Abstract The characteristics and mechanism of microgap breakdown is of great significance to the insulation reliability evaluation of micro devices and plasma generation at this dimension. Based on the theory of micro-scale breakdown, field electron emission and secondary electron emission are two major cathode emission processes which may dominate the breakdown at microscale. Therefore, exploring the role of the cathode in the process of micron breakdown and further understanding the mechanism of micron-scale breakdown are very vital to the evaluation of electrical insulation performance and the application of microplasma. To address these issues, a two-dimensional physical model of microgap breakdown in air is established, and the numerical simulation on electric field distribution and charged particles in the process of microgap breakdown in air is carried out by using particle in cell/Monte Carlo collision (PIC/MCC) method, which are also verified by the experimental results. Firstly, based on the experimental setup, a two-dimensional physical model of microgap breakdown in air is established. Secondly, with the aid of Vsim software, numerical simulation of microgap breakdown is carried out by using PIC/MCC method. Simulation results of electric field distribution characteristics show that the field enhancement factor β decreases from 2.50 to 1.65 when the cathode radius R0 increases from 1μm to 10μm. It indicates that the local electric field distortion on the cathode surface decreases, and the field emission threshold voltage increases with the increase of the cathode radius. A larger voltage is demanded for the field electron emission, so the breakdown voltage gradually increases. Simulation results on current variation characteristics during breakdown show that field emission current accounts for more than 95 % of anode current, which means field emission dominates the breakdown process. Moreover, the amplitude of the field emission current is proportional to the radius of the cathode. When R0 increases from 1 μm to 10 μm, the breakdown time t also increases from 22.4 ps to 30.6 ps, due to the fact that when the radius of the cathode is smaller, the electric field strength near the cathode is higher, so the initial velocity of electrons is larger, which speeds up the breakdown process. Simulation results on spatial distribution characteristics of charged particles show that the width of cathode discharge region increases with the increase of the cathode curvature radius. The same phenomenon is also observed in the experimental results of optical morphology of the breakdown, which shows a good agreement with the simulation results. The following conclusions can be drawn from the simulation and experiment analysis: (1) In the 5 μm gap, the electric field strength on the surface of the needle cathode reaches up to 2×108 V/m, thus the field emission current contributes greatly to the breakdown which becomes one of the main physical processes in breakdown. (2) As the radius of curvature of the tip decreases, the electric field distortion as well as the field emission will be further enhanced, so the gap is more easily to be broken down. Therefore, the breakdown voltage and the breakdown time gradually decreases with the decrease of the cathode radius. (3) The effective area of the field emission increases with the increase of the cathode radius, which will directly lead to the increase of the field emission current. The increase of the breakdown channel width will cause more severe impact ionization in the air gap, which will eventually lead to the increase of the breakdown current.
Chang Zezhou,Meng Guodong,Ying Qi等. Study on the Influence of Cathode Radius on the Breakdown Characteristics across Microgaps in Air[J]. Transactions of China Electrotechnical Society, 2023, 38(4): 1032-1041.
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