HIS Ion Source

Larry Lamm, Research Professor,Technical Director

March, 2009

Nuclear Science Laboratory

Helium Ion Source (HIS)

• 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.

HIS

• 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.

HIS Theory of Operation

• 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 several 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.

HIS Theory of Operation

• The electrons ionize the source gas, and singly charged positive helium ions emerge from a very small (0.013 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.

HIS Theory of Operation

• The beam is then focused by an Einzel lens 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. This charge exchange region is manufactured by High Voltage Engineering Europa (HVEE).

HIS Charge Exchange Cell

HIS Theory of Operation

• Some fraction of the positive beam will acquire 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 A are typical.

HIS Theory of Operation

• The exchange canal is maintained at approximately -20 kV, to provide the incoming positive beam with the energy needed to maximize the cross section for the charge exchange, which is velocity dependent.

HIS Theory of Operation

• As a result, the singly charged negative helium beam which emerges from the charge-exchange canal has an energy of 40 keV, since the positively charged helium ions entering the charge exchange region gain 20 keV energy as they accelerate into the region, and the negatively charged helium ions gain another 20 keV energy as they accelerate away from the region.

HIS HV Platform

• Recent upgrade• Entire HIS moved to a

high voltage platform• Can operate at

approximately 30 kV below ground

• Beams now have about 70 keV energy (40 from charge exchange, 30 from platform)

HIS HV Platform

• Higher energy beams – better optics

• Higher energy beams – better timing for TOF experiments

HIS Ion SourceRemote Computer Control

• In a manner entirely similar to that used to develop remote computer control of the SNICS Ion Source, we have also developed remote computer control for the HIS Ion Source.

• As with the SNICS Ion Source, the control system we use to allow for remote control operations of the HIS Ion Source is known as Group3 with ControlNet, a fibre optically linked control system using distributed I/O modules, available from GMW Associates, Inc.

HIS Ion SourceRemote Computer Control

• Shown here is an old front panel image of the LabVIEW program being used to control the HIS Ion Source.