The Ultra-Stable Magnet of the Mark II Experiment

A new watt balance experiment is under development at the Federal Institute of Metrology (METAS, Switzerland). Its scope is to link the mass unit to the Plank constant in order to set a new definition of the kilogram, which is no longer based on the international prototype. The experiment is performed in two steps with the same setup and requires that the magnetic field produced in the two steps by the magnet fitted in the setup remains stable within few parts per $10^8$. This paper describes the design, manufacture, and characterization of this magnet, which is performed through a collaboration between METAS and CERN.