Explosively formed fuse (EFF) opening switches have been used in a variety of applications to divert current in high explosive pulsed power (HEPP) experiments. Typically, EFF's operate at 0.1 -0.2 W ( c m switch width), and have an -2 ps risetime to a resistance of 10's -100's mR. We have demonstrated voltage standoff of -7KV/(die pattem) in some configurations, and typical switches have up to 100 die patterns. In these operating regimes, we can divert large currents (10-20 MA) to low impedance loads, and produce voltage waveforms with risetime and shape determined by the shape of the resistance curve and amount of magnetic flux in the circuit. Progress in quantitatively modeling EFF performance with magnetohydrodynamic (MHD) codes has been slow, and much of our understanding regarding the operating principles of EFF switches still comes from small-scale experiments coupled with hydrodynamic (hydro) calculations. These experiments are typically conducted at currents of -0.5 MA in a conductor 6.4 cm wide. A plane-wave detonation system is used to drive the EFF conductor into the forming die, and current and voltage are recorded. The resulting resistance profiles are compared to the hydro calculations to get insight into the operating mechanisms. Our original goals for EFF development were limited in scope, and in pursuing specific large systems, we have left behind a valuable body of small-scale test data that has been largely unused.
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We are developing a prototype high explosive pulsed power (HEPP) system to obtain isentropic Equation of State (EOS) data with the Asay technique [l]. Our prototype system comprises a flat-plate explosive driven magnetic flux compression generator (FCG), an explosively formed fnse (EFF) opening switch, and a series of explosively-actuated closing switches. The FCG is capable of producing-10 MA into suitable loads, and, at a length of 216 mm, the EFF will sustain voltages in excess of 200 kV. The load has an inductance of-3 to 10 nH, allowing up to-7 MA to be delivered in times of-0.5 ps. This prototype will produce isentropic compression profiles in excess of 2 Mbar in a material such as tungsten. Our immediate plan is to obtain isentropic EOS data for copper at pressures up to-1.5 Mbar with the prototype system; eventually we hope to reach several tens of Mbar with more advanced systems.
Isentropic compression experiments that utilize intense magnetic fields to compress samples have been designed, developed and performed. The technique has been shown to work to pressures of more than 1 Mbar on Sandia National Laboratory's Z pulsed power machine. We are extending the technique to use high-explosive pulsed power.
We have previously reported on the development of explosively formed fuse (EFF) opening switches for use in applications where very long conduction times (10's or 100's ps) are required and where opening times of 1-10 ps are adequate. In this paper we report on the development of an EFF that allows magnetic flux in the switch to be delivered to the load rather than lost from the circuit. The topology of this device is substantially different from earlier versions and contains new design constraints. In the most strenuous test to date, we delivered 7.5 MA to the EFF from a small helical explosive driven magnetic flux compression generator, and completely turned off the current in the remaining 34-nH circuit in -3 ps producing a 140-kV pulse. The switch in this test is 15 cm in length. We also report on work with EFFs in this configuration tailored for slightly longer opening times.
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