New Materials and New Challenges in Vacuum Technology
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minum (for the chamber) and carefully selected aluminum alloys for appropriately matched thermal and mechanical properties in fittings and flanges. Sealing surfaces are usually coated with TiC. AU exposed aluminum surfaces are subjected to a spécial process which produces a very dense, thin, hydroxidefree aluminum oxide coating. Synchrotrons, especially dedicated synchrotron light sources, hâve unique vacuum problems extending beyond
the need for a low base pressure. The beam lifetime is determined not by the base pressure (beam off), but rather by the pressure obtained when the beam is on. As a resuit of photon- and electroninduced desorption, this pressure may be several orders of magnitude higher than the base pressure. High pumping speeds are therefore required, although the location of the synchrotron magnets and required magnet field uniformity make it difficult to provide sufficient lumped pumping speed to obtain the necessary operating pressure of 10'91010 torr. The photon beam itself provides a cleaning process. However, in order to use the beam-induced desorption to remove the residual gas trapped in near-surface layers, it is first necessary to obtain a beam. The surface must therefore be preconditioned to provide as little gas burden as possible. A number of techniques, including baking, gas discharge and chemical cleaning, hâve been employed. The latter technique applies primarily to préparation of sections prior to assembly. For stainless steel, and especially for aluminum chambers, the formation of water-containing oxide layers upon exposure to atmosphère requires that once treated, the chamber section be vented as little as possible, and that dry nitrogen be used as the vent gas.
Full Width Zr/AI Pumping Strips Extruded Aluminum Chamber
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Water Cooling 4in. 10 cm Figure 1. Cross section of the vacuum chamber for the Argonne National Laboratory Advanced Photon Source. See related article on the APS in this issue in the Research! Researchers section.
MRS BULLETIN/JULY1990
New Materials and New Challenges in Vacuum Technology
Several récent machines hâve made use of strips of Zr-Al alloy running around the circumference of the machine. This alloy acts as a nonevaporable getter (NEG) material,2 providing pumping speeds in excess of 1,000 1 s"1 m"1. The first machines to make use of NEG pumps simply placed the strips inside the beam chamber. More récent designs3 make use of a separate pumping chamber to house the NEG strips, communicating through a relatively narrow channel to the beam chamber. The resulting vacuum chamber has a complicated cross section (Figure 1) which is difficult to fabricate in stainless steel but which may be extruded in aluminum. Unfortunately, the photon-and electron-induced desorption cross sections for aluminum are higher than those of stainless steel, initially by a factor of 100, but dropping with continued beam exposure to a factor of approximately 10.45 In the newer machines using insertion devices which produce highly collimated r
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