Investigations Of Pulse Discharge Propagation Under Varying Gas Pressure

Non-equilibrium plasmas generated by pulsed DC discharge have an electron temperature that is much higher than gas temperature at low, moderate and atmospheric pressure. Owing to some unique technical advantages of the pulsed plasmas, they have come to play an increasing role in many novel applications, such as biological and chemical decontaminations of media, surface modification/functionalization of polymers, deposition of thin films, the synthesis of carbon nanotubes, production of graphene-based materials, absorption and reflection of electromagnetic waves, etc. Although the propagation Characteristics of the pulsed discharge has been studied for decades, some peculiar phenomena depends on many experimental conditions and the nature of the dependences on these conditions is not yet known sufficiently well for reliable extrapolations. As working medium of gas discharge, gas has a direct effect on the characteristics of discharge. In this dissertation, effects of gas pressure and gas composition on discharge are investigated, and the details are presented as follows:1. A DBD system which can exactly control the gas pressure and composition was constructed.2. The transition of propagation mode depended on gas pressure was studied with an high speed ICCD camera,(ⅰ) Pressure between10Pa～200Pa, fast ionization wave (FIW) discharge is generated in the tube by pulse voltage (amplitude7kV, frequency4kHz, and pulse width1μs). The wave velocity is as high as107m/s magnitude. A diffuse and volume-filling column is generated, while the wave front propagates along the walls of the channel. Pressure about1000Pa, pulsed discharge propagates in a plasma segment mode, with a velocity of106m/s. When the pressure is higher than104Pa, a bullet-like plasma with a speed of105m/s, propagates towards the wall.(ⅱ) The electric field in the front of a wave is so high that the front becomes a moving source of a flux of high-energy electrons. The high-energy electrons generates pre-ionization of the gas ahead of the FIW front, which provides sufficient seed electrons. Meanwhile, the preionization strengthen the electric field in the front. The high electric fields propagate wih the FIW front and cause strong ionization which leads to the formation of FIWs and their travelling with a high speed. In the ionization process, high energy electrons impact direct ionization is the dominant process. Moreover, production of Ar2+and Ar2+also play an important role for the ionization.(ⅲ) The channel behind the wave front is a volume-filling plasma column until the end of the pulse width. The reason for this phenomenon are:①Hornbeck-Molnar (HM) process plays an important role.②In the relatively low channel electric field(-5kV/cm), electrons could obtain high energy because of their long mean free Path at low pressure.③The role of metastable state atom can not be ignored.(ⅳ) Pressure about1000Pa, the plasma segment propagation is mainly due to the decrease of the electron energy. According to the change of the electric field in the plasma segment, the conclusion that reduced intensity of the electric field is greater than at least1000Td for generation of a plasma segment could be deduced.3. Diagnostics investigations on the basic Parameters, namely the electron excitation temperature of the plasma under varying pressure,(ⅰ) The distribution of the population density of argon atomic states and ionic states is given through absolute and high-resolution optical emission spectroscopy measurements,(ⅱ) The electron excitation temperature for the argon ionic state distribution represents the energy of the electrons in the tail Part of EEDF.4. The propagation of pulsed discharge in nitrogen and air under varying pressure is studied.(ⅰ) Pressure between10Pa～200Pa, fast ionization wave (FIW) discharge is generated in the tube by pulse voltage in different pure gas. The wave speed is about the order of107m/s. While the pressure is lower than1000Pa, the electron drift velocity could be estimated from the formula.(ⅱ) When the pressure is higher than1000Pa, pulsed discharge in nitrogen is weakened rapidly.(ⅲ) Pressure between1000-1500Pa, pulsed discharge in air with exotically segmented channel phenomenon and bullet propagation are observed. There are three stages in the dynamics of the discharge: plasma segment propagation, the plasma bullet forms and tends to detach from the plasma segment, plasma segment un-propagation such as a suspension electrode while the bullet propagates further. The reason for this novel propagation behavior is mainly because of the electronegative of oxygen. The movement behavior of electrons is changed due to they are attached by oxygen. Anions of oxygen attach to the wall. A radial electric field is formed between the anions and positive charges remaining in the central zone of the tube. The radial electric field sustains the plasma segment as a suspension electrode.5. Pulse width has a great impact on discharge at high pressure. When gas pressure between5×103Pa～104Pa, duty-ratio between20%-90%, a plasma plume is generated along the axis of the tube, with a maximum length of30centimeters. As the same as dynamics of atmospheric pressure plsams jet, the plasma bullet propagation is observed in the plume in pure argon at sub-atmospheric pressure. Moreover, when the duty-ratio between60%-90%, a phenomenon of bright and dark segments in the plume is observed. The drift of positive ions in a long time pulse width, and reactions between excited particles and low energy electrons may be responsible for the discharge phenomenon.