Compact Beamline and Design against Radiation

The present ITER NB system includes two design features: short beamline design and vacuum insulated beam source (ion source and accelerator) which sustain 1 MV high voltage under radiation environment. The short beamline design is achieved by subdividing interior of the neutraliser into narrow channels to reduce gas conductance, and hence, the gas fl ow rate to ensure target thickness required for neutralisation. Optimizing the beam transmission and reionisa-tion loss of neutral beams, the axial length of the ITER NB injector was shortened to be 23.4 m with a reasonably high injection effi ciency. Three-dimensional neutron transport analyses clarifi ed possible excess power dissipation due to radiation-induced conductivity, if the high voltage is insulated by conventional gas such as SF6. Thus, design of vacuum insulated beam source has been established for the present ITER NB system. Keywords: fusion, ITER, neutral beam, accelerator, negative ion, H–, beamline, neutralise