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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