TAMA300, a laser interferometric detector for gravitational waves, involves two vacuum ducts (300 m long, 400 mm in diameter) and eight vacuum chambers (2 m high, 1.2 m in diameter). The required vacuum pressure is 10-6 Pa or less, in order to avoid phase fluctuations in the scattering of the laser beam by the residual gases. Surface treatments, such as TiN coating and electrochemical buffing (ECB), were examined as an outgassing reduction method without baking. The outgassing rates measured for these ducts using a conductance-modulation method, were less than 1 × 10-8 Pa m3s-1m-2 after 50 h pumping. We adopted the ECB treatment on the entire surface of the ducts and chambers. The TAMA300 vacuum system was completed at the beginning of 1998. After 300 h of the initial pump-down procedure without baking, the pressure was of the order of 10-6 Pa, though the outgassing rate was higher than that in the test duct by a factor of over 10.
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