Superconducting Magnet

The magnetic field for HELIX is generated by a 2-coil superconducting magnet designed originally for the HEAT instrument. It has been used in five successful balloon campaigns. The magnet provides an approximately uniform field within a rectangular warm bore measuring 51x51x61 cm³. Two superconducting NbTi coils centered along the cryostat axis produce a 1 T central field at a current of 91.7 A. The coils are cooled to 4.2 K in a liquid-helium (LHe) bath and the cryostat employs a vapor-cooled radiation shield and super-insulation to isolate the inner helium vessel from the ambient surroundings.

To provide a pressurized environment for the tracking system, the bore is sealed with a thin aluminum pressure cap which introduces less than 0.3 g/cm² of material overburden. The total LHe capacity of the magnet is 260 L, which provides a hold time of 7 days, at a mass of 420 kg.