slapper detonator

Detonating high explosives involves numerous requirements for effectiveness and safety. At times, the characteristics of the explosives used will involve placing even higher requirements on the detonation techniques used. For example, it is desirable for high explosives used in both industrial and military applications to be as stable as possible prior to their intended detonation. This means that in order to trigger said detonation, an extremely large amount of energy (ideally) will need to be applied to them extremely quickly.

In the early days of high explosive, it was traditional to use low explosives as detonators for high explosive. That way the detonating explosives could be kept separate from the primary until the arming point, and then themselves ignited using simple methods involving shock or flame. As HE became more sophisticated, however, simple low explosives such as black powder began to fall short of the energy levels required to detonate them.

The next step was to trade stability for safety; detonators would contain high explosives which were simply less stable - and therefore less safe - than primary explosives, allowing the detonators to (again) remain separate until arming. Blasting caps were vulnerable to both shock and flame, but their detonation was capable of triggering the otherwise stable trinitrotoluene and variants which served as a basic high explosive of the mid-nineteenth to mid-twentieth century. TNT itself was famously stable, in that it could be hit with hammers or burned in fire without detonating - but if hit with the energy release of a blasting cap, it would detonate.

Nuclear weapons changed things again. In order to detonate an implosive atomic device, it is not only necessary to detonate a high explosive which has been been maintained stably - it is necessary to do so with incredibly precise timing, as it depends on the simultaneous detonation of explosive around the device. Blasting caps and their ilk were simply too variable in their timing and energy output to rely on. Furthermore, the chemical components of blasting caps were themselves unstable and became more so over time. Chemical fusing was difficult for weapons which had to not only detonate in highly precise sequences but would need to remain 'on the shelf' for long periods of time without regular maintenance.

The original solution to this problem was the exploding-bridgewire detonator, which substituted electrical energy for the chemical energy of a blasting cap or other unstable fusing substance. An EBW detonator could be attached directly to a block of stable high explosive, where it would patiently wait until an extremely high-amperage burst of electricity came across it - high enough that it was vanishingly unlikely to happen by accident. The energy of this pulse would be converted to plasma, and the expansion of that plasma would serve as a small explosion - replacing that of a blasting cap - and detonate the high explosive it was attached to.

There were problems with EBW detonators, however. The EBW detonator itself needed to be placed against the explosive in question, which meant that the energy release into the explosive block occurred in all directions from that point of contact outwards; it was not possible to 'shape' a bridgewire burst as it was possible to shape a chemical blasting or fusing charge into a focused point on the primary explosive block. As a result, much energy was wasted.

There were also problems with durability. With the EBW laid against the primary block, although no inadvertent detonation could occur, there were whole classes of chemical reactions between the high explosive and the necessarily unshielded bridgewire which could (and did) result in corrosion of the bridgewire as well as deformation of the explosive directly in contact with the EBW. While miniscule in absolute terms, these deformations and corrosions are enough to significantly throw off the calculations of energy transferred from the EBW to the primary both in terms of amount and rate. When timing detonations for a physics package, this can be a severe problem.

Enter (finally) the slapper detonator. Devised at Lawrence Livermore National Laboratory, the slapper detonator is a modification of the EBW detonator. A disc with a hole drilled through it, made of a neutral materal, is placed against the primary explosive. A thin film is placed over the hole, with the side facing the explosive coated with a metal such as gold or copper, and the bridgewire of the detonater is placed directly behind the film just opposite the hole.

When the bridgewire is vaporized by electric current, the film and its metal coating are converted to plasma by the plasma burst of the bridgewire. The hole in the disc, a cylinder for our purposes, guides this wave directly against the primary, so that rather than a single point of energy transfer a focused beam of plasma strikes (slaps) a circular section of explosive beneath. This is compressed in a conical shape into the primary explosive block, and more of the resultant energy is captured into the compression (since the initial impulse is directly inwards across a larger face). This results in a more efficient energy transfer, and more reliable, faster detonation. As an additional plus, the metallic components of the detonator (the film and bridgewire) are kept out of contact with the primary explosive, limiting corrosive and other chemical interactions.