Stress and strain measurement of the large helical device during coil excitation

The large helical device (LHD), which has a large superconducting magnet system, has been constructed and the magnetic field at the plasma axis reached to 2.9 T in the third cycle operation in 1999. To carry out the strain measurement without temperature rise by eddy current, slow ramp rate of the magnetic field was employed and the data acquisition system worked continuously during the coil excitation. The results obtained in the experiments showed that the deformation behavior of the cryogenic support structure could be explained qualitatively and it was confirmed that the FEM analysis gave good results in comparing with the stresses measured. Also, it is clarified that the strain measurement will give valuable information on the deformation and the stress condition even at cryogenic temperature, in high vacuum and under high magnetic field, and it is possible to evaluate the soundness of the cryogenic structure.