What is a Fusor? – By Noah Liebnitz

What is a Fusor…?  (By Noah Liebnitz)

We can actually answer that question simply: a fusor is a device designed to fuse atoms. While this gets to the heart of the matter rather quickly, it is not particularly enlightening, nor is it completely correct. In reality, there are a number of devices designed to generate fusion reactions. Typically, when people use the word “fusor,” they are referring to the machine first built by Philo Farnsworth and Robert Hirsch in 1964 (you may recognize the former name, Fig. 1, as the inventor of the television) (“Fusor” 2021). The Farnsworth/Hirsch fusor uses an electric field to accelerate charged gas particles (ions) toward one another. If the ions happen to collide under the right conditions, they may fuse together to form a larger atom, releasing energy and neutrons in the process. So, a more detailed response to the question “what is a fusor?” may go something like this: a fusor is a device that generates fusion reactions by using electrical forces to generate continuous collisions between charged particles.

Fig. 1. Philo Farnsworth -- Inventor of the fusor...and the television.
Although the fusor mechanism may seem complicated (as indeed the detailed physics and mathematics are), the basic concept is relatively simple. Farnsworth originally designed his fusor to have a cylindrical structure, but he ultimately found greater success using concentric spherical grids (“Fusor” 2021; Hull 1997). In the spherical system (see the diagram in Fig. 2), a voltage is applied between the inner grid, which serves as the cathode, and the outer grid, a grounded anode (Klopfer 2012). When ionized (charged) gas particles are injected into the chamber, they interact with the electric field between the two grids. The injected gas is selected to have a net positive charge, so individual atoms accelerate inward—parallel to the electric field. Most of the ions miss colliding with the inner grid, and therefore enter the innermost region of the fusor. There is no electric field in this region, but the ions are still continually pushed inward by other ions streaming in through the pores in the inner grid (Hull 1997). Any ions that happen to escape the inner grid tend to be forced back in by the electric field.
Fig. 2. The Hirsch-Meeks fusor, a closely related model of the original Farnsworth-Hirsch design (Klopfer 2012).
It is in the central region where the interesting, potentially fusion-inducing physics occurs. There, ions collide at high velocities with ions that have been accelerated into the innermost region from opposite sides of the fusor chamber (Hull 1997). At lower operating pressures and voltages these collisions generate heat and form a dense plasma, but ramping up the system increases the temperature and density of the central plasma, endowing some small nuclei with enough energy to fuse together (“Fusor” 2021; Hull 1997). The whole process is depicted neatly in fig. 3.
Fig. 3. “An illustration of the basic mechanism of fusion in fusors. (1) The fusor contains two concentric wire cages: the cathode is inside the anode. (2) Positive ions are attracted to the inner cathode, they fall down the voltage drop. The electric field does work on the ions, heating them to fusion conditions. (3) The ions miss the inner cage. (4) The ions collide in the center and may fuse.” (“Fusor” 2021).

Fusion is often touted as the long-awaited solution to earth’s increasing appetite for energy, but—at least so far—nobody has been able to design a fusor that puts out more energy than it takes in. So, if you’re not trying to optimize the energy production, what then, is the point of building a fusor? Besides producing energy, fusion reactions spur the release of neutrons from atomic nuclei. As neutrally charged, fundamental particles, neutrons are useful in all sorts of physics experiments. So, functionally, a fusor is a low-cost means of producing neutrons and studying nuclear physics! It is our hope that the UPS Reactor Project will provide a learning tool for many future students!


“Fusor.” 2021. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Fusor&oldid=1007442866.

Hull, Richard. 1997. “The Farnsworth/Hirsch Fusor,” 8.

Klopfer, Brannon. 2012. “The Fusor.” 2012. http://large.stanford.edu/courses/2012/ph241/klopfer2/.

“Philo Farnsworth.” 2021. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Philo_Farnsworth&oldid=1008296151.