Coilgun

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (April 2008)

A handheld single stage coilgun with a pistol grip

A coilgun is a type of projectile accelerator that consists of one or more electromagnetic coils in the configuration of a synchronous linear electric motor. These are used to accelerate a magnetic projectile to high velocity. The name Gauss gun is sometimes used for such devices in reference to Carl Friedrich Gauss, who formulated mathematical descriptions of the electromagnetic effect used by magnetic accelerators.

Coilguns consist of one or more coils arranged along the barrel that are switched in sequence so as to ensure that the projectile is accelerated quickly along the barrel via magnetic forces. Coilguns are distinct from railguns, which pass a large current through the projectile or sabot via sliding contacts. Coilguns and railguns also operate on different principles. The first operational coilgun was developed and patented by Norwegian physicist Kristian Birkeland.

Construction

Simplified diagram of a multistage coilgun with three coils, a barrel, and a ferromagnetic projectile

A coilgun, as the name implies, consists of a coil of wire – an electromagnet – with a ferromagnetic projectile placed at one of its ends. Effectively a coilgun is a solenoid: an electromagnetic coil with the function of drawing a ferromagnetic object through its center. A large 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 is switched off and the next electromagnet 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 projectile rides on, with the driver into the electromagnetic coils around the track. Power is supplied to the electromagnet from some sort of fast discharge storage device, typically a battery or high-capacity high voltage capacitors designed for fast energy discharge. A diode is used to protect polarity sensitive capacitors (such as electrolytics) from damage due to inverse polarity of the current after the discharge.

There are two main types or setups of a coilgun, single stage and multistage. A single stage coilgun uses just one electromagnet to propel a ferromagnetic projectile. A multistage coilgun uses multiple electromagnets in succession to progressively increase the speed of the projectile.

Many hobbyists use low-cost rudimentary designs to experiment with coilguns, for example using photoflash capacitors 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.

A superconductor coilgun called a quench gun could be created by successively quenching a line of adjacent coaxial superconducting electromagnetic 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.^[1]

Switching

One main obstacle in coilgun design is switching the power through the coils. There are several main options — 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 IGBTs or power MOSFETs (which can be switched off mid-pulse) and SCRs (which release all stored energy before turning off).^[2] 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, due to 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.

Limitations

This section does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (July 2009)

A multistage coilgun

Despite heavy research and development by the amateur and professional community, great obstacles have yet to be overcome.

Resistance

Electrical resistance is a major limitation of a typical coil gun; due to the extremely large currents used, even a well designed coil will waste the majority of the input energy as heat, dictated by Ohm's Law. This energy is effectively lost as it is not driving the projectile.

Electrical resistance as a design limitation could be overcome through the use of a superconducting material. However, there are no known materials that are superconductive at room temperature.

The magnetic circuit

Ideally, 100% of the magnetic flux generated by the coil would be delivered to and act on the projectile, but this is often far from the case due to the common air-core-solenoid / projectile construction of most coilguns.

Since an air-cored solenoid is simply an inductor, the majority of the magnetic flux is not coupled into the projectile, instead being stored 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; much of it returns to the capacitor when the circuit's electric current is decreasing. As the coilgun circuit is inherently analogous to an LC oscillator, it does this in the reverse direction ('ringing'), which can seriously damage polarized capacitors (such as electrolytics).

The capacitor charging to a negative voltage can be prevented by placing a diode across the capacitor terminals. However, this means that the diode (or diodes, if several are paralleled for increased capacity), along with the coil, must dissipate all of the stored energy as heat. While this is a simple and effective solution, it requires robust and therefore expensive semiconductors, along with increased construction strength and resistance consideration for the coil.

Some designs attempt to recover the energy stored in the magnetic field by using a couple of diodes. These diodes, instead of being forced to dissipate the remaining energy, recharge the capacitors with the right polarity to be used again for the next discharge cycle. This will also prevent the need to recharge the capacitors completely, thus significantly reducing charge times.

In order to reduce component size, weight, durability requirements, and most importantly, cost, the magnetic circuit must be optimized to deliver more energy to the projectile per discharge cycle while still using the same energy input. This has been addressed to some extent with the usage of end iron and back iron, which are pieces of magnetic material that enclose the coil and help reduce the reluctance of the magnetic circuit. The results of this vary widely, due to the use of materials ranging anywhere from actual magnetic steel to video tape. The inclusion of an additional piece of magnetic material in the magnetic circuit also magnifies the problems of flux saturation and other magnetic-related losses.

Projectile saturation

Another significant limitation of the coilgun is the occurence of ferromagnetic projectile saturation. 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 I², increasing current beyond this point eventually decreases efficiency (yet it may further 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

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 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.

See also

• Mass driver
• Railgun
• Linear motor

References

External links

• Coilgun at the Open Directory Project