Microhollow Cathode Discharge Research Articles

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.

The Effect of Air-Water-Plasma-Jet-Activated Water on Penicillium: The Reaction of HNO2 and H2O2 Under Acidic Condition

Plasma-activated water (PAW) that is produced by micro hollow cathode discharge (MHCD), where air is the working gas used. The treatment effect of PAW against Penicillium is expressed by the germination inhibition rate and the sporoderm-breakage rate. The maximum values of the germination inhibition rate and the sporoderm-breakage rate are 82.31% and 66.91%, respectively. In this paper, the importance of the reaction of HNO2 and H2O2, and its product ONOOH in Penicillium inactivation is illustrated. The substantial impact of ONOOH on inactivation beyond the theoretical lifetime is analyzed, and the complexity of peroxynitrite chemistry is responsible for prolonging this effect. Moreover, according to the reaction of HNO2 and H2O2 under acidic conditions, we identify a method for increasing the yield of ONOOH, namely, adding water into the working gas, which further contributes to the improvement of the treatment efficiency.

Preparation and electrochemical performances of silver (alloy) nanoparticles decorated on reduced graphene oxide, using self-polymerization of dopamine in an acidic environment

Self-polymerization of dopamine, in either an alkaline or an acidic environment, to form polydopamine is a material-independent surface coating technique, influencing almost all areas of material science and engineering. We demonstrated a simple, two-step method to prepare in-situ silver or silver-copper alloy nanoparticles on the surface of reduced graphene oxides, using polydopamine formed in an acidic medium. The acidic medium was created by a nonthermal micro-hollow cathode discharge device and the device was operated at atmospheric pressure, using air as the working gas. The nanocomposites were characterized with SEM, EDX, ICP-OES, and FT-IR; the electrochemical catalytic activity was tested using rotating disk electrode. The characterization methods confirmed the formation of the nanocomposites, which contain polydopamine, reduced graphene oxides, and metal nanoparticles or nanoalloy. We hypothesized that by alloying silver and copper on the surface of reduced graphene oxides, the oxygen reduction reaction (ORR) catalytic activity of the nanocomposites will be enhanced through both alloying and substrate effects. The size range of the nanoparticles is between 10 nm and 15 nm. We find that both the silver and alloy samples catalyze the ORR via a four-electron mechanism. The alloy nanocomposites showed better performance indicator parameters than the silver one, in both mass activity and kinetic current density. This preparation method has paved a new way of synthesizing an ORR catalyst in an environmentally friendly manner.

Simulation study of excimer VUV emission from xenon microhollow cathode discharge in the self-pulsing regime

A volume-averaged model is employed to study the dynamics of a microhollow cathode discharge in xenon operating in the self-pulsing regime. The numerical results revealed that the discharge voltage initially increases during the abnormal mode when the discharge current is quite low. Then, the discharge is transferred to a normal mode in which the current shows a steep growth to extremely high values. The results also indicated that the self-pulsing frequency increases with an increase in the input voltage at constant pressure. However, an increase in pressure at a constant input voltage leads to a reduced self-pulsing frequency. The strongest vacuum ultraviolet emission is attributed to xenon triplet excimers at a wavelength of 173 nm. An increase in the input voltage enhances the average densities of the xenon excimers, but has almost no effect on the maximum densities. Compared to the input voltage, an increase of the pressure has a much stronger effect on both the maximum and average densities, such that the maximum density of the triplet excimer is increased from 1.48×1017 m−3 at P = 30 Torr to 2×1021 m−3 at P = 250 Torr. A comparison between the self-pulsing and stationary regimes shows that the maximum densities of the excimers at the self-pulsing regime are an order of magnitude higher than those of the stationary regime. Furthermore, the average densities of the excimers in the self-pulsing regime are higher than those of the stationary regime at higher input voltages. However, the reverse is true for lower voltages.

Study of VUV Emission From Xe–Ne Microhollow Cathode Discharge in the Self-Pulsing Regime

This article by employing a zero-dimensional model investigates vacuum ultraviolet (VUV) emission from a xenon–neon microdischarge. The effects of the pressure, the input voltage, and the neon fraction are carefully studied. For specific input voltages, the microdischarge operates in an oscillatory regime which is called the self-pulsing regime. The self-pulsing frequency is mainly affected by the input voltage as well as the pressure. Increasing the input voltage enhances the average densities but has no effects on the peak densities. However, the pressure has significant effects on both densities. The main peaks of the VUV emission spectrum are seen at $\lambda =173$ nm and $\lambda =147$ nm which are attributed to xenon triplet-excited dimers and resonance-level excited atoms, respectively. Adding the neon gas enhances the emission at 147 nm through reducing the power loss in elastic collisions, but attenuates the emission at 173 nm. The results also show that steady-state densities of the major VUV emitters in the stationary regime are higher than the time average densities in the self-pulsing regime. Although, the lower power consumption of the self-pulsing regime has a more decisive effect such that the VUV emission efficiency in the self-pulsing regime is greater than that of the stationary regime.

Influence of operating parameters on high‐pressure microhollow cathode discharge with a cylindrical hole

AbstractAs a promising candidate for generating large‐scale plasma at high pressure, microhollow cathode discharge (MHCD) with a cylindrical hole has been numerically simulated at an argon pressure higher than 50 Torr by a two‐dimensional particle‐in‐cell Monte Carlo collision method. Results indicate that MHCD is operated in a confined phase at the beginning, which transits to an expanded phase and reaches a steady‐state at about 160 ns. In a steady‐state MHCD, the majority of electrons have a low energy of several eV, and some electrons have a high energy up to 480 eV. High‐energy electrons mainly appear adjacent to the cathode surface. With increasing applied voltage, there is a transition from the confined phase to the expanded phase for a steady‐state MHCD. Moreover, maximal density of charged particles increases for the expanded phase with increasing applied voltage and argon pressure or decreasing hole diameter. Besides, discharge morphology has been investigated in detail with varying operating parameters. All these results have been compared with those simulated by the fluid model.

Range and similarity of hollow cathode discharge in argon

Hollow cathode discharge and micro-hollow cathode discharge have numerous applications in the fields of industry, medical treatment, environmental protection, and analytical chemistry. However, many of them lack the typical features of hollow cathode mode, especially the applications at atmospheric pressure. In order to investigate the underlying basic science of hollow cathode discharge, the hollow cathode discharge in argon was studied by experiments. The range for the operation of the hollow cathode mode in the argon–aluminum device was quantitatively determined to be from 0.8 to 4 Torr cm, no matter how small the cathode cavity is. The atmospheric pressure operation of the hollow cathode mode was realised with the aluminum cathode of a 50 μm cavity. The hollow cathode discharges were consistent with Townsend similarity law when the anode was very close to the cathode and the value of p·D was chosen at the lower limit of the range for hollow cathode mode. In contrast, if the anode was moved a little bit far from the cathode and the value of p D was significantly increased, the results followed Allis–White scaling law. The reason for the deviation of Allis–White scaling law from Townsend similarity law was given.

On the validity of neutral gas temperature by emission spectroscopy in micro-discharges close to atmospheric pressure

The neutral gas temperature (Tg) of a single micro-hollow cathode discharge (MHCD) elaborated on silicon wafer is investigated. The MHCD is continuously powered in DC yielding a plasma in He, Ar and N2 gases close to atmospheric pressure. Values of Tg are determined by means of optical emission spectroscopy inside and in the vicinity of a single cavity of diameter. In this work, the use of the resonant broadening to infer Tg is thoroughly reconsidered taking into account the reevaluation of an empirical coefficient as well as including the weaker contribution of the Van der Waals interaction in the line profile analysis. Tg is found to be around in He while in Ar it ranges from 460 to 860 K depending on the current. A temperature gradient is observed between the cavity and the surrounding gas, which is correlated with the design of the MHCD reactor. A second approach involves the study of the relative rotational population distributions of several N2(C–B) vibrational bands to infer the rotational temperature (Trot). While the latter is commonly assumed to approximated Tg, its validity will be discussed based on experimental results. Discrepancies between Trot from different vibrational bands as well as with values found with the resonant broadening are investigated using different operating regimes of the MHCD. The validity of both approaches will be discussed supported by a comprehensive evaluation of the Tg and Trot uncertainty estimations. The analysis of resonant atomic line profile appears to be a reliable and accurate method to measure Tg in absolute room temperature atmospheric pressure plasma sources used in industrial processes as well as in environmental and biomedical applications.

Experimental characterization of a ns-pulsed micro-hollow cathode discharge (MHCD) array in a N2/Ar mixture

An array of seven micro-hollow cathode discharges (MHCDs) operating in a N2/Ar mixture and ignited by a nanosecond pulsed high voltage is experimentally characterized in this work. Compared to other non-equilibrium discharges, high electron densities can be obtained with this type of microdischarge, and, thus, a high dissociation degree of the molecules is expected to be achieved. This is particularly suited for nitride deposition, which is the targeted application of this work, given that the atomic nitrogen density is a key parameter for this deposition process. The electrical diagnostics reveal that the energy deposited in the plasma during one voltage pulse increases with the Ar fraction in the gas mixture and the voltage pulse repetition frequency. A small effect of the repetition frequency is observed on the voltage and current characteristics of the discharge, while a significant increase in the amplitude of these two electrical parameters is observed with increasing Ar admixture. Consequently, a higher electron density is expected with increasing Ar fraction in the gas mixture and repetition frequency. Time-integrated optical emission spectroscopy is carried out to identify the excited emissive species in the plasma and to determine the time-averaged vibrational and rotational temperatures. For a discharge operating in pure N2, at a repetition frequency of 40 kHz, the time-averaged rotational temperature is found to be 390 K ± 40 K and the time-averaged vibrational temperature is 3580 K ± 200 K, revealing the non-equilibrium nature of the discharge. Both temperatures increase with the Ar fraction in the N2/Ar mixture, while they are almost independent of the repetition frequency. Finally, the different phases of the spatio-temporal evolution of the discharge are mapped using space-time resolved ICCD images. The spatial evolution of the discharge between two consecutive voltage pulses can be decomposed in four phases, corresponding to the different stages of the temporal evolution of the discharge voltage.

Experimental and simulation study of pulsed micro-hollow cathode discharge in atmospheric-pressure helium

In this paper, we present an experimental and numerical study on micro-hollow cathode discharge (MHCD) in helium at atmospheric pressure. The discharge in the micro-hollow cathode is driven by a −700 V pulsed power supply with the μs width and kHz frequency. The electrical characteristics were obtained by the experimental measurement and a self-consistent fluid model. The waveforms of the voltage and discharge current obtained by simulation are highly consistent with those obtained by experiments. The electrical characteristics, the spectra, and the distribution of the discharge plasma indicate that an abnormal glow discharge was formed in the MHCD. The spatial-temporal evolutions of the discharge show that the maximum plasma density can attain about 1015 cm−3. The averaged plasma density is over 1014 cm−3 during the discharge, and the plasma can be maintained for a long time during the afterglow period. The discharge current density can reach 30 A cm−2. All results show that a strong ionization and excitation can be generated by pulsed micro-hollow cathode discharge which is suitable for chemical analysis.

Optical and electrical investigation of DC micro-hollow cathode discharge in He/O2 and He/H2 mixture

A hollow cathode discharge source was developed using a negatively biased micro-hole cathode to generate high density point-like plasma. Through electrical and optical investigations, the properties of gas discharge plasmas such as dissipated power, electronic excitation temperature, and rotational temperature were compared in glow and hollow cathode discharge conditions. In the hollow cathode discharge mode, the power consumption and excitation temperature were larger. It was also observed that the relatively higher intensity of vacuum ultraviolet (VUV) oxygen triple lines in He/O2 plasma and VUV hydrogen Lyman-α line in He/H2 plasma were realized in the hollow cathode discharge mode.

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.

Acoustic emission by self-organising effects of micro-hollow cathode discharges

We designed micro-hollow cathode discharge prototypes under atmospheric pressure and investigated their acoustic characteristics. For the acoustic model of the discharge, we correlated the self-organisation effect of the current density distribution with the ideal model of an acoustic membrane. For validation of the obtained model, sound particle velocity spectroscopy was used to detect and analyse the acoustic emission experimentally. The results have shown a behaviour similar to the ideal acoustic membrane. Therefore, the acoustic excitation is decomposable into its eigenfrequencies and predictable. The model was unified utilising the gas exhaust velocity caused by the electrohydrodynamic force. The results may allow a contactless prediction of the current density distribution by measuring the acoustic emission or using the micro-discharge as a tunable acoustic source for specific applications as well.

Direct current microhollow cathode discharges on silicon devices operating in argon and helium

Microhollow cathode discharges have been produced on silicon platforms using processes usually used for MEMS fabrication. Microreactors consist of 100 or 150 μm-diameter cavities made from Ni and SiO2 film layers deposited on a silicon substrate. They were studied in the direct current operating mode in two different geometries: planar and cavity configuration. Currents in the order of 1 mA could be injected in microdischarges operating in different gases such as argon and helium at a working pressure between 130 and 1000 mbar. When silicon was used as a cathode, the microdischarge operation was very unstable in both geometry configurations. Strong current spikes were produced and the microreactor lifetime was quite short. We evidenced the fast formation of blisters at the silicon surface which are responsible for the production of these high current pulses. EDX analysis showed that these blisters are filled with argon and indicate that an implantation mechanism is at the origin of this surface modification. Reversing the polarity of the microdischarge makes the discharge operate stably without current spikes, but the discharge appearance is quite different from the one obtained in direct polarity with the silicon cathode. By coating the silicon cathode with a 500 nm-thick nickel layer, the microdischarge becomes very stable with a much longer lifetime. No current spikes are observed and the cathode surface remains quite smooth compared to the one obtained without coating. Finally, arrays of 76 and 576 microdischarges were successfully ignited and studied in argon. At a working pressure of 130 mbar, all microdischarges are simultaneously ignited whereas they ignite one by one at higher pressure.

Probe measurements of electron energy spectrum and plasma-wall interaction in Helium/air micro-plasma at atmospheric pressure

It is experimentally demonstrated that a wall probe may be a useful instrument for measurement of electron energy spectrum (EES) and the plasma-boundary interaction in a micro-plasma with a nonlocal electron distribution function at atmospheric pressure. The measurements of the EES have been conducted in the plasma of several micro-hollow cathode discharges of differing sizes. The discharges with a flow of Helium gas were exposed to air. Typical results of measurements demonstrate signature of energetic electrons arising due to plasma-chemical reactions. It is experimentally shown that wall probe potential is associated with energetic electrons rather than the ambient electron kinetic energy. The devices may be applicable for developing analytical sensors for extreme environments, including high radiation and high temperatures.

Photogalvanic self-pulsing of electro-wetting of LC droplet on LN-crystal

Photogalvanic current photoinduced by CW laser illumination of Fe-doped LN reveals quasi-periodic pulsing due to microplasmas discharges. This transformation of constant external influence into periodic response is interesting as example of self-excited systems, resembling pulsations of biological systems (including heart beats). Microplasmas is also interesting for potential applications in various fields, such as surface treatment, sterilization, water splitting, light sources, micro jets. Self-pulsing regime of microplasmas discharges in microhollow cathode discharges was observed and modeled in plasmas physics. In this paper, we analyze self-pulsing of photogalvanic current visualized by dynamic electrowetting in a simple LC-cell, formed by a nematic LC droplet placed on Fe-doped LN crystal. We have developed model of self-pulsing regime based on nonlinear discharge resistance. Experimental phase-space plot may be used to find parameters of the nonlinear discharge resistance. In the dynamic modeling contributions from both photogalvanic and pyroelectric effect are included. The analysis of dynamic regime reveals that a current pulse (in microsecond range) have asymmetric shape with extremely sharp rise with longer decay time. These self-pulsations are visualized by reflection of laser light from a LC droplet placed on LN-crystal. Reflected interference patterned was modulated by the dynamic electro-wetting effect.

Electron heating mechanism in radio-frequency microhollow cathode discharge in nitrogen

A two-dimensional particle-in-cell/Monte-Carlo code has been developed to study the electron heating mechanism in radio-frequency microhollow cathode discharge (rf-MHCD) operated in nitrogen at 100 Torr. The influence of secondary electron emission coefficient (γ) on the electron density and total ionization rate, the occurrence of α ionization rate and γ ionization rate, and the electron heating rate are calculated. The results show that compared with the condition of γ = 0, the maximum electron density at γ = 0.1 shows an increase of 60% and the maximum total ionization rate increases by nearly one order of magnitude, which indicates secondary electron heating in rf-MHCD plays an important role. Through the detailed distribution of γ ionization rate and α ionization rate by two-electron model, it is found that γ ionization rate is about 90% of the total ionization rate and the spatial distribution of γ ionization rate presents the same characteristics of the total ionization rate. Therefore, we can further confirm secondary electron heating is the main heating mechanism in rf-MHCD. From the distribution of electron heating rate, it also shows the decisive role of secondary electron heating. With the increase of γ coefficient, α ionization rate increases. This means the electrons which are from fast electron group transferred into slow electron group in the plasma are heated again by sheath oscillation and do contribute to the occurrence of α ionization collision.

A Three-Electrode Gas Switch Triggered by Microhollow Cathode Discharge With Low Trigger Voltage

The trigger voltage is usually high in traditional three-electrode gas switch, which makes the trigger system large and complex in large-scale pulsed power system such as linear transformer driver. In this paper, a three-electrode gas switch triggered by microhollow cathode (MHC) discharge is put forward, which can be operated stably under low trigger voltage. MHC discharge is a kind of microplasma with the characteristics of low trigger voltage (several thousand volts), high electron density, and high gas pressure discharge. In this switch, three MHC triggers sharing the same cathode are designed as the third electrode of the switch in the middle. Positive and negative high voltages are applied to two main electrodes, respectively. The breakdown characteristics of MHC discharge triggered by pulse voltage are first studied. The trigger voltage can be as low as 8 kV at 0.3 MPa in pure N2 with the discharge peak current up to 80 A. A circuit simulation model describing breakdown process of MHC is also built. Then two operating modes of the switch, three-electrode mode (T-mode) and two-electrode mode (D-mode), are analyzed. The stable operation with the charging voltage from ±35 to ±50 kV is realized under different working coefficients from 64% to 92% and different trigger voltages from 8 to 15 kV. The turn-on parameters including delay time, jitter, and turn-on time are studied that the delay time and jitter of D-mode are longer than that of T-mode under the same condition. At last, the improvement of MHC trigger for the stable operation of D-mode with shorter delay time is described.

Spatio-temporal characteristics of microhollow cathode sustained discharge

Micro hollow cathode sustained discharge (MCSD) is simulated by using a fluid model, and the spatiotemoral characteristics of the electric potential, electron density, ion density and electric field are investigated. Results show that the MCSD acts in different modes at different times. The first stage is the Townsend discharge mode. The second is a transition mode from Townsend discharge mode to a hollow cathode effect mode, and the electron density, ion density and electric field near the cathode rise drastically, in which the MCSD is ignited initially. The third stage is the hollow cathode effect mode, and the MCSD forms generally. The last stage is stable discharge state. At the stable discharge stage, the electron density and the ion density each achieve 1015 cm-3 with a peak density located in the center of hollow cathode chamber. The value of electron density in the MCSD region is on the order of 1013 cm -3. The results also show that the micro-hollow cathode discharge (MHCD) contributes to the formation of MCSD, and the MCSD also facilitates the development of MHCD. In addition, the voltage on the second anode has important influence on the distributions of electric potential, electron density and electric field both inside the hollow cathode and outside the hollow cathode. Moreover, the influence on the MCSD is more apparent than the influence on the MHCD. With the increase of voltage on the second anode, the cathode sheath close to the first anode becomes more and more apparent. The second anode is necessary for the formation of micro-hollow cathode sustained discharge.

Simultaneous simulation of microhollow cathode discharge and its sustained discharge with split third electrodes

Simultaneous simulation of microhollow cathode discharge (MHCD) and its sustained discharge in argon is attempted with a two dimensional, self-consistent, continuum fluid model. Bell-shape enlarged volume discharge is simulated with a single third electrode, and further expansion in the discharge volume of the sustained discharge is demonstrated with split third electrodes. In the sustained discharge region, monomer metastable is a dominant species and the number density of dimer ions is higher than that of monomer ions. The expansion in the sustained discharge does not alter the characteristics of the MHCD observed with a single electrode. But electrons and metastables in the sustained discharge region are expanded when the third electrodes are turned on. The simulated distribution of metastable species shows a rake-like structure that is also observed in experiments. As the pressure increases, a ratio of dimer-to-monomer metastable number density increases.