Storage Ring Readied for High-Current Operation

For high-current operation, the APS must have an effective storage-ring vacuum system to reduce scattering of the positron beam by the residual gases in the accelerator. A variety of pump technologies are being used to provide ultra high vacuums. Sputter ion pumps containing very reactive metal alloys [called non-evaporable getters (NEGs)] are mounted on each section of vacuum chamber around the ring to efficiently remove the gas loads caused by thermal outgassing. Concentrated quantities of NEG material are mounted near photon absorbers to increase the system pumping speed near the highest gas loads. In addition, NEGs bonded to constantan foil are distributed around the ring to pump out distributed gas loads due principally to thermal outgassing of the vacuum vessel.

After the APS vacuum system was installed, however, a problem with the NEG strips became apparent. The NEG strip was designed to be "activated" by means of resistive heating of the constantan foil to 450 C. The foil was isolated from the chamber by carriers that had alumina stand-offs. The carriers were designed to slide freely in a channel of the aluminum extrusions used for the vacuum chamber. However, for a variety of reasons, the friction of some of the carriers in the channel was sufficient to immobilize the carriers. As a result, after resistive heating, the NEG strips expanded and curled like ribbons. The curls were often large enough to touch the chamber, shorting the strip to ground.

A new NEG carrier has now been designed, fabricated, and installed in the vacuum system. In these new carriers, the strip can grow thermally and never touch the chamber. The new carriers are fabricated with the old carrier base but have several new features that correct all the shortcomings of the previous design. In the new carrier, NEGs move freely in the chamber channels as they are heated.

Currently, the new NEGs have been installed in the storage-ring vacuum chambers, and baking will be complete by September 30. Once all chambers are baked and the strips are activated, the vacuum system should have base pressures below 10 Torr, which should be adequate to achieve the design beam lifetime of more than 10 hours for 100 mA of positron current.

-R. Dortwegt, J. Noonan, and S. Sharma
Accelerator Systems Division

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