Cyclic Behavior of Wendelstein 7-X Magnet System During First Two Phases of Operation

The sophisticated large magnet system of the Wendelstein 7-X (W7-X) stellarator has been operated during first two experimental campaigns at the Max-Planck-Institute for Plasma Physics in Greifswald, Germany, for roughly 13 months. Its 70 superconducting coils (NbTi CCIC) are extraordinary not only due to complex 3D shapes of 50 non-planar coils, but also due to a non-linear support system. In addition, five big resistive coils with the aim to correct W7-X error fields are installed on the outer cryostat and use rubber pads in their supports to compensate thermal expansion of the coils. The structural behavior of the W7-X magnetic system is monitored and evaluated on the basis of the analysis of the signals from the extended set of mechanical and temperature sensors. Several cooldown/warming up and thousands of electromagnetic cycles with different loading patterns and with up to 70% design load magnitude have been performed by the system successfully. The focus of this paper is on the cyclic structural behavior of the W7-X magnet system and the comparison with finite element predictions. Several related issues such as bolts and rubber pads prestress degradation, support slippage development, evolution of mutual coil displacement, loading path dependence of stress levels, sliding weight support (cryoleg) adjustment, and sensor failure are addressed. Lessons learned so far are also briefly summarized.