Phase transitions and plasma formation during the explosion of thin point cathodes in a high-current diode operating...

We investigate the processes of plasma formation and phase transitions occurring during the explosion of metal point emitters made from copper and tungsten wires approximately 1 mm in length and ∼10 µm in diameter. The experiments were performed under vacuum-to-forevacuum discharge conditions using nanosecond current pulses of up to ∼1 kA. It was found that an extended region of the copper emitter, with a length of ∼200-300 µm, remains intact for tens of nanoseconds and even after the discharge ends, while the emitter's base explodes completely, and its explosion products become stratified. A transition zone is observed between the base and end regions of the emitter, characterized by a smooth or sharp expansion of the degraded metal without stratification in the explosion products. During the copper emitter explosion, ejections of dense plasma with an electron concentration of at least 10^{20}cm^{-3} can occur. The expansion velocity of the exploded copper emitter material can reach (1-3)×10^{5} cm/s at residual air pressures of 10^{-4} to 10^{-1} Torr. An increase in air pressure to ∼1 Torr leads to a decrease in both the specific energy input and the expansion velocity of the explosion products. The explosion dynamics of copper and tungsten point emitters differ. In the structure of the tungsten emitter's explosion products, no striations are observed, and the length of the exploded region almost entirely coincides with the emitter's total length. Analysis of the point emitters' explosion dynamics and the discharge's electrophysical parameters indicates that the transition of the emitter to the explosion regime is based on two key stages: first, the achievement of the explosive electron emission mode over an extended part of the emitter's end region, followed by the development of a shunting breakdown through the expanding explosion products along the emitter surface.