Magnetic shielding considerations for the ITER neutral beam injectors

As a possible means for heating and current drive, negative ion based neutral beam injection is envisaged for ITER. In order to make the overall volume of the tritium containment as small as possible, the neutral beam injectors have to be placed close to the machine. In this region, the magnetic stray field from the tokamak is rather high (> 1 kG). It has to be shielded to less than 1 G in the source and neutraliser region of the injector. To achieve this reduction, the magnetic shield has to consist of three parts: Several tens of cms thick iron walls on both sides of the injector have to reduce the field by a factor of 2 to 3. A double walled screen of very soft iron around the injector vessel, again with a thickness of tens of cms, reduces the field to less than 10 G. This residual field can be compensated with actively controlled coils. The disturbance of the ITER magnetic field due to this shielding is less than 0.3 %, within the accuracy of the present calculations. (orig.)