Dumpster Television Junkies


Saturday, March 17, 2007

After EXN.ca published Lightsabers are cutting edge technology, an informal discussion group on how to build lightsabers sprung up on our pages. One of the ideas touted as a method of constructing a lightsaber: plasma. Here's a sample letter:

"A plasma field contained in a magnetic field could be constructed to create a lightsaber. The plasma could be contained in the handle, when the switch is flipped the electromagnetic field is energized and the plasma is ejected within it. Although creating the high temperature of the plasma would require a large energy supply, the lightsaber would perform just as in the movies and is not a technical impossibility." -- Michael Ernst, Student, Miami

Sounds good. In response to popular demand, we consulted some experts. Let's take a look at plasma itself, and what it would take to build a plasma lightsaber.

Plasma is the so-called fourth state of matter, right after solids, liquids, and gases. Basically, if you heat up a gas enough (or apply enough of any energy, for that matter) you can get it to turn to plasma. The extra energy blasts negatively-charged electrons off of the atoms that make up the gas, and divides the gas into positively charged ions and loose electrons. Plasma generally radiates a lot of visible light and heat. The electrons continue to be attracted to the ions, and tend to reattach themselves as soon as the energy or heat diminishes, so plasma is inherently unstable and doesn't last very long on Earth.

We see examples of plasma every day. The sun itself is made out of plasma, being a giant ball of superheated glowing hydrogen. Lightning is an example of the gases in the atmosphere igniting into a streak of plasma as an electric discharge crashes from sky to Earth. Neon and fluorescent tubes are also examples of an electric charge igniting a gas, although at lower temperatures. Nuclear fusion is technology that seeks to fuse atoms in a plasma, which would result in a large release of energy.

In this electrodeless plasma gun, a 3 megawatt signal induced in the three coils sparks gas into plasma.

Plasma technology has been commercialized for a number of years. Plasma guns are used extensively to coat surfaces with thin films of other materials. When you're trying to coat something that has a low melting point, like metals, with something that has a high melting point, like ceramics, then plasma guns are the way to go. They're used in the construction of all aircraft engines. Large volumes of gas are blasted through an electric arc, or zapped by radio waves or even microwaves. This energizes the gas to a plasma state. Temperatures of 6,600 C to 16,600 C can be reached, which exceeds the surface temperature of the sun. Pellets of the coating material are shot into the jet, which is going at a very high speed. The pellets melt instantly into droplets, which are propelled in the same direction as the gas. The plasma cools rapidly as the electrons reattach themselves to the ions, but the droplets stay molten longer. They strike the surface of the material to be covered at high speed, and apply an even coat.

The technology is there, so shouldn't it be easy to build a plasma lightsaber? Unfortunately there are a few technical challenges, especially if you want to look more like a Jedi than a tank. Probably the most daunting challenge is that since plasma is the fourth state of matter after gas, it's necessarily extremely hot. Hydrogen plasma is the one of the cooler plasmas, and it comes in at 4,000 degrees Celsius. Might singe the robe cuffs a bit.

Some plasmas are less threatening than others...

A noticeable exception to this rule is that when certain gases are kept under low pressure, like neon tubes or fluorescent light bulbs, they turn into plasma at low voltage and give off quite a bit of light and very little apparent heat. This property disappears when the gas is exposed to atmospheric pressure. By this token, a toy lightsaber made from a neon tube is, in fact, a very real plasma lightsaber. But aside from putting someone's eye out, it's unlikely to do much harm.

There is also something known as coronal discharge. This can be seen on a humid night as a slight glow around a high tension power line. It occurs when a high energy source, such as the power line, starts turning the air around it into a cool plasma. The reason it takes a humid night to bring it out is that dry air itself has too much resistance to become plasma with so little voltage. The gases found in pollution will have the same effect. Coronal discharge isn't very fearsome, and is regularly demonstrated in classrooms. About the only things it can destroy are some gas molecules, and it's used by industry in this way to clean up polluting fumes in power plants and paint factories. It can also be used to increase the surface porosity of things like car bumpers for painting purposes. That requires a good ten to twenty minutes of treatment, though - its hard to imagine an opponent who will stand still that long to have their surface porosity altered.

This AC plasma torch uses 125 to 750 kilowatts and generates a temperature up to 8,000 degrees Celsius.

Since cool plasmas aren't much good, you're back to hot ones and a serious heat problem. Plasma guns that we use today are extensively water- and gas-cooled. They also thrust vast amounts of gases through them to obtain a jet. One researcher suggested that miniaturization had come to the point where you could get a 10,000 degree plasma jet about ten centimetres long from forty kilowatts of power in a unit that would weigh something reasonable, but it would require about 50 litres of air to be pumped through it per minute.

A plasma gun at Aerospatiale in France.

The length of the jet would be another problem. "You might be able to create a lightdagger easier than a lightsaber," suggested one researcher. Most plasma guns only reach a couple of centimetres at atmospheric pressure before the ions and the electrons recombine and the plasma loses energy. Out in space any plasma gun is liable to extend a lot longer because there is nothing to diffuse the energy into, but were talking a couple of meters here, not exactly a Death Star. There are some very long plasma jets out there, however. The French aerospace company Aerospatiale has a plasmatron, or plasma generator, whose torch measures a metre in length. They use it to test re-entry conditions on materials for their Hermes space shuttle. This baby goes through 6 megawatts of power and calls for few cubic metres of gas per minute [1 cubic metre=1000 litres]. The equipment to operate it weighs several tons. They have plans to develop a plasmatron that would be several metres long, but which would require 120 megawatts of energy (of which 50 percent would go to the cooling system).

An image taken of the plasma inside a tokamak, which is a magnetic-confinement plasma chamber used in fusion experiments, at Germany's Max Planck Institute.

Confining the plasma would allow you to lengthen the torch. In some applications, the plasma is confined in a quartz tube which is extensively gas-cooled to prevent it from melting. You can contain plasma using an electromagnetic field, a sort of a magnetic bottle. This is what is used in fusion experiments, where plasma atoms are encouraged to fuse within magnetic confinement. If you contain the plasma, then you can get it to extend itself longer because there's less diffusion of the plasma's energy. You might even be able, under certain conditions, to magnetically repel the lightsaber of an opponent. The problem is that confining something as dangerous as plasma with a magnet in a combat situation might not be such a good idea. As one correspondent put it, "Just throw a kitchen magnet at Mr. Jedi, and he's toast." We have to remember that in laboratory or industrial conditions, no one is trying to kill the plasma researchers or operators. It's just too easy to jam something that's relying on magnetic fields, so we don't recommend this design for dueling.

All in all, we think the unique properties of plasma make it better suited to industry than weaponry - at least for the time being. The biggest limit to plasma research right now is the availability of energy for the plasma generators. If vast new energy sources were to open up, some researchers suggest, then plasma research would probably accelerate to levels we can't imagine. Until then, you'll have to make do with the Force.

Story by:
Brahm Rosensweig

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