Acceleration of 1 MeV, 100 mA Class H- ion beams in a proof-of-principle accelerator for ITER.

Development of an electrostatic accelerator for 1 MeV, 1 ampere class H- ion beams has been carried out as a Proof-of-Principle (PoP) of high current accelerators required in neutral beam system of fusion reactors such as ITER. A unique feature of such fusion oriented accelerators is to utilize vacuum insulation not only inside but also outside of the accelerator for high voltage insulation, since conventional gas insulation is not applicable due to excess radiation induced conductivity in the gas. The PoP accelerator is surrounded by an insulator stack as a vacuum boundary with a vacuum gap of 50 mm all around the accelerator. One of the key technologies to achieve insulation of MV voltage was reduction of electric field strength at triple junction of the insulator (made of FRP, fiber reinforced plastic) stack. By lowering the stress to 1.7 kV/mm, the accelerator sustained 1 MV stably for more than 2 hours. By tuning the ion source with/without cesium under stable accelerator operation, the PoP accelerator generated H- ion beams of 70 mA at 1 MeV, and 110 mA (80 A/m2) at 900 keV stably for several hundreds shots (pulse length, 0.5–1 s limited by the target). The dark current flowed inside/on the surface of the FRP insulator due to photoelectron effect was estimated to be less than μA in case of 0.1 A electron beam acceleration (maximum current observed in the experiment). This is in the same order to that due to conductivity of the insulator material. So far, high voltage breakdown due to photoelectron has never been observed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]