Feasibility study of a flux-gate magnetic field sensor suitable for ITER neutral beam injectors

ITER Neutral Beam Injectors (NBIs) need to be shielded from the relatively strong stray magnetic field
generated by the Poloidal Field Coils of the Tokamak. For this reason both the Heating Neutral Beams
(HNB) and the Diagnostic Neutral Beam (DNB) will be provided with a Passive Magnetic Shield and
with a system of Active Correction and Compensation Coils (ACCC). The ACCC will operate in feedback
control and thus require the measurement of magnetic field inside the NBI vessel, i.e. in an environment
subjected to the neutron flux coming from the Tokamak. To this purpose, magnetic sensors which are
robust, radiation hard, drift-immune and remote-handling compatible are required.
Flux-gate magnetic sensors are a good candidate for this task, but commercial sensors of this kind have
typically a limited measured range (below 0.1 mT).
The feasibility of a flux-gate sensor for the ITER NBI has been studied by developing a numerical model
which includes magnetic core hysteresis, and which demonstrated that, by suitable choice of the core
magnetic properties and geometry, itis possible to increase the measurement range by atleast 2 orders of
magnitude. On this basis, a flux-gate sensor prototype has been realized at Consorzio RFX. Experimental
tests carried out so far have demonstrated that the measurement range can be increased to ~10 mT with
acceptable accuracy and frequency bandwidth.