Micrometer-scale electrical breakdown in high-density fluids with large density fluctuations: Numerical model and experimental assessment

Experimentally observed electrical breakdown voltages (U-B) in high-pressure gases and supercritical fluids deviate from classical theories for low-pressure gas discharges, and the underlying breakdown mechanisms for the high-density fluids making the U-B differ from those in the classical discharges are not yet well understood. In this study, we developed an electrical breakdown model for the high-density fluids taking into account the effects of density fluctuations and ion-enhanced field emission (IEFE). The model is based on the concept that a critical anomaly of the U-B (local minimum near the critical point) is caused by long mean free electron path leading to a large first Townsend coefficient in locally low-density spatial domains generated by the density fluctuations. Also, a modified Paschen's curve considering the effect of the IEFE on the second Townsend coefficient was used to reproduce the U-B curve in the high-density fluids. Calculations based on the novel model showed good agreements with the experimentally measured U-B even near the critical point and it also suggested that the critical anomaly of the U-B depends on the gap distance. These results indicate that both the density fluctuations and the IEFE have to be considered to comprehend the plasmas in high-density and density-fluctuating fluids.