Please choose your delivery country and your customer group
Vacuum integrity of large fusion tokamaks has become increasingly important as the complexity and requirements for cleanliness have increased. Doublet 3 is a large noncircular tokamak designed for a base pressure of 2 x 10-8 T and a leak rate of 5 x 106 T.1/s. Initial leak testing of the vacuum chamber used conventional techniques. After final assembly, leak testing became more difficult as diagnostic systems, coil systems, and heating blankets enveloped the vacuum vessel. Methods were developed to locate and quantify vacuum leaks in this difficult environment. A residual gas anlyzer was used for temporal response to gases flowed at various points outside the vessel. Leaks occurring in the primary vessel wall were measured by pumping on gas cooling channels adjacent to the primary vacuum wall. Air in these channels was also displaced by other gases at a constant rate to give location of leaks to within 50 cm. Vacuum leaks occurred during machine operations due to applied stresses, plasma-material interactions, and diagnostic equipment failures. Machine vents to fix these leaks involve a significant loss of time, particularly in returning to clean wall conditions. Methods other than venting the vessel have been used, such as helium 'patches', if these leaks are sufficiently small.