Low pressure and high power rf sources for negative hydrogen ions for fusion applications (ITER neutral beam injection) (invited).

The international fusion experiment ITER requires for the plasma heating and current drive a neutral beam injection system based on negative hydrogen ion sources at 0.3 Pa. The ion source must deliver a current of 40 A D^- for up to 1 h with an accelerated current density of 200 A/m² and a ratio of coextracted electrons to ions below 1. The extraction area is 0.2 m² from an aperture array with an envelope of 1.5×0.6 m². A high power rf-driven negative ion source has been successfully developed at the Max-Planck Institute for Plasma Physics (IPP) at three test facilities in parallel. Current densities of 330 and 230 A/m² have been achieved for hydrogen and deuterium, respectively, at a pressure of 0.3 Pa and an electron/ion ratio below 1 for a small extraction area (0.007 m²) and short pulses (<4 s). In the long pulse experiment, equipped with an extraction area of 0.02 m², the pulse length has been extended to 3600 s. A large rf source, with the width and half the height of the ITER source but without extraction system, is intended to demonstrate the size scaling and plasma homogeneity of rf ion sources. The source operates routinely now. First results on plasma homogeneity obtained from optical emission spectroscopy and Langmuir probes are very promising. Based on the success of the IPP development program, the high power rf-driven negative ion source has been chosen recently for the ITER beam systems in the ITER design review process. [ABSTRACT FROM AUTHOR]