Permanent magnet rail gun (PMRG)
A railgun is a special kind of linear homopolar electric motor used to accellerate projectiles to high speeds, by lorenz forces generated between two parallel static rails and a moving, conductive projectile between them, after the rails are powered with a powerful DC current pulse.
For several years now I’ve been observing attempts of rail gun construction by other hobbyists. Most of them usually followed the same basic design, involving two copper rails separated by insulating plates to form a square shaped barrel, into which a solid conductive projectile was inserted. The power supplies ranged from multi-kV high energy capacitors, to electrolytic capacitors at voltages of few hundred volts. Most of these designs seemed to demonstrate fairly poor performance and efficiency, and I wanted to gain a better understanding why.
Hence I decided to stray from these classical designs and do some experimental variation in ways I believed could improve efficiency and reusability. Amateur railguns that used solid projectiles tended to erode their rails heavily, wasting large amounts of energy in form of a cloud of plasma and molten metal exiting the barrel along with the projectile. Despite this looked cool on the pictures, it is very hard on the rails and tends to erode them after a few shots.
A while ago, I ran into a very interesting site, describing one of best built railguns on the web: http://www.rapp-instruments.de/index6.htm While the page is in German, a friend helped me to translate the most important points . The design was remarkable for it’s absence of muzzle flash, and the secret was in use of a projectileequipped with copper brushes, instead of solid metal projectile. The design easily surpassed speed of sound, a remarkable feat for amateur railgun.
Further increase in efficiency may be achieved by reducing the total current required by the railgun – in other words, by increasing it’s input impedance. When the projectile moves in a magnetic field, it produces electromotive force much like a generator – and only this voltage times the input current is the power that is actually converted to kinetic energy on the projectile – the rest of voltage drops all count to losses! In amateur railgun design, this electromotive force tends to be tiny – take an example of a 1cm wide projectile moving at speed of sound (330m/s) in 1 Tesla magnetic field, figures typical for an amateur railgun – the resulting EMF is only 3.3 volts! Considering that lowest commonly used supply voltages for railguns are still in hundreds of volts, it can be seen why the efficiencies tended to be so low. Obviously, a low voltage, extremely high current power supply is required for an efficient railgun, and the resistance in the current path needs to be kept to absolute minimum! It makes very little sense to use capacitors in kilovolt range, unless the projectile already moves at hypersonic speeds, as in record-breaking plasma armature railguns. But I believe those are well outside amateur category.
Since the projectile EMF depends on the magnetic field, the effective impedance may be increased by providing external magnetic field, along with the field generated by the rails. This may be done, for example, using extra sets of rails that act as field coils. But, for this design, I decided to investigate usage of permanent magnets as alternative method. Today’s Neodymium magnets are light and powerful, and consume no energy, providing fields up to 1 Tesla. Note that their usefulness drops with increase in current, since force due to magnets rises linearly, while force of non-augmented railgun rises with square of current. At some point, the magnets may even get demagnetized – this project is here to investigate.
A considerable amount of iron, copper and rare earth magnets had to be to acquired for this project.
A railgun is most commonly powered directly from a capacitor bank, that is switched either by a separate switch or by projectile injection. The pulse may be conditioned by an impedance matching network, which is usually a simple inductor, sometimes incorporated as augmenting coil.
A video of “first light” shooting at low power from a car battery:
Like several other projects of mine, this one is on suspension too because I was unable to get the electrolytic capacitors I hoped for, due to some rather weird customs laws in my country. The idea is to switch a high energy capacitor bank at 400V or so into a primary of a specially designed transformer at sensible peak currents of few kA, which will be stepped up to 20kA+ required to power the railgun. A special pre-saturation circuit will be used to double the volt-seconds attainable on the transformer for single pulse operation.
Links and references
 4hv thread http://4hv.org/e107_plugins/forum/forum_viewtopic.php?145747