The Plasma Window is a stabilized plasma arc used as an interface between accelerator vacuum and pressurized targets. There is no solid material introduced into the beam and thus it is also capable of transmitting particle beams and electromagnetic radiation with low loss and of sustaining high beam currents without damage.
In Simple words, if a gas is heated to a high enough temperature, thereby creating a plasma, it can be molded and shaped by magnetic and electrical fields. It can, for example, be shaped in the form of a sheet or window. Moreover, this “plasma window” can be used to separate a vacuum from ordinary air. In principle, one might be able to prevent the air within a spaceship from leaking out into space, thereby creating a convenient, transparent interface between outer space and the spaceship.
The plasma window was invented by physicist Ady Herschcovitch in 1995 at the Brookhaven National Laboratory in Long Island, New York. He developed it to solve the problem of how to weld metals using electron beams. A welder’s acetylene torch uses a blast of hot gas to melt and then weld metal pieces together. But a beam of electrons can weld metals faster, cleaner, and more cheaply than ordinary methods. The problem with electron beam welding, however, is that it needs to be done in a vacuum. This requirement is quite inconvenient, because it means creating a vacuum box that may be as big as an entire room.
Dr. Herschcovitch invented the plasma window to solve this problem. Only 3 feet high and less than 1 foot in diameter, the plasma window heats gas to 12,000°F, creating a plasma that is trapped by electric and magnetic fields. These particles exert pressure, as in any gas, which prevents air from rushing into the vacuum chamber, thus separating air from the vacuum. (When one uses argon gas in the plasma window, it glows blue, like the force field in Star Trek)
The plasma window has wide applications for space travel and industry. Many times, manufacturing processes need a vacuum to perform microfabrication and dry etching for industrial purposes, but working in a vacuum can be expensive. But with the plasma window one can cheaply contain a vacuum with the flick of a button.
Many industrial and scientific processes like ion material modification, electron beam melting and welding, as well as generation of high energy radiation are performed exclusively in vacuum nowadays, since electron guns, ion guns, their extractors and accelerators must be kept at a reasonably high vacuum.
A) Commercial Applications
a) Non-vacuum electron beam welding: With plasma windows, higher production rates, no limit on the size of target objects, and high quality electron beams in atmosphere.
b) Non-vacuum material modifications by ion implantation, and dry etching, or micro-fabrication:
Presently performed only in vacuum, since ion beams at energies used in these applications are completely attenuated by foils and by long differentially pumped sections. Potentially very large yet unexplored market.
c) Electron beam melting: for manufacturing alloys is performed at a pressure of about 10-2 Torr. A major drawback of operating at this pressure range is the loss of elements with low vapor pressure. Consequently, it is desirable to raise the operating pressure to a higher level.
d) Electron beam generation of photo-neutrons for the production of medical isotopes (e.g., Tc-99):
A 40 MeV electron beam strikes a W target. Resultant radiation can dislodge a neutron (via a giant resonance) to create a new element. With a plasma window, the target can be in the air, sufficiently cooled to absorb an intense electron beam to generate photo-neutrons.
e) Windowless gas targets for fast neutron radiography to detect nitrogen (weapons) and carbon (diamonds), as
well as for other forms of neutron tomography and therapy (BNCT).
f) Windows for high power lasers (especially high- pressure gas lasers).
B) Scientific Applications
a) Windowless beamlines for transmission of synchrotron radiation
: Plasma windows offer many advantages over presently used beryllium windows: radiation passes through the window unaffect
ed. A plasma window cannot be damaged by radiation. UV filter for rejection of ‘high-order’ light is of significant benefit to experiments like threshold photoionization spectroscopy, where contamination even at the 10-4 harmonic content can obscure the features of the spectrum of interest. X-ray microscopies, since it is free from the attenuation and spatial structure that attend the use of conventional window materials (e.g. beryllium or SiN), i.e., no scratches.
b) Windowless gas targets for fast (fusion) neutron generation (neutron sources).
c) Radioactive waste transmutation (ATW): protons are accelerated to 2 GeV unto a heavy metal target in air.
Resultant spallation neutrons reduce radioactive waste. Presently, windows limit proton output.
d) APT- tritium production by accelerator: Solid windows limit beam and tritium output.
e) Spallation Neutron Source (SNS): To replace various solid windows with cooling problem.
f) Windows for high power x-ray source (DARHT).
g) Internal (gas or plasma) targets, strippers, lenses in storage rings: A plasma stripper/lens or an internal gas target “sandwiched” between two plasma windows.
Examples: BNCT based on recirculating proton beams and various internal targets (including spin polarized ).
h) Fast acting valves in UHV beamlines: In case of vacuum breach, plasmas can be ignited faster than mechanical valves without damage to beamline (unlike, presently used, msec spring loaded shutters).