Instability and Stability Test Results of the 30kA-NbTi Demo Poloidal Coil (DPC-U)

30kA, NbTi Demo Poloidal Coils (DPC-U1, U2) were fabricated and tested in the Demo Poloidal Coil (DPC) project at Japan Atomic Energy Research Institute (JAERI). The DPC-U has large forced-flow cable-in-conduit conductor and its strand surface is insulated by the formvar. The DPC-U exhibited an instability in pulse and even in DC charging, such as the DPC-U had quenches at the 1/10th current of the conductor critical current. Detailed investigations on the quench currents and stability experiments were carried out in order to clarify the cause of the instability. These experimental results showed existence of the non-uniformity of current distribution in the conductor even in DC charging. And it could be concluded the instability was caused by the non-uniform current distribution and long time constant of current redistribution because of insulation between the strands, However, it was also shown the current distribution could be uniform, depositing energy on the conductor by the inductive heater without quench. Therefore it was suggested stable operation of the DPC-U was possible using inductive heater.