High-fidelity modeling of breakdown in helium: initiation processes and secondary electron emission

Abstract

Understanding the role of physical processes contributing to breakdown is critical for many applications in which breakdown is undesirable, such as capacitors, and applications in which controlled breakdown is intended, such as plasma medicine, lightning protection, and materials processing. The electron emission from the cathode is a critical source of electrons which then undergo impact ionization to produce electrical breakdown. In this study, the role of secondary electron yields due to photons (γ ph) and ions (γ i) in direct current breakdown is investigated using a particle-in-cell direct simulation Monte Carlo model. The plasma studied is a one-dimensional discharge in 50 Torr of pure helium with a platinum cathode, gap size of 1.15 cm, and voltages of 1.2–1.8 kV. The current traces are compared with experimental measurements. Larger values of γ ph generally result in a faster breakdown, while larger values of γ i result in a larger maximum current. The 58.4 nm photons emitted from He(21P) are the primary source of electrons at the cathode before the cathode fall is developed. Of the values of γ ph and γ i investigated, those which provide the best agreement with the experimental current measurements are γ ph = 0.005 and γ i = 0.01. These values are significantly lower than those in the literature for pristine platinum or for a graphitic carbon film which we speculate may cover the platinum. This difference is in part due to the limitations of a one-dimensional model but may also indicate surface conditions and exposure to a plasma can have a significant effect on the secondary electron yields. The effects of applied voltage and the current produced by a UV diode which was used to initiate the discharge, are also discussed.

Effect of dielectric target properties on plasma surface ionization wave propagation

“…It is known from numerical work that secondary electron emission plays a large role in the propagation of SIW and the plasma morphology near the surface [30]. For the SIW interaction with dielectrics the source of secondary electrons is primarily due to ion and photon emission [31]. Vacuum ultraviolet (VUV) radiation can also greatly decrease plasma-induced surface charging of dielectric surfaces by temporarily increasing the local conductivity [32].…”

Improving PIC-DSMC Simulations of Electrical Breakdown via Event Splitting

“…The excitation cross-sections, on the other hand, are significantly smaller, and as a consequence, the probabilities of these processes are minuscule compared to the ionization and elastic cross-sections. However, accurate simulation of these excitation processes is needed not only to account for the energy loss of the electrons in the flow, but also to model the photon emission from the excited states, which may be a significant sources of electrons at the cathode [10,16].…”

“…We have previously shown that such an approach significantly reduces the level of stochastic noise in the computed ionization rate in an unsteady spatially homogeneous setup [8]. The resulting reduction in stochastic noise may be beneficial to unsteady PIC-DSMC simulations such as streamer propagation [9,10], as low-probability events can significantly affect the flow physics, perhaps even qualitatively. Due to the unsteady nature of such problems, reducing stochastic noise levels requires either the use of a larger number of computational particles, or ensemble averaging over multiple simulationsapproaches which may not always be viable due to computational time and/or machine memory constraints.…”

Simulation of the statistical and formative time delay of Townsend‐mechanism‐governed breakdown in argon at low pressure

“…[ 22–24 ] Most of these articles mainly focus on the breakdown criterion. Recently, the time delay has been considered in the state‐of‐the‐art particle‐in‐cell (PIC) modelling of helium discharge, [ 20,21 ] while the time delay of the streamer discharge was investigated in Reference [19]. Three stages in the breakdown process have been identified, and the effect of the various parameters on the delay time is reported in Reference [21].…”

“…Electric breakdown of gases, in general, has been widely studied in the literature. [14][15][16][17][18][19][20][21][22][23][24] For instance, the Monte Carlo method of electron transport has been used to determine the breakdown criterion for the low-pressure Townsend breakdown in helium and investigate electron bursts under the homogeneous electric field. [14] The radio-frequency breakdown in low-pressure argon has been studied in Reference [15], while low-pressure Townsend discharge in hydrogen has been modelled in Reference [16].…”