Directly oil-free machined aluminum ultrahigh vacuum parts for the beamline screen monitor

A compact beam screen monitor (BSM) was produced for the beamline adjustment at the synchrotron light source. The assembly of BSM, composed of a built-in water-cooled BSM-body and a vacuum duct, was made of A6061T651 aluminum alloy. Various special designed diamond-edge gaskets with circular or rectangular shapes, made of A1050O aluminum alloy, were applied for the vacuum-sealing between the duct and the adjacent vacuum components in which the flanges without knife-edge were directly machined. All the aluminum components associate with the BSM were machined by the oil-free in-ethanol CNC machine and obtained a precise and ultra-clean surface that was ready for the installation without any further chemical cleaning process. This assembly is compatible to the ultrahigh vacuum (UHV) quality. The BSM-body was aligned by facing to the incident synchrotron light at 45°, and the image-profile on the surface of BSM was monitored through a glass-viewport located on axis perpendicular to that of the incident light. The assembly of BSM was installed in the 19B beamline of the 1.5 GeV Taiwan Light Source (TLS) for the photon exposure test. The ultimate pressure under 1.0×10⁻⁸ Pa near the BSM has been achieved after vacuum baking at 150 °C for 1 day that approved the leak-tight of the diamond-edge gasket sealing in UHV. When the synchrotron light was introduced to the beamline, the fluorescent image of the beam appeared on the surface of BSM was clearly observed that the upstream chambers of beamline can be aligned accordingly by monitoring the image on the face of BSM moved toward the center. However, the significant increase of the pressure over three decades was occurred as expected due to the photon stimulated desorption (PSD) during the photon exposure. The yield of PSD was quickly reduced from 1.0×10⁻² to 1.0×10⁻⁴ molecules/photon at the accumulated exposure beam dose of 10 Ah in few days, but took another one week for beam cleaning to achieve 1.0× 10⁻⁵ molecules/photon of PSD-yield at about 70 Ah. The aluminum BSM with the innovative design of diamond-edge gasket sealing and the oil-free CNC-machining possesses a superior quality of precise dimensional, UHV-compatible, and high reliable performance. The manufacturing process for the BSM can be applied to other UHV systems with special designed configurations such as the beam diagnostic elements or optical chambers etc. for the accelerators and the beamlines.