In order to increase the cooling stability of the helical coils, an upgrade of the cooling scheme is planned from 4.4 K pool-boiling to 3.0 K sub-cooled helium of flow rate of about 50 g/sec. We have designed and constructed a R&D coil made of the same superconductor as that used for the helical coils and a sub-cooled helium test facility on the assumption of the application for actual Large Helical Device (LHD). This facility is composed of decompression tank for the sub-cooled helium generation, the R&D coil, a current lead storage tank, etc., installed in a cryostat with a liquid nitrogen shield. Two stage cryogenic compressors are installed in the top flange, and sub-cooled helium is generated by decompressing of saturated liquid helium from 0.12 MPa to 24 kPa. Liquid helium of 4.4 K and 0.12 MPa is heat exchanged by the sub-cooled helium in the decompression tank and supplied to the R&D coil from the bottom. The sub-cooling R&D experiment has been carried out. As the results of experiment, we found that the saturated liquid helium could be cooled from 4.4 K to 3.0 K in 8 hours and maintained at 3.0 K. The rotation speed of the first and second stage cryogenic compressors were about 87,000 and 88,000 rpm, respectively. These values agreed well with the designed values of the two-stage cryogenic compressors. Finally, this facility operated stably during 8 days of the two sub-cooling R&D experiment campaigns.
