Breakdown Characteristics of Electrical Discharges in High-Density Helium Near the Critical Point

We present an investigation of the breakdown behavior of micrometer gap direct-current discharges in gaseous, supercritical, and liquid helium, which shows a critical anomaly of the breakdown voltage near the critical point. A discharge model that combines gas- and liquid-like breakdown mechanisms and takes into account the local fluid structure in a fluctuating fluid with the concept of a modified electron mean free path, allows us to reproduce the breakdown behavior. The result of the analysis suggests that the critical breakdown anomaly is caused by long acceleration paths inside low-density domains resulting from the density fluctuation.