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 th