The ALPHA-g Antihydrogen Gravity Magnet System

The ALPHA-g experiment at CERN aims to perform the first-ever precision measurement of the weight of antimatter, using antihydrogen atoms confined in a magnetic trap. In the measurement, anti-atoms are allowed to escape through either a lower or an upper port in the trap, the up-down balance of which depends on gravity and the trap field at the ports. Achieving the initial target of 1% precision in weight requires constructing a magnet system capable of controlling the trap field at the 10 ppm level, as well as creating other field configurations needed for plasma (antiproton and positron) and antihydrogen manipulation. A high precision superconducting magnet system is constructed for this purpose, containing five octupoles and 24 coils enveloped by a shielded solenoid. The number, positioning, layer construction and conductor structure for each element is carefully designed to minimise magnetic asymmetry, taking persistent current, fabrication tolerances and anti-atom orbits into account.