Cryogenics for an HTS degaussing system demonstrator

This paper describes the design, construction and test results of a high temperature superconducting (HTS) degaussing demonstrator system. Such a system compensates the local disturbance in the earth's magnetic field caused by the ferromagnetic hulls of ships, to prevent detection by active or passive magnetic field sensors. This is done by placing coils around the ship, creating a magnetic field opposing the effect of the earth's magnetic field. Degaussing systems for large naval vessels typically need currents of up to 1 or 2 kAturns, which gives rise to sizeable ohmic losses in conventional copper coils. These losses can be reduced if high temperature superconductors are used, since they have no electrical resistance when cooled down to temperatures below 90 K. For the demonstrator, 3 coils able to generate fields in 2 directions were realized both with HTS and copper to get a representative degaussing performance. A dedicatedly designed cooling system maintains the superconductors at a temperature of 77-85K using (subcooled) liquid nitrogen. Due to the relatively small laboratory scale that this first 1:5m long demonstrator system which was produced, the copper degaussing system is still more efficient than the HTS system because of the cooling power needed. A large fraction of this cooling power is needed to cool away parasitic heat loads, that hardly increases if the size of the system increases. Thereafter the performance of both systems was compared to evaluate on what scale HTS degaussing systems become more efficient than copper degaussing systems.