AVS 46th International Symposium
    Vacuum Technology Division Thursday Sessions
       Session VT-ThM

Paper VT-ThM7
LIGO Beam Tube Component and Module Leak Testing

Thursday, October 28, 1999, 10:20 am, Room 610

Session: Outgassing, Leaks, and Mass Flow Controllers
Presenter: W.A. Carpenter, Chicago Bridge and Iron Co.
Authors: W.A. Carpenter, Chicago Bridge and Iron Co.
P.B. Shaw, Chicago Bridge and Iron Co.
R. Weiss, Massachusetts Institute of Technology
L. Jones, California Institute of Technology
Correspondent: Click to Email
LIGO (Laser Interferometer Gravitational-wave Observatory) is a joint project of the California Institute of Technology and the Massachusetts Institute of Technology funded by the National Science Foundation. The project is designed to detect gravitational waves from astrophysical sources such as supernova and the formation of black holes. The LIGO project constructed facilities at two sites in the U.S. Each site includes two perpendicular laser beam tube lines (each 4 km long) which join at one end to form an "L" shape. The beam tube is a stainless steel, ultra high vacuum tube which will operate at a vacuum of 1 x10@super -9@ torr or better. The beam tube was manufactured using a custom spiral weld tube mill and was manufactured with special emphasis on reduced outgassing rates. The integrity of the beam tube was assured by leak testing each component of the beam tube system prior to installation and then to leak test each 2 kilometer isolatable beam tube module after completion. This paper discusses the leak detection procedures used to leak test 16 km of 1.25 m diameter UHV beam tube used in the LIGO project. The beam tube was leak tested in four steps including leak testing fabricated tube sections, local leak testing of 250 mm diameter valve and valve nozzles, leak testing circumferential welds joining tube sections together and final leak testing of the installed beam tube module. The component leak testing included 800 tube sections which were 20 m long, 808 circumferential welds and 72 valved nozzles. Each component was tested to a sensitivity of better than 1x10@super -10@ atm. cc/sec of helium. The leak test of the 2 kilometer beam tube module would have been extremely difficult and expensive using standard helium leak detection techniques. Therefore, a method was developed utilizing a Residual Gas Analyzer (RGA) to measure the leak tightness of the two kilometer long modules. A method was also developed to utilize nine RGAs to locate any detected leaks.