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Plasma Window Technology

Plasma Window Technology for Propagating Particle Beams and Radiation from Vacuum to Atmosphere

Vacuum and atmosphere are separated by a plasma window to facilitate nonvacuum electron-beam welding and nonvacuum material processing.

May 1998

Brookhaven National Laboratory, Upton, New York

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 vacuum regions, and even act as a pump:

1) 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.

2) 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.

3) 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 ofApplied Physics, 78 [9],5283 [1995].)

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 I 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.

Electron-beam welding has many well-known advantages over other techniques: a high depth-to-width ratio of the welds, very high energy efficiency (since electrical energy is converted directly into beam-output energy), low distortion, and the ability to weld reasonably square butt joints without filler-metal addition. Principal components of an electron-beam welding column assembly are: an electron gun, magnetic focusing lens, deflection coils, and the workpiece. The electron gun must be held at a pressure below 10^-4 Torr. In present-day technology, the vast majority of electron-beam welding, with very few exceptions, is done in vacuum. Some of the shortcomings of vacuum welding are low production rates due to required pumping time, and limits the vacuum system sets on the size of assemblies to be welded. Thus the major advantages of nonvacuum welding are the elimination of the vacuum-chamber evacuation time and limits on the weldment size.

However, since the electron gun must be kept in vacuum, in the few nonvacuiim electron-beam welding machines in existence, the beam itself is generated in high vacuums. Then it is projected through several orifices separating a series of differ-endally pumped chambers, before emerging into the work environment that is at atmospheric pressure. This causes large dispersion in the electron beam, which practically nullifies all the advantages of electron-beam welding.

Material modifications by ion implantation, dry etching, and microfabrication are widely used technologies, all of which are performed in vacuum, since ion beams at energies used in these applications are completely attenuated by foils and by long differentially pumped sections. Therefore, no attempts at applying these technologies in atmosphere were ever made. 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 as high a level as possible, preferably reaching operation in atmosphere.

To rectify the shortcomings of present-day vacuum-atmosphere interfaces, orifices and differentially pumped chambers can be replaced by a short high-pressure arc-a plasma window-that interfaces between the vacuum chamber and atmosphere and has the additional advantage of focusing charged particle beams.

For transmission of high-energy synchrotron radiation, conventional beryllium windows can be replaced with plasma windows to avoid many of the problems associated with solid windows. Plasma windows offer many advantages over presently used beryllium windows, since the interaction of high-energy photons with plasmas that form these windows is negligible, i.e., the radiation passes through unaffected. Additionally, a plasma window cannot be damaged by radiation.

This work was done by Ady Hershcovitch at the Department of Energy's Brookhaven National Laboratory,Upton, New York. Inquiries concerning rights for the commercial use of this invention should be addressed to David Langiulli, Office of Technology Transfer, Building 902C, Brookhaven National Laboratory, Upton, NY 11973.

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