Negative hydrogen ion sources for the neutral beam systems at ITER

Large and powerful RF-driven ion sources are the front end component of the neutral beam injection systems of the international fusion experiment ITER. A low-pressure hydrogen plasma (0.3 Pa) is generated via inductively coupling (up to 800 kW RF power at 1 MHz) using a modular concept, i.e. eight drivers from which the plasma expands to a common expansion chamber of the size of 0.9 m×1.9 m. The generation of negative ions is based on the surface conversion process for which caesium is evaporated in the source. Negative ions are formed at the plasma grid, the first grid of the electrostatic accelerator system composed of 1280 apertures to extract a beam of 66 A H⁻ for 1000 s and 57 A D⁻ for 3600 s. In front of the plasma grid an ion-ion plasma evolves which is beneficial for reducing the current of co-extracted electrons often limiting the ion source performance. The presentation summarizes the status of the ion source development, the challenges still to overcome and gives insight into the complex plasma dynamics of these ion sources at the example of the size scaling experiments BATMAN Upgrade (1 driver) and ELISE (4 drivers) in operation at IPP. The route towards NBI systems at ITER, including the task of the neutral beam test facility (NBTF) will be briefly described as well.