Investigations Of Atmospheric Pressure Low-temperature Plasma Jets

As a new gas discharge technology generating low-temperature plasmas at atmospheric pressure, atmospheric pressure low-temperature plasma jets have received vigorous devel-opments in recent years, and become one of the hot research issues in plasma science and technology. As some unique technical advantages of this type of gas discharge, such as gen-eration in open space, low gas temperature, advanced plasma chemistry, pollution-free and etc., it has great application potentials in various fields, like biomedicine, material science, environment, etc. In this dissertation, comprehensive investigations of atmospheric pressure low-temperature plasma jets are carried out and the details are presented as follows:1. Development of atmospheric pressure low-temperature plasma jet setups for biomed-ical applications,(i) Two different types of plasma jet setups are developed for plasma treat-ments in biomedical field (or other fields need low-temperature processing). Based on the mechanism of dielectric barrier discharge, the high-voltage electrode is covered by dielectric material, which successfully avoids the generation of arc discharge and greatly improves the operation safety of plasma jet setups. One of the setups is able to generate a plasma jet with a length up to11cm, which is the longest plasma jet generated in open air ever reported in the world at that time. Human fingers can directly contact with the plasma jets generated by these setups;(ii) Through analyzing the equivalent circuit of the case when a human finger is contacting with the plasma jet generated by sine-wave high-voltages with frequency40kHz and amplitude7kV, it is found that the voltage dropped on the human body is at the safety range (<36V), which proves that the safety of the plasma jet is high enough for biomed-ical applications;(iii) A setup which can simultaneously generate multiple plasma jets is developed based on the configuration with multiple gas channels, which greatly increases the treatment area of plasma jets.2. Investigations of the control of plasma jet length. Experimental research on the effects of the air in open space, parameters of power supply (applied voltage amplitude, pulse duration, frequency), and gas velocity on the plasma jet length is carried out.(i) It is found out that the diffused air has a significant adverse influence on the generation of plasma jet in open space. The jet length generated in a pure working gas environment is much larger than that of the jet in open air. Analysis shows that the adverse effect of diffused air is resulted by the fast quenching of energy-saving species, such as excited or metastable He or Ar atoms (with helium or argon working gas) and high-energy electrons by air molecules;(ii) Through the fluid simulation of working gas, it is found out that when the gas flow is at laminar mode, the plasma jet can continue propagating in the open space only when the working gas in the top-area of plasma jet achieves a specific ratio. When the ratio of working gas is lower than the specific value, the length of plasma jet will not perform an obvious rise when the applied voltage is increased.(ⅲ) The fluid simulation shows that the change of plasma jet length can be divided into three regimes with the rise of the working gas velocity, namely the laminar regime, transition regime from laminar to turbulent, and turbulent regime. The plasma jet achieves its maximal length when the gas flow starts to change to the turbulent mode from laminar mode;(ⅳ) Based on the analysis of the influence of gas velocity on the plasma jet length, it is found out that the curves of jet length versus gas velocity at different diameters of setup nozzles can be unified in a map of the jet Reynolds number versus the dimensionless ratio between jet length and nozzle diameter. The map is helpful for the design of jet setups and allows us to predict the flow pattern of plasma jet in order to estimate and control the plasma jet length at different geometrical size of setup nozzles.3. Research on the generation mechanisms of plasma jets. Detailed information of the generation processes of plasma bullet are obtained through high-speed nanosecond imaging technology and time-and-space resolved optical emission spectroscopy measurements.(ⅰ) It is found out that the plasma bullet achieves its maximal propagation velocity at a similar position in the open space where the emission intensity of N2+391.4nm line also gets its peak. This demonstrates that the propagation velocity of plasma bullet is directly related to the density of charged particles inside the bullet;(ⅱ) When the plasma bullet contacts with a dielectric tube and the tube is fed with working gas, a second plasma bullet is generated inside the tube and propagating fast toward the open space. Analysis shows that the gen-eration of the second plasma bullet is induced by the strong space electric-field formed by the charged particles of the first plasma bullet accumulated on the tube surface;(ⅲ) These results demonstrate that the generation and propagation of the plasma bullet is directly in-duced by the strong local space electric-filed in front of the bullet-like ionization volume, the more charged particles inside the volume, the stronger the local space electric-field is formed, and the faster and larger distance the bullet can propagate, and finally the longer plasma jet is generated.4. Diagnostic investigations on the basic parameters, namely the electron temperature and electron density of the plasma jet, are carried out through absolute and high-resolution optical emission spectroscopy measurements.(ⅰ) It is found out that the electron excitation temperature for the low-part of helium atomic state distribution function (ASDF) is around1.2eV, much higher than that of the high-part of helium ASDF (0.3eV). The jet plasma is working at an extreme non-equilibrium state;(ⅱ) Meanwhile, it shows that the high-part of helium ASDF is almost following the Saha equilibrium, but the low-part is far away from the Saha equilibrium;(iii) The plasma electron density is determined by profile-fitting of the helium and hydrogen lines through a composition profile-fitting method. The spatially non-uniform characteristic of electron density in the plasma jet is studied. A high electron density up to1.2x1021m-3is characterized for the discharge core in the jet center, which is of an order of magnitude larger than that of the discharge area in the jet edge.5. Diagnostic investigations of the plasma jet chemistry are performed through the advanced laser-induced fluorescence (LIF) spectroscopy measurements. The absolute densities of OH and O radicals, and their spatial distribution in the plasma jet are deter-mined,(i) A calculation model is developed in this dissertation for determining the OH density in plasma jet with a higher accuracy. The model includes several important physical mechanisms affecting the generation of LIF photons and is able to provide a true OH density in plasma sources,(ii) It shows that with increase of the admixtures of water or oxygen molecules, the distribution of OH or O radicals in the plasma jet gradually shrink to the discharge core close to the gas outlet. The generation of OH or O radicals perform a up-and-down behavior with the rise of the contents of water or oxygen molecules in working gas;(iii) The density of OH or O radicals achieve their maximum densities of5.5×1019m-3and2.4×1022m-3respectively when the admixtures of water or oxygen molecules are both of0.3%.