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Recent developments in Exploding Foil
Initiator (EFI) based electronic safety,
arming and initiation systems
by Simon Bower mSc ceng miet and Dr Brian M Coaker phd ceng cphys cSci miet minstp whSch
In modern explosive systems, low-voltage
electrically-driven ‘hot wire’ detonator
systems (incorporating primary explosive)
have increasingly given way to highervoltage explosive detonators (such as
Exploding Bridgewire (EBW) and Exploding
Foil Initiator (EFI) types) containing
secondary explosive.1 The high degree of
insensitivity2, reliability, precision and
functionality afforded by electronic in-line
EFI based systems (coupled with their rapid
and repeatable activation time) has been
applied as a key enabling technology into a
growing portfolio of Electronic Safety &
Arming Units (ESAUs) in modern system
applications.
Figure 1. Surface-mount eFi detonator.
Figure 2. Stripline-mount eFi detonator.
Exploding Foil Initiator (EFI) detonators
exploding Foil initiators (eFis) have been
used since the early 1980s3 as a safe and
reliable method of initiating insensitive
explosive material in the first stage of an
explosive chain.4, 5 established e2v designs
comprise a copper-kapton laminate / shortbarrel initiator structure, and are activated
by the application of a fast-rising highcurrent pulse from a capacitor-discharge
Fireset circuit (which typically operates in
the range 2,000v to 3,000v).
low-inductance strip-line interconnect is
used in the Fireset-detonator assembly to
enable optimum switched-energy transfer
from the firing capacitor into the eFi bridge;
this low-loss configuration supports both
(i) surface-mount detonator designs
mounted directly onto the eSau
(figure 1), along with
(ii) stripline-mounted detonators on
interconnect lengths of up to one (1)
metre from the eSau (figure 2).
both eFi detonator configurations have
found application in electronic safety and
arming units (eSaus) in contemporary
system applications. the use of an
insensitive secondary high-explosive pellet
(hexanitrostilbene - hnS iv) combined with
a high-reliability firing circuit has seen the
use of these eFi detonator formats in singleoutput, twin, tandem and multi-point
configurations, with a twin eSau initiation
configuration illustrated in figure 3.
Figure 3. twin eSau
configuration using
surface-mounted (left)
and stripline-mounted
(right) eFi detonator
initiation points.
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Switching of the low-inductance firing
capacitor can be achieved using a triggered
vacuum switch (tvS), with gas-filled spark
gap and silicon switches also used. the tvS
offers electro-magnetic robustness and
interference immunity benefits in the fireset
within the eSau module, whilst delivering
high-speed, low-loss switching of the firing
capacitor and initiation of the eFi
detonator.6
Low-Energy EFI (LEEFI) detonators
in recent years the desire to reduce the size,
cost and mass of the Fireset has driven the
investigation of lower-energy eFi initiators
which can reliably, safely and precisely
operate at reduced firing voltages. physical
embodiment of the 2,000v - 3,000v
operating voltage range of conventional eFi
Fireset technology has been constrained by
operational requirements and also by the
availability of suitably-rated components
(which must survive the extreme
requirements associated with munition and
borehole operating environments).
recent development of low-energy eFi
technology has worked to reduce the all-fire
threshold of the exploding foil bridge
structure, to enable initiators and compact
Firesets that will reliably operate at firing
levels of c. 1,200v or less7; Stanag 4187
requirements currently mandate the no-fire
level to be at 500v (or more)8. Figure 4
illustrates the general arrangement of an
e2v low-energy eFi detonator (leeFi): this
design has no-fire threshold (nFt) and allfire threshold (aFt) characteristics as shown,
with aFt and nFt values also shown for the
standard eFi design as a comparison.
Figure 5. comparative langlie firing trial outcome for standard e2v eFi and e2v leeFi at hot (+85°c) and
cold (-54°c) temperatures [solid markers represent firing events, empty markers denote no-fire events].
Figure 5 goes on to show the outcome of
comparative langlie statistical trials to
determine firing performance over the
temperature range -54°c to +85°c. the test
uses a software algorithm to derive mean
and standard deviation parameters from a
finite sample by pre-determination of
required test levels from prior performance
as the test proceeds; the mean and standard
deviation values are then used to calculate
the aFt and nFt.
For a standard e2v eFi, the aFt was in the
range 1,794v – 1,824v and the nFt 1,398 –
1,568v, compared to ranges for the leeFi of
1,189 – 1,227v (aFt) and 702 – 1,010v (nFt)
respectively: this represents an
improvement in confirmed firing
performance of >30% (dependent upon
operating temperature) of the leeFi
detonator over the standard eFi unit.
with an identical explosive pellet
composition and mass (together with
identical output septum geometries) the eFi
detonator and leeFi detonator designs
shown in figure 1, in figure 2 and in figure 4
each deliver an equivalent explosive output.
Furthermore, careful design of the Fireset
circuit and minimisation of firing circuit
losses has enabled equivalent activation
times from both eFi and leeFi initiator
configurations, recorded in the two series of
sequential 25ns images of figure 69.
Figure 6. Fast-frame images of exploding bridge
foils at t0, (t0 + 250ns) and (t0 + 500ns), for e2v eFi
(left) and leeFi (right) initiators.
t0
(t0 + 250ns)
Figure 4. low-energy
eFi (leeFi) detonator,
with aFt and nFt
values for e2v leeFi
and e2v eFi at +85°c
(hot) and -54°c (cold).
eFi
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(t0 + 500ns
leeFi
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define the original position of the pellet.
Figure 7. burst currents (i) for eFi (left, 1ka/div) and leeFi (right, 500a/div) initiators at 100ns/div. [the
leeFi trace (right) also shows firing capacitor voltage at 500v/div.].
typical burst current-time histories driving
these events are shown in figure 7 for the eFi
and leeFi, showing peak currents of 2,600a
(eFi) and 1,250a (leeFi) respectively.
EFI takeover onto explosive material
continuing from the initiator output
characteristics shown in figure 6, two further
series of fast-frame camera images illustrate
the development of the explosive output
from the eFi detonator. the velocity of the
detonation wave in sleeved and un-sleeved
hnS pellets was investigated by direct
imaging of the pellet: the pellet was
initiated using a single standard e2v eFi,
from a 100nF / 3,000v fire-set. Figure 8
shows the progression of detonation of an
un-sleeved hnS pellet at 25ns frame
exposure and 250ns inter-frame period: a
pre-image of the pellet has been
superimposed on the detonation image to
Figure 8. Fast-frame 25ns
images at 250ns intervals
showing initiation of an unsleeved hnS pellet (top) using
e2v eFi initiator. [original
pellet image superimposed
on subsequent image frames].
the test was repeated using a hnS pellet
with a 1mm thick/5mm long stainless-steel
sleeve, as used in the e2v eFi and leeFi
detonator described figures 1, 2 and 4.
Figure 9 shows an image of the progression
of detonation of a sleeved hnS pellet at
25ns frame exposure and 250ns inter-frame
period; again, a pre-image of the pellet has
been superimposed on the detonation
image to define the original position of the
pellet.
these images allowed an estimation of the
detonation wave velocity to be made,
subject to the uncertainty introduced by the
exposure duration, inter-frame period and
precise initiation time relative to the image
frames.
the clearest data are from the un-sleeved
pellet images of figure 8 (since there was no
sleeve to block the light which signals the
point of initiation). Figure 8 clearly shows
some light at the base of the pellet in frame
1, indicating that the detonation had started
by the end of that frame exposure. in frame
4 the detonation had progressed beyond
the end of the pellet, indicating that at least
Figure 9. Fast-frame 25ns images at 250ns intervals showing initiation of a sleeved hnS pellet using e2v eFi initiator. [original pellet
image superimposed on subsequent image frames].
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5mm (the length of the pellet) had been
travelled in three exposure durations (3 x
25ns = 75ns) plus three inter-frame periods
(3 x 250ns = 750ns), totalling 825ns. the
velocity calculated in this way is 5mm/825ns
= 6.1 mm/µs, or 6.1km/sec: this is
considered an under-estimate since the
detonation has clearly travelled further than
5mm in the time measured, placing the
velocity in the range 6 to 8 km/sec which
agrees well with other published data (7,000
m/sec) for detonation velocity of hnS10, and
confirming full-order detonation of the
explosive material.
comparing the detonation images of the
sleeved and un-sleeved explosive pellets,
the detonation wave front appears to travel
with similar velocity in both cases, reaching
the front face of the pellet (frame 4) and
being fully-developed at one pellet-length
from the front face (frame 8) in the same
respective image frames (within the 25ns
uncertainty of the frame exposure duration).
EFI takeover onto pyrotechnic material
extending the safety and reliability benefits
of insensitivity to rocket motor systems, eFi
detonator based initiation modules have
been used in conjunction with throughbulkhead initiators (tbi)11. the explosivelydriven tbi maintains a pressure bulkhead
seal into the rocket motor casing, where the
explosive shock from the bulkhead septum
initiates a pyrotechnic material (pyrogen)
chain, to provide a deflagrating (igniferous)
output from the tbi. hot-wire initiators have
been used to directly initiate a pyrogen
pellet12, with no intermediate explosive
chain; this arrangement has the advantage
that minimal mechanical shock is imparted
into the rocket motor casing by the
activation of the ignition module (since
there is no explosive event), thereby
simplifying the mechanical design of the
motor casing and enhancing the ignition
reliability of the motor.
this direct initiation approach can also be
evaluated with an eFi initiator within the
pryrogen ignitor, with the eFi ignitor
bringing additional benefits in terms of
insensitivity, ignition reliability, electromagnetic immunity and electro-static
discharge resilience13. preliminary work has
demonstrated the direct initiation of
pyrotechnic material using an eFi initiator, to
provide an igniferous output from an igniter
module with no explosive material within
the ignition chain. Figure 10 shows the
arrangement of an eFi Fireset, initiator and
pyrogen pellet, which provided a
160pSi pressure output (into a 20cm3 test
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Figure 10. eFi ignitor hardware, demonstrating direct initiation of a deflagrative output.
vessel) and achieved peak pressure within
100ms of triggering the ignitor. this directinitiation ignitor configuration can form the
basis of an insensitive munition (im)
compliant motor safety ignition module14.
a low-inductance triggered vacuum switch
is retained within the miniature fireset, to
optimise the operation of the leeFi
detonator, and allow the minimum value
(and size) of firing capacitor. with a leeFi
detonator that produces an equivalent
explosive output to established e2v eFibased designs, the miniature eSau provides
a compact, compliant fuzing solution8 that
can be readily packaged for single, twin/
tandem and multi-point applications.
Miniature ESAUs employing LEEFI
Firesets
with a low-energy leeFi-based Fireset now
operating in the region 1,200v to 1,400v9,
the physical size and mass of the Fireset
components can be reduced (especially the
firing capacitor, along with insulation stand- ESAUs for selectable effects
off spacings around the circuit).
multiple eFi detonators may be driven
simultaneously from a single eSau (such as
consequently the integrated Safety &
the twin output configuration shown in
arming unit can be reduced significantly in
figure 3) or fired individually from an
size and mass, from the nominal 100mm
integrated eSau module comprising
diameter / 1kg of the eFi-based eSau
multiple independent Firesets; figure 12
module shown in figure 11, down to the
illustrates this multi-point eSau concept in a
nominal 40mm diameter / 250 grammes of
four-point eFi initiator array.
the leeFi-based eSau shown in figure 10.
Figure 11. miniature
eSau (left)
exploiting a
compact lowenergy fireset
(centre and right).
Feature
the small physical size and low mass
advantages of leeFi-based miniature eSaus
lend this technology to networks of
independent eSau modules in multi-point
arrays, with a four-module ‘federated’ array
of miniature eSaus depicted in figure 13;
both semiconductor-bridge (Scb) based
eSaus and eFi based eSaus can be ‘arrayed’
as stand-alone modules in this manner15.
Simultaneous or phased switching of the
multi-point initiator array can determine the
explosive output characteristics of the host
system.
Further development of the eFi detonator
and Fireset could also provide the basis for
selectable output from the eFi-based
initiation system itself, allowing the
commanded detonation or deflagration of
the detonator module in order to initiate the
desired effect from the platform system;
such a selectable initiation capability could
support a new generation of selectableeffect / selectable-output explosive
systems16.
Conclusion
eFi detonator based systems have
demonstrated a step improvement in
operational robustness, reliability and
insensitivity in the control and initiation of
energetic systems. the deployment of leeFi
based systems is now enabling these same
higher levels of precision, safety and
insensitivity to be applied to a greater range
(and into smaller calibres) of explosive and
pyrotechnic effector systems.
Further development of these initiator
technologies, and the fireset electronics
used to control and to drive them, is
bringing forward a range of new techniques
to enable the delivery of selectable effects
from selected output - with precision and
with a high level of safety integrity - from
the host system.
Figure 12. multi-point
initiation sub-system:
integrated eSau module with
leeFi detonator array.
1 J o’gorman, ‘ebw and eFi detonators – an
overview’, explosives engineering, September
2001, pp. 14-16
2 r l beauregard, the history of insensitive
munitions,http://www.insensitivemunitions.or
g/, accessed 22 January 2014
3 S. c. Schmidt, w. l. Seitz & Jerry wackerte, an
empirical model to compute the velocity
histories of Flyers driven by electrically
exploding Foils, los alamos Scientific
laboratory, la-6809, July 1977
4 r varosh, ‘electric detonators : ebw and eFi’,
propellants, explosives, pyrotechnics, vol. 21,
pp. 150-154, 1996
5 Schlumberger, Secure detonator, november
2005, accessed 6 February 2014
http://www.slb.com/~/media/Files
/perforating/product_sheets/wireline_perforat
ing/secure_detonator.ashx
6 b m coaker, c bell, r J Seddon & J S bower,
‘miniature triggered vacuum Switches for
precise initiation of insensitive loads in
demanding environments’, 39th ieee int. conf.
on plasma Sci., edinburgh, uk, 8-12 July 2012
7 excelitas technologies, blue chip™ detonator,
2012,http://www.excelitas.com/ downloads
/pn_bluechip.pdf, accessed 6 February 2014
8 nato Standardization agreement, Fuzing
Systems – Safety design requirements,
Stanag 4187, edition 4, march 2007
9 J S bower & b m coaker, a miniature electronic
Safety & arming device for initiation of
insensitive explosive, 8th ordnance, munitions
and explosives Symposium, Shrivenham, uk,
2 – 3 october 2012
10 J akhavan, the chemistry of explosives – 3rd
edition, the royal Society of chemistry, iSbn
978-1-84973-330-4, p. 70, 2011
11 ensign-bickford aerospace & defence, thrubulkhead initiator (tbi),http://www.ebad.com/products/thru-bulkhead-initiator-tbi/,
accessed 28 January 2014
12 F Silverman, the pc-23 nSi commercial
equivalent user’s guide, hi-Shear technology
corporation document no. 9392410-5944,
19 may 2008
13 p Zu et al, ‘improving reliability of Scb
initiators based on al / ni multilayer
nanofilms’, eur. phys. J. appl. phys. (2013) 63:
10302.
Figure 13. multi-point
initiation sub-system:
federated array of
independent eFi-based
eSaus.
14 nato Standardization agreement, ignition
Systems for rocket and guided missile motors,
Safety design requirements, Stanag 4368,
edition 3, 1 august 2011
15 pacific Scientific, Smart energetics architecture
(Seatm), http://www.psemc.com/productfamilies/electronic-ordance- devices/,
accessed 30 January 2014
16 m. graswald, tdw, ‘precise target effects
through Scalable warhead effects’, delivering
precision effects in a complex environment,
paris, 27-29 november 2013
Simon Bower and Dr Brian Coaker are with e2v
technologies, based in the United Kingdom.
http://www.e2v.com/
Further information: [email protected]
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