A dedicated device for measuring the magnetic field of the ND280 magnet in the T2K experiment

This paper describes a dedicated device to map the magnetic field of the T2K near detector (ND280) magnet, which runs at a nominal field value of 0.2 T, with an accuracy of the order of a Gauss. A high accuracy mapping is a key ingredient in order to provide the required momentum accuracy in the ND280 time projection chambers (TPCs) allowing T2K to measure precisely the PMNS matrix parameters ?m232 and ?23. This paper describes the design and realization of the device as well as its performance during operation. The reported results show that the aimed at goals are reached.