Formation Mechanism Of Atmospheric Pressure Plasma Jet In Coplanar Dielectric Barrier Discharge

Atmospheric pressure plasma jet(APPJ) produced in air have been widely used in material processing, environmental management and biomedical fields due to its advantages of simple operation, without vacuum cavity, higher electronic temperature while relative lower gas temperature, etc. Understanding of the propagation mechanism of APPJ is the key issue for its extending applications, processing optimizating and controlling. In this thesis, the formation and propagation mechanism of APPJ in coplanar dielectric barrier discharge(DBD) in helium is investigated numerically and experimentally. The researches include three parts:Firstly, the characteristic of APPJ in coplana DBD are investigated experimentally. The effect of driving voltage, gas flow, tube diameter, electrode width, diameter and length of the dielectric tube, shaped-tube as well as the environmental gas on plasma jet has been tested. The results show that formation and propagation of plasma jet depends strongly on the process of coplanar DBD. Suitable gas channel of helium flow is necessary for the jet. But the jet doesn’t depend on the the direction of gas flow.Secondly, APPJ is interfered by using a controllable electric field, magnetic field or a metal tube, to investigate the behavior of plasma jet in external field. The results show that the plasma jet deflects obviously in the electric field or the magnetic field, accompanied by the reduction of jet length. In the electric field the jet becomes broadening in radial direction or even separates into two plumes. A metal tube inserted in the jet path can impede the jet propagation. But the jet can still propagate through the metal tube when the applied volatage is high enough. A voltage on the metal tube affects the jet in different way, with the positive voltage promoting plasma jet longer while the negative one stopping the jet. The axial electric field for sustaining the jet in helium is estaimated to be about 10kV/cm. The field directs from inner to outside, making electron avalanche to propagate inwards.Finally, a two-dimensional fluid model is used to simulate the formation of APPJ in coplanar DBD in helium. The numerical results illustrate that the plasma jet form initially at the temporary cathode of DBD after the discharge bridges the DBD electrodes, from the near position around the cathode where the axial electric field generated by the positive space as well as wall charges on the cathode is high enough. This group of positive ions actually acts as the initial head of plasma jet. Electrons are accelerated by this strong local electric field, leading to ionization and excitation of helium atoms. The electrons move fast to the DBD electodes, remaining dense ions in space as the new jet head. This makes the jet and its head to develop outwards, until the electric field is reduced below the breakdown value. The wall charges have little effect on the jet, especially away from the DBD cathode. The plasma jet in coplanar DBD is similar to the "positive streamer" in single-electroded positive corona that develops from inner to outside of the dielectric tube.