Xue-Zeng Huang

Cathode-like luminescence from vacuum-dielectric interface induced by self-stabilizing secondary electron emission

A kind of interesting luminescence phenomenon from vacuum-dielectric interface under high electric field is presented in this Letter. It is visible, steady, and continuous, occurring before or accompanied with flashover across dielectric, which is quite different from general electroluminescence from insulator surface reported before. By investigating its optical and electrical characteristics, we consider it as cathode-like luminescence, which is initialized by the field electron emission from cathode triple junction and finally generated by the process of radiative recombination due to the self-stabilizing secondary electron emission and collision on insulator surface.

Theoretical and simulation research on self-stabilizing secondary electron emission process across solid dielectrics in vacuum

Recently, the cathode-like luminescence phenomena are observed from the surface of solid dielectrics under steady-state high voltage excitation in vacuum, which is closely related to the self-stabilizing secondary electron emission (SSEE) occurring on insulator surface. In this paper, the theoretical analysis about surface charging based on the SSEE is made, and a one-dimensional Mont Carlo simulation on surface charge accumulation under DC voltage excitation is provided. The results reveal that surface charging process can be divided into three stages including initial accumulation, fast multiplication, and final stable stage. In the final stage, the SSEE phenomena are achieved and surface charges are invariable and reach up to a stable distribution. The simulated stable surface charge distribution is consistent with the theoretical deduction results, which confirms the existence of SSEE process.

Influence of surface ion exchange on pulsed flashover characteristics of machinable ceramics in vacuum

For pulsed power devices, the surface flashover across their solid insulation becomes a serious problem. Investigators have paid much attention on improving the surface electric withstanding strength of insulator. It is found that surface treatment is very useful to increase the surface flashover voltage. The flashover phenomena along machinable ceramics (MCs) are investigated under pulsed voltage in vacuum. The ion exchange method is used to adjust the surface components of MCs for improving its surface electric strength. CuSO4 is used for ion exchanging with the MC. It is found that ion exchange treatment greatly changes the surface components and microstructure of MCs, which has obvious influence on flashover characteristics of MCs. The MCs with 10 h ion-exchange time show a much improved flashover holdoff. Based on the secondary electron emission avalanche (SEEA) theory, the improvement of flashover withstanding ability can be reasonably explained by electron impact and transport behaviors.

The 3D simulation of high-current vacuum arc under combined effect of actual magnetic field and external transverse magnetic field

Based on a steady 3D Magneto-Hydro-Dynamic (MHD) model, the high-current vacuum arc (HCVA) under combined effect of actual magnetic field (MF) and external transverse magnetic field (ETMF) is simulated. The actual MF is generated by cup-type AMF contact system. The ETMF may cause the deflection of arc column which is the main reason of the contact deflected erosion. According to some experimental results, the electron temperature in HCVA is assumed to be uniform and equal to 3eV. So the MHD model is simplified to improve the simulation efficiency. With the three conservation equations (mass, momentum and energy) of ion flow coupling solved, the spatial distributions of some flow parameters can be obtained. The influence of all three components of the magnetic field is inserted by solving the magnetic transport equations sequentially. Proper boundary conditions are set on the cathode and anode side which separated the cathode spots mixing region and anode sheath region from computation domain respectively. Under the influence of the ETMF, the deflection of the plasma flow can be found which may be helpful to understand the mechanism of the contact deflected erosion.

Cathode-like luminescence on vacuum-dielectric interface under DC voltage based on self-stabilizing secondary electron emission

A particular Cathode-like luminescence (CLL) phenomenon occurring on vacuum-dielectric interface under high DC electrical field before or accompanied with flashover is reported, which is widely existed in vacuum-dielectric system. It is considered that the CLL presented in this paper is initialized by field electron emission at Cathode Triple Junction (CTJ) and finally generated by a process of radiative recombination due to the self-stabilizing secondary electron emission (SSEE) and collision on surface of insulator. The spectrum of CLL is measured and analyzed, and it shows that there are not any characteristic spectrums of C, O, N, which means that the CLL is not produced by discharge of desorption gas. In addition, simulation results also approved that the SSEE process in fact exists. Finally, its relevance to flashover is also discussed. It shows that flashover seldom occurs on the channels of CLL, which seems that the CLL has an inhibition effect on flashover. However, the exact mechanism is still unclear. This work will be researched in near future.

Numerical simulation of anode thermal processes of different materials in vacuum arc

Electrode material seriously influences the characteristics of vacuum arc and further affects the performance of switches. In this paper, thermal processes of six kinds of metal anodes (including pure metal and alloy anodes) are simulated and researched. Two kinds of temperature calculation methods are used. Simulation results show that W and Mo anodes have the higher temperature than Cu, Cr, CuCr25 and CuCr50 anodes. Pure Cr anode has the biggest melting width, and highest saturated vapor pressure. Cu anode has the biggest melting depth. W anode has the smallest melting area. Axial temperature gradient is related to the thermal conductivity, the Cr anode has the largest axial temperature gradient. The thermal characteristics of CuCr25 and CuCr50 anodes are located between the pure Cu and Cr anodes. There are two melting points appear in the results of CuCr alloys, between the two melting points, the alloy anodes are in solid-liquid mixture state.

3D numerical simulation of vacuum arc with anode vapor in actual magnetic fields

Characteristics of vacuum arc seriously influence the interruption ability of vacuum switches. In many cases (especially in commercial electrode systems), vacuum arcs are not completely axial symmetrical configuration. So, three-dimensional simulation of vacuum arc is needed. On the other hand, when the interruption current is very high, the anode will be in active state and becomes another source of inter-electrode plasma. In this paper, a time dependent actual magnetic field generated by commercial cup-shaped axial magnetic field (AMF) electrode is simulated. With this magnetic field and the simulation results of anode temperature, the dynamic characteristics of vacuum arc considering anode vapor pressure (also considering simplified cathode spots diffusion and open process of electrode) are obtained. In the simulation results, the magnetic field distribution of commercial cup-shaped electrode is six leaves shape, and so do other parameters. There are some ions pressed by high anode vapor pressure from anode to arc column.

Effect of different coning angles on flashover characteristics across machinable ceramic in vacuum

Surface flashover in vacuum is a kind of complicated surface and interface phenomena, which seriously restricts the development of high voltage electro-vacuum devices. Introducing a kind of machinable ceramic with excellent machinable performance and good surface electrical capability into vacuum insulation system, and considering the commonly used truncated cone structure, machinable ceramic samples with different coning angles are machined. Their surface flashover voltage are investigated under pulsed voltage in vacuum, and the electric field distributions and surface charge density of different coning angles are numerically simulated. The experimental results indicate that, the flashover voltage of cone insulators is higher than that of cylindrical insulators, the flashover voltage of truncated cone samples with positive coning angle is much higher than that of with negative coning angles, and the flashover voltage with ~45° conical frustum is highest. It is considered that, the coning angle plays an important role on the electric field of cathode triple junction (CTJ) and surface charge distribution. When the coning angle is positive, the electric field of CTJ and surface charge distribution is smaller, and when the coning angle is negative, these values are bigger, so its higher CTJ electric field and surface charge result in a lower flashover voltage than that with positive angle, and also the short free path of electrons and decreased energy of electrons colliding with insulator surface lead to a higher flashover voltage than that of cylindrical insulators.

Measurement of surface charge distribution on insulating material under pulsed voltage

Since the beginning of the 20th century, surface flashover phenomena have been investigated from a viewpoint of scientific interest. The surface flashover can cause serious accident and limit the development of the electric power and high voltage equipment. The measurement of surface charge is important to understand the discharge of insulating materials and may help to find new ways to improve the flashover voltage. Based on the principle of the electrostatic probe, a system for the measurement of charge distribution on insulating material in vacuum is invented; the surface charge distribution on insulating material before and after the surface flashover is compared. The relationship between the flashover characteristics and the surface charge is discussed.

Effect of surface ion exchange of machinable ceramic on its flashover characteristics in vacuum

As well known, the surface conditions of solid materials strongly affect the flashover phenomena under high electric field in vacuum. On the basis of the novel low melting temperature machinable glass ceramics for vacuum insulation system, which has excellent machinable performance and good electrical properties, the ion-exchange method was used to change the samples surface condition, surface electric resistivity and surface trap density. Different alkali metal ions were used in this paper, and this may change the samples surface microscopic structure. We have measured the different samples surface flashover in vacuum, and investigated the relationship between the ion-exchange method and samples surface electricity characteristic. The results indicated that while using suitable ion-exchange method, the samples surface flashover performance can be improved.