Cooling stability test of He II cooled LHD conductor (1) - current supply and measuring method

This paper deals with experimental methods to perform the dynamic stability test of large-scale superconductors cooled by He II. The large helical device (LHD) conductor used in this work has a critical current of dozens of kA. It is difficult to supply such a large current to the conductor immersed in pressurized He II by use of an external power supply in a laboratory scale. The transformer type current supplying method was proposed. The small test coil was wound and short-circuited with the LHD conductor on a stainless steel bobbin. The test coil was set in the center of a superconducting coil coaxially, which supplies an external magnetic field to the test coil. The test coil was inductively coupled with the field coil at a large turn ratio. The large current through the short-circuited test coil was excited by changing the outer field coil current at a constant rate and was kept constant to keep the field coil current. In order to obtain the required set of the test coil DC current and the magnetic fields for the stability tests, the test coil current was attenuated at an appropriate time by energizing a heater that attached on the test coil. The short-circuited test coil current was monitored from measured data of the magnetic field by a Hall element. The calculated current was calibrated by use of a Rogowski coil set to the conductor. The cooling stability test of the LHD conductor can be carried out successfully by use of the proposed method.