Science: Plasma Pinch

Airplanes were still in pinfeathers when Rudyard Kipling wrote his stirring turn-of-the-century story. With the Night Mail, and filled the fictional skies over the Atlantic with swift craft propelled by a vaguely described “Fleury’s Ray.” Present-day jetliners are already three times as fast as Kipling’s night flyers, but his imaginative planes may soon have ray-driven descendants in space. Last week Republic Aviation Corp. demonstrated a “plasma pinch” space engine that uses magnetic force to spit out a fierce blue flash of electrified particles— a 20th century version of Fleury’s Ray.

Swift Flash. The power for Republic’s engine is produced by a plasma* which responds instantly to magnetic forces, thus permitting its particles to be accelerated to extremely high speeds. At the heart of the engine are two aluminum electrodes, 8 in. in diameter and about 1 in. apart. The electrodes are connected to a charged, 3,000-volt capacitor, but as long as the gap between them is a high vacuum (the engine works only in the vacuum of space), no spark of electricity can arc across. Every second or so, when a small amount of nitrogen gas is allowed to leak into the gap, a spark flashes, turning the nitrogen into a plasma by ionizing its atoms. The heavy current, which lasts only a few millionths of a second, creates a magnetic field that gives the plasma a powerful “pinch” and sends it squirting in brief bursts through a carefully shaped hole. The silent blue-white flash of escaping plasma looks harmless and feeble, but its temperature reaches as high as 200,000° F., and its particles shoot into the vacuum at 100,000 m.p.h.

This enormous speed, many times as fast as a chemical rocket’s exhaust, is the secret of the plasma engine’s promise. All space engines must shoot something astern to gain their thrust, and as the exhaust speed rises, the engine becomes more efficient. Republic’s plasma engine can run for a year on 1 Ib. of nitrogen.

Higher Intelligence. The experimental engine’s thrust is only .01 lb., which is less than one grasshopper-power, but no one expects a true space engine to provide the massive thrust that is needed to free a heavy rocket from the earth’s gravitation. Engines designed for use after a vehicle has been lofted into orbit need only a little thrust, but they must exert it for a long time, using only a whiff of fuel. Alfred E. Kunen, director of Republic’s Plasma Propulsion Laboratory, explained that the plasma pinch engine will get its electricity from solar cells and store it temporarily in a battery. When thrust is needed, the engine can work continuously for months or years, consuming only a small amount of nitrogen.

The present engine has been tested in a laboratory vacuum, but it has yet to be made proof against all the malign forces—vibration, acceleration, heat, radiation—that assail components of practical space vehicles. Kunen hopes to have a flyable engine ready for practical testing next June. Its first jobs, he thinks, will be to keep satellites under precise control for long periods of time. Its gentle, cumulative thrust will be quite enough to make a reconnaissance satellite point its cameras continuously toward the earth. It will be able to shift the satellite to a new orbit, helping to foil enemies who would like to molest it. When communication satellites take to space, plasma engines may spiral them to their final orbits. By gentle, watchful hints of thrust, plasma engines will keep the satellites in the proper position to relay commercial TV—or, hopefully, more important intelligence—all the way around the earth.

* In physics, a plasma is a gas in which the atoms have been separated into negatively charged electrons and positively charged ions. Because of its peculiar qualities—it is neither a solid, a liquid nor a normal gas—it is sometimes called “the fourth state of matter.”