Soundness evaluation of cryogenic support structure of large helical device

A fusion magnet system using superconducting magnets must have a cryogenic support structure to sustain huge electro-magnetic force. Since the structure is constructed by welding and it supports static and dynamic forces, the monitoring system for soundness evaluation of the support structure has been desired. In the Large Helical Device project, strain gages and wires were developed and applied to the strain monitoring system. The gages were attached on the weld line to investigate the change in strain due to crack initiation on the weld root. All gages are still active and the monitoring system is working on well. No significant change in strain has been observed and it results in revealing the soundness of the support structure of the Large Helical Device.