High Performance Operation of Negative-Ion-Based Neutral Beam Injection System for the Large Helical Device

摘要

It is a touchstone for the success of ITER and future fusion reactor whether the present high performance negative-ion-based NBI (N-NBI) heating systems work properly. The LHD and JT-60U are only two facilities where N-NBI systems are working for high power plasma heating / current drive in the world. Because handling of negative hydrogen / deuterium ions was amateur technology, it has taken a long time to improve its skill. In LHD, we succeeded in improving the performance of one of three beam lines dramatically in 2003 by adopting a multi-slot grounded grid for the accelerator of ion source. The effort on improving the performance was also done in other beam lines with conventional ion sources in parallel. The guidelines of improving are optimization of magnetic multi-cusp configuration for efficient negative ion production, and increasing the transparency of the grounded grid for reduction of heat load on it. As a result the available beam power has been increased, that is, successive injection power level more than 10MW became possible throughout four-month long experimental campaign, although the maximum injection power has been almost the same. The averaged negative ion beam current density at the exit of ion source, which was evaluated from the port-through injected power, was achieved up to 350A/m^2 up0 nosupersub which is larger than the required value of ITER NBI in hydrogen beam operation. Pulse length at high beam power level has also been extended owing to the reduction of heat load on the grounded grid. These results (increase in available power and pulse length) have contributed to expand the operation region of LHD. By continuous R&D, we also have found the way of solving an associated problem of multi-slot grounded grid system, that is, mismatched conditions of optimum beam optics in vertical and horizontal directions. According to this result, better beam divergence can be realized, and the increase in the total injection power is expected in the next experimental campaign.