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 nonvacuum 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 differentially 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