Long-Term Operation of the Superconducting Triplet Quadrupoles With Small Cryocoolers for BigRIPS In-Flight Separator and RI-Beam Delivery Line at RIKEN

We report long-term operation of the superconducting triplet quadrupoles (STQs) with small cryocoolers for the BigRIPS in-flight separator and RI-beam delivery line. Each STQ is cooled by liquid helium bath cooling method with a small stand-alone cryogenic system in which a Gifford-McMahon (GM)/Joule-Thomson cryocooler and a GM cryocooler are mounted on the cryostat. While the GM/Joule-Thomson cryocooler recondenses evaporating helium gas in the helium vessel, the GM cryocooler cools the 70 K shield and current leads by conduction. We operate 21 STQ systems forming beam lines for more than five years. We report on how we maintain cryocoolers without warming up the magnets. Operational experiences including troubles in the long-term operation are also reported.