The ALPHA-g experiment at CERN aims to be the first-ever to precisely weigh antimatter under Earth’s gravity, by “dropping” antihydrogen atoms with a magnet system. The anti-atoms are initially confined inside a vertical octupole and between two end cap coils. The currents in the coils are then gradually decreased to release the anti-atoms. The up -down balance of the escapes depends on gravity and the relative strength of the coils. By observing the escapes at different coil balances, the weight of antihydrogen is determined.
Achieving our first target of 1% precision in weight requires controlling the measurement field to an unprecedented 10 ppm precision. A sophisticated dual-cryostat magnet system is constructed for this purpose. An inner wet cryostat contains five octupoles, 22 coils and two solenoids (2 m overall height, 48 mm I.D.), and an outer dry cryostat contains a human-sized, shielded solenoid (2 m height, 600 mm I.D.). The centre of the system is used for gravity measurement, while other parts are used to confine, manipulate, transfer and cool the anti-atoms and their constituents. The octupoles and mirror coils are made to high precision at BNL by CNC wire-laying with active correction. The location, geometry and wire stock of each winding and its associated leads and splices are carefully designed using simulation to minimise field error. Wire placement inaccuracy and mechanical deformation of the windings are taken into account. Persistence effect is studied and mitigated by minimising the amount of superconductor, constructing an up-down symmetric magnet system, and using a special high-filament count NbTi cable. Higher-order corrector windings are constructed around the measurement region to provide additional field-shaping flexibility. A DCCT-based, bi-polar PID current control system is used to power the windings with < 10 ppm current precision. An environmental field-cancelling system is planned.