Vacuum leak search on the Wendelstein 7-X cryostat vessel

A new plasma fusion experiment, the Wendelstein 7-X (W7-X) has been built in the Max-Planck Institute for Plasma Physics in Greifswald, Germany. A cryostat vacuum tank, shaped like a torus, hosts 70 superconducting magnetic coils to produce a toroidal magnetic field that will confine a fusion relevant plasma. The cryostat tank has a volume of about 420 m3 and is equipped with several hundreds of vacuum ports, feed-throughs and service openings. Nested inside the cryostat is the plasma vessel with a volume of about 80 m3, also shaped like a torus. This plasma vessel is connected to the outside by 254 ports that traverse the cryostat. These ports are needed as access for plasma diagnostics, plasma heating and the supply of cooling media. We report in this paper on the vacuum leak search on the W7-X cryostat, both for the internal piping as well as the integral leak search of the entire vessel against air. This is the initial leak search prior to the first cool-down of W7-X. Emphasis is put on the description of the methods, rather than on the results. The technical commissioning of the cryostat vacuum system within a limited time was the primary task; the emphasis was therefore much more on achieving practical results (the leak-tightness) rather than answering fundamental questions.