Development of a High-Current Hydrogen-Negative Ion Source for LHD-NBI System
ABSTRACT We have developed a high-current hydrogen-negative ion source for a negative-ion-based NBI system in Large Helical Device (LHD). The ion source is a cesium-seeded volume-production source equipped with an external magnetic filter. An arc chamber is rectangular, the dimensions of which are 35 cm x 145 cm in cross section and 21 cm in depth. A three-grid single-stage accelerator is divided into five sections longitudinally, each of which has 154 (14 x 11) apertures in an area of 25 cm x 25 cm. The ion source was tested in the negative-NBI teststand, and 25 A of the negative ion beam is incident on a beamdump 13 m downstream with an energy of 104 keV for 1 sec. Multibeamlets of 770 are focused on a focal point 13 m downstream with an averaged divergence angle of 10 mrad by the geometrical arrangement of five sections of grid and the aperture displacement technique of the grounded grid. A uniform beam in the vertical direction over 125 cm is obtained with uniform plasma production in the arc chamber by balancing individual arc currents flowing through each filament. Long-pulse beam production was performed, and 1.3 MW of the negative ion beam is incident on the beamdump for 10 sec, and the temperature rise of the cooling water is almost saturated for the extraction and the grounded grids. These results satisfy the first-step specification of the LHD-NBI system.