The Plasma Car Wash
By Kathy A. Svitil
DISCOVER Vol. 18 No. 10 | October 1997 | Technology
The most common form of matter in the universe is neither solid, liquid, nor gas. It is plasma--an electrically charged, high-energy soup of ions and electrons. Plasmas make up the stars, most of the gas in interstellar space, and comet tails. Although on Earth the three other forms of matter predominate, plasmas do exist here, in flames, fluorescent lights, auroras, and lightning bolts. And now plasmas can also be found in a device dubbed the plasma car wash. The inventors--a team of physicists, engineers, and chemists at UCLA and Los Alamos National Laboratory in New Mexico--say they have found a way to harness plasmas to decontaminate radioactive and chemical waste and strip graffiti off walls, among other uses.
The device runs on 300 watts, about half the power used by a microwave oven. It weighs less than 80 pounds, looks like a leaf blower, and essentially consists of a tank of pressurized gas (the type of gas used depends on the application) that is pumped into a six-inch-long tube housing two concentric cylindrical electrodes. As the gas shoots between the electrodes, the electric field pulls off its electrons, creating the charged ions of the plasma. The plasma then boosts the energy of other gas molecules in the tube, which shoot out the nozzle and react with other molecules they encounter--like paint in graffiti--by either pulling them apart or stealing their electrons.
The technology sounds simple, but the description is deceptive. In fact, the Los Alamos team had to overcome a major obstacle: they had to develop a plasma that survives at room temperature outside a vacuum.
Natural plasmas exist only at high temperatures, as in lightning bolts and stars, or in low-temperature vacuums, as in interstellar space. To exist at room temperature, artificial plasmas such as are found in neon signs require both a vacuum and a strong electric field to pump energy into the plasma. Once outside the vacuum, the plasma particles combine with molecules in the atmosphere or with other plasma particles, and the plasma becomes an ordinary, electrically neutral gas. This isn’t the case with natural, high-temperature plasmas. They consist of particles so energetic that they don’t readily combine and settle down as neutral atoms. But such plasmas are so hot--typically at least tens of thousands of degrees--that they annihilate everything they touch. The plasma car wash, however, might be able to operate at temperatures below 400 degrees, so a quick pass of its particle beam could decontaminate carpets, clothes, and other materials without vaporizing them.
The Los Alamos team solved the problem by using an energetic, low- temperature plasma to boost other atoms or molecules into what physicists call a meta-stable state. Unlike plasma particles, which lose their energy within millionths of a second in air, meta-stable particles can survive in air for a few tenths of a second. Meta-stable molecules owe their longevity to a delicate arrangement of their outermost electrons. By carefully tuning the strength of the electric field of the plasma, the Los Alamos team can nudge low-energy electrons into higher-energy meta-stable states. Electrons in these states can’t fall back to a lower energy unless they collide with another molecule with just the right amount of energy. The rarity of such encounters enables meta-stable particles to live long enough to reach and react with, say, a chemical contaminant or a paint molecule on a graffiti- covered wall.
Meta-stable particles are selective in the substances they attack, so the Los Alamos researchers use different gas mixtures depending on the task. To clean radioactive waste, for example, they use a fluorine compound. The beam vaporizes the uranium and plutonium particles, which are then sucked up by a vacuum attachment. You can start with a massive, slightly contaminated object--a truck, tank, or even a building, says Gary Selwyn, head of the Los Alamos team, and decontaminate it and concentrate all the waste into a small filter.