Investigations Of Atmospheric Pressure Low-temperature Plasma Jets

Atmospheric pressure low-temperature plasma jets have recently attracted growing interest, motivated by their advantages such as low temperature, advanced chemistry, safety and briefness, and become one of the hot research issues in plasma science and technology. It has great application potentials in various fields. People want to know more about the relationship between the parameter of plasma jet and device to take advantages of it. In this dissertation, co mprehensive investigations of atmospheric pressure low-temperature plasma jets are carried out and the details are presented as follows:1. Investigation of the diffusion of air and the change of the permittivity of the medium surrounding the plasma plume. Several special experiments are designed to study the mechanism of the acceleration behavior of plasma bullet. These two experiments con?rm that the acceleration behavior of the plasma bullet when it exits the nozzle is due to the air diffusion, hence Pe nning ionization, and the permittivity change when the bullet exits the nozzle, for example, from a tube with high permittivity to air with low permittivity. Besides, electric ?eld measurements show that the electric ?eld in the bullet head increases when the plasma bullet accelerates. This con?rms the electric ?eld driven nature of the plasma bullet propagation.2. To better understand the plasma bullet behavior in dielectric tubes,the propagation of plasma streams in a tube with a part of it covered by a conductor is investigated. It‘s found that the propagation of the plasma is suppressed by the surrounding conductor, and a secondary plasma streamer begins to propagate at the downstream end of the conductor.It‘s reasonable to believe the capacitor formed by the conductor outside the tube and the wall of the tube plays an important role in inhibiting the original plasma stream and generating the secondary stream. This capacitor composes a branch circuit of low impedance, and the majority of the current flows through this branch circuit to the downstream end of the conductor, rather than arriving at the front edge of the original plasma bullet. Thus the original plasma stream is inhibited. Furthermore, a strong electric field, which is necessary for a seconda ry plasma stream, is formed at the downstream end of the conductor. Moreover, the active species generated by the original plasma3. To better understand the propagation mechanism of plasma bullets in capillary tubes, a specially designed U-shaped tube is used to guide plasma plumes. The plasma plume in U-shaped tubes could pass through the bend, and propagating in the opposite direction. The length of plasma plume decreases with the reduction of the distance(d) between the two branched tubes(upper and bottom tubes), and it‘s always shorter than the plasma plume in straight tube under the same external conditions. When d is reduced to 1 mm, it‘s clear to see that the plasma plume in the bottom tube is discontinuous. Dynamic of the plasma plume for d=1 mm shows that the plasma bullet starts to accelerate as soon as the secondary discharge occurs, and propagating along the upper wall of the bottom tube. This behavior is totally different with the propagation of plasma plumes in straight tube4. The ?uid-dynamic characterization by means of Schlieren high-speed imaging of the effluent region of a single electrode plasma jet is presented. Time evolution of ?uctuations generated in a free ?ow regime as been investigated. Plasma ignition causes ?uid-dynamic instabilities moving in the direction of the jet ?ow and correlated with the high- voltage pulses. A key factor for the efficiency of most of these systems is the interaction between the noble- gas channel, where the ?bullets‘(streamers) propagate, and the plasma itself. It is the object of this section to demonstrate this interaction and to provide the explanation on the gas ?ow ?eld medication induced by the plasma ignition. In accordance with the present results, the mechanism leading to the ?ow ?eld alteration is clearly related to the electro hydrodynamic force, and the flow field alteration influence the propagate of the plasma plume.