Two-dimensional electron density visualization over plasma flare in vacuum breakdown process

Spatiotemporal evolution of plasma flares in a vacuum breakdown process was characterized by various optical techniques including Shack-Hartmann type laser wavefront sensors for two-dimensional electron density imaging, high-speed framing photography for speed determination of flare expansion, and optical emission spectroscopy for plasma composition assessment. The experimental results showed that the plasma flares with high electron densities of 10²⁵ m⁻³ and gas temperature of 2 eV were initiated on the copper anode and expanded to the vacuum gap with a propagation speed of 6 × 10³ m/s. Subsequently, the electron densities in the anode flare tips demonstrated a drastic decrease in a short time scale of 50 ns due to a three-body recombination reaction. The anode flare tips combined with the cathode plasma flares initiated on the copper cathode, and the conductive plasma channels containing flare-induced copper vapor were established in the interelectrode gap. In the final stage of the vacuum breakdown, the copper-vapor-contaminating plasmas were deconstructed and they transformed into the vacuum arc discharges.