In order to produce an intense beam of monoatomic negative hydrogen ions from a beam of accelerated positive hydrogen cluster ions, kinetic energy per constituent atom of the hydrogen cluster ions 0.5-1.0 keV is most suitable. For an acceleration voltage of 1 MV, specific sizes of the hydrogen cluster ions should be 10
3 atoms/unit charge. However, the large equivalent current of a beam of cluster ions has the tendency to make the specific sizes much larger than the above value. Therefore, the specific sizes must be controlled to the value.
A fundamental study of controlling the mean specific size of nitrogen cluster ions has been carried out and the initial mean specific size 3×10
5 molecules/unit charge was reduced to 10
3-10
4 molecules/unit charge by multiple ionization of the cluster ions.
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