High-power negative ion sources for neutral beam injectors in large helical device

Large‐scaled hydrogen negative‐ion sources, in which cesium is introduced in the source plasma, have been developed for neutral beam injectors in Large Helical Device, and their operational characteristics are reviewed. For high‐efficient negative ion production, configuration of the magnetic filter field and the cusp magnetic field was optimized, resulting in a high arc efficiency for the negative ion production of 0.23A/kW. With use of a multi‐slotted grounded grid, the gas pressure in the acceleration gap is lowered, leading to reduction of the heat load of the grounded grid. As a result, the voltage holding ability is much improved, and the rated energy of 180 keV is achieved in a short conditioning period of 4 days. The injection power is increased linearly to the 5/2 power of the beam energy and reached 5.7MW with an energy of 184keV, which exceeds the specified value of 180keV–5MW. Beam uniformity has been improved with an individual control of the local arc discharge by adjusting 12‐divided output...