Device for direct-current transmission and cooling process

There is provided a device for DC transmission with a superconducting transmission line having at least a superconducting conductor element and a cooling device for cooling a radially inner portion of the transmission line by means of a fluid coolant to a temperature below a critical temperature of the superconducting conductor element. The superconducting transmission line comprises a vacuum-insulated sleeve for the thermal separation of the inner area of ​​the transmission line from a warmer outside environment. Furthermore, the cooling device comprises at least one feed device for supplying coolant to an end portion of the transmission line in the inner region of the transmission line. The transmission line has a longitudinal dimension of at least 20 km is free from axially inward horizontally arranged feeding devices for feeding coolant to this length. Furthermore, a method for cooling a superconducting conductor element in such a device to a temperature is specified below its critical temperature at which the fluid coolant is fed only in one or two end portions of the transmission line in the inner region of the transmission line.