Investigation On The Characteristics Of Pulsed DC Breakdown Of Metal Material In Vacuum

Research of breakdown of metal radio frequency(RF)structure in vacuum is one of the foundational works of the study of high power microwave(HPM)device.Carrying out the research on this aspect has great signification for avoiding the shortening of the pulse of HPM devices and improving the performance index of device.Considering the similarity between DC breakdown in vacuum and vacuum RF breakdown on microscopic physical mechanism or experimental law of metal material,study of characteristics of pulsed DC breakdown in vacuum on common materials of HPM generator is proposed in this paper by particle simulation and experimental research,which is based on the analyze of mechanisms and laws of metal vacuum breakdown.The main contents are as follows:First,the survey and investigate on the mechanism of metal vacuum breakdown,the main conclusions are as follows: The classical whisker explosion model only shows the simple process,lacking detail description as well as the analysis of the plasma's sustainable development.The main idea of ecton model is that the explosion emission is a periodical process,the change of the surface after breakdown provide the condition of further breakdown.When the vacuum ranges from 10-2Pa to 10-4Pa,the gas desorption from the surface is the main reason of RF breakdown.The main idea of the model is that the gas desorption is necessary for the avalanche development of the emission process.If the local microscopic electric field raises up to 10GV/m,the strong field will lift the cusp,whose main point is that in the initial process of emission,the stretch of the strong electric field will result in the crush of the surface,the field emission current bombard the fragment,which results in the produce and development of plasma.Second,the simulation of development process of breakdown plasma was carried out,and the spatial particle distribution as time goes was got.Besides,the effect of applied magnetic field and vacuum on the developing process of plasma was analyzed.The results of simulation are as bellow: The development of electron and ion are avalanche between cathode and anode.Besides,the decrease of vacuum will result in the raise of the development of plasma from cathode to anode,which will result in the easier form of electric channel between the electrodes.In addition,the magnetic field will constrain the movement perpendicular to the magnetic field,so the effect of magnetic field is decided by the direction of magnetic.Third,the experimental platform to carry out the research was designed and set up.The equivalent impedance of the platform is about 10?.The stable output voltage can reach up to 400 kV by changing the air pressure of the master switch,bottom width of the pulse is about 200 ns.The impedance matching can be realized by changing the water resistance tolerance,and the impedance change can be got directly by the voltage waveform.The distance between electrode can be changed by making use of dynamic sealing device,the vacuum can be changed from 100 Pa to 10-3Pa.triple needle electrode structure is designed,so the breakdown threshold of different materials can be compared by the diagnoses of high speed flash camera(HFSC).Fourth,the breakdown characteristics of the common materials of the HPM devices are got by experimental research.The results are as bellow: When the pulse width of the voltage is 200 ns and the machining accuracy is 0.01 mm,the breakdown threshold of stainless steel can reach up to 46~56MV/m,which is higher than the value of copper whose value is about 38~43MV/m and the value of aluminum whose value is about 36~39MV/m.The distance between electrode when there comes the breakdown is measured in different vacuum,and the result shows that the breakdown electric field in the vacuum of 10-1Pa is the largest,and the overall trend live up to the PD curve.Comparing the result before electrochemical treatment and after electrochemical,it suggests that the electrochemical treatment can improve the DC breakdown threshold.Comparing the surface states before and after breakdown of different material,it suggests that the change breakdown do to copper and stainless steel whose melting point is higher is less,which is restricted in hazardous area,while there are local region ups and downs on the surface of aluminum after breakdown.