Electromagnetic Force Effect on Pressure Drop and Coupling Loss of Cable in Conduit Conductor

Abstract

In the Engineering Design Activities of the International Thermonuclear Experimental Reactor (ITER), a Central Solenoid Model Coil (CSMC) and a CS Insert Coil (CSIC) have been tested successfully. The CSIC conductor consists of 1, 152 superconducting strands bundled on a central cooling channel. As interesting phenomena in the CSIC experiment, it was observed that a pressure drop of the CSIC decreased by about 12% during a current-carrying operation at 40kA, and coupling losses indicated an operating current dependence. It is considered as a hypothesis that an electromagnetic force causes a compressive deformation of superconducting cable in a jacket and that a new flow path was then generated between cable and jacket. Therefore it is also considered that the decreasing of contact resistance between strands as a result of the electromagnetic force derives an increase of coupling losses in the conductor. A pressure drop calculation model with a gap generated by electromagnetic force is constructed. The gap is estimated to be about 1.4mm at nominal operating conditions (13T, 44.3kA). From this calculation, a void fraction as a function of electromagnetic force is evaluated during the current-carrying operation of CSIC. The coupling time constant (nτc) as a function of void fraction is then calculated from the coupling loss measurement result during the pulsed operation of CSMC and CSIC. The evaluated nτc is about 24ms and is close to nτc of 20-30ms of a heat treated short sample having a history of exposure to the electromagnetic force. We used the evaluated nτc as a function of electromagnetic force to calculate the coupling losses, which varied from 24ms to about 50ms during pulsed current operation. These results show a good agreement with measured coupling losses, depending on coil current. To reduce the possibility of strand damage as a result of cable movement, we also here proposed that the void fraction of real ITER conductor should be smaller than that of CSIC, and it is preferable that the void fraction is about 34.5%. In this paper, the quantitative explanation of coupling loss change under the electromagnetic force is described from the viewpoint of the pressure drop change.