Estimation of surface flashover threshold in a vacuum II: flashover phase transition

Abstract

Surface discharge in a vacuum occurs under an intense external field, also called a flashover. The flashover threshold over which the surface flashover occurs is of vital importance in the vacuum-solid interface. It is widely assumed that the flashover is initiated by a single surface multipactor and develops as a plasma discharge along a dielectric surface, namely the secondary electron emission avalanche (SEEA). Yet the SEEA model is not versatile for all practical vacuum-solid interfaces and there remain experiment results contradicting the theory predictions without a resolute answer. Here we further expand on previous theories and consider the conventional SEEA model under arbitrary field distributions. The example of a conical insulator is then given to test the validity of the SEEA model under an arbitrary field. It is found that the obtained flashover threshold is only consistent with the experiment at low cone angles where the field near the cathode triple junction is strong, while remarkable discrepancies appear when the anode field is significantly enhanced. The discrepancies can be understood if a different discharge model, i.e. an anode-initiated flashover (AIF) is included. Both particle-in-cell simulation and optical diagnostics are employed to visualize this unique discharge, which are then formalized on theoretical grounds to elucidate the early stage of the AIF. The transition between two flashover phases is then clarified. The plasma-surface interaction and sheath dynamics during the discharge are analyzed theoretically, showing distinct sheath evolutions in two flashover models. Additionally, a new factor reflecting field distortion is introduced to estimate the discharge threshold under an SEEA framework.

Effect of Bismuth Ferrite Nanometer Filler Element Doping on the Surface Insulation Properties of Epoxy Resin Composites

“…Deep traps make it difficult for electrons to escape. Increasing the energy level and density of deep traps can reduce the number of primary electrons, make it difficult to form secondary electron emissions on the material surface, and thus increase the flashover voltage [ 52 ]. Compared with BFO/EP composites, the depth and density of deep trap energy level and flashover voltage of EP composites are increased after the BFO is doped with La, Cr, or co-doped with La and Cr elements.…”

Design and 3D printing of porous cavity insulation structure for ultra‐high electrical withstanding capability

“…Electrons entering the cavity could further induce secondary electron emission (SEE), if not backscattered, but these SEs will collide more with the side walls of cavity instead of coming back to surface as L increases. Collision with side wall will, however, unlikely to further induce SEE due to low energy of the SEs [8,9]. The parameter L is hence fixed as 3 mm in the following experiments.…”

“…Over the past decades, several models have been proposed to expatiate the vacuum flashover phenomenon, among which the secondary electron emission avalanche (SEEA) model is the most widely accepted [8][9][10][11]. The flashover process, according to the SEEA theory, is approximately divided into three phases: initiation (i.e., field electron emission from cathode), development (i.e., multipactor and surface charging), and breakdown (i.e., gas desorption and plasma discharge) [8,9].…”

“…Over the past decades, several models have been proposed to expatiate the vacuum flashover phenomenon, among which the secondary electron emission avalanche (SEEA) model is the most widely accepted [8][9][10][11]. The flashover process, according to the SEEA theory, is approximately divided into three phases: initiation (i.e., field electron emission from cathode), development (i.e., multipactor and surface charging), and breakdown (i.e., gas desorption and plasma discharge) [8,9]. For the cathode-initiated flashover, the seed electrons originate from the field emission from the junction of the cathode, insulator, and vacuum, that is, the cathode triple junction (CTJ), where the E-field strength is significantly higher.…”

Effect of morphology and particle size of bismuth ferrite filler on the surface insulation of epoxy resin composites

“…According to the second electron emission avalanche (SEEA) theory, [31] when the two ends of the electrode start to pressurize, the electric field at the three-point junction of the metal electrode, air, and polymer surface is severely distorted, which will form field-induced electron emission and generate primary electrons. Some electrons are absorbed by the polymer and deposited on the surface of the sample to form a space charge, while another part of the electrons obtain energy under the action of an external electric field and collide with the surface of the insulator.…”