Coilgun
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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.
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[edit] 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.[1]
[edit] 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.
[edit] Limitations
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Despite heavy research and development by the amateur and professional community, great obstacles have yet to be overcome.
[edit] Projectile saturation
A significant limitation to the coil gun are conditions of ferromagnetic projectile saturation. When a projectile is in linear B(H) dependency area, 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. Once the core is saturated, B does not increase any more with H (and thus with I), so force gain is linear. Since losses are proportional to I2, increasing current beyond this point 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. A common misunderstanding among amateur coilgun builders is that a saturated projectile cannot take more accelerating force, which is basically not true.
[edit] 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 raise 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.
[edit] 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.
[edit] Energy dissipation
The coils have an electrical resistance, and resistive losses are often very significant.
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.
[edit] Fictional references
The coil gun first appeared in literature as the "electric gun" in the 1897 science fiction novel A Trip to Venus by John Munro.[3] The novel described in detail a way to launch vehicles into outer space from the Earth's surface. In the novel, Munro describes in great detail multiple coils fired in sequence by solenoids timed to achieve acceleration without generating g forces that would harm passengers. The gun could be angled on a hillside if desired.
[edit] See also
[edit] References
- ^ "Electromagnetic Guns". http://www.coilgun.info/theorymath/electroguns.htm. Retrieved on February 13 2009.
- ^ "Room 203 Technology". Coil Gun. http://philstechnologyblog.blogspot.com/. Retrieved on October 20 2007.
- ^ Munro, John (1897). A Trip to Venus (2007 IndyPublish ed. ed.). London: Jarrold & Sons. pp. 26-28.
[edit] External links
- Coilgun at the Open Directory Project
