thumb|A homemade fusor
A fusor is a device that uses an electric field to heat ions to a temperature at which they undergo nuclear fusion. The machine induces a voltage between two metal cages inside a vacuum. Positive ions fall down this voltage drop, building up speed. If they collide in the center, they can fuse. This is one kind of an inertial electrostatic confinement devicethe type studied in a branch of fusion research.
A Farnsworth–Hirsch fusor is the most common type of fusor. This design came from work by Philo T. Farnsworth in 1964 and Robert L. Hirsch in 1967. A variant type of fusor had been proposed previously by William Elmore, James L. Tuck, and Ken Watson at the Los Alamos National Laboratory though they never built the machine.
Fusors have been built by various institutions. These include academic institutions such as the University of Wisconsin–Madison, the Massachusetts Institute of Technology and government entities, such as the Atomic Energy Organization of Iran and the Turkish Atomic Energy Authority. Fusors have also been developed commercially, as sources for neutrons by DaimlerChrysler Aerospace Fusors have also become very popular for hobbyists and amateurs. A growing number of amateurs have performed nuclear fusion using simple fusor machines. However, fusors are not considered a viable concept for large-scale energy production by scientists.
Mechanism
Underlying physics
Fusion takes place when nuclei approach to a distance where the nuclear force can pull them together into a single larger nucleus. Opposing this close approach are the positive charges in the nuclei, which force them apart due to the electrostatic force. In order to produce fusion events, the nuclei must have initial energy great enough to allow them to overcome this Coulomb barrier. As the nuclear force is increased with the number of nucleons, protons and neutrons, and the electromagnetic force is increased with the number of protons only, the easiest atoms to fuse are isotopes of hydrogen, deuterium with one neutron, and tritium with two. With hydrogen fuels, about 3 to 10 keV is needed to allow the reaction to take place.
Traditional approaches to fusion power have generally attempted to heat the fuel to temperatures where the Maxwell-Boltzmann distribution of their resulting energies is high enough that some of the particles in the long tail have the required energy.]]
Losses
It is important to consider the actual startup sequence of a fusor to understand the resulting operation. Normally the system is pumped down to a vacuum and then a small amount of gas is placed inside the vacuum chamber. This gas will spread out to fill the volume. When voltage is applied to the electrodes, the atoms between them will experience a field that will cause them to ionize and begin accelerating inward. As the atoms are randomly distributed to begin, the amount of energy they will gain differs; atoms initially near the anode will gain some large portion of the applied voltage, say 15 keV. Those initially near the cathode will gain much less energy, possibly far too low to undergo fusion with their counterparts on the far side of the central reaction area.
The fuel atoms inside the inner area during the startup period are not ionized. The accelerated ions scatter with these and lose their energy, while ionizing the formerly cold atom. This process, and the scatterings off other ions, causes the ion energies to become randomly distributed and the fuel rapidly takes on a non-thermal distribution. For this reason, the energy needed in a fusor system is higher than one where the fuel is heated by some other method, as some will be "lost" during startup.
There are numerous other loss mechanisms as well. These include charge exchange between high-energy ions and low-energy neutral particles, which causes the ion to capture the electron, become electrically neutral, and then leave the fusor as it is no longer accelerated back into the chamber. This leaves behind a newly ionized atom of lower energy and thus cools the plasma. Scatterings may also increase the energy of an ion which allows it to move past the anode and escape, in this example anything above 15 keV.
As a result of these loss mechanisms, no fusor has ever come close to break-even energy output and it appears it is unable to ever do so. However, a fusion research project was not regarded as immediately profitable. In 1965, the board of directors started asking Harold Geneen to sell off the Farnsworth division, but he had his 1966 budget approved with funding until the middle of 1967. Further funding was refused, and Farnsworth became ill and entered medical retirement, which ended ITT's experiments with fusion. New fusors based on Hirsch's design were first constructed between 1964 and 1967. Most recently, the fusor has gained popularity among amateurs, who choose them as home projects due to their relatively low space, money, and power requirements. An online community of "fusioneers", The Open Source Fusor Research Consortium, or Fusor.net, is dedicated to reporting developments in the world of fusors and aiding other amateurs in their projects. The site includes forums, articles and papers done on the fusor, including Farnsworth's original patent, as well as Hirsch's patent of his version of the invention.
Fusion in fusors
Basic fusion
thumb|upright=1.35|The cross-sections of different fusion reactions
Nuclear fusion refers to reactions in which lighter nuclei are combined to become heavier nuclei. This process changes mass into energy which in turn may be captured to provide fusion power. Many types of atoms can be fused. The easiest to fuse are deuterium and tritium. For fusion to occur the ions must be at a temperature of at least 4 keV (kiloelectronvolts), or about 45 million kelvins. The second easiest reaction is fusing deuterium with itself. Because this gas is cheaper, it is the fuel commonly used by amateurs. The ease of doing a fusion reaction is measured by its cross section.
Net power
At such conditions, the atoms are ionized and make a plasma. The energy generated by fusion, inside a hot plasma cloud can be found with the following equation.
: <math>P_\text{fusion} = n_A n_B \langle \sigma v_{A,B} \rangle E_\text{fusion},</math>
where
: <math>P_\text{fusion}</math> is the fusion power density (energy per time per volume),
: n is the number density of species A or B (particles per volume),
: <math>\langle \sigma v_{A,B} \rangle</math> is the product of the collision cross-section σ (which depends on the relative velocity) and the relative velocity v of the two species, averaged over all the particle velocities in the system,
: <math>E_\text{fusion}</math> is the energy released by a single fusion reaction.
This equation shows that energy varies with the temperature, density, speed of collision, and fuel used. To reach net power, fusion reactions have to occur fast enough to make up for energy losses. Any power plant using fusion will hold in this hot cloud. Plasma clouds lose energy through conduction and radiation. The halo mode occurs in higher pressure tanks, and as the vacuum improves, the device transitions to star mode. Star mode appears as bright beams of light emanating from the device center. seen as visible "rays" penetrating the core. These form because the forces within the region correspond to roughly stable "orbits". Approximately 40% of the high energy ions in a typical grid operating in star mode may be within these microchannels. Nonetheless, grid collisions remain the primary energy loss mechanism for Farnsworth–Hirsch fusors. Complicating issues is the challenge in cooling the central electrode; any fusor producing enough power to run a power plant seems destined to also destroy its inner electrode. As one fundamental limitation, any method which produces a neutron flux that is captured to heat a working fluid will also bombard its electrodes with that flux, heating them as well.
Attempts to resolve these problems include Bussard's Polywell system, D. C. Barnes' modified Penning trap approach, and the University of Illinois's fusor which retains grids but attempts to more tightly focus the ions into microchannels to attempt to avoid losses. While all three are Inertial electrostatic confinement (IEC) devices, only the last is actually a "fusor".
Radiation
Charged particles will radiate energy as light when they change velocity. This loss rate can be estimated for nonrelativistic particles using the Larmor formula. Inside a fusor there is a cloud of ions and electrons. These particles will accelerate or decelerate as they move about. These changes in speed make the cloud lose energy as light. The radiation from a fusor can (at least) be in the visible, ultraviolet and X-ray spectrum, depending on the type of fusor used. These changes in speed can be due to electrostatic interactions between particles (ion to ion, ion to electron, electron to electron). This is referred to bremsstrahlung radiation, and is common in fusors. Changes in speed can also be due to interactions between the particle and the electric field. Since there are no magnetic fields, fusors emit no cyclotron radiation at slow speeds, or synchrotron radiation at high speeds.
In Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium, Todd Rider argues that a quasineutral isotropic plasma will lose energy due to Bremsstrahlung at a rate prohibitive for any fuel other than D-T (or possibly D-D or D-He3). This paper is not applicable to IEC fusion, as a quasineutral plasma cannot be contained by an electric field, which is a fundamental part of IEC fusion. However, in an earlier paper, "A general critique of inertial-electrostatic confinement fusion systems", Rider addresses the common IEC devices directly, including the fusor. In the case of the fusor the electrons are generally separated from the mass of the fuel isolated near the electrodes, which limits the loss rate. However, Rider demonstrates that practical fusors operate in a range of modes that either lead to significant electron mixing and losses, or alternately lower power densities. This appears to be a sort of catch-22 that limits the output of any fusor-like system.
Safety
There are several key safety considerations involved with the building and operation of a fusor. First, there is the high-voltage involved. Second, there are the x-ray and neutron emissions that are possible. Also there are the publicity / misinformation considerations with local and regulatory authorities.
Commercial applications
{| class="wikitable floatright" style="text-align: left"
! Production source
| Neutrons
|-
! Energy
| 2.45 MeV
|-
! Mass
| 940 MeV
|-
! Electric charge
| 0 C
|-
! Spin
| 1/2
|}
Neutron source
The fusor has been demonstrated as a viable neutron source. Typical fusors cannot reach fluxes as high as nuclear reactor or particle accelerator sources, but are sufficient for many uses. Importantly, the neutron generator easily sits on a benchtop, and can be turned off at the flick of a switch. A commercial fusor was developed as a non-core business within DaimlerChrysler Aerospace – Space Infrastructure, Bremen between 1996 and early 2001. After the project was effectively ended, the former project manager established a company which is called NSD-Fusion.