Translated Abstract
In the end of 1950s, the pseudospark discharge was discovered by J. Christiansen, a Germany scientist. This discharge looks like glow discharge, but the discharge current is large and the jitter is small which are similar with that of spark discharge. However, the discharge mechanism is different from than of glow discharge and spark discharge. The pseudospark discharge can be widely used in fields of E-beams, X-ray source and so on. Therefore, investigation on the breakdown characteristics and the related mechanism is theoretically and practically significant to the development of the pseudospark discharge under nanosecond pulsed voltages.
The device of pseudospark under nanosecond pulsed voltages is designed. Based on nanosecond pulsed voltage source, high speed framing camera and spectrograph, a photoelectric diagnosis platform of pseudospark discharge under nanosecond pulsed voltages is built, which can be used to study breakdown characteristics, shoot dicharge process photoes and diagnosis plasma spectrum.
Firstly, the space-time distribution characteristics of the pseudospark discharge under different gas pressure, gap distance and gap number are studied. The experimental results show that a strong light area which is called space charge region form near the anode, then the region moves toward the cathode, and into the cathode aperture in the end. The space charge region appeared ahead of time at higher gas pressure and longer gap distance. According to the photoelectric diagnosis and analysis of the space-time distributions of the pseudospark, the discharge process may be divided into three parts, the first is production of initial electrons, the second is collision ionization in the main gap, the last one is electron emission of the cathode. We built the model of the collision ionization in the main gap based on PIC-MCC, and the explosive electron emission model to describe the discharge progress. Based on the simulation results, the collision ionization phase including the formation and the motion of the space discharge region mainly affects the breakdown characteristics.
The breakdown voltages under different conditions have been studied. The influence rules of multi-factor of the device on the breakdown characteristics have been mastered. The breakdown voltage decreases with the increase of the front time and repetition frequency. The breakdown voltage of the pseudospark device decreases with the increase of the gap pressure in N2, air and Ar. However, the attenuation coefficient in Ar is larger than that in N2 and air. And when the pressure is low (16.5Pa), the influence of the pressure on the breakdown voltage becomes small. The breakdown voltage increases with the decrease of the gap pressure as power function form, but when the gas pressure is below a certain value, the breakdown voltage doesn’t depend on the pressure. The value is related with the gap distance, and the product of the pressure and gap distance is about 65Pa?mm. The beakdown voltage also increase with the decrease of the gap distance as power function form in a certain range. When the gap distance decrease to a certain value, the breakdown voltage decreases suddenly. And the breakdown voltage shows independence on the pressure, but affected by the condition of the electrode surface.
The breakdown voltage is also affected by the depth of the hollow cathode and the aperture diameter. The results show that the depth of the hollow cathode has no influence on the breakdown characteristics. The influences of the aperture diameter and depth on the breakdown characterisitcs are different under different gap distances. When the ratio of the gap distance and the cathode aperture diameter is over 4, the breakdown voltage suddenly increases, the aperture on the cathode plays no role in the discharge. When the ratio is in the range of 0.67~4, the breakdown voltage increases with the aperture decreases in the form of power function. When the ratio is below 0.67, the breakdown voltage almostly changes with the pressure.
Based on the model of the collision ionization in the main gap, the delay of the space charge region and the speed of the space charge region moving into the cathode aperture have been studied under different pressures, gap distances and aperture diameters. When the gas pressure is higher, the gap distance is longer and the aperture diameter is larger, it is easier for the space charge region formation. And the speed of the space charge region moving from anode to cathode is faster when the pressure and gap distance are higher and longer. In the end, we can conclude that the gas pressure and gap distance influence the characteristics of the space charge region by affecting the number of the collision ionization. The pressure has greater influence on the space charge region than gap distances and aperture diameters.
The breakdown characteristics of multi-gap pseudospark are similar with those of single-gap pseudospark diacharge, and the breakdown voltage decreases with the increase of the pressure and gap distance as power function form. And the attenuation coefficient decreases with the increase of the gap number. The average voltage in each gap of the multi-gap pseudospark device decreases with the increase of the gap number. The decrease speed in the condition of lower pressure and shorter gap distance is faster than that of higher pressure and longer gap distance.In the condition of multi-gap pseudospark discharge, the collision ionization firstly appeared in the first gap near cathode, then in the next gaps. Electrons produced in the front gap move into the next gaps and cause the collision ionization in the next gaps. The collision ionization in each gap is similar with the single-gap pseudospark discharge.
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