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flash camera

Coilgun

A coilgun is a type of synchronous linear electric motor which is used as a projectile accelerator that consists of one or more electromagnetic coils. 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.

Construction

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 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.

Switching

One main obstacle in coilgun design is switching the power through the coils. There are several main options—the most simple (and probably the 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 (which can be switched off mid-pulse) and SCRs (which release all stored energy before turning off). 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 (though there will be a flash of light) 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.

Limitations

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

Projectile saturation

One of the greatest limitations to the coil gun is the rate at which the ferromagnetic projectile becomes fully saturated by the magnetic field and the rate at which it loses its magnetic saturation. Once a ferromagnetic object becomes completely saturated the amount of force in which it can be attracted stops increasing. The rate at which the projectile loses its saturation is critical; as this rate is constant, greater distances between drive electromagnets are needed to compensate for this rate. As the projectile increases in speed it reaches drive electromagnets at progressively faster rates. Without compensation for desaturation time, there will be less and less effect to the velocity of the projectile, resulting in significantly lower efficiency per drive electromagnet stage as the projectile travels down the line. Once the amount of force exerted to the projectile is less than or equal to the amount of resistance exerted on the projectile due to air friction and friction in the barrel the projectile will no longer gain velocity.

Resistance

Electrical resistance is a major limitation because when dumping large amounts of electrical energy into a conductor the majority of the energy is converted to heat due to resistance and therefore effectively lost as it is not driving the projectile. This could be overcome through the use of a superconducting material.

Energy dissipation

The coils have an electrical resistance, and resistive losses are often very significant indeed.

The energy in the magnetic field itself does not simply dissipate; much of it returns to the capacitor when the electric current is decreasing. Unfortunately it does this in the reverse direction (via a 'ringing' mechanism due to inductance of the coils), which can seriously damage polarized capacitors (such as electrolytics).

In the circuit the magnetic field keeps the current in the coil flowing after the capacitor has discharged, so that it keeps discharging and builds up a negative voltage (see Lenz's law). This is similar to an LC oscillator.

The capacitor charging to a negative voltage can be prevented by placing a diode across the capacitor terminals.

Some designs bypass this limitation by using couple of diodes. Then, diodes reverse polarity to charge capacitors instead with proper polarity again, effectively re-using remaining coil energy.

See also

References

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