Simulation and experimental study on lap joint between YBCO double pancake coils of high field insert magnets

In recent years, YBCO coated conductors (CC) are widely used in high field insert magnets, and YBCO CC joint is inevitable because of the piece length limitation of commercial YBCO tapes. Therefore developing suitable joint and achieving low resistance has become more and more important. In this paper, the equivalent circuit model of YBCO joint was established by simulating the distribution of the joint current. Then a soldering machine capable of soldering on both short YBCO tape joint and double pancake joint was introduced. And in order to find out the best soldering condition, joint samples were made to study the influence of soldering current, time and pressure. V-I curves of joint samples were obtained at 77 K in liquid nitrogen by using 4-probe method. Preliminary tests show that of all experimental soldering conditions, soldering current of 16A, soldering time length of three minutes and pressure of 10 Kg is superior to others. Resistance of joint fabricated under this condition is 80 nΩ and joint resistivity is 64 nΩ-cm².