Effect of magnetic field distribution on recovery currents of NbTi superconducting conductors

NbTi conductors for helical coils of the Large Helical Device were developed to satisfy cold-end stability. Their recovery currents were measured with a conductor test facility with 9 T split coils at the National Institute for Fusion Science. The measured recovery currents were higher by 15 to 20 % than that calculated from Maddock's equal area theorem with the measured conductor resistance and heat transfer. We have proposed an analytical method to estimate the recovery current in a finite magnetic field using the temperature distribution that is calculated with representative thermal conductivity, resistivity, and heat transfer. In order to check the validity of this method, we carried out simulation with a finite-difference method. The results revealed that the proposed analytical method is applicable with slight underestimation as long as the resistivity can be fitted by a function of temperature only. In addition, the necessary length of the magnetic field higher than 95 % is around three times of the temperature characteristic length of the conductor for measurement of recovery currents with an overestimation of less than 5 %. (author)