The plasma window (not to be confused with a plasma shield) is a technology that fills a volume of space with plasma confined by a magnetic field. With current technology, this volume is quite small and the plasma is generated as a flat plane inside a cylindrical space.
Plasma is any gas that has had some of its atoms or molecules ionized and is generally held to be a separate phase of matter. This is most commonly achieved by heating the gas to extremely high temperatures, although other methods also exist. Plasma becomes increasingly viscous (thick) at higher temperatures, to the point where other matter has trouble passing through.
A plasma window's viscosity allows it to separate gas at standard atmospheric pressure from a total vacuum, in fact it is reported that it can withstand a pressure difference of up to nine atmospheres. At the same time, the plasma window will allow radiation such as lasers and electron beams to pass. 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. One of the major applications of this technology is electron beam welding, where it has made EBW practical outside of a hard vacuum.
Further inventions using this principle include the Plasma Valve in 1996. 
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 recompression. This gives researchers time to shut off the particle beam in the accelerator and slowly recompress the accelerator ring to avoid damage.
The only limit to the size of the plasma window are current energy limitations as generating the window consumes around 20 kilowatts per inch (8 kW/cm) in the diameter of a round window.
The plasma window emits a bright glow, with the color being dependent on the gas used.
Similarity to "force fields"
In science fiction, such as the television series Star Trek, a fictional technology known as the "Force shield" is often used as a device. In some cases it is used as an external "door" to hangars on spacecraft, to prevent the ship's internal atmosphere from venting into outer space. Plasma windows could theoretically serve such a purpose if enough energy were available to produce them. The StarTram proposal plans on use of a power-demanding MHD window over a multi-meter diameter launch tube periodically, but briefly at a time, to prevent excessive loss of vacuum during the moments when a mechanical shutter temporarily opens in advance of a hypervelocity spacecraft.
- Shiga, David (2006-07-17). "Plasma bubble could protect astronauts on Mars trip". New Scientist. Retrieved 2008-04-02.
- "Plasma Window Technology for Propagating Particle Beams and Radiation from Vacuum to Atmosphere". Nasa Tech Briefs. 1998-05-01. Retrieved 2008-04-02.
- Kaku, Michio (2008-03-14). "Physics of the Impossible: A Scientific Exploration into the World of Phasers, Force Fields, Teleportation, and Time Travel". The Wall Street Journal. Retrieved 2008-04-02.
- "Hot mettle". New Scientist. 2003-04-12. Retrieved 2008-04-02.
- United States Patent: 5578831:Hershcovitch (November 26, 1996) Method and apparatus for charged particle propagation
- United States Patent: 6528948:Hershcovitch (March 4, 2003) Plasma valve
- "StarTram2010: Maglev Launch: Ultra Low Cost Ultra High Volume Access to Space for Cargo and Humans". startram.com. Retrieved April 28, 2011.
- Ady Hershcovitch (1995). High-pressure arcs as vacuum-atmosphere interface and plasma lens for nonvacuum electron beam welding machines, electron beam melting, and nonvacuum ion material modification, Journal of Applied Physics, 78(9): 5283-5288
- BNL Wins R&D 100 Award for `Plasma Window' 
- Ady Hershcovitch. Plasma Window Technology for Propagating Particle Beams and Radiation from Vacuum to Atmosphere. *
- Official article by Ady Hershcovitch - Inventor of the Plasma Window.
- Brookhaven Lab Wins R&D 100 Award for the "Plasma Window"
- Brookhaven National Laboratory - Where the Plasma Window was invented
- News on the Plasma Window
- Plasma Window Patent
- Plasma Valve Patent
- Diagram of plasma window in Ady Hershcovitch's electron beam welder (NewScientist)