Test of a New Cryogenic System for the Superconducting Triplet Quadrupole Magnet

Radioactive isotope (RI) beam delivery lines in the Radioactive Isotope Beam Factory (RIKEN RIBF) project are characterized by superconducting triplet quadrupoles (STQs) with cryocoolers. The STQ magnet is a large aperture superferric quadrupole triplet, whose LTS coils are cooled by liquid helium bath cooling method. The STQ cryostat is designed to be operated with two cryocoolers, and 22 STQ systems have been operated for more than fifteen years without warm-up. Originally, a Gifford-McMahon (GM)/Joule-Thomson (JT) cryocooler re-condenses evaporating helium gas in the helium vessel and a single stage GM cryocooler cools the thermal shield and current leads by conduction. We recently replaced some of the ageing GM/JT cryocoolers with re-condense type 4 K GM cryocoolers without warm-up the cryostats. Because the cooling capacity (1.8 W) of the 4 K GM cryocooler is smaller than that of the GM/JT cryocooler (2.5 W), we reduce the 4 K heat load to the helium vessel by lowering the thermal shield temperature. Replacing the compressor for the single stage GM cryocooler with a high-powered one, the cooling capacity of the GM cryocooler is improved, and the thermal shield temperature is reduced. Maintenance methods of two different cryocoolers are discussed together with reports on operational experiences in the long-term operation of the STQ systems.