Feasibility of poloidal superconducting coils for a medium size Tokamak

A model pulsed coil (RPC-II) with an average ramp rate of 200 T/a has been built and tested in order to meet the requirements of superconducting poloidal coils for a medium size Tokamak. The inner and outer diameters of the coil are 19.0 and 47.2 cm, respectively, with an axial length of 43.1 cm. The coil has a solenoid winding with a layer to layer cooling spacing of 2.2 mm and insulation sheets between layers. The pulsing tests were performed by a capacitor discharge method with a crowbar circuit. By applying a terminal voltage of 7.6 kV, the field was raised up to 3.9 T in 19 ms, slightly increased up to the maximum field 4.0 T in about 120 ms from the start, and decayed with a time constant of 450 ms. This mode simulates the operation of ohmic heating coils. The average and maximum pulsing rates in charge-up period were 200 T/s and 314 T/s, respectively. The associated a.c. loss per pulse was 0.4 kJ which was only 0.3 % of the energy stored in the coil. After twelve pulses with a repetition period of 15 min., no change in the pulsing characteristics of the coil has been observed. These results showed the scientific feasibility of superconducting poloidal coils to be used in long-pulse medium size tokamaks