Improvements of the Versatile Multiaperture Negative Ion Source NIO1

The ion source NIO1 (Negative Ion Optimization 1) was developed and installed as a reduced-size model of multi-aperture sources used in neutral beam injectors. NIO1 beam optics is optimized for a 130 mA H- current (subdivided in 9 beamlets) at a Vs=60 kV extraction voltage, with an electron-to-ion current ratio Rj up to 2. In an initial phase, NIO1 was operated with Vs=0 to verify the coupling of radiofrequency (rf) power Pf to plasma. The distinction between capacitively coupled plasma (E-mode, consistent with a low electron density plasma ne) and inductively coupled plasma (H-mode, requiring larger ne) was clearly related to several experimental signatures, especially for air or nitrogen plasmas, where E to H transition was observed with Pf about 0.5 kW (with hysteresis). In hydrogen, the same transition may require about 1 kW, so that air cooling of the rf window was improved. Beams of H- and O- were separately extracted (using pure gases as input); since no caesium is yet introduced into the source, the expected ion currents are typically lower than 10 mA; this requires a lower acceleration voltage Vs (to keep the same perveance). Operation at lower voltages and current (as appropriate in this phase) is possible, but use of weaker deflection magnets in the extraction grid (EG) or use of heavier gases (oxygen) is then advisable. Preliminary experiments indicate the need of increasing the ampere-turns in magnetic filter and the bias voltage to reduce Rj (still very large, about 150 for oxygen), in qualitative agreement with theoretical and numerical models. In a separate test stand, NIO1 caesium oven was tested, with deposition on a molybdenum substrate, and advanced Cs dispensers are in development.