1 MV vacuum insulation for the ITER neutral beam injectors

The ITER is an international project which aims to develop an experimental reactor as a step to realize fusion energy. To inject the 1 MeV D⁰ neutral beams of 33 MW for heating and current drive in ITER plasma, D^- ions are accelerated by dc -1 MV. The accelerator consists of five acceleration stages, and each acceleration gap has to withstand 200 kV. Gaps between the accelerator and the vacuum vessel whose potential is ground must also sustain voltages up to 1 MV. Key issues are high voltage vacuum insulation and design studies have been performed. A minimum gap length of > 900 mm was selected as 1 MV insulation design criteria. Development of a high voltage bushing (HVB) which is a bulkhead and a feed-through between gas insulated HV transmission line and the beam source in vacuum is ongoing. The HVB consists of a stack of five large bore ceramic rings (1.6 m diameter), each 0.29 m in height. Five stage insulation concept was applied to both the acceleration gap and the HVB for better voltage holding with multi shorter gaps. R&D for insulation of one stage ceramic with screens was conducted and 5 hr voltage holding of dc 200kV (rated voltage) was confirmed.