View source for Coilgun

{{Short description|Artillery using coils to electromagnetically propel a projectile}}
{{distinguish|Gauss gun|railgun}}

[[File:Coilgun animation.gif|thumb|Simplified diagram of a multistage coilgun with three coils, a barrel, and a [[ferromagnetic]] [[projectile]]]]

A '''coilgun''' is a type of [[mass driver]] consisting of one or more coils used as [[electromagnet]]s in the configuration of a [[linear motor]] that [[accelerate]] a [[ferromagnetic]] or conducting [[projectile]] to high velocity.<ref name="AutoAU-1"/>  In almost all coilgun configurations, the coils and the [[gun barrel]] are arranged on a common axis.  A coilgun is not a [[rifle]] as the barrel is [[smoothbore]] (not [[rifling|rifled]]).

Coilguns generally consist of one or more coils arranged along a barrel, so the path of the accelerating projectile lies along the central axis of the coils. The coils are switched on and off in a precisely timed sequence, causing the projectile to be accelerated quickly along the barrel via magnetic forces.

Coilguns are distinct from [[railgun]]s, as the direction of acceleration in a railgun is at right angles to the central axis of the current loop formed by the conducting rails. In addition, railguns usually require the use of sliding contacts to pass a large current through the projectile or [[Sabot (firearms)|sabot]], but coilguns do not necessarily require sliding contacts.<ref name="AutoAU-2"/> While some simple coilgun concepts can use ferromagnetic projectiles or even permanent magnet projectiles, most designs for high velocities actually incorporate a coupled coil as part of the projectile.

Coilguns are also distinct from [[Gauss gun]]s, although many works of science fiction have erroneously confused the two. A coil gun uses electromagnetic acceleration whereas Gauss guns predate the idea of coil guns and instead consists of ferromagnets using a configuration similar to a [[Newton's Cradle]] to impart acceleration.<ref>{{cite journal|url=https://aapt.scitation.org/doi/10.1119/1.4979653|title=Magnetic cannon: The physics of the Gauss rifle|last=Chemin, Besserve, Caussarieu, Taberlet, Plihon|first=Arsène, Pauline, Aude, Nicolas, Nicolas|date=1904|journal=American Journal of Physics|volume=85 |issue=7 |pages=495–502 |doi=10.1119/1.4979653 |access-date=2023-03-09}}</ref>

== History ==
The oldest electromagnetic gun came in the form of the coilgun, the first of which was invented by Norwegian scientist [[Kristian Birkeland]] at the [[University of Oslo|University of Kristiania]] (today Oslo). The invention was officially patented in 1904, although its development reportedly started as early as 1845. According to his accounts, Birkeland accelerated a 500-[[gram]] projectile to approximately {{convert|50|m/s|ft/s}}.<ref>archive.org: [https://archive.org/stream/PopularMechanics1933/Popular_Mechanics_06_1933#page/n31/mode/2up Popular Mechanics 06 1933 page 819]</ref><ref>{{cite web|url=https://patents.google.com/patent/US754637A/en|title=US Patent 754,637 "Electromagnetic Gun"|last=Birkeland|first=Kristian|date=1904|website=Google Patents|access-date=2019-02-10|archive-date=2018-11-02|archive-url=https://web.archive.org/web/20181102045841/https://patents.google.com/patent/US754637A/en|url-status=live}}</ref><ref>{{Cite journal|last1=Damse|first1=R.S.|last2=Singh|first2=Amarjit|s2cid=34169057|date=October 2003|title=Advanced Concepts of the Propulsion System for the Futuristic Gun Ammunition|journal= Defence Science Journal|volume=53|issue=4|pages=341–350|doi=10.14429/dsj.53.2279}}</ref>

In 1933, Texan inventor Virgil Rigsby developed a stationary coil gun that was designed to be used similarly to a [[machine gun]]. It was powered by a large electrical motor and generator.<ref name="googlepatents"/> It appeared in many contemporary science publications, but never piqued the interest of any armed forces.<ref name="AutoAU-3"/>

== Construction ==
{{Unreferenced section|date=September 2017}}

There are two main types or setups of a coilgun: single-stage and multistage. A single-stage coilgun uses one electromagnetic coil to propel a projectile. A multistage coilgun uses several electromagnetic coils in succession to progressively increase the speed of the projectile.

=== Ferromagnetic projectiles ===
[[File:Single stage coilgun.jpg|thumb|A single stage coilgun]]

For ferromagnetic projectiles, a single-stage coilgun can be formed by a coil of wire, an [[electromagnet]], with a [[ferromagnetic]] projectile placed at one of its ends. This type of coilgun is formed like the [[solenoid]] used in an electromechanical relay, i.e. a current-carrying coil which will draw a ferromagnetic object through its center. A large [[electrical current|current]] is pulsed through the coil of wire and a strong [[magnetic field]] forms, pulling the projectile to the center of the coil. When the projectile nears this point the electromagnet must be switched off, to prevent the projectile from becoming arrested at the center of the electromagnet.

In a multistage design, further electromagnets are then used to repeat this process, progressively accelerating the projectile. In common coilgun designs, the "barrel" of the gun is made up of a track that the projectile rides on, with the driver into the magnetic coils around the track. Power is supplied to the electromagnet from some sort of fast discharge storage device, typically a [[battery (electricity)|battery]], or [[capacitor]]s (one per electromagnet), designed for fast energy discharge. A [[diode]] is used to protect polarity sensitive components (such as semiconductors or electrolytic capacitors) from damage due to inverse polarity of the voltage after turning off the coil.

Many hobbyists use low-cost rudimentary designs to experiment with coilguns, for example using [[photoflash capacitor]]s from a [[disposable camera]], or a capacitor from a standard [[cathode-ray tube]] television as the energy source, and a low [[inductance]] coil to propel the projectile forward.<ref name="AutoAU-4"/><ref name="AutoAU-5"/>

=== Non-ferromagnetic projectiles ===

Some designs have non-ferromagnetic projectiles, of materials such as [[aluminium]] or [[copper]], with the armature of the projectile acting as an electromagnet with internal current induced by pulses of the acceleration coils.<ref name="AutoAU-6"/><ref name="AutoAU-7"/> A [[superconducting]] coilgun called a ''quench gun'' could be created by successively [[Superconducting magnet#Magnet quench|quench]]ing a line of adjacent coaxial superconducting coils forming a gun barrel, generating a wave of magnetic field gradient traveling at any desired speed. A traveling superconducting coil might be made to ride this wave like a [[surfboard]]. The device would be a mass driver or linear synchronous motor with the propulsion energy stored directly in the drive coils.<ref name="Practical Coilgun Design"/> Another method would have non-superconducting acceleration coils and propulsion energy stored outside them but a projectile with superconducting magnets.<ref name="StarTram"/>

Though the cost of power switching and other factors can limit projectile energy, a notable benefit of some coilgun designs over simpler [[railgun]]s is avoiding an intrinsic velocity limit from hypervelocity physical contact and erosion. By having the projectile pulled towards or levitated within the center of the coils as it is accelerated, no physical friction with the walls of the bore occurs. If the bore is a total vacuum (such as a tube with a [[plasma window]]), there is no friction at all, which helps prolong the period of reusability.<ref name = StarTram/><ref name="airforcereport"/>

=== Switching ===
[[File:Multistage coilgun.jpg|thumb|A multistage coilgun]]

One main obstacle in coilgun design is switching the power through the coils. There are several common solutions—the simplest (and probably least effective) is the [[spark gap]], which releases the stored energy through the coil when the voltage reaches a certain threshold. A better option is to use solid-state switches; these include [[IGBT]]s or power [[MOSFET]]s (which can be switched off mid-pulse) and [[Silicon-controlled rectifier|SCRs]] (which release all stored energy before turning off).<ref name="Phil's Technology Blog"/>

A quick-and-dirty method for switching, especially for those using a flash camera for the main components, is to use the flash tube itself as a switch. By wiring it in series with the coil, it can silently and non-destructively (assuming that the energy in the capacitor is kept below the tube's safe operating limits) allow a large amount of current to pass through to the coil. Like any flash tube, ionizing the gas in the tube with a high voltage triggers it. However, a large amount of the energy will be dissipated as heat and light, and, because of the tube being a spark gap, the tube will stop conducting once the voltage across it drops sufficiently, leaving some charge remaining on the capacitor.

=== Resistance ===

The [[electrical resistance]] of the coils and the [[equivalent series resistance]] (ESR) of the current source dissipate considerable power.

At low speeds the heating of the coils dominates the efficiency of the coilgun, giving exceptionally low efficiency. However, as speeds climb, mechanical power grows proportional to the square of the speed, but, correctly switched, the resistive losses are largely unaffected, and thus these resistive losses become much smaller in percentage terms.

=== Magnetic circuit ===
{{unreferenced section|date=January 2024}}
Ideally, 100% of the magnetic flux generated by the coil would be delivered to and act on the projectile; in reality this is impossible due to energy losses always present in a real system, which cannot be eliminated.

With a simple air-cored solenoid, the majority of the magnetic flux is not coupled into the projectile because of the magnetic circuit's high [[reluctance]]. The uncoupled flux generates a magnetic field that stores energy in the surrounding air. The energy that is stored in this field does not simply disappear from the magnetic circuit once the [[capacitor]] finishes discharging, instead returning to the coilgun's electric circuit. Because the coilgun's electric circuit is inherently analogous to an LC oscillator, the unused energy returns in the reverse direction ('ringing'), which can seriously damage polarized capacitors such as [[electrolytic capacitor]]s.

Reverse charging can be prevented by a [[diode]] connected in reverse-parallel across the capacitor terminals; as a result, the current keeps flowing until the diode and the coil's resistance dissipate the field energy as heat. While this is a simple and frequently utilized solution, it requires an additional expensive high-power diode and a well-designed coil with enough thermal mass and heat dissipation capability in order to prevent component failure.

Some designs attempt to recover the energy stored in the magnetic field by using a pair of diodes. These diodes, instead of being forced to dissipate the remaining energy, recharge the capacitors with the right polarity for the next discharge cycle. This will also avoid the need to fully recharge the capacitors, thus significantly reducing charge times. However, the practicality of this solution is limited by the resulting high recharge current through the [[equivalent series resistance]] (ESR) of the capacitors; the ESR will dissipate some of the recharge current, generating heat within the capacitors and potentially shortening their lifetime.

To reduce component size, weight, durability requirements, and most importantly, cost, the magnetic circuit must be optimized to deliver more energy to the projectile for a given energy input. This has been addressed to some extent by the use of back iron and end iron, which are pieces of magnetic material that enclose the coil and create paths of lower reluctance in order to improve the amount of magnetic flux coupled into the projectile. Results can vary widely, depending on the materials used; hobbyist designs may use, for example, materials ranging anywhere from magnetic steel (more effective, lower reluctance) to video tape (little improvement in reluctance). Moreover, the additional pieces of magnetic material in the magnetic circuit can potentially exacerbate the possibility of flux saturation and other magnetic losses.

=== Ferromagnetic projectile saturation ===
{{unreferenced section|date=January 2024}}
Another significant limitation of the coilgun is the occurrence of [[magnetic saturation]] in the ferromagnetic projectile. When the flux in the projectile lies in the linear portion of its material's B(H) curve, the force applied to the core is proportional to the square of coil current (I)—the field (H) is linearly dependent on I, B is linearly dependent on H and force is linearly dependent on the product BI. This relationship continues until the core is saturated; once this happens B will only increase marginally with H (and thus with I), so force gain is linear. Since losses are proportional to I<sup>2</sup>, increasing current beyond this point eventually decreases efficiency although it may increase the force. This puts an absolute limit on how much a given projectile can be accelerated with a single stage at acceptable efficiency.

=== Projectile magnetization and reaction time ===
{{unreferenced section|date=January 2024}}
Apart from saturation, the B(H) dependency often contains a [[hysteresis loop]] and the reaction time of the projectile material may be significant. The hysteresis means that the projectile becomes permanently magnetized and some energy will be lost as a permanent magnetic field of the projectile. The projectile reaction time, on the other hand, makes the projectile reluctant to respond to abrupt B changes; the flux will not rise as fast as desired while current is applied and a B tail will occur after the coil field has disappeared. This delay decreases the force, which would be maximized if the H and B were in phase.

== Induction coilguns ==

Most of the work to develop coilguns as hyper-velocity launchers has used "air-cored" systems to get around the limitations associated with ferromagnetic projectiles. In these systems, the projectile is accelerated by a moving coil "armature". If the armature is configured as one or more "shorted turns" then induced currents will result as a consequence of the time variation of the current in the static launcher coil (or coils).

In principle, coilguns can also be constructed in which the moving coils are fed with current via sliding contacts. However, the practical construction of such arrangements requires the provision of reliable high speed sliding contacts. Although feeding current to a multi-turn coil armature might not require currents as large as those required in a [[railgun]], the elimination of the need for high speed sliding contacts is an obvious potential advantage of the induction coilgun relative to the [[railgun]].

Air cored systems also introduce the penalty that much higher currents may be needed than in an "iron cored" system. Ultimately though, subject to the provision of appropriately rated power supplies, air cored systems can operate with much greater magnetic field strengths than "iron cored" systems, so that, ultimately, much higher accelerations and forces should be possible.

== Formula for exit velocity of coilgun projectile ==
An approximate result for the exit velocity of a projectile having been accelerated by a single-stage coilgun can be obtained by the equation<ref name=":0">{{Cite web |last=Holzgrafe, Lintz, Eyre, Patterson |first=Jeff, Nathan, Nick, Jay |date=December 14, 2012 |title=Effect of Projectile Design on Coil Gun Performance |url=http://phome.postech.ac.kr/user/edulab/phy1032015/coilgun_01.pdf |url-status=live |archive-url=https://web.archive.org/web/20161020104714/http://phome.postech.ac.kr/user/edulab/phy1032015/coilgun_01.pdf |archive-date=October 20, 2016 |access-date=December 14, 2012 |website=postech.ac.kr/eng/}}</ref>

<math> v_{exit}=\sqrt{\frac{2}{m}{V\mu_0\chi_mn^2I^2}}</math>

m being the mass of the projectile, defined as kg

V being the volume of the projectile, defined as m<sup>3</sup>

μ<sub>0</sub> being the [[vacuum permeability]], defined in [[International System of Units|SI units]] as 4π × 10<sup>−7</sup> [[Volt|V]]·[[Second|s]]/([[Ampere|A]]·[[Metre|m]])

χ<sub>m</sub> being the [[magnetic susceptibility]] of the projectile, a dimensionless proportionality constant indicating the degree of [[magnetization]] in a material in response to applied [[magnetic field]]s. This often must be determined experimentally, and tables containing susceptibility values for certain materials may be found in the [[CRC Handbook of Chemistry and Physics]] as well as the Wikipedia article for [[magnetic susceptibility]]

n being the number of coil turns per unit length of the coil, defined as m<sup>−1</sup>

And I being the current passing through the coil, defined as A.

While this approximation is useful for quickly defining the upper limit of velocity in a coilgun system, more accurate and non-linear second order differential equations do exist.<ref name=":0" /> The issues with this formula being that it assumes the projectile lies completely within a uniform magnetic field, that the current dies out instantly once the projectile reaches the center of the coil (eliminating the possibility of coil suck-back), that all potential energy is transferred into kinetic energy (whereas most would go into frictional forces), and that the wires of the coil are infinitely thin and do not stack on one another, all cumulatively increasing the expected exit velocity.<ref name=":0" />

== Uses ==
[[File:EMmortar.jpg|thumb|upright|A M934 mortar round is adapted for experimental coilgun launch with a conformal armature tail kit, to be fired through a barrel composed of short solenoidal electromagnets stacked end to end]]

Small coilguns are recreationally made by hobbyists, typically up to several [[joule]]s to tens of joules projectile energy (the latter comparable to a typical [[air gun]] and an [[order of magnitude]] less than a firearm) while ranging from under one percent to several percent efficiency.<ref name="AutoAU-8"/>

In 2018, a Los Angeles-based company [[Arcflash Labs]] offered the first coilgun for sale to the general public, the [[EMG-01|EMG-01A]]. It fired 6-gram steel slugs at 45&nbsp;m/s with a muzzle energy of approximately 5 joules.<ref>{{Cite news|url=https://hackaday.com/2018/07/12/you-can-now-buy-a-practical-gauss-gun/|title=You Can Now Buy a Practical Gauss Gun|date=2018-07-12|work=Hackaday|access-date=2018-08-07|language=en-US|archive-date=2018-08-07|archive-url=https://web.archive.org/web/20180807094800/https://hackaday.com/2018/07/12/you-can-now-buy-a-practical-gauss-gun/|url-status=live}}</ref> In 2021, they developed a larger model, the [[GR-1 "Anvil"|GR-1 Gauss rifle]] which fired 30-gram steel slugs at up to 75&nbsp;m/s with a muzzle energy of approximately 85 joules,<ref>{{Cite news|url=https://www.newscientist.com/article/2287028-handheld-railgun-as-powerful-as-an-air-rifle-to-go-on-sale-in-the-us/|title=Handheld railgun as powerful as an air rifle to go on sale in the US|date=2021-08-12|work=NewScientist|access-date=2021-10-07|language=en-US|archive-date=2021-10-07|archive-url=https://web.archive.org/web/20211007171951/https://www.newscientist.com/article/2287028-handheld-railgun-as-powerful-as-an-air-rifle-to-go-on-sale-in-the-us/|url-status=live}}</ref> comparable to a [[Pre-charged pneumatic|PCP air rifle]].

In 2022 [[Northshore Sports Club]], an American gun club in [[Lake Forest, Illinois]] began distributing the [[E-shotgun|CS/LW21]], also referred to as the "E-Shotgun", a compact, 15 joule magazine fed coil gun, manufactured by the [[Norinco|China North Industries Group Corp]].<ref>{{Cite news|url=https://www.bloomberg.com/news/articles/2023-04-17/china-rolls-out-electromagnetic-weapon-to-quell-violent-protests|title=China Rolls Out Electromagnetic Weapon to Quell Violent Protests |website=Bloomberg|date=17 April 2023 }}</ref> They project distribution to reach 5000 units per year in the US,<ref>{{Cite web|url=https://www.northshoresportsclub.com/coil-accelerator|title=Coil Accelerator|website=North Shore Sports Club|access-date=2022-07-06|archive-date=2022-07-18|archive-url=https://web.archive.org/web/20220718173123/https://www.northshoresportsclub.com/coil-accelerator|url-status=dead}}</ref><ref>{{Cite web|url=https://e-shotgun.com/reviews/|title=Reviews|date=December 22, 2021|website=e-Shotgun}}</ref> and the manufacturer has also unveiled plans to supply the Chinese police and military with units for "non-lethal riot control".<ref>{{Cite web|url=https://www.scmp.com/news/china/science/article/3217198/china-unveils-electromagnetic-gun-riot-control|title=China unveils electromagnetic gun for riot control |website=South China Morning Post|date=16 April 2023 }}</ref>

Much higher efficiency and energy can be obtained with designs of greater expense and sophistication. In 1978, Bondaletov in the USSR achieved record acceleration with a single stage by sending a 2-gram ring to 5000&nbsp;m/s in 1&nbsp;cm of length,<ref>K. McKinney and P. Mongeau, "Multiple stage pulsed induction acceleration," in IEEE Transactions on Magnetics, vol. 20, no. 2, pp. 239-242, March 1984, doi: 10.1109/TMAG.1984.1063089.</ref> but the most efficient modern designs tend to involve many stages.<ref name="AutoAU-9"/> It is estimated that greater than 90% efficiency will be required for vastly larger superconducting systems for space launch.<ref name = airforcereport/> An experimental 45-stage, 2.1&nbsp;m long [[DARPA]] coilgun [[Mortar (weapon)|mortar]] design is 22% efficient, with 1.6 [[megajoule]]s [[kinetic energy]] delivered to a round.<ref name="EMmortar"/>

[[File:Electromagneticgunlaunch.jpg|thumb|left|A large coilgun concept, a coaxial electromagnetic launcher firing projectiles [[Mass driver#On Earth|to orbit]]]]

Though they face the challenge of competitiveness versus conventional guns (and sometimes [[railgun]] alternatives), coilguns are being researched for weaponry.<ref name = EMmortar/>

The [[DARPA]] Electromagnetic Mortar program is one example, if practical challenges like sufficiently low weight can be achieved. The coilgun would be relatively silent with no smoke giving away its position, though a supersonic projectile would still create a [[sonic boom]]. Adjustable, smooth acceleration of the projectile along the barrel length would allow higher velocity, with a predicted range increase of 30% for a 120mm EM mortar over the conventional version of similar length. With no separate propellant charges to load, the researchers envision the firing rate to approximately double.<ref name = EMmortar/><ref name="ArmyTimes"/>

In 2006, a 120mm prototype was under construction for evaluation, though a tenuous time for deployment was then estimated to be 5 to 10+ years by [[Sandia National Laboratories]].<ref name = EMmortar/><ref name = ArmyTimes/> In 2011, development was proposed for an 81mm coilgun mortar to operate with a hybrid-electric version of the future [[Joint Light Tactical Vehicle]].<ref name="AutoAU-10"/><ref name="AutoAU-11"/>

[[Electromagnetic catapult|Electromagnetic aircraft catapults]] are planned, including on board future U.S. [[Gerald R. Ford class aircraft carrier]]s. An experimental induction coilgun version of an Electromagnetic Missile Launcher (EMML) has been tested for launching [[Tomahawk (missile family)|Tomahawk]] missiles.<ref name="AutoAU-12"/> A coilgun-based [[Active protection system#Hardkill measures|active defense]] system for tanks is under development at [[Harbin Institute of Technology|HIT]] in China.<ref name="AutoAU-13"/>

Coilgun potential has been perceived as extending beyond military applications.

Few entities could overcome the challenges and corresponding capital investment to fund gigantic coilguns with projectile mass and velocity on the scale of [[gigajoule]]s of [[kinetic energy]] (as opposed to [[megajoule]]s or less). Such have been proposed as Earth or Moon launchers:

* An ambitious lunar-base proposal considered in a 1975 [[NASA]] study would have involved a 4000-ton coilgun sending 10 million tons of lunar material over several years to [[Lagrangian point#L4 and L5|L5]] in support of massive [[space colonization]] utilizing a large 9900-ton power plant).<ref name="AutoAU-14" />
* A 1992 NASA study calculated that a 330-ton lunar quench gun (superconducting coilgun) could launch 4400 projectiles annually, each 1.5 tons and mostly [[liquid oxygen]] payload, using a relatively small amount of power, 350 [[Watt#Kilowatt|kW]] average.<ref name="AutoAU-15" />
* After a [[Ames Research Center|NASA Ames]] study of aerothermal requirements for heat shields with terrestrial surface launch, [[Sandia National Laboratories]] investigated electromagnetic launchers for spacecraft and researched other EML applications using both railguns and coilguns. In 1990 a kilometer-long coilgun was proposed for launch of small satellites.<ref name="AutoAU-16" /><ref name="AutoAU-17" />
* Later investigations at [[Sandia National Laboratories|Sandia]] included a 2005 proposal of the [[StarTram]] concept for an extremely long coilgun, one version conceived as launching passengers to orbit with survivable acceleration.<ref name="AutoAU-18" />
* A [[mass driver]] is essentially a coilgun that magnetically accelerates a package consisting of a magnetizable container with a payload. Once accelerated, the container and payload separate, the container is slowed and recycled to receive another payload.

==See also==
* [[Electromagnetic propulsion]]
* [[Carl Friedrich Gauss]]
* [[Helical railgun]]
* [[Light-gas gun]]
* [[Mass driver]]
* [[Edwin Fitch Northrup]]
* [[Plasma railgun]]
* [[Railgun]]
* [[Ram accelerator]]
* [[Solenoid]]
* [[Tubular linear motor]]

== References ==

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}}

== External links ==
{{Commons category|Coilguns}}

* [https://www.youtube.com/watch?v=TWeJsaCiGQ0 Prototype of coil gun. CG42- Full auto coil gun]

{{Electric machines}}
{{emerging technologies|topics=yes|military=yes}}
{{Non-rocket spacelaunch}}

[[Category:Artillery by type]]
[[Category:Electric motors]]
[[Category:Magnetic propulsion devices]]
[[Category:Non-rocket spacelaunch]]
[[Category:Norwegian inventions]]