The ion source/accelerator for ITER-NBI has to produce 40A D- ion beams at an energy of 1MeV for a pulse duration of more than 1000 s with a beam divergence of less than 5 mrad. High D- ion current density of more than 20 mA/cm² is required to make the size of the NBI system compact. One of the important characteristics is the operating pressure; the negative ion source should be operated at a low operating gas pressure of less than 0.3 Pa to reduce the stripping loss of negative ions in the accelerator and to have a reasonable acceleration efficiency.

Although these specifications are far beyond the specification of existing negative ion sources, basic performances have already been achieved individually by the negative ion sources developed at JAERI. Namely, H- ion beam of 18.4A/350keV and D- ion beam of 13.5A/400keV were obtained in the negative ion source for JT-60U N-NBI. H- ion beams have been accelerated up to 805 keV in MeV Test Facility at 150mA drain current for 1 s. D- ion current density of 20 mA/cm² (1.4A/30keV) was achieved in the JAERI/CEA joint experiment at a low operating gas pressure of 0.3 Pa. Continuous operation of the negative ion source for 139 hours, which corresponds to half year operation in ITER-NBI, was also demonstrated in JAERI Long Pulse Negative Ion Source. The beam optics study has showed that the accelerated negative ion beam has the minimum beam divergence of around 2-3 mrad at an optimum perveance and a beam-halo component of about 10%.

In the present paper, the development of these high power negative ion source/accelerator is described with the design of the ion source accelerator for ITER-NBI.