Cooling stability test of He II cooled LHD conductor (2) - experimental results

Cooling stability tests of the Large Helical Device (LHD) conductor immersed in pressurized He I and He II were carried out. A small test coil wound and short-circuited with a LHD conductor on a stainless steel bobbin was used. The test coil was set coaxially in the center of a superconducting magnet (field magnet), which supplies a certain magnetic field to the test conductor. A large current was supplied to the test coil conductor by use of a transformer effect, that is, the test coil current was induced by increasing the current of the field magnet. Stability tests of the LHD conductor at a certain pulse heat input were performed for the magnetic flux densities from 1.2 T to 6.8 T and the bulk liquid He temperatures from 2.0 K to 4.2 K at atmospheric pressure. Experimental results can be classified into three groups. The normal zone arose only around the heater and disappeared after the heat pulse (Group I). The normal zone moved to only one side direction and disappeared (Group II). The normal zone spread on both sides and the conductor current decreased greatly (Group III). The maximum conductor current without a quench at the pulse heat input increased greatly by shifting to He II regime from He I regime. The advances of the He II cooling on the stability of the LHD conductor were confirmed.