Development and test of the poloidal field prototype coil POLO at the Forschungszentrum Karlsruhe

A superconducting prototype (3 m Ø and nominal current 15 kA) of a poloidal field coil was developed, constructed and tested according to the typical specification of the Tokamak magnet system in collaboration with European industry. In order to withstand fast ramping (≈2 T s−1), plasma disruption (80 T s−1, 10 ms) and plasma control (≈ ±0.05 T, 10 Hz) in the superconducting state a low loss conductor and a well developed high voltage technique at 4 K were indispensable attributes of the design. The sc cable of the conductor consists of mixed matrix strands (NbTi in Cu/CuNi) cooled by a dual cooling system (two phase forced flow helium for heat removal at constant temperature, and stagnant helium for good transient stability). The sc cable is inclosed in a thick walled stainless steel jacket. All coil components are designed and constructed according to the rules of the high voltage technique. The coil was tested in the TOSKA facility at FZK-Karlsruhe. For generating the typical positive and negative magnetic field transients a special high power switching circuit of a discharge power up to 700 MW (30 kA, 23 kV) was constructed and used. The coil was investigated considering its superconducting, electromagnetic and mechanical properties. All results were conclusive in agreement with calculations and fulfil the specifications. A highlight is that for this conductor type the stability against transient field changes is only determined by the expected stability margin or the critical current boundary no other limitations of the ramp rate were observed.