Volume and surface effects in Cs-free regimes in NIO1

NIO1 (Negative Ion Optimization phase 1) is a compact multiaperture radiofrequency H- ion source whose design was optimized for sustainable prolonged beam on target operation; for installation economy, its size was drastically reduced with respect to fusion device D- sources, down to a plasma volume of 1.6 × 10-3 m-3. Even if Cesium improves H- production as well known, also Cs-free regimes (and intermediate regimes) well deserve some development effort, in view of avoiding long term contamination of the accelerator and for use as cleaning procedure. Data collected by NIO1 in a true Cs-free regime (before 2020) are thus very important, and need a thorough statistical analysis, with special attention to the technique of gas conditioning that was discovered in NIO1 and to the issues concerned with long term operation; N2, Ar, and Xe comparison to oxygen is here shown; all these gases proved useful for conditioning, to different extents. Gas conditioning macroscopically proves the importance of surface effects, even when the final production of H- happens in the source volume. As regards to the Electron Cyclotron Resonance Ion Sources (ECRIS), exchange of ideas and concepts, such as electron economy and biased disks, liners and wall coatings, is considered. Gas mixing, conditioning and surface material perspectives are envisioned.