Detailed Numerical Simulation of Cathode Spots in Vacuum Arcs—I

Abstract

A model of cathode spots in high-current vacuum arcs is developed, with account of the plasma cloud left over from a previously existing spot, all mechanisms of current transfer to the cathode surface, including the contribution of the plasma produced by ionization of the metal vapor emitted in the spot, and the Joule heat generation in the cathode body. The simulation results allow one to clearly identify the different phases of life of an individual spot: the ignition, the expansion over the cathode surface, and the thermal explosion. The expansion phase is associated with a nearly constant maximum temperature of the cathode, which occurs at the surface and is approximately 4700-4800 K. Thermal explosion is a result of thermal instability (runaway), which develops below the cathode surface when the Joule heating comes into play. The development of the spot is interrupted if the plasma cloud has been extinguished: the spot is destroyed by heat removal into the bulk of the cathode due to thermal conduction. Therefore, different scenarios are possible depending on the time of action of the cloud: the spot may be quenched before having been formed or during the expansion phase, or even at the initial stage of thermal explosion.

Numerical simulation of the initial stage of unipolar arcing in fusion-relevant conditions

“…In other words, the arc plasma is not explicitly considered in the modeling of this work: the parameters of the nearelectrode plasma layer evaluated in the first step are introduced as boundary conditions describing the energy and current transfer in the arc spot and pressure acting on the electrode surface in the model of the second step. (This is the same procedure employed in [40,43]. )…”

Numerical investigation of AC arc ignition on cold electrodes in atmospheric-pressure argon

“…The reported results refer to the atmospheric-pressure argon plasma, electrodes made of tungsten or copper, the temperatures at the bottom of the electrodes T s1 = T s2 = 300 K, and the effective secondary electron emission coefficient γ = 0.1. The temperaturedependent mass density ρ s , isobaric specific heat C ps , and thermal conductivity κ s have been taken from [32] for tungsten and from [33][34][35], respectively, for copper. (C ps was corrected to include the latent heat of melting.)…”

Revisiting Theoretical Description of the Retrograde Motion of Cathode Spots of Vacuum Arcs

“…Neglecting the kinetic energy with which the electrons are emitted from the cathode compared to the electrostatic energy eU , one obtains, similar to (9) w e = ω e δ( − ξ), u e = −u (s) e 1 − − θ e ( − ξ) 2 (18) where ω e = eB/m e is the electron gyrofrequency, u (s) e = (2eU/m e ) (1/2) and R Le = u (s) e /ω e are the characteristic electron speed and Larmor radius, respectively, and θ e = (δ/R Le ) 2 . (Note that alternative forms of the last expression are θ e = (ε 0 eB 2 /m e j i )(eU/m i ) (1/2) and θ e = θ i m i /2m e .)…”