Vacuum leak search on Wendelstein W7-X using a numerical regression analysis

• The vacuum leak search in the stellarator W7-X is described.
• The numerical regression analysis for leak localization is described.
• The results of the regression analysis are presented.
• The limitations of the regression analysis are discussed.
• The regression analysis is compared to the standard leak search techniques.

The stellarator Wendelstein W7-X, a fusion plasma experiment, is almost ready to be set into plasma operation. Seventy superconducting coils produce the magnetic field that is required to confine the plasma. The coils are arranged in a cryostat vacuum vessel in shape of a torus. The superconductors are cooled by supercritical helium down to the nominal operating temperature between 3 K and 4 K. Before operation, vacuum leak search was performed on the machine, both the cryostat and the plasma vessel. Most of the leaks could be fixed; however not all. But the residual helium leak rate is too small to hamper the successful coil operation. A refined leak search is pending for the future, in order to localize and fix all the remaining leaks. This paper describes a numerical regression analysis, performed to localize these remaining leaks. The regression analysis could gain information on the size and position of the leak, and, at least in principle, on the hydraulic helium flow behavior inside the leak channel. It could be a helpful complement to the standard leak search techniques that had been used before.