A sheath is the transition region from plasma to a solid surface, which also plays a critical role in determining the behaviors of many lab and industrial plasmas. However, the cathode sheath properties in arc discharges are not well understood yet due to its multiscale and kinetic features. In this letter, we have adopted an implicit particleincell Monte Carlo collision (PICMCC) method to study the cathode sheath in an atmospheric arc discharge plasma. The cathode sheath thickness, number densities and averaged energies of electrons and ions, the electric field distribution, as well as the spatially averaged electron energy probability function (EEPF), are predicted selfconsistently by using this newly developed kinetic model. It is also shown that the thermionic emission at the hot cathode surface is the dominant electron emission process to sustain the arc discharges, while the effects from secondary and field electron emissions are negligible. The present results verify the previous conjectures and experimental observations.
Recent studies have shown that thermo-field emission is a dominating electron source of microdischarges at cathode temperatures far above room temperature. However, little research has focused on the post-breakdown nature of microdischarges. In order to explore the post-breakdown characteristics of thermo-field emission-driven microplasms, a one-dimensional implicit particle-in-cell with Monte Carlo collision method is adopted and updated by using thermo-field electron emission to investigate the kinetic characteristics of direct-current argon microdischarges at atmospheric pressure. The fundamental properties of microplasmas, such as electric field, particle number density, averaged particle temperature and current density are analyzed in the post-breakdown regime. In addition, sheath behavior is investigated to further observe how the space charge affects the thermo-field emission. The results indicate that thermo-field emission-driven micro-scale discharges can produce high current density and high-energy particles with a low applied voltage of 20 V. The impact of cathode temperature on enhancing the thermo-field emission is more pronounced, compared to the applied voltage and the electrode spacing. The electron energy probability function shows a multi-peak distribution.
The near-cathode region plays a crucial role in exploring the transport characteristics of the transition from arc column to the hot cathode in atmospheric-pressure arc discharges because of the existing non-equilibrium phenomena. A one-dimensional unified model including the near-cathode region and the cathode body is developed for an argon arc discharge with tungsten cathode at atmospheric pressure in this paper. The electrostatic model coupled with an external circuit in the near-cathode region is solved based on the implicit Particle-In-Cell coupled Monte-Carlo Collision method without any assumptions of thermal or ionization equilibrium or quasi-neutrality. A detailed description of the arc plasma-cathode and cathode-gas interactions is obtained by calculating the nonlinear heat conduction equation in the cathode. It is shown that the space-charge sheath strongly affects particle transport in the near-cathode region and energy transport from arc plasma to the thermionic cathode. The total current density has significant effects on the kinetic characteristics of arc plasma by feedback-like mechanisms. The Joule heating by the external circuit and charged particles deposited into the cathode are dominating mechanisms of energy transfer from the near-cathode region to the cathode, while energy loss by radiation is more significant compared with natural convection.
A voltage-driven cathode sheath model in an atmospheric-pressure argon arc discharge is developed in the framework of an implicit particle-in-cell Monte Carlo collision (PIC–MCC) method. Plasma transport processes are solved numerically in one dimension without any local-equilibrium hypotheses, in particular, without explicitly dividing sheath and a quasi-neutral plasma region. The right boundary of the computational domain located at the pre-sheath is determined first by observing the variation in typical parameters. A comparison of results is given with different positions of the right boundary to study the plasma transport processes in the cathode sheath. Number densities, spatially averaged energies, electric field and potential, collision frequency, heating rate of electrons, as well as the spatially averaged electron energy probability function inside the sheath, are predicted self-consistently based on this newly developed kinetic model. It is shown that both excitation collisions and ionization collisions occur inside the sheath, and collision frequency of the former is larger than the latter. The collision frequency of charge exchange is higher than that of elastic collision for ions. In addition, the effects of different electron emission processes are described. It is indicated that the thermionic emission on the hot cathode surface is not the only significant emission mechanism to sustain the arc discharges.
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