Location and repair of air leaks in the ATF vacuum vessel

On the basis of partial pressure rate-of-rise and base pressure measurements, it was determined that the Advanced Toroidal Facility (ATF) vacuum vessel had an air leak in the low 10^-4-mbar-L/s range. Large portions of the outside of the vessel are covered by the helical field coils and a structural shell, making the leak difficult to pinpoint. Various alternative leak-detection schemes that were considered are summarized and their advantages and disadvantages noted. In the method ultimately used, gum-rubber patches of various sizes ranging from 12.7 by 12.7 cm to 20.3 by 30.5 cm were positioned on the inside surfaces of the vessel and evacuated by the leak detector (LD). After roughly 5% of the surface was inspected in this way, a leak of >10^-5 mbar-L/s was discovered and localized to an area of 5 by 5 cm. Dye penetrant applied to this area disclosed three pinholes. Two small slag pockets were discovered while these points were being ground out. After rewelding, no further leakage could be found in the repaired area. Global leak rates measured after the machine was reevacuated indicated that this leak was about 30% of the overall leak rate