Ballooning of an Ultra-Thin Superconducting Solenoid for Particle Astrophysics in Antarctica

An ultra-thin superconducting solenoid has been developed to provide a magnetic field of 0.8 T in a balloon-borne spectrometer for cosmic ray research, which is named BESS-Polar. The coil with a diameter of 0.9 m, a length of 1.4 m and a thickness of 3.5 mm was fabricated by using a mechanically strengthened aluminum stabilized superconductor. The coil winding is strong enough to eliminate the outer support cylinder which is necessary in the former thin solenoid type coils. Consequently the coil weight and material thickness are 40 kg, and 2.52 g/cm², respectively. The BESS-Polar was launched near the US McMurdo Station in Antarctica on December 13th 2004, floated at an altitude of 37000 m around the South Pole for nine days. The solenoid was charged up on the ground and kept the field in a persistent current mode during launch and floating. This report describes the flight performance of the solenoid