A Magnetic Residual Ion Removal System with In-Line Ion Dumps for the ITER Neutral Beam Injection System

An alternative residual ion removal concept for the ITER neutral beam system is presented. It consists of magnetic deflection of the residual ions to in-line ion dumps. The target plates are hit from one side and form a 0.5-m-wide opening to the beam. First calculations show that for the most severe case of a 3-mrad beam, the maximum power load can be kept below 15 MW/m², using a different horizontal focal length. However, this different beamlet optic increases the beam peak power density changing the plasma deposition profile and increasing the shine-through power during low-density operation. First calculations showed that using a passive screening, the additional stray field created by the magnet could be kept below the required 1 gauss within the neutralizer. The overall beamline transmission increases by ~10% (i.e., an additional 1.7-MW injected power for each beamline for a 3-mrad beam) due to the open structure of the magnet and the ion dumps. Furthermore, the concept offers a larger operating window regarding beam alignment, divergence, steering, and transmission, and it avoids creating accelerated secondary electrons.