro-hollow cathode sustained discharge (MCSD) in helium are investigated. Three kinds of electrodes with different geometries are used as the second anode. The results show that when needles are inserted into the plate to be used as the second anode, the formation of MCSD can be promoted, and MCSD can be obtained at a low cathode current. Moreover, the distribution of needles has an important influence on the formation of MCSD. The cathode breakdown threshold currents for the three cases of the second anode from low to high are as follows: 13 needle-plate, five needle-plate, and plate with no needles. At the same cathode current, a stronger MCSD and a higher second anode current can be obtained when needles are inserted into the second anode plate compared with that when only a plate is used as the second anode. In the present experiment, the volume of MCSD reaches approximately 10 cm3 with an electron density of 4–5 × 1017 m−3 and an electron temperature of 2–3 eV. The experimental and simulated results show that the MCSD originates from cooperative formation between the micro-hollow cathode discharge in the cavity and the discharge around the second anode. Compared with that when only a plate is used as the second anode, when needles are inserted into the second anode, the electric field, electron temperature, and electron production rates near the second anode increase, and a stronger discharge is generated near the second anode. When the needle-plate is used as the second anode, the electron density in both the axial and radial directions in the MCSD region is higher than that when the plate without needles is used as the second anode. Before the formation of MCSD, the electrons generated by the strong discharge near the second anode extend toward the first anode, thus promoting the formation of MCSD.

Efficient hydrophilicity improvement of titanium surface by plasma jet in micro-hollow cathode discharge geometry

Surface hydrophilicity improvement of titanium (Ti) is of great significance for the applications of the important biomaterial. In this study, efficient hydrophilicity on the Ti surface is improved by an air plasma jet generated by a micro-hollow cathode discharge (MHCD) geometry. Elementary discharge aspects of the plasma jet and surface characteristics of the Ti surface are investigated by varying dissipated power (P d). The results show that the plasma jet can operate in a pulsed mode or a continuous mode, depending on P d. The plume length increases with P d and air flow rate increasing. By optical emission spectroscopy, plasma parameters as a function of P d are investigated. After plasma treatment, water contact angel (WCA) of the Ti sample decreases to a minimum value of 15° with P d increasing. In addition, the surface topography, roughness, and content of chemical composition are investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS) with P d increasing. The results show that Ti–O bond and O–H group on the Ti surface are beneficial to the improvement of the hydrophilicity of Ti surface.

Ozone Concentration Measurement of an Atmospheric Pressure Air Plasma Jet by Ultraviolet Absorption Spectroscopy

Atmospheric pressure plasma jets have great potentials for various biomedical applications due to abundance with active oxygen species. In this article, an air plasma jet with a geometry of microhollow cathode discharge has been developed, which operates in a pulsed mode or a continuous mode depending on the dissipated power of the plasma jet. There are some continuous spectra emitted from the plasma jet, whose integrated intensity indicates that electron density increases with the increase of dissipated power, while decreases with increasing air flow rate for both of the two modes. Moreover, the electron density in the continuous mode is higher than that in the pulsed mode. Gas temperature of the plasma jet is obtained by the best fitting of the simulated and the experimental optical emission spectrum of OH (A-X), and ozone concentration produced by the plasma jet is measured by means of ultraviolet absorption spectroscopy. Results indicate that both ozone concentration and gas temperature decrease with increasing distance from the jet nozzle. At the jet nozzle, both of them increase with increasing dissipated power. However, ozone concentration decreases, while gas temperature increases with increasing air flow rate. The results are very important for the biomedical application of plasma jet.

Numerical simulation on the characteristics of a micro-hollow cathode discharge with external surface of the cathode covered by a dielectric layer

Being able to generate a plasma denser than that in the usual micro-hollow cathode discharge (MHCD) device, a MHCD with the cathode outer surface covered by a dielectric layer has been numerically simulated in atmospheric pressure helium with a two-dimensional fluid model in this paper. Results indicate that before reaching a steady state, the covered MHCD sequentially undergoes three phases. In the initial phase, a strong discharge and a weak discharge appear in the vicinities of the cathode and the anode, respectively. Then, a positive streamer propagates along the axis from the anode to the cathode. After that, the entire gap between the two electrodes is filled with discharges. In addition, the evolution of averaged densities is given for various species. Moreover, voltage and current curves are compared for the covered and uncovered MHCDs in the steady state. As expected, the covered MHCD has an averaged electron density higher than that of the uncovered MHCD in the steady state. These results are helpful in generating an atmospheric pressure plasma with high electron density, which is important for the extensive applications of MHCD.

Absolute N-atom density measurement in an Ar/N2 micro-hollow cathode discharge jet by means of ns-two-photon absorption laser-induced fluorescence

In this work, nanosecond two-photon absorption laser-induced fluorescence (TALIF) is used to probe the absolute density of nitrogen atoms in a plasma generated using a micro-hollow cathode discharge (MHCD). The MHCD is operated in the normal regime, and the plasma is ignited in an Ar/N2 gas mixture. First, we study a MHCD configuration having the same pressure (50 mbar) on both sides of the electrodes. A good agreement is found between the density of N atoms measured using TALIF in this work and previous measurements using vacuum ultraviolet Fourier transform absorption spectroscopy. Then, we introduce a pressure differential between the two electrodes of the MHCD, creating a plasma jet. The influence of the discharge current, the percentage of N2 in the gas mixture, and pressures on both sides of the MHCD is studied. The current has a small impact on the N-atom density. Furthermore, an optimal N-atom density is found at around 95% of N2 in the discharge. Finally, we demonstrate that the pressure has a different impact depending on the side of the MHCD: the density of N atoms is much more sensitive to the change of the pressure in the low-pressure side when compared to the pressure change in the high-pressure side. This could be due to several competing phenomena: gas residence time in the cathodic region, recirculation, or recombination of the N atoms at the wall. This study contributes to the optimization of MHCD as an efficient N-atom source for material deposition applications.

Characterization of Novel ALD Process for the Synthesis of CaO for 12CaO·7Al2O3 electride Applications

In this work we report on a novel CaO ALD process development with a little used solid ALD precursor for Calcium. This forms part of a larger effort to synthesize 12CaO·7Al2O3 [C12A7], which is a new electronic oxide compound with very low work function and applications potentials in displays and Field-Emission (FE) Cathodes. Currently there are no existing thin film technologies for the conformal deposition of C12A7 electride films on complex device surfaces with nanoscale precision. Use of ALD to obtain precise nanoscale interleaving of CaO and Al2O3 layers over Micro-Hollow Cathode Discharge devices MHCD or FE devices allows conversion to correct alloying of C12A7 in the desired nano-layers. Success in reproducibly obtaining these layers will significantly benefit applications using cathodes, such as light sources, nano-sat thrusters, vacuum electronics and small-format microwave amplifiers. For the final C12A7 film, the challenge is to develop two ALD processes for two binary films in this case CaO and Al2O3 which must be synthesized as alternating nanolaminates in the correct stoichiometric ratio and are subsequently reacted under high temperature annealing. Since ALD of Al2O3 is already a well-known established ALD process, we had to focus on developing an ALD process for CaO. For this study we decided to utilize the relatively novel solid ALD precursor Bis(N,N-di-iso-propylformamidinato)calcium(II) dimer [Ca(famd)2], which is a low vapor pressure solid Ca amidinate precursor and which has to be reacted with ozone or DI H2O vapors. The chemical structure of the Calcium precursor is shown schematically in Fig. 1. The Al2O3 binary compound will be synthesized with trimethylaluminum (TMA) and DI water vapor. The low vapor pressure of Ca amidinate precursor Ca(famd)2 in solid form constitutes a major technical challenge and cannot be reacted by ALD in vapor draw mode but necessitated as new hardware a 150mL booster bubbler with dip tube for bubbling the molten precursor and the installation of a Low Vapor Pressure Delivery (LVPD) kit with bypass. For this presentation we will report the physical characterization of the ALD CaO film with FE-SEM surface morphology studies, SEM EDS elemental analysis of the final composition, X-ray diffraction XRD analysis and atomic force microscopy AFM. Fig. 1: Schematic of chemical structure of the novel solid ALD Ca amidinate precursor Bis(N,N-di-iso-propylformamidinato)calcium(II) dimer [Ca(famd)2] Figure 1

Ignition Conditions for a Self-Pulsing Microhollow Cathode Discharge in Air

Ignition Conditions for a Self-Pulsing Microhollow Cathode Discharge in Air

Plasma-based microwave power limitation in a printed transmission line: a self-consistent model compared with experimental data

Plasma-based microwave power limitation in a suspended microstrip transmission line integrating a micro hollow cathode discharge (MHCD) in its centre is experimentally and numerically studied. Transient and steady state microwave power measurements exhibit a limitation threshold of 28 dBm and time responses of 25 microseconds. Intensified charge-coupled device imaging shows that microwave breakdown occurs at the top of the MHCD. The plasma then extends towards the microwave source within the suspended microstrip transmission line. Besides, a self-consistent model is proposed to simulate the non-linear interaction between microwave and plasma. It gives numerical results in agreement with the measurements, and show that the plasma expansion during the transient response is related to a shift between the ionization source term and the electron density maximum. The propagation speed, under the tested conditions, depends mainly on the stepwise ionization from the excited states.

Cross-comparison of diagnostic and 0D modeling of a micro-hollow cathode discharge in the stationary regime in an Ar/N2 gas mixture

A micro-hollow cathode discharge (MHCD) operated in Ar/N2 gas mixture, working in the normal regime, was studied both experimentally and with a 0D (volume-averaged) model in this work. This source provides high electron densities (up to 1015 cm−3) at low injected power (1 W). To understand the mechanisms leading to the production of N atoms, the densities of electrons, N atoms and argon metastable atoms (Ar*) were monitored over a wide range of experimental conditions. Electrons, N atoms and Ar* densities were probed by means of optical emission spectroscopy, vacuum ultra violet Fourier transform spectroscopy and tunable diode laser absorption spectroscopy, respectively. Measurements showed that using a smaller hole diameter enables to work with less injected power, while increasing the power density inside the hole and, subsequently, increasing the densities of excited species. Varying the percentage of N2 in the gas mixture highlighted that, up to 80%, the density of N atoms increases although the dissociation rate drops. Looking at the processes involved in the production of N atoms with the help of the 0D model, we found that at very low N2 fraction, N atoms are mostly produced through dissociative electron-ion recombination. However, adding more N2 decreases drastically the electron density. The density of N atoms does not drop thanks to the contribution of Ar* atoms, which are the main species dissociating N2 between 5% and 55% of N2 in the gas mixture. A reasonable agreement is found between the experiments and the model results. This study shows that, with this MHCD, it is possible to significantly modify the production of N atoms when modifying the physical parameters, making it particularly relevant for applications requiring a N atoms source, such as nitride deposition.

Hole size effect on microhollow cathode discharge in air

In this work the values of main electrical parameters of the microhollow cathode discharge in atmospheric air (peak discharge current, discharge ignition voltage) depending on the diameter of the hole in the cathode were obtained. It was experimentally found that at currents up to 2 mA, the discharge operated in a self-pulsing mode. According to the experimental results it was concluded that the value of the discharge ignition voltage at average discharge current exceeding 2 mA didn’t depend on the hole diameter.

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.

Numerical study of light emission from a micro-hollow cathode discharge; the effects of different working gases

This work addresses the ultraviolet, infrared, and visible light emissions from a micro-hollow cathode discharge operating at the self-pulsing regime. Three different gas mixtures, i.e., He–Xe, Ne–Xe, and He–Ne–Xe, are investigated. A volume-averaged approach with a comprehensive chemistry model is used to simulate the discharge. The effects of different gas mixtures, the total gas pressure, and the concentration of xenon atoms on the generation of different species, as well as on the light emissions, are carefully studied. The results show that the infrared efficiency of the micro-hollow cathode discharge is dramatically low (less than 0.001%), but its ultraviolet efficiency, with a maximum of 38% for a pure Xe discharge, is even higher than that of plasma display panels, which have a maximum of 35%. The ultraviolet efficiency of the Ne–Xe discharge is higher than that of the He–Xe at low xenon concentrations, whereas the reverse is true at higher concentrations. It is also found that there is an optimum pressure that maximizes the excimers’ densities and consequently the amount of ultraviolet radiation. The optimum pressure depends on the hole diameter of the hollow cathode structure. A comparison between different gas mixtures revealed that the ultraviolet efficiency of the He–Ne–Xe mixture at certain mixing ratios, e.g., 0.45He–0.45Ne–0.1Xe, can be much higher than those of He–Xe and Ne–Xe mixtures.

Direct current microplasma formation around microstructure arrays

We demonstrate the formation and transition behaviors of a microplasma around microstructure arrays at different gas pressures via two-dimensional particle-in-cell/Monte Carlo collision simulations. It is found that the microdischarge occurs outside the cathode microcavities at the lowest pressure and starts penetrating the microcavities with a curved sheath edge as the pressure increases. At higher pressure, coupled periodic microhollow cathode discharges (MHCDs) are formed inside the microcavities. Further increasing the gas pressure results in the disappearance of the MHCDs, and the dominant discharge shifts outside of the microcavity, locating above the protrusion tips. The effect of the space charge shielding on the discharge and the conditions for MHCD formation are discussed. The macroscopic discharge parameter scalings with the gas pressure and the electron kinetics are also examined. The results are helpful for deeply understanding the microplasma formation with nonplanar electrodes, which inform the scaling, design, and optimization of microplasma array devices across a wide range of pressure regimes in practical applications.

Diagnostics of a microhollow cathode discharge at atmospheric pressure

Based on a sandwich-like structure, a microhollow cathode discharge device is designed, and a stable discharge is realized by injecting helium into the discharge region of the device at atmospheric pressure. A wall probe is used to determine the relevant parameters of the plasma generated by the device, such as particle density, electron temperature, and the electron distribution function. At the same time, a sink parameter is used to correct the electron distribution function of the wall-probe diagnostics, and to further study the relationship between electron density and the electron temperature of the corrected electron distribution function.

Study on the characteristics of helium plasma jet by pulsed micro-hollow cathode discharge

In this paper, the helium plasma jet generated by micro-hollow cathode discharge (MHCD) was studied. The MHCD was driven by a square-wave pulsed power source, and the characteristics of discharge and plasma jet were measured experimentally. The influences of the gas flow rate on the MHCD and the plasma jet were investigated. And the propagation mechanisms of the plasma jet were analyzed. The results show that within 100–1000 sccm of the gas flow rate, the breakdown delay time of the MHCD increases with the helium flow increasing. It is considered that the gas flow affects the density of seed electrons and thus the breakdown delay time. With the helium flow rate increasing, the whole plasma jet length increases firstly and then decreases. A detailed investigation shows that during one discharge pulse, two distinguishable propagation processes of the plasma jet are observed. It is found that the jet of the first stage is formed during the rising edge of the current pulse, while the other is generated after the discharge current becomes stable. The propagation velocity of jet in the first stage is on the order of several km s−1, which is similar to that of the discharge evolution obtained by simulation. And the propagation speed of the jet in the second stage is on the order of several hundred m s−1, which is close to the velocity of gas flow. The spatial–temporal distributions of light emission show that high-energy electrons can only be observed during the jet propagation in the first stage, and low-energy electrons can be detected in both the first and second stages. The results show that the electric field plays an important role on the jet propagation in the first stage, and the jet propagation during the second stage is mainly promoted by the thermal gas expansion.

Transition characteristics and electron kinetics in microhollow cathode discharges

We demonstrate the transition characteristics and electron kinetics of microdischarges in a microgap with a cathode having microstructures using two-dimensional particle-in-cell/Monte Carlo collision (2d3v) simulations. It is found that the microdischarge is inside the hollow cavity at lower pressures, forming microhollow cathode discharges (MHCDs), while the dominant discharge moves outside the cavity at higher pressures, locating above the cathode rectangular protrusion tip. The spatial distributions of the microdischarge parameters (e.g., electric potential and electron density) are presented by capturing the transition characteristics. The electron kinetics of the microdischarges are examined based on the moment analysis of the electron Boltzmann equation. The collisional and non-collisional components of the electron power absorption are decomposed, which were seldomly investigated for microdischarges previously. The results provide fundamental insights into MHCD formation with a structured electrode at varying pressure conditions, which could be beneficial for the design and fabrication of microplasma devices in practical applications.

Diagnostics and comparative analyzes of plasma parameters in micro hollow cathode discharges with an open and covered external surface of cathode in helium using an additional electrode

In this work, diagnostics and comparative analysis of microhollow cathode discharges with an open and covered external surface of the cathode in helium at atmospheric pressure were carried out. It is shown that by covering the external surface of the cathode, it is possible to achieve the effect when the current–voltage characteristics of the discharge grows in a wider range of discharge currents. Obviously, in this case, the negative glow (NG) plasma of the hollow cathode occupies a significant part of the discharge cell, as evidenced by the measurements carried out with the additional electrode. The electron temperature values determined from the I–U characteristic of the additional electrode are 0.3–0.4 eV for a microhollow cathode discharge with a covered external surface, which indicates that the plasma of such discharge is similar to NG plasma of glow discharge. The fast parts of the second derivatives of the I–U characteristic of the additional measuring electrode demonstrate the detection of the spectra of fast electrons produced as a result of Penning ionization reactions. Moreover, by covering the external surface of the cathode, conditions are created under which the entire discharge volume is occupied by a NG plasma, which makes it possible to achieve a better resolution for recording the spectra of fast electrons.

Formation of large-volume plasmas in a microhollow cathode sustained discharge with a needle-plate second anode

A large-volume glow discharge is produced using a microhollow cathode sustained discharge (MCSD) with a needle-plate second anode in argon. The length and volume of the glow discharge are approximately 3 cm and 7 cm3, respectively, and the electron density is on the order of 1011 cm−3. The MCSD is also simulated using a two-dimensional self-consistent fluid model with a single plate as the second anode. Both the experimental and simulated results show that MCSD generation originates from cooperative formation that involves the microhollow cathode discharge and a discharge that arises from the second anode. The second anode plays an important role in MCSD generation. The electric field and the average electron energy clearly increase in the region near the second anode. The influence of the structure of the second anode on the MCSD characteristics is investigated. When the second anode has a needle-plate structure instead of a single plate structure, the sustaining voltage, breakdown, and annihilation current of the cathode for MCSD decrease substantially, a higher discharge current is obtained, and a more stable, uniform MCSD is produced. The results also show that both needle and plate contribute to glow discharge formation in the region near the second anode.