Outgassing Rate of the Copper-Plated Beam Tube for ISABELLE

The ultrahigh vacuum system of the intersecting storage accelerator, ISABELLE, will consist of two interlaced rings of stainless steel beam tubes with a circumference 212 miles each. To obtain a good heat conduction during bakeout and to reduce the resistive wall instability during beam operation, a 1mm thick copper coating will be electroplated to the outer surface of this 1.5mm thick beam tube. To minimize the beam loss due to beam-gas collision, the pressure inside the beam tube is required to be 1 × 10-11 Torr (N2 equivalent) or less. To achieve this ultrahigh vacuum, the outgassing rate of the 304 LN stainlefs steel tubes has been reduced to ~1 × 10-13 Torr·l/cm2· sec by vacuum firing at 950°C for one hour. However, during acid-bath electroplating of copper, significant amount of hydrogen will be reintroduced and trapped in stainless steel which will substantially increase the outgassing rate (to ~2 × 10-12 Torr·l/cm2 sec). The outgassing characteristics of these copper-plated beam tubes are studied and discussed within the scope of diffusion and energy of activation. Methods to reduce the outgassing rate to an acceptable level (~l × 10-13 Torr·l/cm2·sec) are also given.