A study of Coulomb explosion in a dense plasma focus hotspot using EPOCH particle-in-cell simulation code

Hotspots – ultra-hot, ultra-dense microscopic regions within the pinch plasma column of a dense plasma focus (DPF) device – are environments of extreme conditions that make the DPF a fascinating and powerful source of radiation and particles. In this paper, we present a detailed particle-in-cell (PIC) simulation of the Coulomb explosion of a dense, fully ionised plasma region (hotspot) against a background of cold, fully ionised plasma. We model a two-dimensional (2D) circular deuterium plasma inspired by observations reported in DPF experiments. By introducing a temperature difference between hotspot electrons and other plasma species, our simulations successfully model Coulomb explosion dynamics at picoseconds resolution. Our analysis indicates that, although Coulomb explosion accounts for increase in ion kinetic energy, this mechanism alone does not fully account for the observed levels of energetic radiation and particles as is often suggested throughout literature. In addition, we identify a novel plasma feature triggered by the Coulomb explosion: the formation of a distinct ring sheath surrounding the dense hotspot. Short-lived instabilities, lasting less than 20 ps , are also observed during the early stages of the simulation. The results are validated through an extension to three-dimensional (3D) simulations, which show good agreement with our 2D model.