Jia Pengying

Simulation of transition from Townsend mode to glow discharge mode in a helium dielectric barrier discharge at atmospheric pressure

The dielectric barrier discharge characteristics in helium at atmospheric pressure are simulated based on a onedimensional uid model. Under some discharge conditions, the results show that one discharge pulse per half voltage cycle usually appears when the amplitude of external voltage is low, while a glow-like discharge occurs at high voltage. For the one discharge pulse per half voltage cycle, the maximum of electron density appears near the anode at the beginning of the discharge, which corresponds to a Townsend discharge mode. The maxima of the electron density and the intensity of electric eld appear in the vicinity of the cathode when the discharge current increases to some extent, which indicates the formation of a cathode-fall region. Therefore, the discharge has a transition from the Townsend mode to the glow discharge mode during one discharge pulse, which is consistent with previous experimental results.

Characteristics of a Normal Glow Discharge Excited by DC Voltage in Atmospheric Pressure Air

Atmospheric pressure glow discharges were generated in an air gap between a needle cathode and a water anode. Through changing the ballast resistor and gas gap width between the electrodes, it has been found that the discharges are in normal glow regime judged from the current-voltage characteristics and visualization of the discharges. Results indicate that the diameter of the positive column increases with increasing discharge current or increasing gap width. Optical emission spectroscopy is used to calculate the electron temperature and vibrational temperature. Both the electron temperature and the vibrational temperature increases with increasing discharge current or increasing gap width. Spatially resolved measurements show that the maxima of electron temperature and vibrational temperature appeared in the vicinity of the needle cathode.

Investigation on the Micro-Discharge Characteristics of Dielectric Barrier Discharge in a Needle-Plate Geometry

In this study, a dielectric barrier discharge device with needle-plate electrodes was used to investigate the characteristics of the micro-discharge in argon at one atmospheric pressure by an optical method. The results show that there are two discharge modes in the dielectric barrier discharge, namely corona mode and filamentary mode. The corona discharge only occurs in the vicinity of the needle tip when the applied voltage is very low. However, the filamentary discharge mode can occur, and micro-discharge bridges the two electrodes when the applied voltage reaches a certain value. The extended area of micro-discharge on the dielectric plate becomes larger with the increase in applied voltage or decrease in gas pressure. The variance of the light emission waveforms is studied as a function of the applied voltage. Results show that very narrow discharge pulse only appears at the negative half cycle of the applied voltage in the corona discharge mode. However, broad hump (about several microseconds) can be discerned at both the negative half cycle and the positive half cycle for a high voltage in the filamentary mode. Furthermore, the inception voltage decreases and the width of the discharge hump increases with the increase in applied voltage. These experimental phenomena can be explained qualitatively by analyzing the discharge mechanism.

Influence of Ionization Degrees on the Evolutions of Charged Particles in Atmospheric Plasma at Low Altitude

A zero-dimensional model which includes 56 species of reactants and 427 reactions is used to study the behavior of charged particles in atmospheric plasmas with different ionization degrees at low altitude (near 0 km). The constant coefficient nonlinear equations are solved by using the Quasi-steady-state approximation method. The electron lifetimes are obtained for afterglow plasma with different initial values, and the temporal evolutions of the main charged species are presented, which are dominant in reaction processes. The results show that the electron number density decays quickly. The lifetimes of electrons are shortened by about two orders with increasing ionization degree. Electrons then attach to neutral particles and produce negative ions. When the initial electron densities are in the range of 1010 ~ 1014 cm−3, the negative ions have sufficiently high densities and long lifetimes for air purification, disinfection and sterilization. Electrons, O2−, O4−, CO4− and CO3− are the dominant negative species when the initial electron density ne0 ≤ 1013 cm−3, and only electrons and CO3− are left when ne0 ≥ 1015 cm−3 · N+2, N+4 and O+2 are dominant in the positive charges for any ionization degree. Other positive species, such as O+4, N+3, NO+, NO+2, Ar+2 and H3O+·H2O, are dominant only for a certain ionization degree and in a certain period.

Study on the transition from filamentary discharge to diffuse discharge by using a dielectric barrier surface discharge device

Discharge characteristics have been investigated in different gases under different pressures using a dielectric barrier surface discharge device. Electrical measurements and optical emission spectroscopy are used to study the discharge, and the results obtained show that the discharges in atmospheric pressure helium and in low-pressure air are diffuse, while that in high-pressure air is filamentary. With decreasing pressure, the discharge in air can transit from filamentary to diffuse one. The results also indicate that corona discharge around the stripe electrode is important for the diffuse discharge. The spectral intensity of N2+ (391.4 nm) relative to N2 (337.1 nm) is measured during the transition from diffuse to filamentary discharge. It is shown that relative spectral intensity increases during the discharge transition. This phenomenon implies that the averaged electron energy in diffuse discharge is higher than that in the filamentary discharge.

Spatial-Temporal Patterns in a Dielectric Barrier Discharge under Narrow Boundary Conditions in Argon at Atmospheric Pressure

Pattern formation phenomena are investigated in a dielectric barrier discharge under narrow boundary conditions in argon at atmospheric pressure. The discharge shows various scenarios with the increasing applied voltage. This is the first observation of alternating single spot and pair spots pattern and of a moving striation pattern in a dielectric barrier discharge system. The spatial-temporal correlations between discharge filaments in these patterns are measured by an optical method. The results show that the zigzag pattern is an interleaving of two sub-structure patterns, which ignites once for each sub-pattern per half cycle of the applied voltage. There is a temporal sequence inversion in consecutive half-cycles for the two sub-patterns. The pattern of alternating single spot and pair spots is also an interleaving of two sub-structure patterns. However, the pair spots sub-pattern ignites twice and the single spot sub-pattern ignites once per half cycle of the applied voltage.

Influence of square wave duty ratio on pattern formation of atmospheric pressure glow discharge

Excited by a square wave voltage, atmospheric pressure glow discharge is generated in a needle to liquid configuration. With decreasing duty ratio of the square voltage, bright disk, bright disk with outer ring, triple ring, triple ring with concentric spot are observed on the water surface. Voltage and current waveforms indicate that the discharge occurs in the negative polarity. The discharge current consists of two parts, the pulse region and the constant region, which has a good periodicity. It is found that the inception voltage and the peak current increase with decreasing the duty ratio. Optical emission spectra are detected. It contains the second positive band of N2, the first positive band of N2, the first negative band of N2+ and the OH (309 nm). Using the Lifbase, emission spectra of the OH are used to estimate rotational temperature by fitting the experimental spectra to the simulated spectra. It is found that the gas temperature increases with increasing the duty ratio. By synchronously triggering discharge current and ICCD, temporally resolved images of the patterns are captured on the water surface. Results show that self-organized pattern gradually evolves in the pulse region, and it does not change any more in the constant region. Taking into account the space negative charge, above phenomena are analyzed and explained by using the basic theory of discharge.

Experimental investigation on the discharge modes of a direct-current voltage excited plasma jet in a needle-to-ring geometry

Plasma jet can generate a remote non-equilibrium plasma out of its intra-electrode region, which has application potentials in a large variety of fields. Now, it has become a key issue in low-temperature plasma research. It is important to decrease the gas temperature of the plasma plume for industrial applications, especially for biomedical application. In this paper, with ambient air used as working gas, an atmospheric pressure plasma jet in a needle-to-ring electrode geometry is developed to generate a non-thermal plasma plume excited by a direct current voltage, which has a so low temperature that can be touched directly by human body. Through electrical and optical methods, it is found that the discharge can operate in two distinct modes: a pulsed mode or a continuous mode. For the pulsed mode, the discharge frequency increases with increasing the gas flow rate or the power output voltage. For the continuous mode, the voltage-ampere characteristics are investigated for different gas flow rates, which indicate that the discharge is in an abnormal glow regime. Moreover, the gas temperature of the plasma plume is investigated through an optical fiber thermometer. It is found that the gas temperature of the plasma plume increases with increasing the power voltage or decreasing the gas flow rate. Compared with the continuous mode, the pulse mode has a lower gas temperature, so that its plume can be touched directly by a human finger. Therefore, the plasma jet in the pulsed mode is more important for the application in biomedical field. The optical emission spectrum from the two discharge modes is detected to calculate the molecular vibrational temperature. Result indicates that the vibrational temperature as a function of the experimental parameters is similar to the variance of the gas temperature. The physical phenomena mentioned above are discussed and qualitative explanations are given. These results are important for the further investigation of discharge mode and application of direct-current voltage excited plasma jet.

Effect of the angle between the electric field and the gas flow one on the plasma plume dynamics at atmospheric pressure

A novel argon plasma jet driven by an alternating current voltage is developed to generate atmospheric pressure plasma plume in the downstream region of a rod electrode. The novel jet is neither a linear-field jet, nor a cross-field jet, whose angle between the electric field and the flow one can be changed easily. Result indicates that the downstream plume length decreases with increasing the angle under constant peak voltage. For different angles, the positive discharge or the negative one per half voltage cycle corresponds to a discharge pulse. Moreover, the pulse intensity decreases with increasing the angle. By fast photography, the effect of the angle is investigated on the discharge dynamics of the plasma plume. It is found that every discharge pulse corresponds to the propagating process of a plasma bullet. The bullet velocity increases firstly, then decreases during its propagating process. Moreover, its maximal velocity decreases with increasing the angle. Based on the Collisional-Radiation model, the intensity ratio of two spectral lines is investigated to obtain the spatial distribution of excited electron temperature, which corresponds to the spatial distribution of net electric field. Result suggests that the spatial distribution of the net electric field is similar with that of the bullet velocity.

Comparative investigation on the atmosphericpressure plasma jet excited by three types of waveforms

By using a rod-to-ring electrode geometry, an argon plasma jet excited by sinusoidal, triangular and square waveforms is developed to generate cone-shaped plasma plumes at atmospheric pressure. By electrical method, it is found that the discharge pluses excited by the three waveforms only appear in the positive half cycle of the applied voltage, however, there are no discharge pulses in the negative half cycle. Moreover, the discharge pulse number per half voltage cycle increases with increasing the peak voltage. For the multi-pulse discharge, the pulse intensity of the discharge increases in the rising edge of the applied voltage for the sinusoidal and triangular waveforms, while it keeps constant in the positive half cycle of the square voltage. Furthermore, the time interval of the adjacent pulses is larger for the square voltage than that for the sinusoidal or triangular voltages. The temporal evolutions of the plasma plumes excited by the sinusoidal and triangular voltages are investigated by high-speed photography. Result indicates that both of them are composed of a breakdown stage (a streamer developing stage) and an afterglow stage. The aforementioned experimental phenomena are discussed qualitatively by analyzing the discharge process.