AC Breakdown Performance Of High Speed Argon Flow

Gas discharges are the main technique for plasma formation in technological and engineering fields. The discharge characteristics in gas flow are the fundamental issues for the application of plasma. The gas flow can interact with the discharge observably. On the one hand, the gas flow can affect the discharge behavior by constraining and guiding the breakdown process; On the other hand, the spatial distribution and the mode of the flow field can be changed by the discharge through the electromagnetic interactions between the flow and the discharge current. And in turn the gas discharge can also change the dynamic behavior of the flow field. Therefore, the combination of gas flow and discharge is widely used in many fields, such as plasma jet formation, aircraft ignition and so on. However, by now, the study on the interaction of gas flow and the gas breakdown behavior is mostly limited by low speed, while few researches on discharges in subsonic and supersonic gas flows are published systematically and deeply. This paper studied the breakdown performance of high speed Argon flow and explored the dependence of the breakdown voltage of gas flow in subsonic to supersonic speed on gas flow speed and then on Reynolds number.By using argon as working gas, a kind of high speed gas flow is generated by a novel design using quartz capillary tubes and molecular beam nozzles. The resultant gas flows cover subsonic to supersonic velocity via varying the gas stagnation pressure and adjusting the hole of capillary tube and nozzle. As the gas flow is formed in capillary tube and nozzle through pneumatic effect, the gas acceleration process can be regarded as an adiabatic expansion. Based on the postulation, the end speed, temperature and density of the gas flows and local sonic speed are calculated from the initial and the downstream pressure and the dimension of the tube and nozzle. To verify the calculation, two types of Pitot tube are designed to measure the flow speed of the gas flows.To be suitable to the dimensions of the capillary and nozzle, a needle-plane asymmetrical electrode configuration was proposed to form the breakdown channel in the gas flows. A AC power supply with frequency of 40KHz is used to study the breakdown of Ar flows with speed ranging from 35m/s to 400m/s and pressure from 0.2 atm to 0.4 atm. The breakdown is indicated using a high voltage probe through the sudden shrink in applied voltage waveform, which resulted from the breakdown. The voltage amplitude just ahead the breakdown shrink is taken as the gas flow AC breakdown voltage. Measurement for Ar gas flow shows that the breakdown voltage rises with higher flow speed and higher pressure. The breakdown voltage dependency on flow speed appears into three different sections with different slopes.Based on the flow speed, temperature and density calculated for the gas flows, the Reynolds numbers are obtained in order to relate the breakdown voltage dependency to the gas flow mode. The results show that the breakdown voltage of gas flow increases with Reynolds numbers too and the tendency curves appear with more obvious sections. The second section corresponds to the Reynolds number 16000-22000 and is the transition phase between the modes of the first section and the third section. For the gas flows formed at different pressures, the transition Reynolds number is close to each other, which hints that the mode transition in gas flow is critical to the gas breakdown and the breakdown voltage. By compared the breakdown results of the gas flows from capillary tube and the Laval nozzle, it is found that Laval nozzle can make the flow keep laminar within much larger speed scope and effectively avoid the gas flow transition from laminar to torrent mode.