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New Trends in Research of Energetic Materials, Czech Republic, 2010

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Velocity measurements of exploding foil initiators (EFIs) using high speed photography

Hannah R. Davies*, Tracy A. Vine**, and David M. Williamson*

* University of Cambridge, Cambridge, UK ** QinetiQ Ltd., Fort Halstead, UK [email protected] Abstract: Exploding Foil Initiators (EFIs) are highly insensitive to mechanical shock and electrical interference, requiring a specific high current pulse for initiation. This allows the use of insensitive secondary explosives, making EFIs a safe and reliable means of initiating explosives. When a high current is passed through the polymer-encapsulated metal bridge, a contained plasma is formed. This causes the film to expand rapidly to form a bubble or shear to form a flyer. This flyer can then impact the secondary explosive and cause initiation. Due to the very high speed at which these systems operate, a streak photography system was designed to characterise the behaviour of the polymer film flyers and determine the velocity. This paper will report the effect of bridge size, barrel length and stripline design on flyer speed. Keywords: EFI; exploding foil initiator; initiators; high speed photography

1

Introduction

A common method to initiate explosives is to use an explosive train with an electrically activated detonator. However, if this detonator contains sensitive explosives or can be initiated below 500 V, then safety regulations require that there must be a physical barrier between the detonator and the rest of the train [1]. Exploding Foil Initiators (EFIs) or Slapper Detonators contain only secondary explosives and are operated by a very specific electrical pulse. This makes the system insensitive to accidental initiation by static electricity etc. and allows the detonator to be used without the need for a physical barrier. Stroud invented EFIs in 1965 at Lawrence Livermore National Laboratory (LLNL) [2]. The EFI contains an explosive pellet pushed against a thin barrel, Fig. 1. Below this barrel is an insulating film, such as Kapton polyimide, covering a thin metal strip with a `bridge' at the centre. When a high current pulse is passed through the bridge the metal strip is vaporised. The metal plasma is confined by the film base and so it expands into the barrel. This causes the insulating foil to form a bubble or `flyer' which is accelerated down the barrel. The flyer produced can reach velocities greater than 4 km s-1 in 200 ns, impacting the explosive with pressures up to 15 GPa and causing detonation [1]. In the 1980s EFIs were further studied and optimized. Thomson Brandt Armaments (TBA) [1] determined the flyer velocity using its flight time. Sandia National Laboratories (SNL) [3] developed a low inductance fireset (< 50 nH) for slapper detonators, which increased the efficiency. Los Alamos National Laboratories (LANL) [4] demonstrated that short pulses give the highest efficiency in converting electrical energy to kinetic energy (KE) of the flyer.

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New Trends in Research of Energetic Materials, Czech Republic, 2010

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This paper investigates the effect of barrel length, stripline design and bridge size on EFI velocity. Samples were mounted in a highly stable optical system and the EFI bubble velocities were measured using streak photography.

1.1 Bridge design

This paper investigated `infinite barrel' EFIs. These EFIs have barrel diameters two or more times greater than the bridge width, Fig. 1. In this infinite case the dielectric layer forms a bubble which impacts the explosive. `Finite barrel' designs have a barrel width close to the bridge width, which causes the film to shear off and form a discrete flyer. Meyers [4] determined that shorter length barrels are preferable for `infinite' barrels. Long length barrels lead to increased `bubbling', which reduces the effective spot size of the flyer for only a small gain in KE and risks fracture of the bubble. The shape of the power pulse input to the bridge is critical, as short pulses produce the highest efficiency in converting electrical energy to flyer kinetic energy (KE). The rate of current rise can be improved by minimising the circuit's inductance as to reduce the discharge time. Richardson [4] demonstrated that the stripline inductance and capacitance can be reduced by laminating the stripline copper films very closely together and reducing the stripline width. A variety of stripline types and thicknesses were investigated in this paper, Table 1. Stripline Type 1 had separate bridges and was much thicker than Type 2 and 3, with Type 3 being the thinnest.

Table 1: Bridgewire samples tested. Two EFIs were tested of each sample type.

Stripline type 1 1 1 1 2 2 2 3

Bridge width/µm 100 200 300 400 200 200 200 200

Barrel length/µm 155 155 155 155 100 360 380 180

Figure 1: Cross section of an `infinite barrel' EFI before and after application of a large current pulse. (Not to scale.)

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2

Experimental

To determine the EFI bubble velocities, high speed streak photography was used with a Hadland 675 camera [5]. Streak photography images an event through a thin slit in one direction and streaks this image across the film in the other direction, allowing temporal information to be captured, Fig. 2. Due to the high magnification and alignment accuracy required, a mount was custom made for the EFI. This consisted of a three-axis spatial mount, rotation mount and sample holder, Fig. 3. A 2 mm streak slit was manufactured to aid in alignment of the centre of the bridge with the optical axis of the camera and lens system, Fig. 4. The calibrated streak speed of (6.07±0.01) ns mm-1 provided a suitable streak window. Different stripline type, bridge size and barrel length EFIs were developed by QinetiQ at Fort Halstead to determine the optimum design parameters, Table 1. The streak photographs were scanned and converted into line images using MATLAB. These images were converted into displacement histories with the appropriate magnification and streak speed factors. By fitting a straight line to the required section, the velocity of the fastest point of the EFI bubble was calculated, Fig. 5.

Figure 2: Demonstration of how streak imaging works for a constant velocity object.

Figure 3: Experimental set up for imaging EFIs.

Figure 4: An example after shot overlaid with the streak slit position.

Figure 5: A linear fit to the displacement data of EFI01 gave a velocity of 4.44 km s-1.

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3

Results and discussion

The EFI velocity was calculated from a linear fit between 100 and 300 µm from the end of the barrel, with the streak slit aligned with the centre of the EFI bridge. Some variation in velocity was seen between bridges of the same quoted size, due to possible variations in the true dimensions of the bridges, and slight misalignment of the streak slit. Increasing the EFI barrel length was seen to decrease the velocity of the EFI bubble, Fig. 6. After firing the current drops causing the plasma to cool, the pressure to drop and the flyer velocity to decrease rapidly [1]. Therefore, the longer the barrel the slower the flyer exits. Type 1 EFIs underperformed when compared with Type 2 and 3, Fig. 6. This may be due to the lower inductance of the Type 2 and 3 striplines with their integrated bridges. The bridge burst of an EFI is very rapid and the resultant resistance peak has a width of only a few nanoseconds, during which time most of the useful work is done. Energy transferred during this short period will contribute towards expanding the plasma and accelerating the flyer. After the flyer has moved a few bridge widths away from rest, this bridge expansion rate rapidly decreases and any further power input no longer accelerates the flyer. Therefore, it is more efficient to have a high rate of rise of current prior to burst, which is achieved with low inductance striplines [4]. The effect of increased bridge size was to reduce the velocity, as seen in Fig. 7 for 150 µm barrel, Type 1 stripline EFIs. Increasing the bridge width increased the volume of copper to be converted into a plasma. As the energy delivered to the EFIs was kept constant, this reduced the speed at which the plasma expanded, resulting in slower flyers. The 400 µm bridges produced slower, dimmer and more delayed streak images, similar to that of the slower 300 µm sample. This indicates that the large difference in velocity between the two 300 µm samples may be due to variations in the actual bridge dimensions.

Figure 6: Velocity as a function of barrel length and stripline type (200 µm bridges).

Figure 7: Velocity as a function of bridge size (150 µm barrel length, Type 1 stripline).

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New Trends in Research of Energetic Materials, Czech Republic, 2010

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4

Conclusion

The optical system successfully photographed the high speed EFI bubbles with the 2 mm streak slit allowing accurate alignment of the EFI bridge with the optical axis of the system. From the analysis of the streak images the EFI velocities were obtained. EFIs with separate bridges produced slower flyers when compared to those with integrated bridges. This is believed to be due to the separate bridges increasing the inductance, leading to longer current bursts and slower plasma production. Increasing the barrel length reduced the EFI velocity, due to cooling of the propelling plasma. EFIs with larger bridge widths resulted in slower flyers. The increased copper volume with a fixed input energy slowed down the plasma production and led to slower flyers.

Acknowledgments

H.R.Davies would like to thank QinetiQ for her PhD funding. D. Townsend of BAE for the Hadland 675 streak camera. D. Johnson, D. Appleby and others from the Cavendish Laboratory workshop for equipment manufacture. This work was funded by the UK MOD through the UK-Energetics research programme.

References

[1] C. Riviere, The slapper detonator. principle. measurements. applications, Revue Technique, 24(1), p.125-141, 1992. [2] J.R. Stroud, A new kind of detonator-the slapper, UCRL-7739, Lawrence Livermore Laboratory report, 1976. [3] P.G. Grohmann, Divisional working paper 5/86 (XM3), RARDE report, 1986. [4] P.G. Grohmann, Divisional working paper 6/86 (XM3), RARDE report, 1986. [5] J.E. Field, High-speed photography: its history and application, Royal Institution Proceedings, 60, p.195­222, 1989.

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