Cryogenic Performance of Ultra-Thin Superconducting Solenoids for Cosmic-Ray Observation With Ballooning

Ultra-thin superconducting solenoids have been developed for cosmic-ray spectrometer ballooning around South Pole, BESS-Polar. The coils with a diameter of 0.9 m, a length of 1.4 m and a thickness of 3.5 mm, were wound with a mechanically strengthened aluminum stabilized superconductor and provide 0.8 T in the spectrometer. The BESS-Polar with the first solenoid was launched in Antarctica and observed cosmic rays for nine days in 2004. The solenoid system successfully provided the magnetic field of 0.8 T during the flight. Since then, the second solenoid has been developed for the next flight. A longer liquid helium life time, which determine the observation period, is a major improvement in the second solenoid with (1) adding a third vapor gas cooled radiation shield, (2) installing high Tc current leads at cold ends of gas cooled leads, (3) increasing tank volume. Consequently, the liquid helium life in the second solenoid is extended to 21 days which is double-long life than the one of the first solenoid.