Cathode position effects on microwave discharge cusped field thruster

The microwave discharge cusped field thruster is a novel concept of electric micropropulsion device,which operatesμN level thrust in low mass flow rate conditions,making use of a coaxial transmission line resonator.With its advantages of low thrust noise and high thrust resolution over a wide range of thrust,the thruster has emerged as a candidate thruster for the space-borne gravitational wave detection mission.The cathode effects commonly exist in many kinds of electric propulsion,and they are typically significant in micropropulsions.In order to find out the cathode position effects on a microwave discharge cusped field thruster,a thermionic cathode is mounted on a cross-slider for coupling.Under different cathode positions,the plume is analyzed by a Faraday probe and a retarding potential analyzer to analyze the performance and discharge characteristics.The results show that the magnetic mirror effect leads to significant degradation of anode current and an increase in low-energy ion ratio as the cathode moves away from the thruster exit.The electron conduction route also significantly impacts anode current efficiency,related to the cathode-exit distance and the thruster magnetic topology.

Effective technology for processing industrial volatile organic compounds by the atmospheric pressure microwave plasma torch

In this study,we investigated the abatement of volatile organic compounds(VOCs)by the atmospheric pressure microwave plasma torch(AMPT).To study the treatment efficiency of AMPT,we used the toluene and water-based varnish to simulate VOCs,respectively.By measuring the compounds and contents of the mixture gas before/after the microwave plasma process,we have calculated the treatment efficiency of AMPT.The experimental results show that the treatment efficiency of AMPT for toluene with a concentration of 17.32×10^(4) ppm is up to 60 g/kWh with the removal rate of 86%.For the volatile compounds of water-based varnish,the removal efficiency is up to 97.99%.We have demonstrated the higher potential for VOCs removal of the AMPT process.

Use of a Microwave Plasma Process at Atmospheric Pressure for Bacterial Inactivation without Thermal Effects

An atmospheric microwave plasma argon was used for the inactivation of bacteria E. coli. The employed device, called Axial Injection Torch (or TIA for Torche à Injection Axiale), consisted of a microwave power source, a waveguide and a gas supply system. Using this argon plasma source, we studied the effects of the exposure time, the exposure distance, the input power, and the gas flow rate on the reduction rate of Escherichia coli cells. The first part of the study was carried out with a static sample exposed to the plasma and then in the second part the sample was set in motion relative to the plasma jet. A log reduction number of E. coli of 4 (10-4 CFU/mL) was obtained with UV and active species, for UV only a log of 1 (10-1 CFU/mL) was obtained.

Studies on Excitation and Rotational Temperatures of an Oxygen-shielded Argon Microwave Plasma Torch Source

Excitation（ Texc ） and rotation（ Trot ） temperatures were determined under different conditions for an oxygen-shielded argon microwave plasmsa torch source（OS-Ar-MPT）. The Texc value, which was shown to be between 4300 and 5250 K under different operating conditions, was calculated from the slope of the Boltzmann plot with Fe as the thermometric species. The Trot value, which was in the range of 2100-2500 K, was measured with OH molecular spectra. The influences of microwave power, flow rates of the support gas, cartier gas, and shielding gas, as well as the observation height on Texc and Trot were investigated and discussed. The detailed results of Texc and Trot provided a better understanding of the performance of an OS-ArMPT as a source for atomic emission spectrometry.

Microwave Plasma Chemical Vapor Deposition of Diamond Films on Silicon From Ethanol and Hydrogen

Diamond films with very smooth surface and good optical quality have been deposited onto silicon substrate using microwave plasma chemical vapor deposition (MPCVD) from a gas mixture of ethanol and hydrogen at a low substrate temperature of 450 ℃. The effects of the substrate temperature on the diamond nucleation and the morphology of the diamond film have been investigated and observed with scanning electron microscopy (SEM). The microstructure and the phase of the film have been characterized using Raman spectroscopy and X-ray diffraction (XRD). The diamond nucleation density significantly decreases with the increasing of the substrate temperature. There are only sparse nuclei when the substrate temperature is higher than 800 ℃ although the ethanol concentration in hydrogen is very high. That the characteristic diamond peak in the Raman spectrum of a diamond film prepared at a low substrate temperature of 450 ℃ extends into broadband indicates that the film is of nanophase. No graphite peak appeare

Electron field emission characteristics of nano-catkin carbon films deposited by electron cyclotron resonance microwave plasma chemical vapour deposition

This paper reported that the nano-catkin carbon films were prepared on Si substrates by means of electron cyclotron resonance microwave plasma chemical vapour deposition in a hydrogen and methane mixture. The surface morphology and the structure of the fabricated films were characterized by using scanning electron microscopes and Raman spectroscopy, respectively. The stable field emission properties with a low threshold field of 5V/μm corresponding to a current density of about 1μA/cm^2 and a current density of 3.2mA/cm^2 at an electric field of 10V/μm were obtained from the carbon film deposited at CH4 concentration of 8%. The mechanism that the threshold field decreased with the increase of the CH4 concentration and the high emission current appeared at the high CH4 concentration was explained by using the Fowler-Nordheim theory.

Field emission characteristics of nano-sheet carbon films deposited by quartz-tube microwave plasma chemical vapour deposition

Nano-sheet carbon films are prepared on Si wafers by means of quartz-tube microwave plasma chemical vapour deposition （MPCVD） in a gas mixture of hydrogen and methane. The structure of the fabricated films is investigated by using field emission scanning electron microscope （FESEM） and Raman spectroscopy. These nano^carbon films are possessed of good field emission （FE） characteristics with a low threshold field of 2.6 V/μm and a high current density of 12.6 mA/cm^2 at an electric field of 9 V/μm. As the FE currents tend to be saturated in a high E region, no simple Fowler-Nordheim （F-N） model is applicable. A modified F N model considering statistic effects of FE tip structures and a space-charge-limited-current （SCLC） effect is applied successfully to explaining the FE data observed at low and high electric fields, respectively.

On the Characteristics of Coaxial-Type Microwave Excited Linear Plasma:a Simple Numerical Analysis

To unveil the characteristics and available propagation mechanism of coaxial-type microwave excited line-shape plasma, the effects of parameters including microwave power, working pressure, dielectric constant, and external magnetic field on the plasma distribution were numerically investigated by solving a coupled system of Maxwell＇s equations and continuity equations. Numerical results indicate that high microwave power, relatively high working pressure, low dielectric constant, and shaped magnetic field profiles will help produce a high-density and uniform plasma source. Exciting both ends by microwave contributed to the high-density and uni- form plasma source as well. Possible mechanisms were analyzed by using the polarization model of low temperature plasma. The generation and propagation processes of the line-shape plasma mainly depend on the interaction of three aspects, i.e. the transmitted part, penetration part and absorptive part of the electromagnetic field. The numerical results were qualitatively consistent with available experimental results from literature. More elaborate descriptions of the three aspects and corresponding interactions among them need to be investigated further to improve the properties of the line-shape plasma.

Study of Spectral Character of Alkali Metals Using Microwave Plasma Torch Simultaneous Spectrometer

A microwave plasma torch （MPT） simultaneous spectrometer was used to study the spectral character and the matrix effect on alkali metal ions in solution. The main parameters were optimized. The microwave forward power was 100 W. The argon flow rate that was used to sustain the Ar-MPT included the flow rate of carrier gas and the flow rate of support gas, which were 0. 8 and 1.0 L/min, respectively. The HC1 concentration in the solution was 0.02 mol/L. The observation height was 9. 0 ram. The detection limits of Li, Na, K, Rb, and Cs were 0. 0003, 0. 0004, 0. 009, 0.07 and 2.4 mg/L, respectively, and the resuhs obtained by the Ar-MPT were compared with those obtained by argon inductively coupled plasma（Ar-ICP） and argon microwave induced plasma（Ar-MIP）. The interference effects of several matrix elements were also studied.

Preparation of Iron Nanoparticles from Iron Pentacarbonyl Using an Atmospheric Microwave Plasma

A novel method is introduced for preparing iron nanoparticles from iron pentacar- bonyl using an atmospheric microwave plasma. The prepared iron nanoparticles were characterized by transmission electron microscopy and X-ray diffraction. The results show that the size of the particles can be controlled by adjusting the microwave power and the flow rate of the carrier gas. The magnetic properties of the synthesized iron particles were studied and a saturation magnetiza- tion of ~95 emu/g was obtained. The convenient preparation process and considerable production rate were also found to be satisfactory for industrial applications.

Growth of 4＂ diameter polycrystailine diamond wafers with high thermal conductivity by 915 MHz microwave plasma chemical vapor deposition

Polycrystalline diamond（PCD） films 100 mm in diameter are grown by 915 MHz microwave plasma chemical vapor deposition（MPCVD） at different process parameters,and their thermal conductivity（TC） is evaluated by a laser flash technique（LFT） in the temperature range of230-380 K.The phase purity and quality of the films are assessed by micro-Raman spectroscopy based on the diamond Raman peak width and the amorphous carbon（a-C） presence in the spectra.Decreasing and increasing dependencies for TC with temperature are found for high and low quality samples,respectively.TC,as high as 1950 ± 230 W m-1 K-1 at room temperature,is measured for the most perfect material.A linear correlation between the TC at room temperature and the fraction of the diamond component in the Raman spectrum for the films is established.

A Study on Microwave Induced Plasma Ionization Detector for Gas Chromatograph

Among various ionization detectors for gas chromatography(GC), the most promising one is perhaps the nonradioactive ionization detector which makes use of a microwave induced plasma(MIP). The use of MIP for gas chromatography was first studied by McCormack, et al. in 1965, Freeman developed a photoionization detector utilizing helium microwave discharge. In 1971, we developed a microwave

Quantitative Determination of Density of Ground State Atomic Oxygen from Both TALIF and Emission Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

Two experimental techniques have been used to quantify the atomic oxygen density in the case of hot air plasma generated by a microwave （MW） resonant cavity. The latter operates at a frequency of 2.45 GHz inside a cell of gas conditioning at a pressure of 600 mbar, an injected air flow of 12 L/min and an input MW power of 1 kW. The first technique is based on the standard two photon absorption laser induced fluorescence （TALIF） using xenon for calibration but applied for the first time in the present post discharge hot air plasma column having a temperature of about 4500 K near the axis of the nozzle. The second diagnostic technique is an actinometry method based on optical emission spectroscopy （OES）. In this case, we compared the spectra intensities of a specific atomic oxygen line （844 nm） and the closest wavelength xenon line （823 nm）. The two lines need to be collected under absolutely the same spectroscopic parameters. The xenon emission is due to the addition of a small proportion of xenon （1% Xe） of this chemically inert gas inside the air while a further small quantity of H2 （2~） is also added in the mixture in order to collect OH（A- X） and NH（A-X） spectra without noise. The latter molecular spectra are required to estimate gas and excitation temperatures. Optical emission spectroscopy measurements, at for instance the position z=12 mm on the axis plasma column that leads to a gas measured temperature equal to 3500 K, an excitation temperature of about 9500 K and an atomic oxygen density 2.09× 1017＋ 0.2×1017 cm-3. This is in very good agreement with the TALIF measurement, which is equal to 2.0×101T cm-3.

A New Atmospheric Pressure Microwave Plasma Source (APMPS)

An atmospheric pressure microwave plasma source （APMPS） that can generate a large volume of plasma at an atmospheric pressure has been developed at Tsinghua University. This paper presents the design of this APMPS, the theoretical consideration of microwave plasma ignition and the simulation results, including the distributions of the electric field and power density inside the cavity as well as the accuracy of the simulation results. In addition, a method of producing an atmospheric pressure microwave plasma and some relevant observations of the plasma are also provided. It is expected that this research would be useful for further developing atmospheric pressure microwave plasma sources and expanding the scope of their applications.

Catalyst-Free Growth of Graphene by Microwave Surface Wave Plasma Chemical Vapor Deposition at Low Temperature

Catalyst-free graphene films has been synthesized by microwave (MW) surface wave plasma (SWP) chemical vapor deposition (CVD) using hydrogenated carbon source on silicon substrates at low temperature (500℃). The synthesized process is simple, low-cost and possible for application on transparent electrodes, gas sensors and thin film resistors. Analytical methods such as Raman spectroscopy, transmission electron microscopy (TEM) and four points prove resistivity measurement and UV-VIS-NIR spectroscopy were employed to characterize properties of the graphene films. The formation of multilayer of graphene on silicon substrate was confirmed by Raman spectroscopy and TEM. It is possible to grow graphene directly on silicon substrate (without using catalyst) due to high radical density of MW SWP CVD. In addition, we also observed that the hydrogen had significant role for quality of graphene.

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.

EVALUATON OF MICROWAVE PLASMA TORCH FOR ATOMIC FLUORESCENCE SPECTROMETRY WITH AN ULTRASONIC NEBULIZATION SAMPLE INTRODUCTION SYSTEM

In this paper, a new MPT(microwave plasma torch) device has been used as a atomizer for atomic fluorescence spectrometry. Spme elements, such as Zn, Cd, Hg, Pb, As, Co, Mg, Cu, Ag, Mn, Fe have been investigated in detail.

An investigation on improving the homogeneity of plasma generated by linear microwave plasma source with a length of 1550 mm

To develop a larger in-line plasma enhanced chemical vapor deposition(PECVD)device,the length of the linear microwave plasma source needs to be increased to 1550 mm.This paper proposes a solution to the problem of plasma inhomogeneity caused by increasing device length.Based on the COMSOL Multiphysics,a multi-physics field coupling model for in-line PECVD device is developed and validated.The effects of microwave power,chamber pressure,and magnetic flux density on the plasma distribution are investigated,respectively,and their corresponding optimized values are obtained.This paper also presents a new strategy to optimize the wafer position to achieve the balance between deposition rate and film quality.Numerical results have indicated that increasing microwave power and magnetic flux density or decreasing chamber pressure all play positive roles in improving plasma homogeneity,and among them,the microwave power is the most decisive influencing factor.It is found that the plasma homogeneity is optimal under the condition of microwave power at 2000 W,chamber pressure at 15 Pa,and magnetic field strength at 45 mT.The relative deviation is within−3.7%to 3.9%,which fully satisfies the process requirements of the equipment.The best position for the wafer is 88 mm from the copper antenna.The results are very valuable for improving the quality of the in-line PECVD device.

Similarity principle of microwave argon plasma at low pressure

In order to validate the similarity principle of microwave breakdown, a two-dimensional （2D） fluid model of low- pressure microwave argon plasma is established and solved by the finite-element method. Proportional conditions are used in this model to build three different breakdown processes that meet the premise of a similarity principle, and these breakdown processes are called ＂similar cases＂ in this paper. Similar cases have proportionately sized breakdown regions, where the ratio of frequency of incident microwave f to gas pressure p （f/p）, and the reduced field E/p in them are kept the same. All the important physical parameters such as electron density, electron temperature, and reduced electric field can be obtained from the simulation of this model. The results show that the parameters between similar cases are in constant ratio without changing with time, which means that the similarity principle is also valid in microwave breakdown.

Reduction of Ilmenite Through Microwave Plasma

Ilmenite was reduced through microwave plasma-assisted chemical vapor deposition with continuously flowing hydrogen and methane gas. The reduction products were analyzed by XRD and SEM technology, and the component products were TiO 2 -Carbon Nanotubes （CNTs） composite powders. The reduction process was in good agreement with the Jander equation. Compared with other reduction process by kinetics analysis, microwave plasma could significantly facilitate the reduction process at low temperature.