Modelling of exploding foil initiator (EFI) bridge-flyer behaviour
Creators: name: Borman, Alexander
Publisher: name: University of Lincoln
Year: 2023
Subjects: H890 - Chemical, process &amp; energy engineering not elsewhere classified

description: 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.