Exploding-bridgewire detonator
Encyclopedia
Detonator
A detonator is a device used to trigger an explosive device. Detonators can be chemically, mechanically, or electrically initiated, the latter two being the most common....
used to initiate the detonation
Detonation
Detonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases...
reaction in explosive materials, similar to a blasting cap
Blasting cap
A blasting cap is a small sensitive primary explosive device generally used to detonate a larger, more powerful and less sensitive secondary explosive such as TNT, dynamite, or plastic explosive....
in that it is fired using an electric current. EBWs use a different physical mechanism than blasting caps, using more electricity delivered much more rapidly, and explode in a much more precise timing after the electric current is applied. This has led to their common use in nuclear weapons.
The slapper detonator
Slapper detonator
A slapper detonator, also called exploding foil initiator , is a relatively recent kind of a detonator developed in Lawrence Livermore National Laboratory...
is a more recent development along similar lines.
History
The EBW was invented by Luis AlvarezLuis Alvarez
Luis W. Alvarez was an American experimental physicist and inventor, who spent nearly all of his long professional career on the faculty of the University of California, Berkeley...
and Lawrence Johnston for the Fat Man
Fat Man
"Fat Man" is the codename for the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945. It was the second of the only two nuclear weapons to be used in warfare to date , and its detonation caused the third man-made nuclear explosion. The name also refers more...
-type bombs of the Manhattan Project
Manhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...
, during their work in Los Alamos National Laboratory
Los Alamos National Laboratory
Los Alamos National Laboratory is a United States Department of Energy national laboratory, managed and operated by Los Alamos National Security , located in Los Alamos, New Mexico...
. The Fat Man Model 1773 EBW detonators used an unusual, high reliability detonator systems with two EBW "horns" attached to a single booster charge, which then fired each of the 32 explosive lens units.
Description
EBWs 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 be ejected 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
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
— 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 vaporizes the wire in several microseconds. 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 quickly 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 electric 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 cap
Blasting cap
A blasting cap is a small sensitive primary explosive device generally used to detonate a larger, more powerful and less sensitive secondary explosive such as TNT, dynamite, or plastic explosive....
s 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 quickly the explosive is heated up, and minor electrical variations in the wire or leads will change how quickly 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.
Use in nuclear weapons
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 low tolerance applications such as nuclear weapon explosive lensExplosive lens
An explosive lens—as used, for example, in nuclear weapons—is a highly specialized explosive charge, a special type of a shaped charge. In general, it is a device composed of several explosive charges that are shaped in such a way as to change the shape of the detonation wave passing through it,...
es.
Due to their common use in nuclear weapons, 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. In the US, EBWs are on the US State Department Munitions Control List, and exports are highly regulated.
Civilian use
EBWs have found uses outside nuclear weapons, such as the Titan IVTitan IV
The Titan IV family of space boosters were used by the U.S. Air Force. They were launched from Cape Canaveral Air Force Station, Florida, and Vandenberg Air Force Base, California. At the time of its introduction, the Titan IV was the "largest unmanned space booster used by the Air Force."The...
, 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.
Mechanism of operation
The bridgewire is usually made of goldGold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
, but platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
or gold/platinum alloys can also be used. The most common commercial bridgewire diameter and length is 1.5 and 40 mils (0.038 mm and 1 mm), but lengths ranging from 10 mils to 100 mils (0.25 mm to 2.5 mm) can be encountered. From the available explosives, only PETN
PETN
Pentaerythritol tetranitrate , also known as PENT, PENTA, TEN, corpent, penthrite , is the nitrate ester of pentaerythritol. Penta refers to the five carbon atoms of the neopentane skeleton.PETN is most well known as an explosive...
at low densities can be initiated by sufficiently low shock to make its use practical in commercial systems as a part of the EBW initiator. It can be chained with another explosive booster
Explosive booster
An explosive booster acts as a bridge between a low energy explosive and a low sensitivity explosive such as TNT. It increases the explosive shockwave from an initiating explosive to the degree sufficient to detonate the secondary charge.Unlike C4 plastic explosive, not all explosives can be...
, often a pellet of tetryl
Tetryl
2,4,6-Trinitrophenylmethylnitramine commonly referred to as tetryl is a sensitive explosive compound used to make detonators and explosive booster charges....
, RDX
RDX
RDX, an initialism for Research Department Explosive, is an explosive nitroamine widely used in military and industrial applications. It was developed as an explosive which was more powerful than TNT, and it saw wide use in WWII. RDX is also known as cyclonite, hexogen , and T4...
or some PBX
Polymer-bonded explosive
A polymer-bonded explosive, also called PBX or plastic-bonded explosive, is an explosive material in which explosive powder is bound together in a matrix using small quantities of a synthetic polymer...
(e.g. PBX 9407). Detonators without such booster are called "initial pressing detonators" (IP detonators).
During initiation, the wire heats with the passing current until melting point is reached. The heating rate is high enough that the liquid metal has no time to flow away, and heats further until it vaporizes. During this phase the electrical resistance of the bridgewire assembly rises. Then an electric arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
forms in the metal vapor, leading to drop of electrical resistance and sharp growth of the current, quick further heating of the ionized metal vapor, and formation of a shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
. To achieve the melting and subsequent vaporizing of the wire in time sufficiently short to create a shock wave, current rise rate of at least 100 amperes per microsecond is required.
If a current is supplied in lower rate, the bridge may burn, maybe with deflagrating the PETN pellet, but will not cause detonation. PETN-containing EBWs are also relatively insensitive to a static electricity discharge. Their use is limited by the thermal stability range of PETN. (Cf. slapper detonators, which can use high density hexanitrostilbene
Hexanitrostilbene
Hexanitrostilbene , also called JD-X, is a heat resistant high explosive developed at the Naval Ordnance Laboratory in the 1960s. Other names include 1,1'-bis[2,4,6-trinitrobenzene]; 1,2-bis--ethylene; hexanitrodiphenylethylene.It is commercially produced by oxidizing trinitrotoluene with a...
, allowing their use in temperatures up to almost 300 °C and at both vacuum and at high pressures.)
Firing system
The EBW and the slapper detonator are the safest known types of detonators, as only a 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-inductanceInductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
, high-capacitance, high-voltage capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
(e.g. oil-filled, Mylar-foil, or ceramic) through a suitable switch (spark gap
Spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms,...
, thyratron
Thyratron
A thyratron is a type of gas filled tube used as a high energy electrical switch and controlled rectifier. Triode, tetrode and pentode variations of the thyratron have been manufactured in the past, though most are of the triode design...
, krytron
Krytron
The krytron is a cold-cathode gas filled tube intended for use as a very high-speed switch, and was one of the earliest developments of the EG&G Corporation. It is somewhat similar to thyratron...
, 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
Marx generator
A Marx generator is an electrical circuit first described by Erwin Otto Marx in 1924. Its purpose is to generate a high-voltage pulse. Marx generators are often used to simulate the effects of lightning on power line gear and aviation equipment....
. Low-impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
capacitors and low-impedance coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
s are required to achieve the necessary current rise rate.
A possible alternative for bulky (*see note below) capacitors is the flux compression generator. When fired, it creates a strong electromagnetic pulse
Electromagnetic pulse
An electromagnetic pulse is a burst of electromagnetic radiation. The abrupt pulse of electromagnetic radiation usually results from certain types of high energy explosions, especially a nuclear explosion, or from a suddenly fluctuating magnetic field...
, 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
Boosted fission weapon
A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. The neutrons released by the fusion reactions add to the neutrons released in the fission, as well as inducing the fission reactions...
source of fission neutron
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
s.
The energy in such a capacitor would be 0.5xCxV2, which for the above mentioned cap is 12.5J. The capacitor would be small even if a low energy density one was used (soda can size).
By comparison, a defibrillator delivers ~200J from 2Kv and perhaps 20 uF. The flash-strobe in a disposable camera is typically 3J from a 300V capacitor of 100uF.
See also
- Nuclear weapon design - explosive lens
- Triggering sequenceTriggering sequenceA triggering sequence, also called an explosive train, is a sequence of events that culminates in the detonation of explosives. For safety reasons, most widely used high explosives are difficult to detonate. A primary explosive of higher sensitivity is used to trigger a uniform and predictable...