Monitoring of W7-X cryostat commissioning with cryostat system FE model

The Wendelstein 7-X stellarator started its first operational phase in October 2015 at the Max-Planck-Institute for Plasma Physics in Greifswald with the goal to verify that a stellarator magnetic confinement concept is a viable option for a fusion power plant. The main components of the W7-X cryostat system are the plasma vessel (PV), outer vessel (OV), 254 ports, thermal insulation, vessel supports and the machine base. The main task of the cryostat system is to provide an insulating vacuum for the cryogenic magnet system, UHV conditions within the PV and to provide external access to the PV through ports for diagnostic-, supply- and heating systems. The updated finite element (FE) Global Model of the Cryostat System (GMCS) has continued to be used for predicting and assessing the behavior of W7-X as measured during its commissioning and operational phase. The measurements with strain, temperature and displacement sensors as positioned on the OV, Ports, PV and its supports form the basis of the cryostat system monitoring. After successful evacuation of the OV [1] commissioning continued and in 2015 the PV has been evacuated and baked for the first time. The measurements show good correspondence with the predictions of the GMCS and allowed for continuation of the commissioning. This paper gives an overview of analyses performed with the GMCS in support of cryostat commissioning and operation. In addition, the assessment performed for optional PV position adjustment is presented. A PV adjustment might be required in case plasma operation reveals problems with the plasma heat load distribution on critical in-vessel components.