Lab Course: Ion Sources Larry Lamm Research Professor Technical Director of the NSL Winter 2008

Lab Course: Ion Sources

Larry LammResearch Professor

Technical Director of the NSLWinter 2008

Accelerators in the lab

• Van de Graaff accelerators– KN (3.7 MV) and JN (1.5 MV) are “single-

ended”, meaning the ion source is inside the terminal, producing positively charged beams.

– FN (10.6 MV) is a “tandem” accelerator, with the terminal electrode in the center of the accelerator, requiring negatively charged beams external to the accelerator.

AcceleratorsFN (10.6 MV)

KN (3.7 MV) JN (1.5 MV)

Helium Ion Source (HIS)

• Produces 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.

Helium Ion Source (HIS)

Helium Ion Source

• Filament Legs (Ni) used to supply high current (60A) to Filament (W)

Filament Legs (Ni)

Filament (W)

Helium Ion Source

• The filament is surrounded by low pressure helium gas (3He or 4He)

Low Pressure Gas (He)

Helium Ion Source

• The filament glows white hot, emitting electrons which ionize the helium gas.

Filament (W)

Helium Ion Source

• Solenoidal magnet field helps focus electrons to enhance the plasma production.

Solenoid Magnet

Helium Ion Source• Positively

charged helium ions are “pulled” out of the source through a small aperture.

Beam Aperture

Helium Ion Source

• This is done using an electric field produced by the Extractor Electrode, at about (-)15 kV.

Extractor Electrode (-15 kV)

Helium Ion Source

• The diverging,positively charged helium beam is then focused by the Einzel Lens.

Einzel Lens

Helium Ion Source• The beam is

focused into the lithium charge exchange cell, where Li metal is heated to form a vapor.

Lithium Charge Exchange Cell

Helium Ion Source• The lithium

charge exchange cell operates at -15 kV, so the beam accelerates into the cell.

Lithium Charge Exchange Cell (-15 kV)

Helium Ion Source• The lithium

vapor charge exchanges with the positively charged helium beam, giving up electrons to the beam particles.

Lithium Charge Exchange Cell (-15 kV)

Helium Ion Source• Some beam

particles become negatively charged (-1), accelerating out of the charge exchange cell.

Lithium Charge Exchange Cell (-15 kV)

Helium Ion Source• An electrode

at ground helps focus the beam for transmission to the FN Tandem accelerator.

Ground Electrode

• Brute force technique

• Difficult environment to stabilize

• Not much beam (a few microAmps at best), but better than none

• Other materials are available for charge exchange (alkali metals), but Li is probably safest.

Helium Ion Source

SNICS Ion Source

• Source of Negative Ions by Cesium Sputtering

• Workhorse source for FN Tandem, producing all beams except helium

• Highly reliable, sufficient beam output, recently upgraded from single to multi-cathode (major upgrade)

SNICS Ion Source (Old)• We have

recently upgraded our SNICS Ion Source with a Multi-Cathode version. This is a picture of the old, single cathode SNICS.

MC-SNICS Ion Source• This is a

picture of an MC-SNICS Ion Source (not ours). Principle of operation is the same as the old, single cathode version.

MC-SNICS Ion Source• This is 40

element cathode wheel. We have a 40 element and a 20 element wheel. More about this later.

SNICS Ion Source• The cathode is

loaded with a powder containing the material from which a beam is desired. We make these as needed in the lab.

SNICS Ion Source• Cesium is

heated in the reservoir to produce vapor, which rises up into the source.

SNICS Ion Source• Some of the

cesium adheres to the tip of the cathode, which is cooled. Some hits the hot surface of the ionizer.

SNICS Ion Source• The cesium that

hits the ionizer is “boiled” away and an electric field focuses this cesium to strike the tip of the cathode, which sputters material out of the powder.

SNICS Ion Source• The sputtered

material leaves the cathode tip and passes through the cold cesium on the surface.

SNICS Ion Source• The sputtered

particles pick up electrons from the cold cesium, and some become negatively charged and are extracted from the source.

SNICS Ion Source

• Easy to use, very reliable.

• Prolific beam production by FN standards

MC-SNICS Ion Source• The multi-

cathode version allows us to switch quickly (a few seconds) to another cathode, a process that once took several hours.

MC-SNICS Ion Source• This is

especially important for the AMS program. Work still needs to be done for this process.

RF Ion Sources

• KN and JN accelerators use RF (Radio Frequency) ion sources

• Glass bottles (cylindrical) with low presure gas, excited with power from oscillating electric field

• Plasma discharge, much like neon signs, but more intense.

RF Ion Sources

• Positively charged beams

• Significantly (1000 times) more beam than negative ion sources.

• But, much lower energies.