Design of a Cryogen-Free HTS Excitation Coil for a Homopolar Alternator

For the transition to future electric flight high-power-density superconducting machines may be an enabling technology. This also requires accurate thermal models to simulate superconductor cooling. A thermal model was designed for a cryogen-free, high-temperature superconducting coil in the temperature range $\mathbf{20}\ldots \mathbf{30}\ \mathbf{K}$. The coil will be used to excite a high-speed homopolar inductor alternator. Data for heat transfer at low temperatures is presented, particularly for the thermal connections, which make up a large part of the temperature gradient along the heat-conducting cooled system, which does not need any cryogenic fluid (“cryogen-free”). Hence, particular attention is given to the conduction-cooled current leads, as they are responsible for the main heat inflow from outside. A novel approach to thermally connect the current leads to the cold head is presented.