Opening Mechanisms in AN Explosively Formed FUSE Opening Switch

MACH2 is a general purpose MHD code that has been used to simulate the evolution of an explosively formed fuse consisting of a thin cylindrical aluminum shell that is driven into a Teflon die by an explosion. The simulations included the initiation and propagation of the explosive detonation wave and tracked the extrusion of the fuse into the Teflon die. The code predicted that the fuse resistance increased by approximately three orders of magnitude due to the lengthening and thinning of the fuse material and the rise of temperature caused by the ohmic heating. Comparisons with recent experimental measurements performed at Los Alamos National Laboratory indicate that the predicted resistance is approximately one order of magnitude too small and suggest that the fuse must undergo a transition to a plasma in order to reach the observed switch voltage. Additional simulations of a short fuse segment were performed to investigate this transition and to evaluate the effect of spent explosive resistivity on the maximum electric field obtained. These simulations revealed a rapid expansion and heating of the vaporized fuse material that allows current to diffuse into the spent explosion products and limit the maximum value of electric field obtained.