Feasibility study of a flux-gate magnetic field sensor suitable for ITER Neutral Beam Injectors

The ITER Heating Neutral Beam (HNB) injectors shall be protected from stray magnetic field (several hundreds of mT) produced by the ITER PF coils and plasma current. Such stray field would hamper the production of negative ions, deflect ion trajectories in the accelerator and cause intolerable heat load on neutralizer and beam line components. In order to keep the residual magnetic field below acceptable levels (1 mT in the ion source and accelerator and 0.3 mT in the neutralizer), each injector will be surrounded by Passive Magnetic Shield and by six Active Correction and Compensation Coils (ACCC). The ACCC will be feedback controlled using magnetic field sensors located inside the HNB vessel, an environment subjected to considerable neutron flux (~ 1099n/cm22/s) during the ITER operation. Therefore, magnetic sensors that are robust, radiation hard, drift-immune and remote-handling compatible are required. Flux-gate magnetic sensors are good candidate, as their active part includes no semiconductor or other radiation-sensitive component, and consists of a ferromagnetic core and two insulated-wire coils. Commercial flux-gate sensors, used for precise measurements of weak magnetic fields, have very good sensitivity, but measurement range below 0.1 mT, and also include on-board electronics. A flux-gate sensor for the ITER HNB has been studied using a numerical model of the magnetic core hysteresis, essential to describe the sensor operation, sensitivity and measurement range. This model indicated that, by suitable choice of the core magnetic properties and gap geometry, the measurement range can be extended by at least 2 orders of magnitude. On this basis, a prototype flux-gate sensor has been realized at Consorzio RFX. Experimental tests carried out so far have confirmed the results of the numerical model and have demonstrated that the measurement range can be increased to ~ 10 mT with acceptable accuracy and frequency bandwidth.