Effective Vacuum Pumping of a Long HTS Power Transmission Line Insulated With Heat Radiation Shield

The energy loss of an HTS power transmission line is highly correlated to the properties of the vacuum thermal insulation. However, the proper evacuation of the very long cryostat requires a number of equally spaced pumping stations. The distance between pumping stations is determined by the pumping speed, the conductance of the cryostat, and the outgassing rate from inner surfaces. In this paper, the pumping of long cryostat is considered in the example of the Ishikari-2 project, Japan. This 1000-m-long HTS dc test facility rated for 50 MW was developed in order to obtain data for the next generation of power transmission lines. The vacuum pumping of the cryogenic pipe is carried out through ports located at the pipe ends. A 475-m-long straight section No. 1 is equipped with a heat radiation shield providing excellent thermal insulation of the cable pipe, whereas 137-m-long U-bend section No. 2 and 379-m-long straight section No. 3 are protected with conventional multilayer insulation. Experimental data allow us to estimate the lower limit of the pumping length. When using a cryogenic pipe with heat radiation shield "as is,'' it is possible to provide the required level of vacuum for a length of about 8 km in the nighttime and about 5 km in the daytime, because outgassing rate correlates well with the temperature of the outer pipe, varied from 4 °C to 41 °C. Therefore, if we take measures against the heating of the outer pipe by direct sunlight, and redesign the radiation shield to improve pipe conductance, the possibility to evacuate 10-km-long cryostat by two vacuum pumps installed at the ends can be achieved.