Farnsworth Fusor (“Carl’s Jr.”)

February 17, 2008

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.


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

Star dischargeCarl’s JrCathodeFusor systemModeratorCockroft-Walton

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


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.

OldFusorPoissor OldFusorTopViewxray machine sparking

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


Fusor Experiments


  • 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.


  1. […] RF Ion Source February 8, 2009 I have been working on an ion source to support my next fusor and other small accelerator projects.  Criteria for this source were that it had to be easy and […]

  2. nice toys, like those 🙂

  3. […] alchemists, [Carl Willis] might not be poisoning himself in a multitude of ways, but he did build a Farnsworth fusor that’s capable of turning Hydrogen into […]

  4. Vacuum pumps are a major headache if you on a budget.

    I did wonder if a circuit boards with fine tracks etched on it, then covered with cyanoacrylate (aka Superglue) or other HV resistant material such as sodium metasilicate with a sheet of glass placed next to the pcb with about a 0.05mm air gap would work.

    Idea here is to use the same sequential drive method as Jean-Louis Naudin’s ARDA, but with a very narrow air gap so that the overall airflow is in one direction.

  5. […] alchemists, [Carl Willis] might not be poisoning himself in a multitude of ways, but he did build a Farnsworth fusor that’s capable of turning Hydrogen into […]

  6. […] nu s-ar putea să fie el însuşi intoxicaţii într-o multitudine de feluri, dar a construi o Farnsworth fusor care este capabil de a se transforma hidrogenul în […]

  7. […] starters, Carl Willis will show you how to construct a farnsworth fuser to make helium from hydrogen. To fuse Hydrogen in […]

  8. […] source intensity is comparable to contemporary hobby fusion neutron sources, like well-constructed Farnsworth fusors.  After irradiation, a 2.5-hour gamma spectrum of the sample was collected with an HPGe […]

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