Study of hydrogen plasma in the negative-ion extraction region

The effect of the plasma electrode (PE) bias on the plasma characteristics near the extraction aperture in a large volume hybrid multicusp negative-ion source, driven at 2.45 GHz microwaves, is reported. Spatially resolved negative-ion and electron density measurements were performed under various pressures (1–4 mTorr) by means of an electrostatic probe and photodetachment technique. The analysis of the results provides an explanation for the existence of the negative-ion density peak and the optimized extraction of the corresponding current in the same PE bias range, i.e. just above the plasma potential. The diagnostics employed are also discussed.