Improvement of negative ion source with multi-slot grids for LHD-NBI

In this article, we describe injection results on a negative ion source newly designed for one of the neutral beam injectors (NBI) in large helical device (LHD). The ion source consists of an accelerator with a combination of steering grid (SG) and multi-slot grounded grid (MSGG), whose transparency is about twice as large as that of conventional multi-hole grounded grid (MHGG). Due to the high transparency, the MSGG reduces the heat load carried by beam was much reduced comparing to the load onto MHGG. The maximum injection power increased drastically up to 4.4 MW at the energy of 180 keV in 2002 fiscal. In the ion source consisting of accelerator with MSGG, degradation of injection power was observed above the energy of more than 160 keV. The degradation was considered caused by saturation of yield of hydrogen negative ion (H-). In order to obtain further injection power, temperature of plasma grid rises up from 210 deg. C to 240 deg. C, and the arc balance was adjusted after changing total number of filaments from twenty-four to twenty-six. The injection power, consequently, reached 5.7 MW at the energy of 186 keV after improving the H- yield. The maximum beam energy of 189 keV was obtained, and the power and pulse duration were 5.0 MW and 2 second at that time. (author)