Stratification and filamentation instabilities in the dense core of exploding wires

We report experiments characterizing the stratified and filamentary structures formed in the dense core of nanosecond electrical explosion of aluminum wires to understand the physical scenario of electrothermal instability. Direct experimental observations for stratification and filamentation instabilities, as well as the coexistence state of azimuthal strata and vertical filament in the dense plasma column, are presented. The wire core exhibits remarkable different patterns of instability with the decreasing wire length. The shadowgram of shorter wires demonstrates that the instability is transformed from stratified structures to filamentary structures. According to a radial magnetohydrodynamic computation, the wire enters a phase state of negative temperature dependence of resistivity before voltage breakdown. However, filamentary structures are only observed in exploding wires of 1 cm and 0.5 cm in length. The analyses based on experimental and computational results indicate that the increase in internal energy determines the manifestation of instability in the dense core. Filamentation instability occurs when the total energy input is no less than 1.5 times the vaporization energy at the moment of voltage breakdown. The lower limit of energy deposition ensures that the increase in internal energy covers vaporization energy.