Abstract

This work analytically and numerically predicts key variables relevant to the
development of ejecta material from an Exploding Foil Initiator (EFI) which would
typically be utilised in explosive initiation through impact with an explosive pellet.
Key co-dependent output variables identified and investigated within this study (the
EFI ejecta material’s velocity, duration of impact on the explosive and mass) were
chosen due to their direct influence on impulse delivered during impact in normal
operation.
Empirical and modelling approaches have been employed to correlate principal
design and operational parameters to predict the measurands of interest. Herein the
significance of the time of arrival duration of the ejecta is identified, this parameter
bounds the period over which an EFI can apply an impulse.
Furthermore, the impulse delivered is proportional to the mass of ejecta. This work
demonstrates the variable nature of mass ejecta due to modulation of plasma
development across the bridge as a result of voltage dependent eddy currents.
The evidence compiled through this work demonstrates that the impulse, defined by
velocity, mass and time of arrival duration (ToAD) of ejected material, delivered by
an EFI is dependent upon the nature of the CDC input signal. Both the electrical signal
and EFI bridge geometry should be controlled in unison, contributing to efficient EFI
and controlled timing of initiation in systems such as multi-point initiators.
A laboratory-based velocimetric measurement arrangement recorded EFI ejection
velocities in the order of 6km.s-1 and facilitated comparison with respect to the CDC
voltage. These observations were in line with those predicted by models developed
for this work and the empirical findings of other researchers.
Increased capacitor voltages yielded a delayed ejection of greater mass. The
variability of mass ejection and the duration of impact both corroborate the results
of the numerical modelling.
Impulse upon the target explosive is proportional to ToAD. Explosive response to EFI
is a dependent upon ejectile impulse. The correlation of initial capacitor voltage to
ToAD facilitates the generation of specific impulses tuned for a particular explosive.
Doubling the mass of ejecta (a design variable) yielded 57% of the effect of halving
the initial capacitor voltage (an operational parameter) on ToAD, indicating the
relative inefficiency of flyer thickness optimisation.
Numerical modelling carried out for this work elucidated regions of plasma formation
within the bridge during capacitor discharge. Both the rate and volume of bridge
ejected are influenced by the energy stored within the CDC and by bridge geometry.
The model was validated by laboratory measurement which observed saturation of
the current passing through the bridge at high voltages. The volume of bridge
material converted to plasma directly influences ejecta mass. The evolution of the
plasma region is of critical importance for consideration during EFI design, ultimately
influencing ToAD.
This work demonstrates the real but differing modes of modulation that design and
operational variables contribute to EFI system behaviour.