The effect of local conductor failures on the operation of superconducting magnet

''There is a real danger during the manufacturing of the magnets based on niobium-tin, after the method react and wind'' to cause local mechanical failures to brittle intermetallic, superconducting wires. The presence of such failures was, as we believe, one of the most probable reasons for reduction in the current-carrying capacity and in the emergence of additional heat losses in some units of the superconducting magnet in T-15 tokamak. This assumption is confirmed by a number of the experimental data obtained at various stages of its study. In particular, in the process of testing of operating modules of the magnet at the SIMS-test facilities RRC KI a non-linear active voltage increase attaining 2-10 mv/unit at the test current limit 6.9-7 kA was revealed in all the units. The absolute values of active voltage, and, even more, their spread in various units cannot be explained by a smooth transition volt-amp characteristics of the wires used for manufacturing of a superconducting current-carrying element and by a voltage drop across the joints. Moreover, measuring an active resistivity, it has been found out that the main contribution is made by one of the pancakes. In some cases, such pancakes initiated the unit transition into a normal state at a current lower that 6.9 kA. For the majority of units with a relatively-low resistivities an active voltage difference between their halves is considerably lower than the whole voltage. Given facts can probably be explained by the presence of failures in superconducting wires of a current-carrying element in a number of pancakes within the coil. Under certain conditions, the transport current is forced to flow in copper around these failures. In this case probably there are both small defects-slightly affecting the current-carrying capability and large local ones in separable pancakes of some coil which can considerably reduce the magnet operation, as a whole. (orig.)