Grid-controlled extraction of low-charged ions from a laser ion source

We developed a grid-controlled laser ion source using a KrF excimer laser and investigated the effects of grid-controlled extraction on beam current waveform and beam quality. By using grid-controlled extraction in the over-dense regime, the beam current waveform was found to be stabilized. The beam current increased in proportion to V-3/2, where V is the extraction voltage, which proves that the grid control worked properly and that the ion beam was extracted in a one-dimensional space-charge-limited mode. Beam quality measurement using a pepper-pot imaging method revealed that the beam optics was strongly disturbed by the grid potential in the source-limit regime. Conversely, in the over-dense regime there were no deleterious effects of the grid bias on the beam patterns. This result indicated that the virtual anode in the gap acted as a kind of momentum filter and improved the beam quality.