Improvements of the versatile multiaperture negative ion source NIO1 Free

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 135 mA H⁻ current (subdivided in 9 beamlets) at a V_s = 60 kV extraction voltage, with an electron-to-ion current ratio R_j up to 2. Depending on gas pressure used, NIO1 was up to now operated with V_s < 25 kV for beam extraction and V_s = 60 kV for insulation tests. The distinction between capacitively coupled plasma (E-mode, consistent with a low electron density plasma n_e) and inductively coupled plasma (H-mode, requiring larger n_e) was clearly related to several experimental signatures, and was confirmed for several gases, when applied radiofrequency power exceeds a given threshold P_t (with hysteresis). For hydrogen P_t was reduced below 1 kW, with a clean rf window and molybdenum liners on other walls; for oxygen P_t ≤ 400 W. Beams of H⁻ and O⁻ were separately extracted; since no caesium is yet introduced into the source, the expected ion currents are lower than 5 mA; this requires a lower acceleration voltage V_s (to keep the same perveance). NIO1 caesium oven was separately tested and Cs dispensers are in development. Increasing the current in the magnetic filter circuit, modifying its shape, and increasing the bias voltage were helpful to reduce R_j (still very large up to now, about 150 for oxygen, and 40 for hydrogen), in qualitative agreement with theoretical and numerical models. A second bias voltage was tested for hydrogen. Beam footprints and a spectral emission sample are shown.