The effect of current rise time on the acceleration of thick flyers to hypervelocities using an electric gun

The electric gun is a projectile launcher which utilises both the rapid expansion of an ohmically heated exploding foil and strong electromagnetic forces to accelerate an insulating flyer up to 20 km/s. The gun's acceleration mechanism is highly efficient at converting stored electrical energy in the pulsed-power device driving the load to kinetic energy in the flyer, with values of up to 25% reported (Osher et al., 1990). This high efficiency would allow capacitor banks to use less energy than conventional drives to generate higher pressure states in materials of interest to extreme state research. Despite its promising efficiency, the electric gun is rarely used, as the process of launching flyers above 0.5 mm thickness in this manner is highly variable, often resulting in uncontrolled launch characteristics and premature failure of the flyer. The gun is also difficult to optimise without use of a sophisticated multi-physics hydrocode, further limiting its take-up. This work presents experimental results from the successful launch of 24 x 24 mm flyers up to 2.0 mm thick to 10 km/s using an electric gun load on 1.2 MJ pulsed-power device: Machine 3. In combination with results from a 0D model, these findings are used to identify the mechanism for the destruction of thick flyers accelerated using electric guns, including strategies for mitigating their break-up. The results support the existing idea that flyer failure can be avoided by limiting the maximum pressure within the flyer. They also reveal a second mechanism by which the flyer integrity can be maintained when the current rise time is longer than the flight time; high pressures in thick flyers caused their density to remain high enough to prevent the driving foil plasma from breaking through, averting their disassembly. This second mechanism provides a route to accelerate thick electric gun flyers to higher velocities with efficiencies up to around 9% whilst lengthening the shock duration on impact, broadening the gun's potential applications in extreme state research.

Automated summary

This study investigates the mechanism for the destruction of thick flyers accelerated using electric guns and proposes strategies for mitigating their break-up based on experimental results and mathematical models. The findings suggest that limiting the maximum pressure within the flyer and extending the current rise time can prevent flyer failure, increasing the efficiency and shock duration of the electric gun.