Cooldown performance of an inner vertical field coil for the Large Helical Device

A single cooldown test of an inner vertical (IV) field coil for the Large Helical Device (LHD) was started on February 1 of 1995, and the superconducting transition of the coil was confirmed on February 23. The coil is a forced-flow type using a cable-in-conduit (CIC) superconductor and weighing about 16 tons. The total cooldown time was about 250 hours not including suspended time. Pressure drop characteristics were measured during the cooldown, and the same results were obtained as the R&D coil previously tested. The cooldown time could be reduced by indirect cooling using stainless steel sleeves with cooling pipes. The indirect cooling is effective for the CIC conductor which has no subchannel because otherwise an adequate mass flow cannot be obtained due to a high pressure drop in the high temperature region. The temperature distribution in the vertical direction was also examined during the indirect cooling.