High-current negative-ion beam acceleration with a large negative-ion source for large helical device-neutral beam injection system

A large hydrogen negative-ion source was constructed as a prototype one for the negative-ion-based neutral beam injection (NBI) system in large helical device. The ion source is designed to produce 180 keV-40 A of the negative ion beam with a current density of 40mA/cm(2), and is a cesium-seeded volume production source characterized by the external magnetic filter. The are chamber of multicusp bucket is rectangular of 35 cm x 145 cm in cross section and 21 cm in depth. The accelerator is a three-grid single-stage one, and the total grid area is 25 cm x 125 cm, which is divided into five sections. The ion source was installed on the negative-ion-based NBI test-stand, and using one section of the grid 5.5 A of the negative-ion beam was produced at a gas pressure of 1.8 mTorr corresponding to a current density of 27.5 mA/cm(2). The extracted electron current is low of 50% of the negative ion current, and the acceleration efficiency, defined as the negative ion current divided by the acceleration drain current, is more than 75%. The ion source operation with the full grid area has just started, and, so far, 21 A of the negative ion beam was obtained. The large area beam has been successfully focused by both the geometrical arrangement of five grid sections and the aperture displacement technique of the grounded grid. (C) 1998 American Institute of Physics.