The heating sources of spots on the vacuum arc cathodes

With the development of DC transmission and aerospace technology, the demands on the breaking capacity of vacuum circuit breakers under high di/dt condition are getting higher and higher. The breaking capacity of vacuum circuit breakers is determined by cathode spots, which provide electrons and metal vapor to maintain the arc. In this paper, the experiments of cathode spot observation were carried out based on the detachable vacuum interrupter and the cup-shaped axial magnetic field contacts. The images of cathode spot distribution were obtained by an ultra high-speed camera. In the initial expansion stage of arcing, it was discovered that the distribution of cathode spots is ring-shaped and symmetrical. The initial expansion velocity of cathode spot ring is approximately proportional to the value of (di/dt)1/2 and is independent of frequency. During the whole arcing process, the expansion velocity of cathode spot ring rises first and then drops with arcing time. The effects of the magnetic field and metal vapor density on the expansion process of cathode spot were analyzed. Based on the experimental conditions and results, the values of magnetic field and metal vapor density were calculated which can reasonably explain the variation of expansion velocity of cathode spot.

Section: Study On the Initial Expansion Velocity Of Cathode Spot Ringmentioning

confidence: 99%

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

Liu¹,

Yuan²,

Chen³

et al. 2022

“…Under different values of the electron current fraction s, the results of the electric field E as a function of current density J are shown in figure 7. The range of current density J used for the calculation is selected as 10 9 -10 11 A m −2 , since a wide range of values from 10 9 to 10 12 A m −2 have been reported for this quantity [25]. It can be seen from figure 7 that, for a given value of s, the electric field E increases with the increase of current density inside the cathode spot.…”

Section: What Causes the Generation Of Group Spot And Individual Cath...mentioning

confidence: 99%

Dynamic characteristics of cathode spots during expansion process in a vacuum arc

Liu¹,

Yuan²,

Chen³

et al. 2022

Cathode spot is the source of plasma and metal vapor which affects the breaking capacity of vacuum circuit breakers. In this paper, the cup-shaped axial magnetic field contacts were used and the images of cathode spots were obtained by an ultra high-speed camera in the experiment. The dynamic characteristics of cathode spots during expansion process in vacuum arc were investigated. Under the conditions of different current amplitudes, different distributions of cathode spots were observed. During the expansion process of cathode spot, there are group spots and individual spots. The group spot only occurs in the cold electrode. The electrons emission characteristics of cathode spot as a function of temperature and electric field was calculated. The formation of group spot is related to the current and electron emission of cathode spot. The cathode spots were identified by Matlab. It was found that the variation of number of cathode spots is consistent with the current. However, at different stages, the reasons for the increase in the number of cathode spots are different, such as the increase in the area of cathode spots in the initial arcing stage and the splitting of cathode spot. It was obversed that the cathode spots are not completely evenly distributed on the cathode surface. The farther away from the center of the contact, the longer the distance between the cathode spots. It is mainly effected by the transverse magnetic field which is generated by the arc column. The closer to the contact edge, the higher the transverse magnetic field at the cathode spot and the higher the expansion velocity of cathode spot.

“…coating et al [1][2][3][4][5]. As device size decreases, there has been greater interest in understanding the kinetic characteristics of microarc discharges.…”

Section: Introductionmentioning

confidence: 99%

Energy and mass transport properties in the near-cathode region of atmospheric thermal plasma

Sun,

Li,

2023

As the main region where charged particles are accelerated by the electric field and accumulated near the cathode, the near-cathode region has significant non-local equilibrium characteristics and plays a crucial role in exploring the energy and mass transport properties of the thermal plasma from the arc column region to the hot cathode surface. However, there is a lack of universally adopted models and accepted theories for the study of the near-cathode region due to the complexity of the physical mechanisms involved in the sheath. According to the physical characteristics of the arc discharge, an external circuit is usually used to regulate the total current applied to the electrodes to maintain stable discharge. Therefore, the energy and mass transport properties of atmospheric thermal plasma driven by a current source coupled to an external circuit are investigated in this work to explore the transport mechanism of charged particles in the near-cathode region based on an implicit particle-in-cell Monte Carlo collision method. Firstly, the current-driven model in this work is compared with the present voltage-driven model and fluid model to verify the correctness of this model. Then, the collisions between charged particles and gas atoms inside the sheath are analyzed, and the spatial distributions of particle current density and particle heating rate inside the sheath are also studied. Finally, the variations of typical parameters of thermal particles under different current densities (106–107 A m−2) are analyzed, including maximum particle number density, maximum particle spatially-averaged temperature, sheath thickness, charge density and electric field strength in the cathode.