Positive double-pulse streamers: how pulse-to-pulse delay influences initiation and propagation of subsequent discharges

Residual charges and species created by previous streamers have a great impact on the characteristics of the next discharge. This is especially pronounced in repetitively pulsed discharges, where the physical and chemical reactions during the decay phase play a very important role. We have performed double-pulse streamer experiments in artificial air and pure nitrogen with a varying pulse delay (Δt) from 0.45 μs to 20 ms. We have observed morphological transformations of the 2nd-pulse streamer as a function of Δt and classified six typical stages by streamer length. The propagation distance of the 2nd-pulse streamer can be 66% l

A computational study of positive streamers interacting with dielectrics

We use numerical simulations to study the dynamics of surface discharges, which are common in high-voltage engineering. We simulate positive streamer discharges that propagate towards a dielectric surface, attach to it, and then propagate over the surface. The simulations are performed in air with a two-dimensional plasma fluid model, in which a flat dielectric is placed between two plate electrodes. Electrostatic attraction is the main mechanism that causes streamers to grow towards the dielectric. Due to the net charge in the streamer head, the dielectric gets polarized, and the electric field between the streamer and the dielectric is increased. Compared to streamers in bulk gas, surface streamers have a smaller radius, a higher electric field, a higher electron density, and higher propagation velocity. A higher applied voltage leads to faster inception and faster propagation of the surface discharge. A higher dielectric permittivity leads to more rapid attachment of the streamer to the surface and a thinner surface streamer. Secondary emission coefficients are shown to play a modest role, which is due to relatively strong photoionization in air. In the simulations, a high electric field is present between the positive streamers and the dielectric surface. We show that the magnitude and decay of this field are affected by the positive ion mobility

Enhancing ion extraction with an inverse sheath in negative hydrogen ion sources for NBI heating

Negative hydrogen ion (H−) sources employed in neutral beam injection (NBI) systems are subject to extraction efficiency issues due to the considerable volumetric losses of negative hydrogen ions. Here, we propose to improve the H− extraction by activating an alternative sheath mode, the electronegative inverse sheath, in front of the H− production surface, which features zero sheath acceleration for H− with a negative sheath potential opposite to the classic sheath. With the inverse sheath activated, the produced H− exhibits smaller gyration, a shorter transport path, less destructive collisions, and therefore higher extraction probability than the commonly believed space-charge-limited (SCL) sheath. Formation of the proposed electronegative inverse sheath and the SCL sheath near the H–-emitting surface is investigated by the continuum kinetic simulation. Dedicated theoretical analyses are also performed to characterize the electronegative inverse sheath properties, which qualitatively agree with the simulation results. We further propose that the transition between the two sheath modes can be realized by tuning the cold ion generation near the emissive boundary. The electronegative inverse sheath is always coupled with a plasma consisting of only hydrogen ions with approximately zero electron concentration, which is reminiscent of the ion–ion plasma reported in previous NBI experiments

An alternative simulation approach for surface flashover in a vacuum using a 1D2V continuum and kinetic model

Surface flashover across an insulator in a vacuum is a destructive plasma discharge which undermines the behaviors of a range of applications in electrical engineering, particle physics and space engineering, etc. This phenomenon is widely modeled by the particle-in-cell (PIC) simulation, here the continuum and kinetic simulation method is first proposed and implemented as an alternative solution for flashover modeling, aiming for the prevention of unfavorable particle noises in PIC models. A one dimension in space, two dimensions in velocity kinetic simulation model is constructed. Modeling setup, physical assumptions, and simulation algorithm are presented in detail, and a comparison with the well-known secondary electron (SE) emission avalanche analytical expression and existing PIC simulation are made. The obtained kinetic simulation results are consistent with the analytical prediction, and feature noise-free data of surface charge density as well as fluxes of primary and SEs. Discrepancies between the two simulation models and analytical predictions are explained. The code is convenient for updating and to include additional physical processes. The possible implementations of outgassing and plasma species for the final breakdown stage are discussed. The proposed continuum and kinetic approach are expected to inspire future modeling studies for the flashover mechanism and mitigation

The influence of O2 on positive streamer initiation in supercritical CO2 with field ionization using a 3-D particle model

In this work, the influence of varying mixing ratios of O2 on positive streamer initiation in supercritical (SC) CO2 at different applied voltages was investigated using a 3-D particle model. Zener's model was included in the 3-D particle model to calculate the field ionization rate during streamer formation and propagation. The evolution of positive streamer was classified into three main stages namely inception cloud, primary streamer channels, and successive streamer branching. The effect of O2 with different mixing ratios (2%, 10%, and 20%) on the morphology, average electron density, electric field distribution, streamer length, and electron energy distribution function (EEDF) for positive streamer in SC-CO2 at four different voltages (12, 12.5, 13, and 13.5 kV) is then investigated. The 2-D cross sections of the initial stage of the streamer and the electric field distribution are presented in detail. At the same applied voltage, the size of inception cloud, average electron density, electric field, and streamer length with the maximum electric field decreased with increasing oxygen concentration in SC-CO2. With increasing applied voltage, the apparent size of inception cloud, primary streamer channels, successive streamer branching, average electron density, electric field, and streamer length increased. The simulation results showed that adding O2 to SC-CO2 can significantly affect the streamer initiation and propagation

Characterizing streamer branching in N2-O2 mixtures by 2D peak-finding

The stochastic nature of streamers and the manual identification of features in 2D discharge images together cause great ambiguities when analysing streamer branching characteristics. Here we present the development of streamer image diagnostics by a 2D peak-finding method to obtain accurately quantified extensive statistics on streamer branching. And we present quantitative results on the growth of the streamer head number as a function of time in N2-O2 mixtures at 100 and 200 mbar. Decreasing the oxygen concentration decreases the nonlocal photoionization, and hence allows for local instabilities and more branching. The oxygen concentration in N2-O2 mixtures affects streamer branching not only by smoothening the electron number density in front of streamer heads but also by the creation of an inception cloud. Streamers in pure nitrogen have no noticeable inception cloud, which gives the nitrogen streamers a longer effective propagation time during a voltage pulse of 550 ns; they branch more both as a function of space and of time. However, the statistical results show that the number of streamer heads in high purity N2 is less than in mixtures with 0.1% O2, and it depends on pressure