Analysis of Dissociation Mechanism of CO_2 in a Micro-Hollow Cathode Discharge

Considering the feature of distributions of parameters within the micro-hollow cathode discharge, we use a simple method to separate the sheath region characterized by drastic changes of plasma parameters and the bulk plasma region characterized by smooth changes of plasma parameters. A zero-dimensional chemical kinetic model is used to analyze the dissociation mechanism of CO2 in the bulk plasma region of a micro-hollow cathode discharge and is validated by comparisons with previous modeling and experimental results. The analysis of the chemical kinetic processes has shown that the electron impact dissociation and heavy species impact dissociation are dominant in different stages of the rnicro-hollow cathode discharge process for a given applied voltage. The analysis of energy consumption distributions under different applied voltages reveals that the main reason of the conversion improvement with the increase of the applied voltage is that more input energy is distributed to the heavy species impact dissociation.

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.

Cylindrical Micro-hollow Cathode Discharge at Atmospheric Pressure

The mechanism of micro-hollow cathode discharge at atmospheric pressure is investigated through simulations using two-dimensional fluid model combined with a transport model for metastable atoms.In the simulations,electric potential,electric field,particle density,and mean electron energy of the discharge are calculated.The results show that the two characteristic regions of the discharge,i.e.cathode drop and negative glow can be distinguished in the simulation.The cathode drop is characterized by strong electric field and high mean electron energy,while quasi-neutral plasma of high density and exists in the negative glow.The peak value of electron density can reach the order of 1017cm-3.The electron temperature varies from several eV to tens of eV.The influence of cathode dimension on the discharge characteristics is also investigated.

Effects of discharge parameters on the micro-hollow cathode sustained glow discharge

The effects of parameters such as pressure,first anode radius,and the cavity diameter on the micro-hollow cathode sustained glow discharge are investigated by using a two-dimensional self-consistent fluid model in pure argon.The results indicate that the three parameters influence the discharge in the regions inside and outside of the cavity.Under a fixed voltage on each electrode,a larger volume of high density plasma can be produced in the region between the first and the second anodes by selecting the appropriate pressure,the higher first anode,and the appropriate cavity diameter.As the pressure increases,the electron density inside the hollow cathode,the high density plasma volume between the first anode and second anodes,and the radial electric field in the cathode cavity initially increase and subsequently decrease.As the cavity diameter increases,the high-density plasma volume between the first and second anodes initially increases and subsequently decreases;whereas the electron density inside the hollow cathode decreases.As the first anode radius increases,the electron density increases both inside and outside of the cavity.Moreover,the increase of the electron density is more obvious in the microcathode sustained region than in the micro cavity region.The results reveal that the discharge inside the cavity interacts with that outside the cavity.The strong hollow cathode effect and the high-density plasma inside the cavity favor the formation of a sustained discharge between the first anode and the second anodes.Results also show that the radial boundary conditions exert a considerably weaker influence on the discharge except for a little change in the region close to the radial boundary.

Deposition of Diamond-Like Carbon on Inner Surface by Hollow Cathode Discharge

A cylindrical hollow cathode discharge （HCD） in CH4/Ar gas mixture at pressure of 20-30 Pa was used to deposit diamond-like carbon （DLC） films on the inner surface of a stainless steel tube. The characteristics of the HCD including the voltage-current curves, the plasma im- ages and the optical emission spectrum （OES） were measured in Ar and CHn/Ar mixtures. The properties of DLC films prepared under different conditions were analyzed by means of Raman spectroscopy and scanning electron microscopy （SEM）. The results show that the electron exci- tation temperature of HCD plasma is about 2400 K. DLC films can be deposited on the inner surface of tubes. The ratio of sp3/sp2 bonds decreases with the applied voltage and the deposition time. The optimizing CH4 content was found to be around CH4/Ar =1/5 for good quality of DLC films in the present system.

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 ram. The electricM characteristics showed an abnormal glow mode. Spectroscopic measurements were carried out to examine the plasma and radicals. An analysis of the spectral profile of Ha 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 L[FBASE, the gas temperature was deduced to be 900 K for 4 kPa of He at a discharge current of 50 mA.

Current self-limitation in a transverse nanosecond discharge with a slotted cathode

A high-voltage transverse pulsed nanosecond discharge with a slotted hollow cathode was found to be a source of high-energy （few kV） ribbon electron beams. Conditions for the formation and extinction of electron beams were experimentally studied in discharges in helium at pressures of 1-100 Torr. It was found that interaction of fast electrons with a non-uniform electric field near the slotted cathode led to limitation of the magnitude of the discharge current. A physical model was developed to describe the discharge current self-limitation that was in satisfactory agreement with the experimental results. Some technical solutions that are expected to increase the upper current limits in transverse nanosecond discharge are discussed.

The Effect of Gas Flow Rate on Radio-Frequency Hollow Cathode Discharge Characteristics

It is known that gas flow rate is a key factor in controlling industrial plasma processing. In this paper, a 2D PIC/MCC model is developed for an rf hollow cathode discharge with an axial nitrogen gas flow. The effects of the gas flow rate on the plasma parameters are calculated and the results show that： with an increasing flow rate, the total ion（N＋2, N＋） density decreases, the mean sheath thickness becomes wider, the radial electric field in the sheath and the axial electric field show an increase, and the energies of both kinds of nitrogen ions increase;and, as the axial ion current density that is moving toward the ground electrode increases, the ion current density near the ground electrode increases. The simulation results will provide a useful reference for plasma jet technology involving rf hollow cathode discharges in N2.

Impact of exterior electron emission on the self-sustaining margin of hollow cathode discharge

Hollow cathode researches used to focus on the inner cavity or downstream plume,however,rarely on the gap between the throttling orifice plate and the keeper plate(T-K gap),which was found to impact the self-sustaining margin of hollow cathode discharge in this paper.Near the lower margin,the main power deposition and electron emission and ionization regions would migrate from inner cavity and downstream plume to the T-K gap,in which case,the source and destination of each m A current therein matter for the self-sustaining capability.Changing the metal surfaces in the T-K gap with emissive materials proved effective in lowering the lower margin by supplementing auxiliary thermionic emission,compensating electron loss on cold absorbing walls and suppressing discharge oscillations.By doing so,the lower margin of a 4 A hollow cathode was lowered from 1 to 0.1-0.2 A,enabling it to couple with low power Hall thruster without extra keeper current.

Determinations of plasma density and decay time in the hollow cathode discharge by microwave transmission

The microwave （MW） transmission method is employed to measure both the plasma density and the plasma decay time in the hollow cathode discharge （HCD） in argon at low pressure. The plasma density in DC-driven or pulsed HCD is on the order of 1012 cm-3, which can block the X-band MW effectively. In the case of pulsed HCD, the MW transmittance shows the same waveform as the pulsed current during the rising edge if the driving frequency is low, but with a longer delay during the falling edge. The MW transmittance reaches a constant low level when the driving frequency is high enough. The plasma decay time in the HCD system is measured to be about 100 μs around a pressure of 120 Pa. The ambipolar diffusion is considered to be the major mechanism in the decay process.

PIC/MCC Simulation of Radio Frequency Hollow Cathode Discharge in Nitrogen

A two-dimensional PIC/MCC model is developed to simulate the nitrogen radio frequency hollow cathode discharge（rf-HCD）.It is found that both the sheath oscillation heating and the secondary electron heating together play a role to maintain the rf-HCD under the simulated conditions.The mean energy of ions（N＋_2,N＋）in the negative glow region is greater than the thermal kinetic energy of the molecular gas（N2）,which is an important characteristic of rf-HCD.During the negative portion of the hollow electrode voltage cycle,electrons mainly follow pendulum movement and produce a large number of ionization collisions in the plasma region.During the positive voltage of the rf cycle,the axial electric field becomes stronger and its direction is pointing to the anode（substrate）,therefore the ions move toward the anode（substrate）via the axial electric field acceleration.Compared with dc-HCD,rf-HCD is more suitable for serving as a plasma jet nozzle at low pressure.

Fast Electrons Behind the Plane-grid Cathode at Nanosecond Discharge in Atmospheric Pressure Air

Breakdown of atmospheric pressure air gaps with combined design of the cathode at the voltage pulse rise rate equal to 1014V/s has been studied with picoseconds’temporal resolution.Cathode assembly has a structure consisting of a flat thin foil and a grid of parallel foil thin wires.In the space behind the cathode foil a fast electron flow was recorded.The current value of the fast electron beam recorded behind the cathode is essentially influenced by the anode material.At a grid cathode and flat anode,the spectra of fast electrons generated both in direct(towards the anode)and reverse directions have been reconstructed by the attenuation curves.

High Pressure Micro-Slot Hollow Cathode Discharge

A direct current glow discharge source structure operating at high pressure based on the micro-slot hollow cathode is presented in this article. A 100 μm width slot cathode was fabricated of copper, and a stable DC glow discharge with an area of 0.5 mm^2 was produced in noble gases （He, Ne） and air over a wide pressure range （kPa - 10 kPa）. The current-voltage characteristics and the near UV radiation emission of the discharge were studied.

Experimental investigations of enhanced glow based on a pulsed hollow-cathode discharge

In this work, the pulsed hollow cathode discharges at low pressure argon with an axial magnetic field were studied. The results indicate that the pulsed discharge is operated in an enhanced glow(EG) mode. Under the same conditions, the discharge current of the pulsed discharge is two or three orders higher than that of the direct current discharge. The spatial and temporal evolution of the light emission shows that, the current fluctuation at the rising edge of the pulse plays an important role for the EG discharge of pulsed hollow cathode, which forms a high-density, highcurrent and long-distance plasma column outside the cavity.

Exploration of a MgO cathode for improving the intensity of pulsed discharge plasma at atmosphere

With regard to the lower density and energy of electrons in pulsed discharge plasma （PDP） at atmosphere, leading to the lower energy utilization of plasma, we propose a MgO cathode to enhance the plasma intensity according to field emission principle. The MgO cathode is prepared by an electro-depositing MgO film on a stainless steel plate. This way, the positive charges come to the cathode and accumulate on the surface of the MgO film, leading to the enhancement of the electric field intensity between the cathode and MgO film, and result in the strong emission of secondary electrons from the MgO cathode. As a result, the intensity of plasma can be enhanced. Herein, the effect of the MgO cathode on the intensity of PDP is investigated. It was shown that the discharge peak current was improved by 20% compared with that of without the MgO cathode. With increasing the MgO film thickness, discharge intensity, including the peak current, transforming charge and spectrum intensity first increased and then decreased. Higher enhancement of peak current, transforming charge and spectrum intensity were acquired with a higher peak voltage. Compared to a cathode without MgO film, the ozone production is higher with MgO cathode employed. The research proposes a novel approach for improving the intensity of discharge plasma, and also provides a reference for further application of PDP.

A N_2 Plasma Light Source Generated in Hollow Cathode Discharge and Its Application in Lithographic Plate Making

Computer to conventional plate （CTCP） technology is getting more and more attention in printing industries. In this paper we report a nitrogen plasma light source generated in hollow cathode discharge （HCD）, Which is used for pre-sensitivity （PS） plate exposure. The N2 molecule emits abundant spectrum ranging from 350 nm to 460 nm. With the voltage of 580 V, current of 1.8 A and pressure of 70 Pa in the discharge an optical power density of 0.46 mW/cm2 is obtained. The optical power density could be further increased with optimizing the lens system. The phototonus efficiency of this source is discussed in detail based upon the chemical principle and the FTIR analysis on the coating material.

Transition characteristics from radio-frequency discharge to arc in hollow cathode configuration

The technique of glow discharges in radio frequency configuration was applied to ignite hollow cathode vacuum arc discharge. The effect of discharge parameters on the building up of hollow cathode arc discharge was investigated. The emission spectrum during the vacuum arc ignition process was measured to disclose the discharge dynamics. There exists a threshold radio frequency power （300 W）, beyond which hollow cathode is in T mode discharge status while radio frequency discharge changes into the arc discharge. With the increase of the radio frequency power, the plasma temperature and electronic density increase, and the discharge mode transits more rapidly. The ignition time of hollow cathode vacuum arc discharge is less than 4 s with a radio frequency power of 700 W.

Simulation of the spatio-temporal evolution of the electron energy distribution function in a pulsed hollow-cathode discharge

This article presents the 2D simulation results of a nanosecond pulsed hollow cathode discharge obtained through a combination of fluid and kinetic models.The spatio-temporal evolution of the electron energy distribution function(EEDF)of the plasma column and electrical characteristics of the nanosecond pulsed hollow cathode discharge at a gas pressure of 5 Torr are studied.The results show that the discharge development starts with the formation of an ionization front at the anode surface.The ionization front splits into two parts in the cathode cavity while propagating along its lateral surfaces.The ionization front formation leads to an increase in the fast isotropic EEDF component at its front,as well as in the anisotropic EEDF component.The accelerated electrons enter the cathode cavity,which significantly contributes to the formation of the highenergy EEDF component and EEDF anisotropy.

Influence of heating on the discharge characteristics of a hollow cathode

Hollow cathodes are widely used as electron sources and neutralizers in ion and Hall electric propulsion.Special applications such as commercial aerospace and gravitational wave detection require hollow cathodes with a very wide discharge current range.In this paper,a heater is used to compensate for the temperature drop of the emitter at low current.The self-sustained current can be extended from 0.6 to 0.1 A with a small discharge oscillation and ion energy when the flow rate is constant.This is also beneficial for long-life operation.However,when the discharge current is high(>1 A),heating can cause discharge oscillation,discharge voltage and ion energy to increase,f urther,combined with a rapid decline of pressure inside the cathode and an increase in the temperature in the cathode orifice plate,electron emission in die orifice and outside the orifice increases and the plasma density in the orifice decreases.This leads to a change in the cathode discharge mode.

Experimental Study on RF Hollow Cathode Discharge

By using a longitudinal static magnetic field, we have shown that it is possible to excite an intensive plasma in a simple stainless steel tube which is connected with a RF power supply. Under certain conditions, the very bright Ar II lines were excited. The emission intensities of Ar II lines were increased with the increase in RF power, magnetic field, and the decrease in argon pressure. As the plasma-sheath boundary oscillating under the RF voltage, the plasma column is periodically compressed by the oscillating boundary.