Non-uniform current distribution in CICC (Cable-In-Conduit Conductor) is one of the major concerns for developing a successful magnet system. The three strand CICC magnet has been constructed for the study of current non-uniformity. The magnet is equipped with a forced flow supercritical helium system at variable temperature to simulate the various thermohydraulic conditions of superconducting tokamak. Each strand of the magnet has its own shunt resistor to measure the current it carries during the dc and ac operations. This paper describes an experimental apparatus, current measurement results of the individual CICC strands and 3 strand loop current model.
During the current ramp up, the unbalanced current distribution has been obtained directly from the shunt resistor voltage data. The surface contact resistance of the superconducting strands was small enough to allow the loop current to flow inside the conduit rather than at the terminal joints of the magnet. During the current ramp up at the unstable temperature, local quench occurred and the current was redistributed through the inlet shunt resistors and contact resistance.
To explain the generation of the loop current during the current ramp up, the three strand loop current model was proposed. This model gives an explanation for the relation between the loop current and the twist geometry of the strands. According to this model, the twist geometry of the strand has significant influence on the generation of the loop current especially in the large superconducting magnet.