The Farnsworth fusor is one of the earliest practical designs to generate fusion. It was first proposed by Philio T. Farnsworth in the 1930's to create nuclear fusion with the ultimate goal to develop it for fusion power. The design has however several inherent constraints to the efficiency which makes it unusable for designs with a high net power production. The relatively simple Farnsworth fusor design has however proven to be very usable as a source for fast neutrons (when using D-D or D-T).

As an initial step to nuclear fusion Radiant Matter Research decided first constructed a Farnsworth fusor to build, design and test all the necessary equipment later required for the Polywell project. Also the Farnsworth fusor has been build by several people and a good information resource on these fusion reactors is available at

So far the RMR Farnsworth fusor has undergone several iterations. The main properties of each version are listed in the table below:

RMR-FF1 reactor RMR-FF2 reactor RMR-FF3 reactor RMR-FF4 reactor RMR-FF5 reactor
Lowest pressure $6\cdot10^{-2}$ mbar $6\cdot10^{-2}$ mbar $<1\cdot10^{-3}$ mbar $<1\cdot10^{-3}$ mbar $3\cdot10^{-7}$ mbar

Vacuum chamber

smallest diameter

76mm 130mm 130mm 130mm 300mm
Radiation monitoring $\beta+\gamma$ $\beta+\gamma$

$\alpha + \beta + \gamma$


fast neutrons

$\alpha + \beta+\gamma$

fast, thermal neutrons

extensive dosimetry

Maximum voltage ~1000V 8kV (4kV usable) 8kV (4kV usable) 70kV (35kV usable) 50kV
Maximum power unknown 400W 400W 350W 300W