[Glow Discharge Characteristics of Hollow Needle-Plate Electrode in Atmospheric Pressure Argon].

Atmosphere pressure uniform plasma has the broad application prospect in the industrial field. Using hollow needle cathode-plate anode device excited by direct-current voltage, a uniform and stable glow discharge is generated at atmospheric pressure in ambient air with argon used as working gas. The influence of the experimental parameters (including gas flow rate and the gas gap width) on discharge has been investigated by optical method. It can be found that a glow-discharge plasma column can bridge the two electrodes. The plasma column is uniform, and no filaments can be discerned. Near the plate electrode, the diameter of the plasma column is largest of all positions. The maximal diameter of the plasma column increases with increasing the discharge current or the gas flow rate. Through electrical method, the voltage-current characteristic has been investigated. It has been found that the discharge voltage decreases with increasing the current which is similar with the characteristic of glow discharge in low pressure. It increases with increasing the gas gap width or the gas flow rate. By analyzing the optical emission spectrum scanning from 330 to 450 nm emitted from the direct-current glow discharge, the molecular vibrational temperature and the intensity ratio of spectral lines I391.4/I337.1 have been investigated as functions of the gas flow rate and gas gap width. Results indicate that both the vibrational temperature and the intensity ratio of spectral lines I391.4/I337.1 decrease with increasing the gas flow rate or the gas gap width. In addition, the molecular vibrational temperature and the intensity ratio of spectral lines I391.4/I337.1 have been investigated in spatial resolution along the direction of gas flow (plasma column axial), and give a qualitative analysis as well. It is found that the vibrational temperature and the average electron energy increase with increasing the distance from the hollow needle cathode. These results are important to the industrial applications of glow discharge.

[Investigation on Spectral Characteristics of Micro-Gap Dielectric Barrier Uniform Discharge in Air].

The plasmas generated by dielectric barrier discharge in atmospheric pressure air have wide application prospect in industry. In order to study generation condition and mechanism, the dielectric barrier uniform discharge in atmospheric pressure air has been studied experimentally with a micro-gap discharge device. Results of electrical characteristics indicate that it exist several current pulses with short width in half period of the applied voltage at a low voltage, a large number of micro-discharge filaments are observed. The discharge power increases with increasing peak value of applied voltage, the micro-discharge filaments increase meanwhile. When the peak of applied voltage reaches to 9.2 kV, only a discharge hump with a width of about 5.5 µs appears in a half period of the applied voltage, micro-discharge filaments cannot be discerned. The uniform discharge has been obtained finally as the micro-discharge filaments extend and superimpose randomly. The emission spectrum of dielectric barrier discharge scanning from 330 to 420 nm is collected. It is found that the intensity of 337.1 nm is stronger than that of 391.4 nm. If the intensity of 337.1 nm is considered as the datum reference, the intensity of 391.4 nm shows the magnitude of electron average energy. The molecule internal energy is evaluated by vibration temperature. The electron average energy and molecule internal energy have been investigated by optical emission spectra. It is found that both of them decrease with increasing the applied voltage. Results indicate that it is not easy to form filamentary discharge when the electrical energy is lower. The average electron energy of uniform discharge is lower than that of the filamentary discharge. These results are of great significance to the application of dielectric barrier uniform discharge obtained in air at atmospheric pressure.

[Investigation of a jet operated in atmospheric pressure argon by optical emission spectroscopy].

A uniform plasma plume was generated in a coaxial dielectric barrier discharge jet through blowing argon into the ambient air at atmospheric pressure. The plasma plume was uniform along the direction of the gas flow. The length of the plasma plume was investigated as a function of the peak voltage, the driving frequency and the gas flow rate. It was found that with increasing the gas flow rate, the plume length increases when the flow rate is lower than 4 L x min(-1), and decreases when it is higher than 4 L x mic(-1). Under constant gas flow rate, the length of the plasma plume increases with the increase in the peak value of the applied voltage and the driving frequency. According to the discharge theory and based on the analysis of the turbulence and the advection, a qualitative explanation was given for the variance of plume length as functions of the experimental parameters. Results also show that there is a discharge pulse for the plasma plume in every positive half cycle, while there is no pulse in negative half cycle. The coaxial dielectric barrier discharge shows two pulses in every positive half cycle and a pulse in every negative half cycle. Analyzing these experimental phenomena mentioned above, a formation mechanism of the plasma plume was proposed. The optical emission spectra were obtained for both the coaxial dielectric barrier discharge and the plasma plume. There was no apparent difference except that some emission lines from reactive species such as OH and N2 were found in the plasma plume. Using the first negative band of, the rational temperature of the plasma plume was measured. Results show that the rational temperature of the plasma plume decreases away from the jet nozzle, and increases with increasing the peak value of the applied voltage.

[Spectroscopic characteristics of DC excited atmospheric pressure glow discharges generated in a needle-plate electrode device and a needle-water electrode one].

Glow discharge characteristics in two discharge devices, i.e. in a needle-plate electrode geometry and a needle-water electrode one were compared by using spectroscopic method. Results show that the different emission regions were found in both discharges generated by the two devices. From the cathode to the anode, there are a cathode glow region, cathode dark glow, a positive column, and an anode glow region. The anode dark region can be clearly discerned in the glow discharge in the needle-plate electrode device, while it almost cannot be found in the needle-water electrode discharge. Comparing the current-voltage characteristics of the two glow discharges, it was found that the voltage across the electrodes decreases with increasing the discharge current in both discharge devices, while the voltage in the needle-water glow discharge is higher than that of the needle-plate one at the same current value. The current-voltage curves have a negative slope and their current densities lie in the range from 10-5 to 10-4 A? cm-2, which indicates that a normal glow discharge mechanism was involved in the two discharges. Comparing the optical spectra scanning from 300nm to 800nm and emitted from the whole normal glow discharge in the two electrode devices, similar spectral lines from the two discharges can be found on the optical emission spectrum, including the second positive system of nitrogen molecules (337.1nm) and the first negative system of nitrogen molecular ions (391.4nm). However, the intensity ratio of spectral lines is different. The intensity ratio (391.4nm to 337.1nm) and vibrational temperature were investigated at different locations. It was found that the intensity ratio of the needle-water electrode discharge is larger than that of the needle-plate electrode discharge at the same location. Furthermore, the vibrational temperature in the needle-water electrode discharge is higher than that of the needle-plate one at the same location.

[Spatial distribution of electrons with high energy in atmospheric pressure glow discharge excited by DC voltage].

Atmospheric pressure glow discharge excited by a DC voltage was realized in a 6 mm air gap by using a needle-water electrode discharge device. The atompheric pressure glow discharge has characteristic regions such as a cathode fall, a negative glow, a Faraday dark space, a positive column and an anode glow. The discharge is a normal glow through analyzing its voltage-current curve. The emission intensity of 337.1 nm spectral line from the second positive system of N2 was investigated because it can indicate the electron density with high energy. Results show that the maxima of high energy electrons appears in the vicinity of the needle tip, and it almost remains constant at other locations. The density of high energy electrons decreases with increasing the voltage. Similarly, it decreases with increasing the value of the ballast resistor. Oxygen atom is important for the sterilization and disinfection. The distribution of oxygen atom was also investigated by optical emission spectroscopy. It was found that the oxygen distribution is similar with the distribution of high energy electrons. These results are important for the application of atmospheric pressure glow discharge in environmental protection and biological treatment.

[Vibrational temperature of plasma plume in atmospheric pressure air].

A tri-electrode discharge device was designed in a dielectric barrier discharge configurations to generate a fairly large volume plasma plume in atmospheric pressure air. The discharge characteristics of the plasma plume were investigated by an optical method. The discharge emission from the plasma plume was collected by a photomultiplier tube. It was found that the number of discharge pulse per cycle of the applied voltage increased with increasing the peak value of the applied voltage. The emission spectra of the plasma plume were collected by a spectrometer. The vibrational temperature was calculated by fitting the experimental data to the theoretical one. Results showed that the vibrational temperature of the plasma plume decreases with increasing the U(p). Spatially resolved measurement of the vibrational temperature was also conducted on the plasma plume with the same method. Results showed that the vibrational temperature increases firstly and then decreases with increasing distance from the nozzle. The vibrational temperature reachs its maximum when the distance is 5.4 mm from the nozzle. These experimental phenomena were analyzed qualitatively based on the discharge theory. These results have important significance for the industrial applications of the plasma plume in atmospheric pressure air.

[Measurement of active particles generated in a coaxial barrier discharge by spectral method].

Coaxial dielectric barrier discharge has extensive application prospects. A dielectric barrier discharge device with water electrode was used to investigate the discharge properties and spectral intensity emitted from active particles in the air by optical method. Results indicate that the optical emission spectra consist of spectral lines from oxygen atoms (777.5 and 844.6 nm), which implies that oxygen atoms with high chemical activity were generated in the discharge plasma. Through spatially resolved measurements, spectral intensities from oxygen atoms were given as functions of the experimental parameters such as the value of the applied voltage, the gas flow rate and argon content. Results show that the spectral line intensity from oxygen atom increases with increasing the peak value of the applied voltage, increases with increasing the gas flow rate, reaches its maximum with a gas flow rate of 30 L x min(-1) and then decreases with further increasing the gas flow rate. Similarly, the spectral line intensity increases firstly and then decreases with increasing argon content (in a mixture of argon and air) and a maximum is reached when argon content is 16.7%.

[Comparative investigation of the discharge characteristics of a single needle jet and needle-plate jet].

In the present paper, discharge characteristics were studied in atmospheric pressure argon by a single needle jet and needle-plate jet through combination of optical measurement and electrical one. Results show that the length and cross-sectional area of the plasmas generated in the two jets increase with increasing the peak value of the applied voltage. The cross-sectional area generated by needle-plate jet is bigger than that of the single needle jet at the same voltage. A lower inception voltage is needed for the needle-plate jet compared with the single needle jet at the same U(p). Through the spectra emitted from the two jets, electron temperature and vibration temperature wee compared for the plasmas generated by the single needle jet and needle-plate jet, respectively. It can be found that the electron temperature and the vibrational temperature of the two jets increase with increasing U(p). The needle-plate jet has higher values of electron temperature and vibrational temperature than the single needle jet at the same U(p). These results have significant values for the industrial application of the atmospheric pressure plasma jet.

[Spectral intensity distribution of oxygen atom in a plasma plume].

Low temperature plasma generated in plasma plume discharge at atmospheric pressure has prosperous application fields in industry because the vacuum device can be dispensable and some complex materials can be treated in three dimensions by this plasma plume. A stable plasma plume was generated in atmospheric pressure air in the present paper by using a plasma needle discharge device. It was found by spectral measurement that there are some spectral lines emitted from oxygen atom such as 777.5 and 844.6 nm in the optical emission spectra of the plasma plume. This phenomenon indicates that oxygen atom with high chemical activity was generated in the air discharge at atmospheric pressure. The spatial distribution of the spectral intensity from the oxygen atom was investigated by spectroscopic method. Results show that the spectral intensity from oxygen atom near the needle electrode was much higher than that in other regions. In order to explain this experimental phenomenon, spatial-resolved signals emitted from the plume were detected by using photomultiplier tubes. It was found that the width of light pulse near the needle electrode was much bigger than that in other regions. These results are important for the application of plasma plume in industry such as sterilization and disinfection fields.

[Investigation of the vibrational temperature and gas temperature in gas discharge generated by plasma needle].

Low temperature plasma generated by plasma needle in atmospheric pressure air has extensive application prospects in industry because the vacuum device can be dispensable. In the present paper a stable plasma plume was generated in air by using a plasma needle device. The vibrational temperature and gas temperature were investigated for the plasma plume by optical spectroscopic method. Research results show that the plasma plume generated in atmospheric pressure air can be distinguished as a strong emission area near the needle followed by a weak emission area. The light emission signal from the discharge is a pulse per half cycle of the applied voltage with a time width of several microseconds. Results also indicate that the vabrational temperature varies from 2 500 to 3 000 K for different emission locations. The vibrational temperature increases with increasing the distance from the needle point in the strong emission area and it reaches a peak value at a distance of about 5mm from the needle point. The vibrational temperature decreases with increasing the distance from the needle. Similarly, the gas temperature decreases from 640K to 540K with increasing the distance from the needle point. These results are of great importance for the industrial applications of air discharge at atmospheric pressure.

[Comparative study on the gas temperature of a plasma jet at atmospheric pressure].

A plasma jet of a dielectric barrier discharge in coaxial electrode was used to produce jet plasma in flowing work gas (argon mixed with trace nitrogen) at atmospheric pressure. The relation between the plasma length and the gas flow rate was obtained by taking the images of the jet plasma. A high-resolution optical spectrometer was used to collect the optical emission spectrum. The emission spectra of the first negative band of N(2+) (B2 Sigma(u+)-->Chi2 Sigma(g+), 390-391.6 nm) were used to estimate the rotational temperature of the plasma plume by fitting the experimental spectra to the simulated spectra. The gas temperature was investigated by this optical method and results show that the gas temperature increases with increasing the applied voltage. For comparison, a thermometer was used to measure the temperature of the gas emitted from the jet. The results also show that the gas temperature increases with increasing the applied voltage. The gas temperatures obtained by the two methods are consistent. The difference was analyzed.

[Investigation on the gas temperature of a plasma jet at atmospheric pressure by emission spectrum].

A plasma jet of a dielectric barrier discharge in coaxial electrode was used to produce plasma plume in atmospheric pressure argon. Spatially and temporally resolved measurement was carried out by photomultiplier tubes. The light emission signals both from the dielectric barrier discharge and from the plasma plume were analyzed. Furthermore, emission spectrum from the plasma plume was collected by high-resolution optical spectrometer. The emission spectra of OH (A 2sigma + --> X2 II, 307.7-308.9 nm) and the first negative band of N2+ (B2 sigma u+ --> X2 IIg+, 390-391.6 nm) were used to estimate the rotational temperature of the plasma plume by fitting the experimental spectra to the simulated spectra. The rotational temperature obtained is about 443 K by fitting the emission spectrum from the OH, and that from the first negative band of N2+ is about 450 K. The rotational temperatures obtained by the two method are consistent within 5% error band. The gas temperature of the plasma plume at atmospheric pressure was obtained because rotational temperature equals to gas temperature approximately in gas discharge at atmospheric pressure. Results show that gas temperature increases with increasing the applied voltage.

[Determination of electric field distribution in dielectric barrier surface glow discharge by spectroscopic method].

In the present paper, stable glow discharges were obtained in air at low pressure with a dielectric barrier surface discharge device. Light emission from the discharge was detected by photomultiplier tubes and the research results show that the light signal exhibited one discharge pulse per half cycle of the applied voltage. The light pulses were asymmetric between the positive half cycle and the negative one of the applied voltage. The images of the glow surface discharge were processed by Photoshop software and the results indicate that the emission intensity remained almost constant for different places with the same distance from the powered electrode, while the emission intensity decreased with the distance from the powered electrode increasing. In dielectric barrier discharge, net electric field is determined by the applied voltage and the wall charges accumulated on the dielectric layer during the discharge, and consequently, it is important to obtain information about the net electric field distribution. For this purpose, optical emission spectroscopy method was used. The distribution of the net electric field can be deduced from the intensity ratio of spectral line 391.4 nm emitted from the first negative system of N2+ (B 2sigma u+ -->X 2sigma g+) to 337.1 nm emitted from the second positive system of N2 (C 3IIu-B 3IIg). The research results show that the electric field near the powered electric field is higher than at the edge of the discharge. These experimental results are very important for numerical study and industrial application of the surface discharge.

[Study on the characteristics of dielectric barrier discharge at atmospheric pressure by spectroscopic method].

In the present paper, stable discharges were respectively obtained in air and helium at atmospheric pressure with a dielectric barrier discharge device consisting of water electrodes. By comparing the discharges in the two gases, it was found that the discharge in air at atmospheric pressure is not uniform and micro-discharges can be discerned which are distributed on the electrode stochastically, while the discharge in helium at atmospheric pressure is quite uniform and no discharge filament can be discerned. Comparing the two kinds of discharges, the plasma generated in helium discharge at atmospheric pressure has better prospect for application in industry because of its uniformity. The discharge current waveforms in air and helium at atmospheric pressure were compared, and the results show that the discharge current pulses in air are stochastically distributed in time, while the current pulses in helium discharge appear periodically. The discharge duration in helium is much longer than that in air at atmospheric pressure. The discharge pulse length is about tens of nanoseconds in air and is about one microsecond in helium. Meanwhile the spectra emitted from the two kinds of discharges were studied, and the results show that the spectral line of 391.4 nm emitted from N2+ (B2Sigmau+ --> X2Sigmag+) is quite strong in helium discharge. On the contrary, the strength of 391.4 nm spectral line is very weak in air discharge. These experimental results are significant for the realization of uniform discharge at atmospheric pressure and industrial application of dielectric barrier discharge.