A MERGING PREACCELERATOR FOR HIGH-CURRENT H- ION-BEAMS

The high power ion beams used in the next generation thermonuclear fusion reactors require high current negative ion beams accelerated to high energy, with high efficiency. One way to meet these requirements is to merge multiple low current density H- beamlets into a single high current beam. The feasibility of a high current merging preaccelerator was demonstrated in this experiment by merging 19 beamlets of H- ions distributed over a circular area 80 mm in diameter from a Japan Atomic Energy Research Institute negative ion source. H- ions were extracted at a current density exceeding 10 mA/cm(2) at the ion source which operates at 0.13 Pa (1 m Torr), with a low are power density (70 V x 250 A). Spherically curved grids (with built-in magnetic electron suppression) were used in the preaccelerator to focus the extracted beamlets into a single 104 mA, 100 keV beam. The merged beam has a diameter of 23 mm and a converging angle of +/-30 mrad at the beam envelope. The rms emittance of the 104 mA merging beam was 1.00 pi mrad cm, which is a condition acceptable to the electrostatic quadropole accelerator for further acceleration. (C) 1995 American Institute of Physics.