Radiation Effects in Superconducting Quadrupoles for BigRIPS In-Flight Separator at RIKEN

Abstract

The BigRIPS in-flight fragment separator in the RIKEN Radioactive Isotope Beam Factory project has been operated for nine years. Intense beams of heavy ions up to uranium with the beam power of 1-13 kW and the energy of 230-45 MeV/nucleon were used to produce the radioactive isotope beams by the BigRIPS fragment separator. The first superconducting triplet quadrupoles (STQ1) is in close proximity of the production target in the BigRIPS fragment separator and has been exposed to high radiation. The STQ1 consists of three air-core NbTi coils, wound by the wet-winding method, assembled in the liquid-helium bath cryostat. Based on the operation records of the superconducting magnet and its cryogenic system, the accumulated dose to the coils is evaluated to be order of 1 MGy. The observed impurities in the helium gas, such as tritium and hydrocarbons in the STQ1 cryostat, are presented as a possible radiation effect. Based on the excitation data of training ramps, we discuss the movement of the coils and a possible degradation of coils. No distinct degradation in the coil performance can be observed after nine years of operation.