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How to build railguns and coilguns, continued...
   That Darned Periodic Table of the Elements

OK, so we run to the Periodic Table of Elements for help. This is the list of all the most basic raw materials from which we make everything else.

Researchers have tested every magnetic element and their alloys for use in magnets, and it turns out that we can’t get anything better than soft iron for a core. It is important for iron to be a soft as possible because it has to be as easy as possible for a magnetic field to twist all the iron atoms into the same direction, and for them to twist back to random directions when the electrical current that catalyzes the field is turned off.

Newbie note: Depending upon the way we form iron, it can be super hard or so soft you can bend a big piece of it with your hands. When talking about soft iron for magnets, though, we aren’t talking about how easy it is to bend or dent the iron, but how easy it is for the atoms to all line up their magnetic poles in the same direction. That’s why an ordinary iron nail can make a good electromagnet even though it’s hard enough to pound it into wood. Even so, you can easily bend or dent a nail with a hammer, so you can see it’s sort of soft. It’s much harder to bend or dent a steel knife blade and knife blades make poor electromagnets. There are many ways to make iron hard, by either adding other materials to it when it is melted, by hammering on it as it cools, and by cooling it fast. To make iron not be hard, it must be refined to be as pure as possible. This is done while the iron is hot enough to be a liquid. Then the iron has to be cooled slowly. Sudden cooling (quenching) makes iron and its alloys such as steel get harder.

Evil genius tip: You can’t buy the softest iron at the hardware store, but you can get it for free. Find any sort of junk that has an electrical motor or transformer inside and take it apart. Those thin flat metal things inside coils of wire, often shaped like the letter E or U, are soft iron.

The next way the Periodic Table of Elements gives us grief is with the conducting wire we use for electromagnets. If we have to use an air core, then we need thousands of times more windings than you find in any iron core electromagnet. We need a wire that can carry as much current as possible while heating up as little as possible.

Newbie note: Unless a wire is a superconductor, the more electricity you run through a wire, the hotter it gets.

Besides the obvious problem of too much electricity melting the wire, we also have the problem that the hotter a conductor gets, the more waste heat any electrical current will create. You can solve the problem by using a thicker wire and cooling it as fast as possible. Indeed, this is what researchers are doing in order to build the most powerful possible pulsed power supplies and electromagnets. (More below on pulsed power supply problems.)

However, we can get something better for the conductor that must loop around the core of an electromagnet. We can use superconductors. Or so we hope.

Magnetic resonance imaging devices (MRIs) achieve extreme magnetic fields by using low temperature superconductors to carry far higher currents than normal conductors such as room temperature copper. The “air core” of an MRI unit is where the patient lies. Because superconductors can carry so much more electricity than ordinary wires, they don’t take up as much space.

The Trouble with Superconductors

Now maybe someday superconductors will solve our problems with electromagnetic guns, but we aren’t there yet. We can’t use superconductors because, unlike MRI, guns rely upon huge, fast field changes, and superconductors generate heat whenever their fields change.
So far, we haven’t discovered any superconductors that can get rid of heat fast enough. All of them that can take the high magnetic fields we need for guns are just elements, and they all require extremely low temperatures to work. It is really, really hard to shed heat at those temperatures because the laws of thermodynamics reveal that the colder something gets, the more work it takes to cool it yet another degree further. Sorry, I couldn’t find a newbie-friendly website that explains the laws of thermodynamics. Looks like newbies will have to study physics and math in order to get smart on thermodynamics.

It’s that darned Periodic Table of the Elements again. If a superconductor overheats, it “quenches,” instantly converting all its electrical and magnetic energy into heat. This heat – we’re talking lots and lots of heat -- causes an explosion by making the liquid helium that was keeping the superconductor cold.  So far, nobody has found a way to remove this heat fast enough to make a low temperature superconductor gun work.

An alternative is to find a superconductor that works at a high enough temperature that it is easy enough to remove this heat. When high temperature superconductors were first discovered in 1986, us gun researchers were super excited. But then it turned out that every one of the new high temperature superconductors was unable to operate in strong magnetic fields. Boy, was that a disappointment. So unless somebody discovers a high temperature superconductor that works in magnetic fields of, say, 4 Teslas, we are out of luck.

That is why, instead, today’s weapons grade electromagnetic guns use conventional conductors, air cores, and dump a prodigious amount of heat with each firing. The engineering challenges are great, but solvable, say researchers. Unsolvable and a waste of money, say their critics. Impossible for the home gun enthusiast.

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       © 2013 Carolyn Meinel