The Nuclear Science Laboratory maintains
and operates a helium ion source known as HIS, which is used to produce
negatively charged 3He and 4He beams for injection
into the FN Tandem Van de Graaff accelerator. The heart of the HIS helium
ion source is the duoplasmatron, manufactured by Peabody Scientific.
The details of ion beam production within the duoplasmatron are a bit
complicated, but the basic idea can be simply described. A small diameter
tungsten wire, known as the filament, is heated by passing a large current
through the filament. The filament is housed within a cavity containing
the source gas (in this case, helium) at a pressure of a few hundred
microns. Thermionic emission from the hot filament produces electrons
that are confined along the axis of the cavity by a magnetic field from
a coil surrounding the cavity.
The electrons ionize the source gas, and singly charged positive helium ions emerge from a very small (0.010 inch diameter) aperture known as the "button". Immediately downstream of the button aperture is the "extraction" electrode, maintained at approximately -20 kV so as to extract the positive helium beam from the duoplasmatron. The beam is then focused into the charge exchange region, where the beam passes through a small (0.25 inch diameter) canal filled with lithium vapor from a heated lithium reservoir. Some fraction of the positive beam will acquire one or more electrons in collisions with the lithium atoms in the charge exchange region, resulting in a small quantity of singly charged negative helium ions available for injection into the FN Tandem Van de Graaff accelerator. Helium beam currents of 1 mA are typical.
The exchange canal is maintained at approximately -22 kV, to provide the incoming positive beam with the energy needed to maximize the cross section for the charge exchange, which is velocity dependent. As a result, the singly negatively charged helium beam which emerges from the charge-exchange canal has an energy of 44 keV, since the positively charged helium ions entering the charge exchange region gain 22 keV energy as they accelerate into the region, and the negatively charged helium ions gain another 22 keV energy as they accelerate away from the region. Note that this charge exchange region acts as a "mini Tandem accelerator", because of the charge exchange process.
In principle, the source could be used to
produce beams other than helium, using other gases in the filament region.
However, the SNICS II Sputter Ion Source
is used for all other FN Tandem beams in the Nuclear Science Laboratory,
and the HIS is used only for the production of helium beams.
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