LIGO Document P990023-x0

LIGO Beam Tube Component and Module Leak Testing

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P - Publications
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 black holes. The LIGO project constructed observatories at two sites in the U.S. Each site includes two beam tubes (each 4 km long) joined to form an L shape. The beam tube is a 1.25 m diameter 304L stainless steel, ultra high vacuum tube that will operate at 1 x 10-0 torr or better. The beam tube was manufactured using a custom spiral weld tube mill from materials processed to reduce the outgassing rate in order to minimize pumping costs. The integrity of the beam tube was assured by Helium Mass Spectrometer (HMS) leak testing each component of the beam tube system prior to installation. Each 2 kilometer long, isolatable beam tube module was then lead tested after completion.

This paper discusses the leak detection procedures used to test the 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 20 m long tube sections
Local leak testing of 250 mm diameter valves and valve nozzles
Leak testing circumferential welds joining tube sections together
Final leak testing of the installed 2 km long beam tube modules.
The component leak testing included 800 tube sections (20 m long), 808 circumferential welds and 72 valved nozzles. Each component was tested to a sensitivity of better than 1 x 10-10 atm. Cc/sec of helium.

The leak test of the 2 km long beam tube modules 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 beam tube module. Another method was developed to locate leaks from the pressure gradients in the beam tube using nine RGAs distributed along the length of the beam tube.

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Migrated document information from the old DCC:
- Full document number: LIGO-P990023-00-B
- Author(s): Warren A. Carpenter; Larry Jones; Brad Shaw; Rai Weiss
- Document date: 1999-10-28
- Document received date: 2000-09-22
- Document entry date: 2000-09-22
- Publcation info: J. Vac. Sci. Technol. A, 18, 1794-1799 (2000)
- Publication author(s): Carpenter, W.A., Shaw, P.B., Jones, L., and Weiss, R.
- Citation reference: American Vacuum Society Paper #1708, Program #VT-thM7 - 46th International Symposium - 28 October 1999
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