Developments and Applications for All-Aluminum Alloy Vacuum Systems
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TIN/JULY1990
The TRISTAN électron-positron collider constructed at the National Laboratory for High Energy Physics in Japan is the first all-aluminum alloy accelerator, and the first to use UHV technology. The collider has shown that beam lifetimes up to six hours for 9 mA, 29 GeV beams can be achieved without baking or discharge cleaning. Other applications using thèse advanced technologies, such as an XHV System with attainable pressures on the order of 10"13 torr, vacuum degassing fumaces, vacuum coaters, précision X-Y-9 tables, piezo stepping devices, honeycomb structures, and molecular beam epitaxy and chemical vapor déposition chambers are also described in this article. Materials and Surface Treatment Récent advances in the design of électron synchrotrons hâve placed severe requirements on the pumping and outgassing characteristics of the vacuum System materials. In order to meet thèse demands, new, complex cross-sectional geometries which require innovative fabrication technologies hâve been developed. In particular, aluminum extrusion technology has been developed to the point where extremely long beam chambers with very complex cross sections can be fabricated. During a conventional extrusion process, the end of the aluminum alloy chamber is open to a contaminating atmosphère. Therefore, the active inner surface, at a température of about 500°C, is immediately covered with a porous aluminum oxide/hydroxide layer about 120 Â thick. A spécial extrusion technique is utilized in the XHV process to prevent the growth of this
porous layer which, with trapped contamination, becomes a source of sustained outgassing. In this technique, an atmosphère of 7% oxyen and 93% argon is maintained in the chamber during both extrusion and tempering processes" as shown in Figure 1. Both ends of the extruded chamber are sealed vacuum tight during processing. As a resuit, the inner surface of the extrusion is covered by a fine, nonporous, dense oxide layer about 30 Â thick. The spécial extrusion process is available only for pipes with diameters less than 300 mm. For larger structures, final machining is done under an argonoxygen atmosphère as shown in Figure 2. For a surface finish with good vacuum properties it is essential that the water content of the oxygen be kept in the parts-per-million (ppm) range. Argon is not essential for a high-quality surface, but serves as a dilution gas for the oxygen to avoid the possibility of explosion. The inner surface of the vacuum component body is treated in this atmosphère. The treated surface is called an "EX processed surface."4 Depth profiles of the oxide layer of the EX processed surface were analyzed by Auger électron spectroscopy (AES) and it was found that the oxide layer for the EX processed surface was about 30 Â thick. In contrast, the oxide layer for an ordinary machined aluminum surface is 120-180 Â thick. The EX process is complicated and expensive. For larger diameter vessels, it was necessary to develop other methods to obtain a similarly qualified aluminum oxide layer.
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