Residual magnetic field coils for the neutral beam test facility (NBTF)

The neutral beam test facility (NBTF) will be the first ITER-like injector to be built with the objective of testing all the components under operating conditions equivalent to ITER. During the EDA phase of ITER beam transmission calculations were performed to study the effects of the residual magnetic field of ITER. As a result of these studies, a set of limits was found for the acceptable magnetic field along the beam path. A magnetic field reduction system of the ITER NBH&CD injectors was designed such as to reduce the stray magnetic field of ITER down to the required values, and finite element magnetic analyses were performed in the design process, which acknowledged an associated error greater than the acceptable magnetic field values at some critical areas.In view of the critical effect of even a small magnetic field, and the uncertainty of the predicted values, it seems necessary to install at the NBTF a set of coils able to reproduce the estimated residual magnetic field inside the ITER injectors at different plasma and heating scenarios. These coils will permit also to study the beam behaviour under a wider range conditions, including the new issues raised by the recent simulations.This paper presents the design of the residual magnetic field coils (RMFC) to be installed at NBTF. It consists of 12 rectangular coils electrically grouped into 6 pairs, located above and below the beam line vessels and covering all the areas relevant for beam transport. The paper presents a discussion of the criteria entering the choice of coil design, of which the most relevant is the flexibility of achievable magnetic field scenarios. With the proposed set of coils the magnetic field can be varied independently at the beam source, gap, neutralizer and residual ion dump areas. The final coil arrangement is presented and the coil and support structures design are described.