Cooling of superconducting machines

A machine such as a ship's engine has a superconducting component 15 requiring cooling for its operation, and includes a cooling system 40. The cooling system is operable in first and second modes. In a cool-down phase the cooling system is run in the first mode providing relatively high heat transfer from the superconducting component. On attainment of a desired operating temperature the cooling system is run in the second mode, providing lower heat transfer. This enables a reduced cool-down time of the machine, while allowing economical operation in normal service. The higher level of cooling in the first mode used during the start-up procedure can involve a colder cryogen, or a greater flow of coolant. One way of achieving the latter is to circulate the coolant in normally evacuated regions 16 during the cool-down phase, and then re-establishing the vacuum in these regions for normal service operation.