Exploding-bridgewire detonator
From Wikipedia, the free encyclopedia
The exploding-bridgewire detonator (EBW, also known as exploding wire detonator) was invented by Luis Alvarez and Lawrence Johnson for the Fat Man-type bombs of the Manhattan Project, during their work in Los Alamos National Laboratory. The slapper detonator is a more recent development along similar lines. These devices are subject to the Nuclear Control Authorities in every state, according to the Guidelines for the Export of Nuclear Material, Equipment and Technology. Not to be confused with detonating cord or blasting cap.
They were developed as a means of detonating multiple explosive charges simultaneously, mainly for use in plutonium-based nuclear weapons in which a plutonium core (called a "pit") is compressed very rapidly. This is achieved via conventional explosives placed uniformly around the pit. The implosion must be highly symmetrical or the plutonium would simply squirt out at the low-pressure points. Consequently, the detonators must have very precise timing.
An EBW has two main parts; a piece of fine wire which contacts the explosive, and a "strong" source of high-voltage electricity — strong, in that it holds up under sudden heavy load. When the wire is connected across this voltage, the resulting high current melts and then vapourises the wire in several nanoseconds. The resulting shock and heat initiate the high explosive.
This accounts for the heavy cables seen in photos of the Trinity "Gadget"; they had to deliver a large current with little voltage drop, lest the EBW not achieve the phase transition fast enough.
The precise timing of EBWs is achieved by the detonator using direct physical effects of the vaporized bridgewire to initiate detonation in the detonator's booster charge. Given a sufficiently high and well known amount of electrical current and voltage, the timing of the bridgewire vaporization is both extremely short (a few microseconds) and extremely precise and predictable (standard deviation of time to detonate as low as a few tens of nanoseconds).
Conventional blasting caps use electricity to heat a bridge wire rather than vaporize it, and that heating then causes the primary explosive to detonate. Imprecise contact between the bridgewire and the primary explosive changes how fast the explosive is heated up, and minor electrical variations in the wire or leads will change how fast it heats up as well. The heating process typically takes milliseconds to tens of milliseconds to complete and initiate detonation in the primary explosive. This is roughly one to ten thousand times longer and less precise than the EBW electrical vaporization.
Since explosives detonate at typically 7-8 kilometers per second, or 7-8 meters per millisecond, a one millisecond delay in detonation from one side of a nuclear weapon to the other would be longer than the time the detonation would take to cross the weapon. The time precision and consistency of EBWs (0.1 microsecond or less) are roughly enough time for the detonation to move 1 millimeter at most, and for the most precise commercial EBWs this is 0.025 microsecond and about 0.2 mm variation in the detonation wave. This is sufficiently precise for very high tolerance applications such as nuclear weapon explosive lenses.
EBWs have found uses outside nuclear weapons, such as the Titan IV [1], safety conscious applications where stray electrical currents might detonate normal blasting caps, and applications requiring very precise timing for multiple point commercial blasting in mines or quarries.
[edit] Firing system
The EBW and the slapper detonator are the safest known types of detonators, as only very high-current fast-rise pulse can successfully trigger them. However, they require a bulky power source for the current surges required. The extremely short rise times are usually achieved by discharging a low-inductance, high-capacity, high-voltage capacitor (eg. oil-filled, Mylar-foil, or ceramic) through a suitable switch (spark gap, thyratron, krytron, etc.) into the bridge wire. The ballpark figures are 5 kilovolt and 1 microfarad rating for the capacitor, and the peak current required ranges between 500 and 1000 amperes. The wire used in the bridge tends to be highly pure gold or platinum, 0.02-0.05 mm in diameter, and 1 mm long. The high voltage may be generated using a Marx generator.
A possible alternative for bulky capacitors is the flux compression generator. When fired, it creates a strong electromagnetic pulse, which is inductively coupled into one or more secondary coils connected to the bridge wires or slapper foils.
In a fission bomb the same or similar circuit is used for powering the neutron trigger, the additional booster source of fission neutrons.
