3D PIC-DSMC Simulation of Arc Initiation: A Cautionary Tale of Strongly Coupled Plasmas*

Using the Particle-In-Cell Direct Simulation Monte Carlo code EMPIRE, we have performed simulations of a cathode spot plasma during initiation and encountered a variety of numerical challenges and accuracy concerns. Prior models of electrode vaporization have found that the typical electrode-gas density is near solid density close (<100 nm) to the electrode [1]. An ionization mean free path of <10 nm results in nearly complete ionization as the gas expands away from the near-electrode region. The ion densities (~10²⁶ m^-3) and temperatures (~2000K) result in a strongly coupled plasma (Γ_i > 10) and, for explicit PIC calculations, a sub-nm mesh size is required to avoid numerical grid heating. However, this results in less than one physical particle per element volume which results in “late” (~100ω_p^‑1) time numerical heating. If we use computational particle weights less than one then we will not capture the physical disorder induced heating (DIH) that should occur [2]. Furthermore, at these densities the DSMC assumption of a dilute gas is violated and the ionization rates will be wrong due to unphysical density spikes where a single computational particle of weight one represents an element density >10²⁷ m^-3. We find that charge exchange coupled with DIH seems to be an important mechanism by which the cathode spot plasma expands into the vacuum at velocities ~10× greater than the initial thermal velocity; similar to experimental measurements. Similar challenges exist for initiation of atmospheric pressure arcs and this talk will examine Artificial Correlation Heating in the streamer causing the plasma density to increase if one naively refines the mesh [2].

 

[1] Koitermaa et al, Vacuum 224 113176, 2024

[2] Acciarri et al, PSST 33 035009, 2024