A coilgun (or Gauss gun) is a type of cannon that uses a series of electromagnets to accelerate a magnetic shell to very high velocities. Coilguns are often mistakenly called railguns by many sources, and while they are similar in general concept (that is, a magnetic gun), they differ in operation, as a railgun accelerates projectiles down two parallel conducting rails. Coilguns are essentially identical to mass drivers, though on a smaller scale.

Kristian Birkeland is commonly considered the inventor of the electromagnetic coilgun, which he obtained a patent for in 1900. The attempts to turn his invention into a usable weapon failed however, and the idea was more or less forgotten for many years.

Many hobbyists use low-cost rudimentary designs to experiment with coilguns. One such design would incorporate the use of photoflash capacitors from a disposable camera as the energy source, and a low inductance coil to propel the projectile forward.

Coilguns, as the name implies, consist in a coil of wire or solenoid with a ferromagnetic projectile placed at one of its ends. A large electrical current is pulsed on the coil and a strong magnetic field forms, pulling the projectile to the center of the coil. When the projectile nears this point, the coil is switched off and a next coil can be switched on, progressively accelerating the projectile down successive stages.

In common coilgun designs, the "barrel" of the gun is made up of a track that the shell rides on, with the driver magnets around the track. Power is supplied to the magnets from some sort of "fast dump" storage device, typically a battery of high-capacity capacitors.

The power must be delivered to each successive electromagnet with precise timing, due to hysteresis. Magnets take some time to "charge up" after power is delivered to them, so the power supply must start before the shell has reached a particular magnet. The same is true after the power is turned off, and if the shell is on the "far side" of the magnet at that time, the magnet will continue pulling on it, slowing it down.

One obvious solution would be to trigger the magnets long before the shell reaches them, but because magnetic force drops off with the cube of distance (that is, very quickly) too much power would be lost with such a solution. For this reason most coilguns that use more than one magnet include some sort of electronic timing device for powering the magnets, one that can be adjusted for various parameters such as power of the shot, and the mass of the shell. The gun starts with all of the magnets turned on, and then turns them off one by one before the shell reaches them.

A superconducting version of the coilgun is called the quench gun. Resistors attached to superconductive coils waste energy in the coil, which is turned into heat. After a time this heats the superconductor up to the point where it is no longer a superconductor, thereby changes its state to normal (non-superconducting). When this happens the resistance of the coil as a whole suddenly increases, dumping all of the power as heat at a very rapid rate. By carefully controlling the heating rates, the magnets can be "turned off" in sequence at the proper rates to make a coilgun, one that generates very powerful magnetic fields with high efficiency, and tends to have lower hysteresis due to the rapid "burn out" of the energy in the coil.

One advantage of the coilgun over the railgun is that it can be made arbitrarily long. This has a number of side effects, but the main one is that the acceleration can be much slower over a longer length, meaning that the power needed in any one section of a coilgun is much lower. However this advantage is offset by the cost and complexity of the switching system needed to supply a longer gun.

Potential uses

Like railguns and ram accelerators, coil guns have been proposed for use in delivering payloads to space.

As a weapon, the coilgun's advantages include the fact that it has no moving parts, apart from the projectile, and the fact that the only noise heard is the movement of the projectile when it reaches very high speeds.

Coil guns are a popular device in science fiction, espcially sci-fi role playing and video games, where they go under such names as Gauss cannon or Gauss rifle (e.g., in Battletech, Syndicate).

In Wing Commander, the coil gun (called a "Mass Driver" in the game, even though the term mass driver implies a much larger object) is used as a primary weapon in some fighter spaceships.

Metal Gear REX's primary weapon in the videogame Metal Gear Solid is identified as a railgun, but the description of its functionality would make it a coilgun. It is intended to launch an uninterceptable, untraceable nuclear weapon (as the coilgun would leave no propellant trail or engine flares, unlike an ICBM).

The Necrons from Warhammer 40,000 use gauss rifles to flay the skin off of their enemies.

The University of Texas at Austin's Center for Electromechanics (http://www.utexas.edu/research/cem/) has been working on this technology (http://www.utexas.edu/research/cem/programs/pulsed-ac.html) for years for the United States Department of Defense.

External links

• Another Coilgun Site _ Single / Multiple and portable coilguns (http://www.anothercoilgunsite.com)
• Joe's Coil Gun Site (http://www.joe.us.tt)
• PowerLabs Coil Gun Page; 3 different kinds of coilguns (http://www.powerlabs.org/coilguns.htm)
• Coilgun Systems (http://www.coilgun.com/)
• Overview of Electromagnetic Guns (http://www.oz.net/~coilgun/theory/electroguns.htm)