Shielding of the NBI boxes against W7-X magnetic stray fields

Neutral Beam Injection (NBI), besides ECRH, is foreseen as one of the main heating devices at the W7-X stellarator currently under construction at IPP Greifswald, Germany. In a final stage 20 MW of NBI heating power will be installed generated by two NBI boxes of the ASDEX Upgrade (AUG) type. magnetic fields generally affect the trajectories of charged particles, essentially all the NBI boxes – including ion sources, acceleration sections, neutralisers and deflection magnets – must be shielded against the stray fields of W7-X. magnetic stray fields of W7-X there exist significant radial and toroidal components whereas at tokamaks the vertical components are dominant. The power loads on the ion dump and the protecting structures of the deflecting magnets and the beam lines caused by residual beam ions, therefore, will be strongly different. Thus the shielding concept of AUG cannot simply be taken over, but must be carefully redesigned in order to remain below the critical power limits. delling calculations of the magnetic shielding, the ion trajectories and the resulting power loads have been carried out for the “high iota” and “low shear” experimental scenarios of W7-X. The fields taken for these calculations are modelled by averaging the calculated W7-X stray fields on the one hand, and by fields generated by two-hypothetical-planar coils perpendicular to the x–y plane, on the other hand. ielding concept for W7-X mainly consist of iron plates in the outer side regions of the boxes and as little magnetic material as possible inside the boxes.