Improvements of the NIO1 Installation for Negative Ion Sources

In view of the long term operation of neutral beam injectors (NBI) used for stellaratorand tokamak heating and current drive, the negative ion source must be carefully optimized,especially because not only plasma but also surface wall conditions are involved in H- or D-production. The NIO1 (negative ion optimization phase 1) installation was developed and isoperated since 2014 (in close collaboration between Consorzio RFX and INFN), as a convenientbenchmark to study innovative solutions and to address physical questions. The comparativelysmaller size of NIO1 (a relatively compact and modular 9 beamlet H- source) is of evidentadvantage for detailed modeling. The apparent plasma impedance is well in agreement withnegligible rf reflection at plasma on, and reflections with plasma off (about 25 %) are well withinlimits tolerable by rf amplifier; transition between E and H modes of rf coupling can be controlledby increasing rf power or by decreasing gas pressure. The filter field intensity, Bx, has beenextended to span the [-12, 5] mT range, and as a trend, source performances improve with |Bx|.Status and results of a first NIO1 cesium oven are reported. Installation of the Cavity Ring DownSpectrometer (CRDS) and recent years progress of beam diagnostics and of the quality of thevolume-produced H- beam are briefly discussed, together with the status of the NIO1 full power”end calorimeter/beam imaging attenuator”, to be placed at the end flange with auxiliary flangesfor infrared observation. Conceptual tests of energy recovery system, perhaps using this fullpower calorimeter are also summarized.