The Polo coil, a prototype tokamak poloidal field coil, design features and test results

A prototype tokamak poloidal field coil of 3 m diameter was successfully tested. The coil was designed to withstand typical field transients like fast ramping, plasma controlling and plasma disruptions. In order to achieve these requirements, a low loss conductor was developed. The coil with its components was designed according to rules of high voltage technology. A realistic test of the magnet requires at 23 kV a current of 15 kA, that means a peak power of 345 MW. Therefore, a fast switching circuit was used combined with an additional current lead at the middle of the magnet winding. This allowed the performance of fast field changes with increasing or decreasing field. In one operation mode, the coil was discharged into a dump resistor. In another operation mode, only one coil half is discharged which generated a field increase in the second short circuited half of the coil. Discharging time constants of 1 ms and local field sweep rates of about 240 T/s were obtained without quench. A reproducible and sharp quench boundary was found. The results showed clearly that ramp rate limitations are only given by the expected stability limit or the load line boundary.