Polar distribution of ions and electrons in extraction region of a large-scaled caesium seeded ion source

The first results of saturation-currents polar distribution, which is measured with a directional Langmuir probe, at the beam extraction region of our caesium (Cs) seeded negative ion source have been reported. The line from maximum to minimum of the distribution tilts by 40° to the normal direction of the plasma grid (PG) surface. The maximum intensity is one order of magnitude larger than the minimum one. Depth distribution of the saturation currents is also measured along the axis of the PG aperture and the middle line between a pair of the axes of nearest neighbor PG apertures. Ionic plasma with quite low electron density is generated within a distance of 10 mm from PG. With a bias voltage lower than plasma potential, the difference between negative to positive saturation currents, which corresponds to electron current, increases rapidly beyond the boundary of ionic plasma. The electron current decreases by a factor of 6 with increasing the bias voltage from 2.8 to 6.4 V.