Study of cathode-spot crater and droplet formation in a vacuum arc

Abstract

A 3D transient model of a cathode-spot crater and droplet formation in a vacuum arc is developed. The model includes mass, momentum, heat transfer (energy), current continuity and potential equations. Using the energy flux density, current density and pressure as external parameters, the shape of the cathode-spot crater and the temperature, velocity, potential and current density distributions at each time step are determined via numerical simulation. Under symmetrical conditions, the 3D simulation results are highly consistent with those from the previous 2D model. The new cathode spot tends to appear in the direction in which the protrusion is largest because the liquid-metal velocity is lower and the liquid-metal ridge radius is larger in this direction. The effect of the external transverse magnetic field is considered by using a symmetric space function to represent the pressure and energy flux density of the plasma cloud. Simulation results show that even small changes in the plasma cloud distribution have a significant impact on the cathode-spot crater process and droplet formation. Since the pressure is asymmetric, the crater becomes asymmetric and the new cathode spot tends to appear opposite to the direction of the Ampere force. Based on this phenomenon, a possible explanation for the retrograde motion phenomenon of cathode spots is proposed.

Investigation into the mechanism of surface atom emission from an individual cathode spot using molecular dynamics simulation

“…The crater profile is relatively flat, with the temperature overall high in most of the cathode spot area. This also agrees well with the temperature distribution calculated in [15]. As a function of the surface temperature, the electron current density is as expected high to maintain the current continuity of the cathode spot.…”

“…In [12] it appeared along the crater expansion, while in [13] it only occurred after the crater expansion stopped. Moreover, limited by the method, the previous HD models of cathode spot were incapable of observing the other route of surface atom emission, which was commonly assumed by the theoretical evaporation expressed by the Langmuir equation [15]. Therefore, the mechanism of surface atom emission is still unclear and requires further investigation.…”

Mie scattering based on-line measurement of droplet from vacuum arc

“…E. Daalder 测量了液滴数量沿阴极表面角度分布情况，发现液滴主要分布 在 10°和 30°之间 [13] 。 D. I. Proskurovsky 等人测量了轴向磁场对液滴直径的影响， 发现在轴向磁场作用下液滴数量及其最大直径均减少 [14] 。收集法作为一种离线 取样方法，只能采集收集板处液滴，无法获得整个空间的液滴信息。此外，收集 法需要多次放电来累积液滴数量，从而获得具有统计意义的结果，没法获得单次 放电的液滴信息以及液滴的运动演化过程。P. Siemroth 等人于 2018 年采用连续 光束照射 Cu 电极喷射液滴，通过阴影成像获得了液滴直径分布，结果表明液滴 数量随直径增大呈指数衰减 [15] 。该方法通过探测器上电信号宽度反推液滴直径， 受杂散发射光影响，具有较大误差。而且液滴不能重叠，每次需要测量单个液滴 信号，过程繁琐复杂。目前液滴实验研究并不多，反而是模拟研究方兴未艾，正 在蓬勃发展 [16][17][18] 。有鉴于此，本文提出了通过 Mie 散射信号来反演获得真空弧 液滴信息的在线测量方法，并对此方法的可行性进行了探索研究。 Mie 散射是光线通过颗粒物时发生的一种散射现象，对颗粒物的散射信号进 行处理，并采用算法反演即可得到颗粒的粒径分布信息，这一方法已在颗粒测试 领域获得了广泛应用，如粉尘、煤炭、炸药、食品等 [19][20][21][22][23]…”

Experimental investigation on the velocity of cathode spots in a vacuum arc with high di/dt

“…The values of i(t) and r(t) increase. According to equations ( 10) and (11), the value of TMF B T is proportional to i r and the value of AMF B z is proportional to i since r ′ is a constant. At this stage, the value of AMF increases gradually as well as the values of TMF and metal vapor density decrease.…”

“…Substantial experimental and theoretical researches have been carried out to investigate the physical parameters of cathode spot. Some 2D and 3D simulation models were developed to describe the formation of a crater, plasma jets and the development of cathode spot [6][7][8][9][10][11]. Lots of experiments were conducted to study the characteristic parameters as type of spot [12], the spot lifetime [13], the spot size [14,15], the spot splitting phenomenon [16,17], and the velocity of retrograde motion in low current [18][19][20].…”