A Farnsworth-Hirsch fusor is a device for generating nuclear fusion reactions. Farnsworth-Hirsch fusors use strong electric fields to contain the fusion plasma, unlike the more common tokamak design, which uses magnetic fields. Farnsworth fusors are generally quite simple to build, and numerous amateurs have built their own, often from spare parts. Although a Farnsworth fusor cannot generate net energy from fusion, several related designs show potential, and research is currently underway to determine if any of them are viable as energy sources.
The basic principle behind the Farnsworth fusor, inertial electrostatic confinement (IEC), was first noticed in television vacuum tubes. If you apply an electric field to a plasma, the positively-charged ions in the plasma will be attracted by the electric field, and the ions will all congregate around the region with the lowest electrical potential. By arranging wires into a “grid” shape and then applying a voltage, the Farnsworth fusor attracts the ions into a central hollow. If the voltage is high enough, the electrical attraction will cause the ions to zoom through the central hollow at high speed; the ions then collide with other ions also moving through the hollow, inducing nuclear fusion.
Because the confinement mechanism is so simple, requiring only wires and a high-voltage power supply, Farnsworth fusors are fairly simple to build. Blueprints and instructions for building a fusor are available on the Internet, often requiring nothing more than a basic knowledge of the techniques used and a few thousand dollars worth of spare parts. Note that these home-built fusors can not be used as power sources due to the low number of fusion reactions, although they can be used as sources of neutron radiation.
The power produced by a fusor is limited by the density of the ions; as more ions are injected into the fusor, the ions begin to repel each other, setting an upper limit on the number of collisions. Ions also collide with the grid, sapping their energy and producing unwanted heat. Many workarounds have been proposed to solve these problems, and there are several active research efforts to develop a viable fusion power source. Some of the alternative designs proposed include the Polywell system, which uses a network of magnetic coils as well as an electric field, and a modified Penning trap, which uses a constant electric and magnetic field to confine charged particles. Although none have yet produced usable power, researchers are hopeful that a viable device can be built.