Operation of Superconducting Quadrupoles in a Radioactive Environment

The first superconducting triplet quadrupole (STQ1), located close to the production target of the BigRIPS in-flight fragment separator, has been exposed to high radiation for 17 years. Intense heavy ion beams from deuteron to uranium with the beam power of 1–20 kW and the energy of 230–345 MeV/nucleon were used to produce the radioactive isotope beams. Based on the operation record of the BigRIPS fragment separator and radiation transport calculations, the accumulated dose to the STQ1 coils is evaluated to be approximately 2 MGy. The STQ1 is a combination of three air-core quadrupoles with flat racetrack-shape coils made of copper-stabilized NbTi wire wound orderly in layer with epoxy resin. The maximum current in the largest coil of the STQ1 magnet is 628 A, but quenching occurred four times. The quench current gradually decreased from 625 A to 600 A as the accumulated dose increased. Since the commissioning of the STQ1 coil, voltage spikes were observed and recorded in the first excitation after precooling operations. From the excitation data of “mechanical training” ramps, we discuss a possible degradation of the coils using statistics of the voltage spikes.