Parametric study of the TF coil design for the European DEMO

In addition to a reduced number of feeders, which would simplify their integration in the cryostat of the tokamak, the present study considers the possibility to reduce the voltage during safety discharge of the toroidal field (TF) coils. A lowered discharge voltage would ease the manufacture of the TF coils and reduce the operational risks. Both aspects closely related to the discharge time constant, determined by the ratio L-TF/R-d, where R-d is the resistance of the dump resistor and L-TF the inductance of the TF coil. The supply of current to two or more TF coils in series requires a reduced discharge voltage per TF coil. The present study considers a fixed total TF current of 14.9 MA-turn provided by the DEMO baseline 2018. The discharge voltage can be reduced by increasing the TF conductor current, and hence lowering the inductance, or by an enlarged discharge time constant. For an increased discharge time constant, the copper cross-section in the TF winding pack needs to be increased to limit the hot spot temperature to acceptable values. A parametric study of the impact of increased TF conductor currents and discharge time constants on the radial build of the TF winding pack has been performed. The advantages of a possible reduction of the TF discharge voltage need to be compared with the disadvantages of an increased radial build.