Statistical Investigation of Helium Breakdown Voltage: Critical Anomalies and Gas-Liquid Transition

Plasmas generated in fluids near the thermodynamic critical point (CP)—including supercritical fluid plasmas—enable unconventional materials synthesis [1]. Although a local decrease in the average breakdown voltage at CP has been reported and summarized in [1], the influence of density fluctuations on the statistics of electrical breakdown and the gas-liquid crossover in them near the CP is still unexplored. We performed a systematic study of breakdown-voltage distributions in cryogenic helium near CP conditions (T_c = 5.20 K, ρ_c = 69.6 kg m⁻³). Using a 2.7 µm tungsten micro-gap and a 4–5 V s⁻¹ ramp, we recorded 800–1200 breakdown events for each of eight isotherms from 5.10 to 5.50 K (densities 5–115 kg m⁻³).

Single-mode Weibull analysis reveals a pronounced minimum of the shape parameter k at ρ_c, signifying remarkably uniform breakdown voltages where density fluctuations are strongest. At approximately 0.02 MPa above the pressure on the critical-density isochore (ρ = ρ_c), the data require a dual-mode Weibull model: a gas-like avalanche component (Mode I, k ≈ 0.02) coexists with a bubble-mediated, liquid-like component (Mode II, k ≈ 0.10). Furthermore, Shannon entropy was effective to discuss the magnitude of variability among the different discharge modes. The simultaneous appearance of these two statistical modes provides evidence of a gas–liquid crossover in the discharge mechanism.

[1] S. Stauss et al. Plasma Sources Sci. Technol. 27 (2018) 023003.