Plasma Window
Science
The plasma window is a technology that fills a volume of space with plasma confined by a magnetic field. With current technology, this volume is a flat "window" cut into some more permanent structure (like a metal wall).
Plasma is a state of matter similar to gas except that all of the particles in a plasma are ionized, i.e. carrying an electrical charge. As a consequence, the plasma state is very high energy compared to an un-charged gas. This tends to mean that plasmas only exist at very high temperatures. Gases become increasingly viscous (thick) with higher temperature, and consequently plasmas are extremely viscous.
A plasma window's viscosity allows it to separate air at standard atmospheric pressure from a total vacuum. At the same time, the plasma window will pass radiation such as lasers and electron beams. This property is the key to the plasma window's usefulness -- the technology of the plasma window permits for radiation that can only be generated in a vacuum to be applied to objects in an atmosphere.
A related technology is the plasma valve, invented shortly after the plasma window. A plasma valve is a layer of gas in the shell of a particle accelerator. The ring of a particle accelerator contains a vacuum, and ordinarily a breach of this vacuum is disastrous. If, however, an accelerator equipped with plasma valve technology breaches, the gas layer is ionized within a nanosecond, creating a seal that prevents the accelerator's decompression. This gives researchers time to shut off the particle beam in the accelerator and slowly decompress the accelerator ring to avoid damage.
History
The plasma window was published in the November 1, 1995 edition of the Journal of Applied Physics published by the AIP. The article was written by Ady Herschcovitz and entitled "High-pressure arcs as vacuum-atmosphere interface and plasma lens for nonvacuum electron beam welding machines, electron beam melting, and nonvacuum ion material modification".
Applications
The most important application for the plasma window at the turn of the 21st century is as a lens that can pass energy but enforce a strict boundary between atmosphere and vacuum. This attribute of the plasma window enables the design of new types of advanced radiation sources, and makes existing radiation sources cheaper to use.
At present, electron beam welding and melting are industrial processes that are superior, in many situations, to various alternatives, but too expensive to deploy for most projects. The original expense of electron beam technology was due to the lack of an efficient means for separating a vacuum, in which an electron source can operate, from atmosphere. Before the plasma window, the only solution to that problem was to operate a large, expensive vacuum chamber large enough to contain the metal parts being machined. The plasma window, by easily separating air from vacuum but still passing electrons, enables electron beam welding without any vacuum chamber except for the small tube wherein the electron beam is generated.
The plasma window also has important applications in scientific research, where it reduces the expense of exotic radiation sources.
Star Trek
Adherents of the ubiquitous science fiction TV series Star Trek, and to a lesser extent fans of other sci-fi literature, viewed the announcment of the plasma window with an excited fervor. This is because the plasma window is exceedingly similar to the force field, a fictional technology which is a mainstay in science fiction in general and star trek in particular.
The parallels between Star Trek and the plasma window go even beyond its similarity to the force field. Star Trek's plots are littered with exotic particles and radiation that scientists use to manipulate and probe physical phenomena. The plasma window, with its extraordinary utility in enabling better, cheaper artificial sources for radiation, is a bright promise of a future where we are able to produce that plethora of particles as readily as the crew of the USS Enterprise.