Refrigerator operation during commissioning and first plasma operations of Wendelstein 7-X

In February 2015 began the cool-down of about 450 tons cold mass of Wendelstein 7-X i.e. 70 superconducting coils, 14 currents leads, massive support structure and the thermal shield, enclosed within a vacuum vessel of about 16 m outer diameter. After a smooth cool-down, the temperatures around 5 K were achieved within about 4 weeks, in the short standby mode with the thermal shield return temperature <80 K. Later, for more than a year, these components were kept cold at different temperatures within various operating modes of helium refrigerator. The operation of helium refrigerator for the controlled cool down of such massive components and operating them at various cooling modes using the cold compressors and cold pumps was quite challenging. The paper describes the details of these operations and the problems faced.