View source for Exploding-bridgewire detonator

{{Short description|Detonator fired by electric current}}
[[Image:Exploding bridgewire detonator.png|right|thumb|300px|Image from the exploding-bridgewire detonator patent. Fig. 2 is a detail of Fig. 1.{{ordered list
| Housing
| High explosive
| Fuse wire
| Lead-in wire
| Lead-in wire
| Insulating support
| Cambrick tubing
| Dividing portion of the support
| (Nothing labeled)
| Condenser (capacitor)
| Switch
| Battery
}}]]

The '''exploding-bridgewire detonator''' ('''EBW''', also known as '''exploding wire detonator''') is a type of [[detonator]] used to initiate the [[detonation]] reaction in [[explosives|explosive materials]], similar to a [[blasting cap]] because it is fired using an electric current. EBWs use a different physical mechanism than blasting caps, using more electricity delivered much more rapidly. They explode with more precise timing after the electric current is applied by the process of [[exploding wire method|exploding wire]].  The precise timing of exploding wire detonators compared with other types of detonators has led to their common use in [[nuclear weapons]].<ref name="Cooper">
{{cite book
 |last=Cooper
 |first=Paul W.
 |title=Explosives Engineering
 |year=1996
 |publisher=Wiley-VCH
 |pages=353–367
 |chapter=Exploding bridgewire detonators
 |isbn=0-471-18636-8
}}</ref>

The [[slapper detonator]] is a more recent development along similar lines.

== History ==
The EBW was invented by [[Luis Walter Alvarez|Luis Alvarez]] and [[Lawrence H. Johnston|Lawrence Johnston]] for the [[Fat Man]]–type bombs of the [[Manhattan Project]], during their work in [[Los Alamos National Laboratory]].  The Fat Man Model 1773 EBW detonators used an unusual, high reliability detonator system with two EBW "horns" attached to a single booster charge, which then fired each of the 32 explosive lens units.<ref name="Coster-Mullen">
{{cite book
 |last=Coster-Mullen
 |first=John

 |author-link=John Coster-Mullen 
 |title=Atom Bombs: The Top Secret Inside Story of Little Boy and Fat Man
 |year=2002
 |publisher=John Coster-Mullen
 |chapter=Chapter 5: Fat Man
 |pages=59–66, 218–220
 |oclc=51283880
 |id=ASIN B0006S2AJ0
}}</ref><ref name="RISI-History">{{cite web |title= RISI Industries Technical Topics 05-93 History |url=http://www.teledynerisi.com/1techtopics/pdf/0593.pdf |publisher=RISI Industries |date=May 1993 |archiveurl=https://web.archive.org/web/20111006013132/http://www.teledynerisi.com/1techtopics/pdf/0593.pdf |archivedate=October 6, 2011 |accessdate=July 14, 2017}}
</ref><ref name=la>{{cite book |last1=Alvarez |first1=Luis |title=Alvarez: Adventures of a Physicist |date=1987 |publisher=Basic Books, Inc. |location=New York |isbn=0465001157 |pages=131-136}}</ref>

== Description ==

EBWs were developed as a means of detonating multiple explosive charges simultaneously, mainly for use in [[Nuclear weapon design#Implosion-type weapon|plutonium-based nuclear weapons]] in which a plutonium core (called a ''[[plutonium pit|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.<ref name=la/>

An EBW has two main parts: a piece of fine wire which contacts the explosive, and a [[high-voltage]] high-current low-impedance [[electricity]] source; it must reliably and consistently supply a rapid starting pulse. When the wire is connected across this voltage, the resulting high [[current (electricity)|current]] melts and then vaporizes the wire in a few microseconds. The resulting shock and heat initiate the [[high explosive]].<ref name="Cooper"/>

{{gallery
|Image:HD.4G.053 (10540204545).jpg|Trinity Gadget
|File:Trinity-Gadget-Closeup-Detonators.jpg|Closeup of a detonator set.  The EBW is the Y-shaped device with two wires coming in at angles along the surface.  The larger round objects with two wires coming out perpendicular to the surface are diagnostic equipment.
}}

This accounts for the heavy cables seen in photos of the [[Trinity site|Trinity]] "[[The gadget|Gadget]]"; high voltage cable requires good insulation and 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-controlled 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]]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 1,000 to 10,000 times longer and less precise than the EBW electrical vaporization.

[[Image:Exploding-bridgewire detonators in case.png|frame|right|Modern exploding-bridgewire detonators arranged in a tray.]]

=== Use in nuclear weapons ===

High explosives such as [[RDX]] and [[Primacord]] have a [[detonation velocity]] of 8 [[millimeters]] per [[microsecond]]. A one microsecond delay in detonation from one side of a nuclear weapon to the other would mean a travel distance of four millimeters. The [[Fat Man]] development eventually brought the timing spread for hundreds of detonations down to a few billionths of a second.<ref name=la/>

In the US, due to their use in nuclear weapons, these devices are subject to nuclear control authorities, according to the Guidelines for the Export of Nuclear Material, Equipment and Technology. EBWs are on the [[United States Munitions List]], and exports are highly regulated.<ref name="RISI-Export">{{cite web |url=http://www.teledynerisi.com/1techtopics/pdf/1193.pdf |title=RISI Technical Topics 11-92 ATF Licenses
|archive-url=https://web.archive.org/web/20070226071620/http://www.teledynerisi.com/1techtopics/pdf/1193.pdf |archive-date=26 February 2007 |url-status=dead}}</ref>

=== Civilian use ===

EBWs have found uses outside nuclear weapons, such as the [[Titan IV]] missile,<ref>{{Cite web|url=http://neyersoftware.com/Papers/AIAA96/HVD.htm|title = AIAA 96-2874 Development and Qualification Testing of the High Voltage Detonator}}</ref> 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.<ref name="RISI-Models">{{cite web |url=http://www.teledynerisi.com/products/0products_1ebw_page10.asp |title=Teledyne RISI - Selecting the Right EBW Detonator
|archive-url=https://web.archive.org/web/20090106140347/http://www.teledynerisi.com/products/0products_1ebw_page10.asp |archive-date=6 January 2009}}</ref> EBW detonators are much safer than regular electric detonators because, unlike regular detonators, EBWs do not have primary explosives. Primary explosives such as [[lead azide]] are very sensitive to static electricity, radio frequency, shock, etc.

== Mechanism of operation ==

The bridgewire is usually made of [[gold]], but [[platinum]] or gold/platinum alloys can also be used. The most common commercial wire size is 0.038&nbsp;mm (1.5 [[thousandth of an inch|mils]]) in diameter and 1&nbsp;mm (40 mils) in length, but lengths ranging from 0.25&nbsp;mm to 2.5&nbsp;mm (10 mils to 100 mils) can be encountered. From the available explosives, only [[PETN]] 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]], often a pellet of [[tetryl]], [[RDX]] or some [[polymer-bonded explosive|PBX]] (e.g., PBX 9407). Detonators without such booster are called ''initial pressing detonators'' (IP detonators).<!-- Paul Cooper: Introduction to the Technology of Explosives, chapter 4.5 -->

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]] 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]]. To achieve the melting and subsequent vaporizing of the wire in time sufficiently short to create a shock wave, a current rise rate of at least 100 amperes per microsecond is required.

If the current rise rate is lower, the bridge may burn, perhaps causing deflagration of the PETN pellet, but it 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.  [[Slapper detonator]]s, which can use high density [[hexanitrostilbene]], may used in temperatures up to almost {{Convert|300|C|F}} in environments ranging from vacuum to high pressures.<!-- Paul Cooper: Introduction to the Technology of Explosives, chapter 4.5 --><ref name="Cooper-Intro">
{{cite book
 |last=Cooper
 |first=Paul W.
 |title=Introduction to the Technology of Explosives
 |year=1996
 |publisher=Wiley-VCH
 |chapter=Chapter 4.5 Exploding bridgewire detonators
 |isbn=978-0471186359
}}</ref>

==Firing system==
<!-- See the NWFAQ link below, raw data taken mostly from there. -->
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-[[inductance]], high-capacitance, high-voltage [[capacitor]] (e.g., oil-filled, Mylar-foil, or ceramic) through a suitable switch ([[spark gap]], [[thyratron]], [[krytron]], etc.) into the bridge wire. A very rough approximation for the capacitor is  a rating of 5 kilovolts and 1 microfarad, and the peak current ranges between 500 and 1000 amperes.<ref name=Cooper /> The high voltage may be generated using a [[Marx generator]]. Low-[[electrical impedance|impedance]] capacitors and low-impedance [[coaxial cable]]s are required to achieve the necessary current rise rate.

The [[flux compression generator]] is one alternative to capacitors. 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. A low energy density capacitor equivalent to a compression generator would be roughly the size of a soda can. The energy in such a capacitor would be {{frac|1|2}}·C·V{{sup|2}}, which for the above-mentioned capacitor is 12.5&nbsp;J. (By comparison, a defibrillator delivers ~200&nbsp;J from 2&nbsp;kV and perhaps 20&nbsp;μF.<ref>{{Cite web
|url=http://class.phys.psu.edu/251Recitations/02_Electric_Potential/Capacitance/Capacitance-Defibrillator.pdf |title=Capacitance and Currents — Defibrillator |access-date=2011-05-17 |archive-url=https://web.archive.org/web/20110813164037/http://class.phys.psu.edu/251Recitations/02_Electric_Potential/Capacitance/Capacitance-Defibrillator.pdf |archive-date=2011-08-13 |url-status=dead}}</ref> The flash-strobe in a disposable camera is typically 3&nbsp;J from a 300&nbsp;V capacitor of 100&nbsp;μF.)

In a fission bomb, the same or similar circuit is used for powering the [[neutron generator]], the initial source of [[nuclear fission|fission neutron]]s.

== See also ==
*[[Nuclear weapon design#Implosion-type weapon|Nuclear weapon design – Implosion-type weapon]]
*[[Triggering sequence]]
*[[Slapper detonator]] (A further development of the exploding-bridgewire detonator)
*[[Electrothermal-chemical technology]]

== References ==
{{reflist}}

== External links ==
*[http://nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html Elements of Fission Weapon Design, section 4.1.6.2.2.6]

{{DEFAULTSORT:Exploding-Bridgewire Detonator}}
[[Category:Detonators]]
[[Category:Nuclear weapon design]]
[[Category:Los Alamos National Laboratory]]