Vacuum and atmosphere are separated by a plasma window to facilitate nonvacuum electron-beam welding and nonvacuum material processing.
Plasma windows can separate vacuum and atmosphere, or regions of high and low vacuums, in a way which facilitates transmission of various particle beams and/or radiation from low- to high-pressure regions. In a prototype device, a stabilized plasma arc was properly configured to function as a nonsolid plasma window. Since charged particles can be focused by these arcs, focusing of such beams is a secondary function of this device.
Charged particle beams (ions, electrons) and intense x-rays are used in many applications. These beams must be generated in a high vacuum. However, in most applications (like electron-beam welding and material modification by ion implantation, dry etching, and microfabrication), it is desirable to utilize these beams at atmospheric pressure, since vacuum operation greatly reduces production rates and limits workpiece size. Also, it is desirable to raise the pressure at which electron-beam melting for manufacturing alloys is performed to facilitate manufacturing of superalloys from scrap recovery. This plasma window can facilitate nonvacuum ion material modification, manufacturing of superalloys, and high-quality nonvacuum electron-beam welding.
In vacuum systems that are used in various industrial devices, and in space vehicles, vacuum-atmosphere separation is accomplished with solid walls and windows. In the plasma window, confined ionized gas accomplishes this separation.
Three effects can enable a plasma to provide for a rather effective separation between vacuum and atmosphere, as well as between vacua regions, and even act as a pump:
- Gas pressure effect: Pressure P (the product of density N, temperature T, and the universal gas constant R) is described by the equation P=NRT. In the plasma window T=12,000 °K compared to room temperature of 300 °K. Therefore, the plasma window matches atmospheric pressure with only 1/40 of its density.
- Viscosity effect: Viscosity (friction, resistance to flow) of a gas increases with temperature. Consequently gas flow through a hot plasma-filled channel is substantially reduced compared to a room-temperature gas-filled channel.
- Pumping effect: Gas atoms and molecules are ionized by plasma electrons and are trapped by the fields confining the plasma window. Experimentally, these effects contributed to a factor of 228.6 pressure reduction over differential pumping. (Details of the theory and the experiments can be found in A. Hershcovitch, Journal of Applied Physics, 78 , 5283 .)
Plasmas act as lenses on charged particle beams in general. The currents in this plasma window generate azimuthal magnetic fields which deflect (focus) the particles radially inward (via the Lorentz force). This plasma lens is stronger than the general case where the beam-generated field focuses. More details can be found in the paper cited above.
Curiously, the plasma window functions in a way which very superficially resembles the force field in the Star Trek TV series. For example, there is an area on the Enterprise (the Shuttle Bay) from which shuttle crafts leave for flights into space. At the edge of that area there is a force field (with a bluish glow at its perimeter) which separates the atmosphere (air) on the Enterprise and the vacuum (space) outside. Similarly, in the plasma window, a plasma (which is an ionized gas confined by electric and magnetic fields) separates air from a vacuum by preventing the air from rushing into the vacuum. A variety of gases can be used to operate this window. When argon is used, the window color is blue, similar to that shown in Star Trek.