Evaluation of minimum anode area for rapid ion extraction from photo-ionized plasma

The characteristics of ion extraction from photo-ionized plasma (ion species: Gd+, typical density: 1 x 1016 m-3; electron temperature: 0.4 eV) in a simplified M-type electrode configuration were measured when the ratio of the anode area to the cathode one (α) was changed between 0.04% and 5%. The ion extraction was as rapid as the original M-type electrode configuration when α≥0.5%, but it was longer when α≤0.1%. The plasma potential was measured by analyzing the energy of the ions extracted from the plasma. The rise of the plasma potential at α=0.1% was delayed more than that at α=0.5%. Therefore, the delay of the rise of the plasma potential caused the delay of the ion extraction when α≤0.1%. The anode area for rapid ion extraction is evaluated to be more than 0.5% of the cathode area. An anode area large enough to raise the plasma potential to the anode one ensures that the ion extraction is as rapid as that of the original M-type electrode system. (author)