Farnsworth Fusor (“Carl’s Jr.”)

This post will serve as a documentation hub for my Farnsworth fusor hobby projects. I have used these devices as neutron generators for doing activation experiments at home and at college. The fusor is a simple spherical ion source / accelerator / collider that can be built on a shoestring budget with a minimum of technical background, enabling hobbyists to access some nuclear fusion reactions. This blog post isn’t intended to provide an adequate overview of the fusor’s physics or serve as a base for technical discussion–there are already some good websites out there with those purposes. Readers with questions or interest in this technology are encouraged to engage in the discussions on the Fusor Forum.

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“Carl’s Jr.” is my current fusor project, first operated in 2006.

Specifications of “Carl’s Jr.” are provided below, with links to more detail and commercial suppliers for some components.

• Typical operating conditions: 75 kVDC, 12 mA, 12 mtorr, > 2E+06 neutrons / sec.
• Vacuum chamber: 2 x 6″ 304SS hemispheres, 8″ equatorial CF flanges
• Chamber ports: 2 x QF25, 2 x 2.75″ CF
• Cathode: 5 x 1.5″ loops of 0.021″ dia. tungsten wire, construction by “Rosenstiel method“
• Ion source: DC magnetron in QF25 nipple (often not in use)
• HV feedthrough: Modified BeO naval antenna feedthrough, under mineral oil
• Gas system: 5o-liter LB cylinder of deuterium, 27″ 0.005″-bore capillary, needle valve
• High voltage system: homebrew switchmode 100 kV Cockroft-Walton generator
• Vacuum system: Edwards EO50/60 diffusion pump, backed by Edwards E2M-1
• Cooling system: forced-convection water in external copper coils
• Neutron irradiator: 120 lb. UHMW polyethylene moderator / flux trap

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While a student at Guilford College, I built the predecessor to Carl’s Jr., a larger but less innovative fusor with which I did some novel neutron activation experiments.

Specifications of my Guilford fusor are provided below.

• Typical operating conditions: 67 kVp, 15 mA, 10 mtorr, ~3E+06 neutrons / sec.
• Vacuum chamber: 2 x 8″ 304SS hemispheres, 10″ equatorial CF flanges
• Chamber ports: 2 x QF25, 1 x 2.75″ CF, 1 x special feedthrough solder lip
• Cathode: 6 x 2.5″ loops of 0.025″ dia. 316SS wire, construction by spot welding
• Ion source: none
• HV feedthrough: surplus 100 kV vacuum-rated feedthrough, air-insulated
• Gas system: 5o-liter LB cylinder of deuterium, 2 series needle valves
• High voltage system: 135 kVp x-ray supply, controlled by Variac and magnetic amplifier
• Vacuum system: Varian 50 LPS baby turbopump system
• Cooling system: forced-convection air cooling (ShopVac)
• Neutron irradiator: Water in VHS cases; paraffin canning wax

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Fusor Experiments

• Production of fission products in natural uranium
• Production of radioactive gallium
• Production of radioactive antimony
• Production of radioactive europium
• Production of radioactive bromine
• Production of radioactive indium
• Production of radioactive manganese
• Production of radioactive gold
• Production of radioactive vanadium
• Production of radioactive aluminum
• Production of radioactive iodine

Documents

• My undergraduate thesis for the physics department of Guilford College, entitled “Neutron Activation Using a Farnsworth Fusor” (2003) can be downloaded here. An accompanying PowerPoint presentation can be downloaded here.