Chemical cleaning of aluminum alloy surfaces for use as vacuum material in synchrotron light sources

Abstract

Photon and electron desorption from the vacuum chamber walls of electron storage rings such as the proposed advanced photon source (APS) are sometimes responsible for the production of large gas loads during operation, even in systems with very good static vacuum. The gas released by beam-induced desorption results in scattering of the beam electrons, and a consequent reduction in the beam lifetime. In extreme cases, the beam-induced outgassing may cause so much scattering that it is not possible to obtain the design goals with regard to obtainable beam current. Consequently, it is important that the surfaces that are exposed to the electron beam and photon fluxes contain as little trapped gas as possible, and that the gas burden during operation be kept as low as possible. The current study investigates the effectiveness of a chemical cleaning treatment developed at the Center for European Nuclear Research (CERN) for the large electron-positron (LEP) storage ring [A. G. Mathewson, LEP Vacuum Tech, Note Jan. 20, 1986; A.G. Mathewson, LEP Vacuum Tech. Note Jan. 15, 1986; A. G. Mathewson, et al., J. Vac. Sci. Technol. A 7, 77 (1989).] as applied to the 6063 alloy to be used in APS. Additionally, depth profiling of the chemically-cleaned samples and samples that were vapor-degreased only, provides new insight on the cleaning process as it applies to the alloy proposed for use in APS. To reduce the beam-induced outgassing of the APS vacuum chamber, a two-step cleaning process has been investigated, based on the treatment utilized for the 6060 Al used in the LEP vacuum chamber at CERN. The treatment is based on an understanding of the near-surface composition as follows: The surface consists of a superficial MgO layer, which is also contaminated with hydrocarbons, lying on top of another layer that is primarily Al2O3 (A.G. Mathewson, LEP Vacuum Tech. Note Jan. 20, 1986). In the first step of the cleaning process, MgO and C are removed by Almeco 18, an alkaline detergent manufactured by Henkel A. G. In the second step, Amklene, a modified KOH solution which is not capable of removing magnesium oxide, is believed to reduce the thickness of the remaining aluminum oxide layer. The current results suggest a modified interpretation of the mechanism by which these chemical cleaning agents alter the surface of the 6063 aluminum alloy. The surface contains both Mg (as MgO) and Al2O3, and the MgO concentration extends much deeper into the bulk than the aluminum oxide. Despite profiling reveals that the underlying aluminum is largely metallic, rather than Al2O3. The aluminum oxide layer observed after the Almeco 18 cleaning step is largely the native aluminum oxide layer that forms during room temperature exposure to air between cleaning operations. The Amklene step does little to remove the native aluminum oxide layer and at the same time acts to restore some of the original MgO layer. The present results do not show any obvious benefits arising from the use of Amklene after treatment in Almeco 18.