An investigation is conducted regarding the influence of surface roughness on the flashover strength of an insulator in vacuum. A series of experiments is first presented, showing the relationship between surface roughness and flashover voltage, combined with measurement of surface potential distribution. It is found that surface charging on a roughened dielectric is suppressed remarkably; also the flashover voltage threshold depicts a rise-and-fall trend with roughness increasing, exhibiting a voltage summit at a certain roughness value. In addition, optical observation is implemented to draw a parallel with an electron trajectory in vacuum. A proposal can therefore be made that multipactor expansion tends to be mitigated as electron bombardment on a dielectric occurs with a lower secondary electron yield. Then a theoretical model considering the microscopic physical process is established to explain the above phenomena, consisting of an internal charge migration layer, an interfacial electron absorption layer and an external electron obstruction layer. The validity of this theoretical model can be confirmed by obtaining the surface trap distribution, microscopic morphology and net secondary electron yield.
We propose a novel theoretical formula to estimate the surface flashover threshold in vacuum using the intrinsic properties of dielectric material as well as dielectric-vacuum configuration. The method is based on an above-surface model called secondary electron emission avalanche (SEEA), where flashover development can be divided into multipactor discharge, outgassing and an ionization avalanche which eventually leads to a plasma discharge. The transition from SEE-dominated regime to discharge plasma is supposed to be of significance where gas breakdown takes place within a nonuniform desorbed gas cloud in the vicinity of dielectric surface. Flashover occurs when specific space charge multiplication is achieved, yielding the critical pressure and breakdown voltage. The depiction of previous stages involves the secondary electron emission of dielectric materials, desorption and diffusion of adsorbed gas, ambient temperature, electrode structure, etc. Both particle-in-cell simulation and experiment data can corroborate the obtained theoretical results.
A 2D simulation based on particle-in-cell and Monte Carlo collision algorithm is implemented to investigate the accumulation and dissipation of surface charges on an insulator during flashover with outgassing in vacuum. A layer of positive charges is formed on the insulator after the secondary electrons emission (SEE) reaches saturation. With the build-up of local pressure resulting from gas desorption, the incident energy of electrons is affected by electron-neutral collisions and field distortion, remarkably decreasing the charge density on the insulator. Gas desorption ionization initiates near the anode, culminating, and then abates, followed by a steady and gradual augmentation as the negatively charged surface spreads towards the cathode and halts the SEE nearby. The initiation of flashover development is discussed in detail, and a subdivision of flashover development is proposed, including an anode-initiated desorption ionization avalanche, establishment of a plasma sheath, and plasma expansion. The transform from saturation to explosion of space charges and dissipation of the surface charge are revealed, which can be explained by the competition between multipactor electrons and ionized electrons.
In this paper, we address two main topics: steady propagation fields for positive streamers in air and streamer deceleration in fields below the steady propagation field. We generate constant-velocity positive streamers in air with an axisymmetric fluid model, by initially adjusting the applied voltage based on the streamer velocity. After an initial transient, we observe steady propagation for velocities of 3×104 m/s to 1.2×105 m/s, during which streamer properties and the background field do not change. This propagation mode is not fully stable, in the sense that a small change in streamer properties or background field eventually leads to acceleration or deceleration. An important finding is that faster streamers are able to propagate in significantly lower background fields than slower ones, indicating that there is no unique stability field. We relate the streamer radius, velocity, maximal electric field and background electric field to a characteristic time scale for the loss of conductivity. This relation is qualitatively confirmed by studying streamers in N2-O2 mixtures with less oxygen than air. In such mixtures, steady streamers require lower background fields, due to a reduction in the attachment and recombination rates. We also study the deceleration of streamers, which is important to predict how far they can propagate in a low field. Stagnating streamers are simulated by applying a constant applied voltage. We show how the properties of these streamers relate to the steady cases, and present a phenomenological model with fitted coefficients that describes the evolution of the velocity and radius. Finally, we compare the lengths of the stagnated streamers with predictions based on the conventional stability field.
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