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CP1195, Shock Compression of Condensed Matter - 2009, edited by M. L. Elert, W. T. Buttler, M. D. Furnish, W. W. Anderson, and W. G. Proud © 2009 American Institute of Physics 978-0-7354-0732-9/09/$25.00

CHARACTERISATION CREDIT LINEEXPLODING FOIL INITIATORPP. 65-68, 83-86, OF AN (BELOW) TO BE INSERTED ON ONLY THE PAPERS ON (EFI) 95-98, 173-176, 185-189, 201-204, 209-212, 233-236, 237-240, 279-282, 297-300, 327-330, SYSTEM 349-352, 369-372, 400-403, 537-540, 541-544, 635-638, 639-642, 651-654, 703-706, 707-710,

H.R. Davies ,

ATTACHMENT II

D.J. Chapman , T.A. Vine and W.G. Proud

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Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge, CB3 0HE, UK CP1195, Condensed Matter QinetiQ Ltd, Fort Halstead, Sevenoaks,Shock CompressionButtler, M.UK - W. W. Anderson, and W. G. Proud Kent, TN14of7BP, D. Furnish, 2009, edited by M. L. Elert, W. T.

2009 American Institute of Physics 978-0-7354-0732-9/09/$25.00

Abstract. Exploding Foil Initiators (EFIs) provide a safe and reliable means of initiating explosives. They 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, eliminating more sensitive primary explosives. When a high current is passed through a polymer-encapsulated metal bridge, a contained plasma is formed. This causes the film to expand rapidly forming a bubble or shear to form a flyer. These flyers can then impact the secondary explosive. Due to the very high speed at which these systems operate, a streak photography system was used to characterise the behaviour of the polymer film flyers. This paper will report the preliminary findings on the mechanical, electrical and velocity changes seen in some proprietary systems. Keywords: Exploding Foil Initiator, EFI, Slapper Detonator PACS: 52.80.Qj, 07.68.+m

INTRODUCTION The traditional way to initiate explosives is to use an explosive train with an electrically activated detonator. The shockwave produced by the detonator is reinforced by detonation relays to increase the shockwave pressure. 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 less sensitive secondary explosives and are operated by a very specific electrical pulse. This makes the system insensitive to accidental initiation by static electricity discharge etc. and allows the detonator to be used without the need for a physical barrier [1]. EFIs were invented by John Stroud of Lawrence Livermore National Laboratory (LLNL) in 1965 [2]. The common construction of the EFI consists of an explosive booster pellet pushed against a thin barrel. Below this barrel is a insulating film, such as Kapton

polyimide, covering a thin metal strip with a `bridge' at the centre, Fig. 1. When a high current pulse is passed through the bridge it is vaporised. The metal plasma is confined by the conducting film base and expands into the barrel, causing the insulating foil to form a bubble or `flyer' which is accelerated down the barrel. The resulting `flyer' 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]. Considerable investigation into EFIs was undertaken in the 1980s. Thomson Brandt Armaments (TBA) [1] determined the flyer velocity using the flight time of the projectile and designed a slapper detonator capable of initiating military anti-aircraft or anti-tank charges. Sandia National Laboratories (SNL) [3] developed a low inductance fireset for slapper detonators, < 50 nH, which increased the efficiency. Los Alamos National Laboratories (LANL) [4] determined that the shape of the power pulse input to the bridge is very important as short pulses give sharp thresholds and the highest efficiency in converting electrical energy to kinetic energy (KE)

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

TABLE 1. Bridgewire samples tested. Stripline Type 1 have separate bridges and are much thicker than Type 2 and Type 3, with Type 3 being the thinnest. Two EFIs were tested of each sample type. Stripline 1 1 1 1 2 2 2 3 Bridge size/µm 100 200 300 400 200 200 200 200 Barrel length/µm 155 155 155 155 100 360 380 180

BRIDGE DESIGNS If the barrel diameter is equal to the bridge length, then it is defined as a `finite barrel' design. If the barrel is two or more times greater than the bridge length, it is an `infinite barrel' design, such as those studied in this paper, Fig. 1. In the `infinite' case the dielectric layer forms a bubble which impacts the explosive. For the `finite' design a disc of dielectric film is sheared off, which then accelerates down the barrel and initiates the explosive. Meyers [4] determined that shorter length barrels are preferable for `infinite' barrels, as long barrels allow the flyer to `bubble' with the centre impacting the explosive first. This reduces the effective spot size of the flyer for only a small gain in KE. The shape of the power pulse input to the bridge is critical, as short pulses give sharp thresholds and the highest efficiency in converting electrical energy to flyer KE. The rate of current rise can be improved by minimising the circuit's inductance as this reduces the discharge time. Richardson [4] showed that the stripline inductance and capacitance could be reduced by laminating copper films very close together and reducing the stripline width. A variety of stripline types and thicknesses will be investigated in this paper, Table 1.

METHODS AND RESULTS The EFI bubble velocities were measured using high speed streak photography [5] with a Hadlands 675 camera. Due to the high magnification and alignment accuracy required a three-axis spatial mount, rotation mount and sample holder were combined to hold the EFI in place and align the centre of the bridge with the streak slit. A K2 microscope lens provided 10.5 times magnification. The calibrated streak speed of (6.07 ± 0.01) ns mm-1 was determined to give accurate velocity data and provide a large enough streak window to allow for any `jitter' in the firing system. Different stripline, bridge size and barrel length samples were developed by QinetiQ at Fort Halstead to determine the optimum design parameters, Table 1. To achieve accurate velocity information the fastest point of the bubble must be imaged. 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. The streak photographs were scanned and converted into line images within MATLAB. The appropriate magnification and streak speed factors were used to convert these line images into displacement histories. By fitting a straight line to the required section, the velocity of the fastest point of the EFI bubble was calculated. An example of such a fit is seen in Fig. 2. The custom-made streak slit allowed easier alignment of the centre of the bubble with the optical system, as shown by the `after shot' in Fig. 3.

FIGURE 1. Cross section of a wide or `infinite barrel' design EFI before and after application of the current pulse. As the barrel is much wider than the bridge the film forms a bubble which then impacts the explosive. (Not to scale.)

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FIGURE 2. A linear fit to the displacement data between 100 µm and 300 µm from the end of the 100 µm barrel of EFI01. From this fit the velocity was determined to be 4.44 km s-1 .

FIGURE 3. EFI14 after shot: The bubble formed by EFI14 can be seen in this after shot, and was successfully captured by the 2 mm streak slit.

few nanoseconds, during which most of the useful work is done. Energy transferred during this short period will help expand the plasma and accelerate the flyer plate. After the flyer has moved a few bridge widths away from rest, this bridge expansion rate will rapidly decrease and any further power input will no longer accelerate the flyer. Therefore it is much more efficient to have a high rate of rise of current prior to burst, which is achieved with low inductance circuits. Type 2 and 3 striplines have integrated bridges which reduced their inductance and improved their performance compared with Type 1, Fig. 4. The thinner stripline in the Type 3 samples would be expected to produce higher velocity flyers than Type 2, however there was no evidence of this from the samples tested here. The effect of bridge size on EFI velocity is less clear, Fig. 5. The 155 µm barrel length samples indicate a reduction in velocity with bridge size. This could be due to increased lateral expansion of the bubble or greater flyer breakup with larger plasma volumes. The 400 µm bridges produced slower, dimmer and 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 a fundamental difference in behaviour. There may be some variation in the dimensions of these two bridgewires.

DISCUSSION Increasing the barrel length was seen to decrease the velocity of the EFI bubble, Fig. 4. This may be due to increased lateral expansion of the bubble due to the longer time to impact. The highest velocities were achieved for the 100 µm barrel length. Type 1 striplines have removable bridges and they underperformed compared with Type 2 and Type 3. This may have been due to the increased inductance of the Type 1 striplines [4]. If the inductance is too high, the striplines will store significant energy which will take a relatively long time to discharge. Longer discharge times reduce the efficiency of the energy transfer from the plasma to the flyer and result in a lower velocity. The bridge burst is very rapid and the resultant resistance peak has a width of only a

FIGURE 4. Velocity as a function of barrel length and stripline type: Type 1 striplines have removable bridges and were thicker than types 2 and 3. The Type 1 striplines underperformed, which may be due to their higher inductance [4]. For the Type 2 striplines the longer the barrel the slower the flyer, which could be due to increased lateral expansion of the bubble.

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4. Grohmann, P. G., Divisional working paper 6/86 (XM3), Royal armament research & development establishment (RARDE) internal report (1986). 5. Field, J. E., Proc. Roy. Inst. Lon., 60, 195­222 (1989).

FIGURE 5. Velocity as a function of bridge size: No firm relationship can be inferred between the bridge size and velocity from the current data set. The 155 µm barrel data imply a decrease in flyer velocity as the bridge size is increased, possibly due to increased lateral expansion of bubbles with larger bridge sizes.

CONCLUSIONS The optical system successfully photographed the high speed EFI bubbles. The 2 mm streak slit allowed accurate alignment of the sample with the optical axis of the system, and the resulting images were analysed successfully to obtain the EFI velocities. EFIs with separate bridges under perform when compared to those with integrated bridges, probably due to increased inductance. Increasing the barrel length reduces the EFI velocity. The effect of bridge size is less clear, but further studies should provide a better relationship between velocity and bridge size.

ACKNOWLEDGMENTS The authors would like to thank QinetiQ for funding and the workshop staff of the Cavendish for their invaluable help.

REFERENCES

1. Riviere, C., Revue Technique, 24, 125­141 (1992). 2. Stroud, J. R., UCRL-7739 Lawrence Livermore Laboratory (1976). 3. Grohmann, P. G., Divisional working paper 5/86 (XM3), Royal armament research & development establishment (RARDE) internal report (1986).

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