An Approach to Calculating the Deterministic Safe Firing Stimulus for Exploding Foil Initiators

For any detonator it is necessary to characterise how the detonation probability changes with the firing stimulus, for safety and reliability assurance. For exploding foil initiators (EFIs) there is a maximum (non-zero) “lift-off” stimulus which can be delivered to the detonator which will not under any circumstances cause a detonation or a lower-level explosive response. The lift-off (typically voltage or energy) detonator can be used to provide a:

• a] voltage or energy which can be used in safety analyses,
  
  b] zero probability stimulus which can be used as a boundary condition in statistical analyses.

To understand the electrical and physical processes determining the lift-off stimulus experimental, magneto-hydrocode modelling and statistical studies have been performed..

A cable assembly, comprising a stripline cable with an integral EFI bridge and flyer, was mounted into an experimental fixture a preset distance away from a perspex target. Flyer velocities were recorded over a range of voltages.

The velocities were used in two ways. Firstly, to validate the magneto-hydrocode, ALEGRA and to predict flyer velocity as a function of voltage. Secondly, to estimate the lift-off stimulus by extrapolating a fitted monotonic spline back to zero velocity. Starting from this lift-off stimulus, ALEGRA simulations were used to investigate various features including the behaviour of the bridge, the shape and orientation of the flyer, and the kinetic energy delivered to the HE pellet.

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