Low energy ion beam with several-hundred eV is required in surface modification or implantation. However, it is difficult to transport low energy ion beam because of the divergence of the ion beam due to its space charge. Reduction of divergence of low energy ion beam could be achieved by space charge neutralization (SCN) with electron supply.

Low energy ion beam was generated with a compact microwave ion source. The generated ion beam was once accelerated to several keV and was decelerated to 0.5 keV. Ion extraction voltage, distance between the plasma and ion extraction electrodes, and the dimensions of electrode apertures were optimized to produce less divergent beam. The ion beam was decelerated by a deceleration system with multiple electrodes. As a result, 130 μA - 0.5 keV Ar⁺ beam was obtained.

SCN experiments were performed with a tungsten filament that emitted 60 ～ 100　μA electrons. The ion beam with the initial diameter of 30 mm was transported for 700 mm. Ion current density distributions were observed by Faraday cups located at separated positions. When the ion current was less than 50 μA, it was shown that the ion space charge was compensated and nine tenth of ion beam was transported for 700 mm, within the diameter of 160 mm. However, transported ion current was at most half of initial value, when the ion current increased to more than 100 μA.