RBS Instrumentation: Source of Negative Helium Ions

The He+ or He++ ions required for a single ended accelerator come from plasma ion sources. The duoplasmatron starts with a low voltage arc burning between cathode and anode. The helium plasma is geometrically and magnetically confined, and ions are extracted by a strong electric field. The radio frequency plasma source also generates He+ ions. Radio frequency sources are more common in single-ended accelerators, because they produce He++ more efficiently than duoplasmatrons. A typical source produces 1 mA of He+.

Tandem accelerators require negative helium ions. Helium is the most inert of the inert gases; it tends not to gain or lose electrons. The ion source must overcome this natural tendency of helium not to form negative ions. The source operates in two stages. First, positive ions are produced as described above. An alkali metal channel converts He+ into He- in a second stage. This charge exchange happens as He+ passes through hot alkali metal vapor. Rubidium is used in the illustration, but all of the alkali metals have sufficient reducing power to form He-. The charge exchange process is inefficient; 1 milliamp of He+ leads to 1 microamp of He-. The negative ions are extracted from the alkali metal channel and injected into the tandem accelerator at 20 to 30 keV.

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