The NIO1 negative ion source: investigation and operation experience

The ion source NIO1 (Negative Ion Optimization 1) is a versatile multiaperture H- source, capable of continuous regime operation, with the plasma generated by a 2 MHz/2.5 kW radiofrequency (rf) power supply and nine beamlet extraction, operated since 2014 in close collaboration between Consorzio RFX and INFN-LNL. It aims to partly reproduce the conditions of the much larger ion sources built or in construction for neutral beam injectors of fusion devices in a compact and modular ion source, where effects of individual source components can be rapidly verified and compared to simulation code results. Due to the need of assessing negative ion production limits (especially without cesiation) in continuous operation for the perspective of future fusion reactors and to the large importance of magnetic field for source optimization, several configuration changes were tried (both inside the ion source and for the embedded magnets inside the accelerator grids) and investigated, with material ranging from hard ferrite to SmCo to NdFeB. Evidence of the effect of the accelerator fringe field inside the ion source is discussed here and in companion presentations. The radiofrequency system takes full advantage of the frequency tuning amplifier capability (+/- 150 kHz installed, 0/+20 kHz used), and it is worth noting, that no adjustment of matching box capacitors was necessary in the last 3 years. Major diagnostic systems (including CCD cameras and BES measurement) have been integrated with the acquisition system along with the date measured by several power supplies (including the high voltage supplies, the RF generator and bias supplies, the pressure measurements and the beam currents).