Plasma Porthole

Saturday, February 01, 1997
Nature may abhor a vacuum, but physicists are pretty fond of them: many of their experiments require an airless environment, completely sealed and isolated from the outside world. But as crucial as vacuum chambers are to physics and technology, working with them can be cumbersome, which is why Ady Hershcovitch, a physicist at Brookhaven National Laboratory on Long Island, New York, developed a vacuum chamber with a unique feature. Surprisingly, his device is not completely sealed off; it has a window that keeps air out but allows particle beams in.

Hershcovitch’s device consists of a small tube on the wall of the chamber. Inside the tube, an electric field divides helium atoms into protons and electrons and accelerates the particles until they reach atmospheric pressure. At that point a valve in the tube opens, exposing the inside of the tube and vacuum chamber to the outside. But the plasma of charged particles rushing through the tube and past the small opening knocks away any slow-moving air molecules that try to proceed through it, much like a powerful waterfall deflecting a tossed ball.

Hershcovitch says his plasma window can preserve a vacuum as well as a solid window, but with the advantage that high-energy beams of radiation or charged particles can pass through with little trouble, like bullets penetrating water.

The plasma window could simplify the electron-beam etching of circuit patterns on computer chips. Instead of constantly inserting chips into and removing them from the vacuum chamber where the electron beam is generated, the chips could pass by a plasma window in assembly-line fashion outside the chamber. The electron beam inside the chamber, which would typically be about a thousand times as energetic as the plasma-window particles, could easily shoot through the opening. This is all done in a vacuum now, Hershcovitch says. But with a plasma window, you can do these applications in a normal atmosphere.
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