Speaker
Description
As HPC computational resources increase, 3D simulations of vacuum arc initiation via the Particle-In-Cell (PIC) Direct Simulation Monte Carlo (DSMC) method are becoming more and more feasible since typically the initiation is modeled as starting from an extremely small region (e.g., the cathode spot). Using Sandia’s PIC-DSMC code EMPIRE, we have performed simulations of a cathode spot plasma during initiation and encountered a variety of numerical challenges and physical accuracy concerns. Prior models of electrode ablation/vaporization have found that the typical electrode-gas density is approximately solid density very close (~100 nm) to the electrode. Thus, the ionization mean free path is <10 nm with an ionization frequency of ~1 ps-1 resulting in nearly complete ionization as the neutral gas expands away from the near-electrode region. The ion densities (1027-1028 m-3) and temperatures (~2000K) result in a strongly coupled plasma (Γi > 10) and, for explicit PIC calculations, the required mesh resolution to avoid numerical heating is dx<10-10 m. This results in less than one physical particle per element volume which results in “late” (~100ωp-1) time numerical heating. If we use computational particle weights less than one or accept some numerical heating at longer times by increasing the mesh size, then we will not capture the physical disorder induced heating (DIH) that should occur [1]. Furthermore, at these densities the traditional DSMC method’s assumption of a dilute gas is, at best, extremely questionable and even if that error is small, the ionization rates will be wrong due to unphysical density spikes. This talk will discuss these limitations and challenges in more depth as well as the role that DIH might play in the evolution of the cathode spot plasma.
[1] Acciarri, Moore, Beving, and Baalrud, “When should PIC simulations be applied to atmospheric pressure plasmas? Impact of correlation heating”, PSST, submitted.
This work was supported in part by the US Department of Energy under award no. DE-SC0022201, and in part by Sandia National Laboratories. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under Contract No. DE-NA0003525.
