Pressure-threshold explosive cathode processes as the dominant source of a high-density plasma in nanosecond air discharges

We report a pressure-threshold phenomenon in nanosecond air discharges: explosive cathode processes become the dominant source of a dense highly ionized plasma at air pressures above ∼100 Torr. Picosecond laser imaging reveals that these explosive processes, while competing with air ionization, result in the ejection of cathode material over a time scale shorter than 1 ns to create a plasma with electron densities as high as ≈6 × 1019 cm−3, significantly exceeding values achievable through air ionization alone. The explosive processes exhibit a sharp onset, disappearing when the air pressure drops by as little as tens of Torr relative to 100 Torr. Our findings demonstrate that, at high air pressures, cathode vaporization—rather than gas ionization—initiates the generation of such a dense plasma and enables electric spark formation. At intermediate pressures, below ∼100 Torr, yet above vacuum conditions, this explosive mechanism is suppressed, and the discharge transitions to a regime governed by volumetric gas ionization. This behavior contrasts with vacuum discharges, where cathode material is the dominant plasma source. The revealed threshold mechanism is crucial for controlling plasma in applications ranging from nanoparticle synthesis to pulsed power systems.