The Farnsworth–Hirsch fusor, or simply fusor, is an apparatus designed (~1964) by Philo T. Farnsworth to create nuclear fusion. It has also been developed in various incarnations by researchers including Elmore, Tuck, and Watson, and more recently by George H. Miley and Robert W. Bussard. Unlike most controlled fusion systems, which slowly heat a magnetically confined plasma, the fusor injects high energy ions directly into a reaction chamber, thereby avoiding a considerable amount of complexity. The approach is known as inertial electrostatic confinement (IEC).
Hopes at the time were high that it could be quickly developed into a practical source of fusion power. However, as with other fusion experiments, development into a generator has proven to be difficult. Nevertheless, the fusor has since become a practical source of free neutrons, and it is produced commercially for this purpose. Fusors have been assembled in low-power forms by hobbyists.
The fusor was originally conceived by Philo T. Farnsworth, better known for his pioneering work in television. In the early 1930s he investigated a number of vacuum tube designs for use in television, and found one that led to an interesting effect. In this design, which he called the multipactor, electrons moving from one electrode to another were stopped in mid-flight with the proper application of a high-frequency magnetic field. The charge would then accumulate in the center of the tube, leading to high amplification. Unfortunately it also led to high erosion on the electrodes when the electrons eventually hit them, and today the multipactor effect is generally considered a problem to be avoided.
What particularly interested Farnsworth about the device was its ability to focus electrons at a particular point. One of the biggest problems in fusion research is to keep the hot fuel from hitting the walls of the container. If this is allowed to happen, the fuel cannot be kept hot enough for the fusion reaction to occur. Farnsworth reasoned that he could build an electrostatic plasma confinement system in which the "wall" fields of the reactor were electrons or ions being held in place by the multipactor. Fuel could then be injected through the wall, and once inside it would be unable to escape. He called this concept a virtual electrode, and the system as a whole the fusor.
Farnsworth's original fusor designs were based on cylindrical arrangements of electrodes, like the original multipactors. Fuel was ionized and then fired from small accelerators through holes in the outer (physical) electrodes. Once through the hole they were accelerated towards the inner reaction area at high velocity. Electrostatic pressure from the positively charged electrodes would keep the fuel as a whole off the walls of the chamber, and impacts from new ions would keep the hottest plasma in the center. He referred to this as inertial electrostatic confinement, a term that continues to be used to this day.
Work at Farnsworth Television labs 
All of this work had taken place at the Farnsworth Television labs, which had been purchased in 1949 by ITT Corporation, as part of its plan to become the next RCA. However, a fusion research project was not regarded as immediately profitable. In 1965 the board of directors started asking 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 that ended ITT's experiments with fusion.
Things changed dramatically with the arrival of Robert Hirsch, and the introduction of the modified Hirsch-Meeks fusor patent. New fusors based on Hirsch's design were first constructed in the late 1960s.
The team then turned to the AEC, then in charge of fusion research funding, and provided them with a demonstration device mounted on a serving cart that produced more fusion than any existing "classical" device. The observers were startled, but the timing was bad; Hirsch himself had recently revealed the great progress being made by the Soviets using the tokamak. In response to this surprising development, the AEC decided to concentrate funding on large tokamak projects, and reduce backing for alternative concepts.
Recent developments 
In the early 1980s, disappointed by the slow progress on "big machines", a number of physicists took a fresh look at alternative designs. George H. Miley at the University of Illinois picked up on the fusor and re-introduced it into the field. A low but steady interest in the fusor has persisted since. An important development was the successful commercial introduction of a fusor-based neutron generator. From 2006 until his death in 2007, Robert W. Bussard gave talks on a reactor similar in design to the Fusor, now called Polywell, that he stated would be capable of useful power generation. 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, 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. 
Basic fusion 
Nuclear fusion refers to reactions in which lighter nuclei are combined to become heavier nuclei. Several such reactions release energy that can, in principle, be harnessed to provide fusion power. The lowest energy reaction occurs in a mix of deuterium and tritium, when the ions have to have a temperature of at least 4 keV (kiloelectronvolts), equivalent to about 45 million kelvins. At such temperatures, the fuel atoms are ionized and constitute a plasma. In a practical fusion power plant, fusion reactions have to occur fast enough to make up for energy losses. The rate of reaction varies with the temperature and the density of the fuel and the loss rate is characterized by the energy confinement time τE. The minimum conditions required are expressed in the Lawson criterion. In magnetic confinement fusion, the necessary conditions are approached by heating a plasma contained by magnetic fields. This has proven to be very difficult in practice. The complexity of the systems applied detracts from the usefulness of the design for a practical generator.
Fusor fusion 
In the original fusor design, several small particle accelerators, essentially TV tubes with the ends removed, inject ions at a relatively low voltage into a vacuum chamber. In the Hirsch version of the fusor, the ions are produced by ionizing a dilute gas in the chamber. In either version there are two concentric spherical electrodes, the inner one being charged negatively with respect to the outer one (to about 80 kV). Once the ions enter the region between the electrodes, they are accelerated towards the center.
In the fusor, the ions are accelerated to several keV by the electrodes, so heating as such is not necessary (as long as the ions fuse before losing their energy by any process). Whereas 45 megakelvins is a very high temperature by any standard, the corresponding voltage is only 4 kV, a level commonly found in such devices as neon lights and televisions. To the extent that the ions remain at their initial energy, the energy can be tuned to take advantage of the peak of the reaction cross section or to avoid disadvantageous (for example neutron-producing) reactions that might occur at higher energies.
Various attempts have been made at increasing deuterium ionization rate, including heaters within "ion-guns", (similar to the "electron gun" which forms the basis for old-style television display tubes), as well as magnetron type devices, (which are the power sources for microwave ovens), which can enhance ion formation using high-voltage electro-magnetic fields. Any method which increases ion density (within limits which preserve ion mean-free path), or ion energy, can be expected to enhance the fusion yield, typically measured in the number of neutrons produced per second.
The ease with which the ion energy can be increased appears to be particularly useful when "high temperature" fusion reactions are considered, such as proton-boron-11, which has plentiful fuel, requires no radioactive tritium, and produces no neutrons in the primary reaction.
Power density 
Because an electrostatic potential well cannot simultaneously trap both positively charged ions and electrons, there must be some regions of charge accumulation, which will result in an upper limit on the achievable density. The corresponding upper limit on the power density, even assuming D-T fuel, may be too low for power production.
Thermalization of the ion velocities 
When they first fall into the center of the fusor, the ions will all have the same energy, but the velocity distribution will rapidly approach a Maxwell-Boltzmann distribution. This would occur through simple Coulomb collisions in a matter of milliseconds, but beam-beam instabilities will occur orders of magnitude faster still. In comparison, any given ion will require a few minutes before undergoing a fusion reaction, so that the monoenergetic picture of the fusor, at least for power production, is not appropriate. One consequence of the thermalization is that some of the ions will gain enough energy to leave the potential well, taking their energy with them, without having undergone a fusion reaction.
There are a number of unsolved challenges with the electrodes in a fusor power system. To begin with, the electrodes cannot influence the potential within themselves, so it would seem at first glance that the fusion plasma would be in more or less direct contact with the inner electrode, resulting in contamination of the plasma and destruction of the electrode. However, the majority of the fusion tends to occur in microchannels formed in areas of minimum electric potential, 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 IEC devices, only the last is actually a "fusor".
"Bremsstrahlung", the German word for "braking radiation", describes radiation which is generated as a result of sub-atomic particles impacting something - another particle, or more typically, impacting a surface. One oft-presented concern for fusor operation is bremsstrahlung. In Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium, Todd Rider shows 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.
Use as a neutron source 
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Regardless of its possible use as an energy source, the fusor has already been demonstrated as a viable neutron source. Fluxes are not as high as can be obtained from 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.
- Bennett, W. H., U.S. Patent 3,120,475, February 1964. (Thermonuclear power)
- P.T. Farnsworth, U.S. Patent 3,258,402, June 1966 (Electric discharge — Nuclear interaction)
- P.T. Farnsworth, U.S. Patent 3,386,883. June 1968 (Method and apparatus)
- Hirsch, Robert, U.S. Patent 3,530,036. September 1970 (Apparatus)
- Hirsch, Robert, U.S. Patent 3,530,497. September 1970 (Generating apparatus — Hirsch/Meeks)
- Hirsch, Robert, U.S. Patent 3,533,910. October 1970 (Lithium-Ion source)
- Hirsch, Robert, U.S. Patent 3,655,508. April 1972 (Reduce plasma leakage)
- P.T. Farnsworth, U.S. Patent 3,664,920. May 1972 (Electrostatic containment)
- R.W. Bussard, "Method and apparatus for controlling charged particles", U.S. Patent 4,826,646, May 1989 (Method and apparatus — Magnetic grid fields).
- R.W. Bussard, "Method and apparatus for creating and controlling nuclear fusion reactions", U.S. Patent 5,160,695, November 1992 (Method and apparatus — Ion acoustic waves).
See also 
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- blogspot.com - Will's Amateur Science and Engineering: Fusion Reactor's First Light!, Feb 2010 (from blog)
- "Biography of Philo Taylor Farnsworth". University of Utah Marriott Library Special Collections. Retrieved 2007-07-05.
- "The Advent of Clean Nuclear Fusion: Super-performance Space Power and Propulsion", Robert W. Bussard, Ph.D., 57th International Astronautical Congress, October 2–6, 2006
- "UWFDM-1267 Diagnostic Study of Steady State Advanced Fuel (D-D and D-3He) Fusion in an IEC Device" (PDF). Retrieved 2009-09-16.
- "The IEC star-mode fusion neutron source for NAA--status and next-step designs". Appl Radiat Isot 53 (4-5): 779–83. October 2000. PMID 11003520.
Further reading 
- Reducing the Barriers to Fusion Electric Power; G.L. Kulcinski and J.F. Santarius, October 1997 Presented at "Pathways to Fusion Power", submitted to Journal of Fusion Energy, vol. 17, No. 1, 1998. (Abstract in PDF)
- Robert L. Hirsch, "Inertial-Electrostatic Confinement of Ionized Fusion Gases", Journal of Applied Physics, v. 38, no. 7, October 1967
- Irving Langmuir, Katharine B. Blodgett, "Currents limited by space charge between concentric spheres" Physical Review, vol. 24, No. 1, pp49–59, 1924
- R. A. Anderl, J. K. Hartwell, J. H. Nadler, J. M. DeMora, R. A. Stubbers, and G. H. Miley, Development of an IEC Neutron Source for NDE, 16th Symposium on Fusion Engineering, eds. G. H. Miley and C. M. Elliott, IEEE Conf. Proc. 95CH35852, IEEE Piscataway, NJ, 1482–1485 (1996).
- "On the Inertial-Electrostatic Confinement of a Plasma" William C. Elmore, James L. Tuck, Kenneth M. Watson, "The Physics of Fluids" v. 2, no 3, May–June, 1959
- PDF (142 KB); R.P. Ashley, G.L. Kulcinski, J.F. Santarius, S. Krupakar Murali, G. Piefer; IEEE Publication 99CH37050, pg. 35-37, 18th Symposium on Fusion Engineering, Albuquerque NM, 25–29 October 1999.
- G.L. Kulcinski, Progress in Steady State Fusion of Advanced Fuels in the University of Wisconsin IEC Device, March 2001
- Fusion Reactivity Characterization of a Spherically Convergent Ion Focus, T.A. Thorson, R.D. Durst, R.J. Fonck, A.C. Sontag, Nuclear Fusion, Vol. 38, No. 4. p. 495, April 1998. (abstract)
- Convergence, Electrostatic Potential, and Density Measurements in a Spherically Convergent Ion Focus, T. A. Thorson, R. D. Durst, R. J. Fonck, and L. P. Wainwright, Phys. Plasma, 4:1, January 1997.
- R.W. Bussard and L. W. Jameson, "Inertial-Electrostatic Propulsion Spectrum: Airbreathing to Interstellar Flight", Journal of Propulsion and Power, v 11, no 2. The authors describe the proton — Boron 11 reaction and its application to ionic electrostatic confinement.
- R.W. Bussard and L. W. Jameson, "Fusion as Electric Propulsion", Journal of Propulsion and Power, v 6, no 5, September–October, 1990 (This is the same Bussard who conceived the Bussard Ramjet widely used in science-fiction for interstellar rocketry)
- Todd H. Rider, "A general critique of inertial-electrostatic confinement fusion systems", M.S. thesis at MIT, 1994.
- Todd H. Rider, "Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium", Ph. D. thesis at MIT, 1995.
- Todd H. Rider, "Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium" Physics of Plasmas, April 1997, Volume 4, Issue 4, pp. 1039–1046.
- Could Advanced Fusion Fuels Be Used with Today's Technology?; J.F. Santarius, G.L. Kulcinski, L.A. El-Guebaly, H.Y. Khater, January 1998 [presented at Fusion Power Associates Annual Meeting, 27–29 August 1997, Aspen CO; Journal of Fusion Energy, Vol. 17, No. 1, 1998, p. 33].
- R.W. Bussard and L. W. Jameson, "From SSTO to Saturn's Moons, Superperformance Fusion Propulsion for Practical Spaceflight", 30th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, 27–29 June 1994, AIAA-94-3269
- Robert W. Bussard presentation video to Google Employees — Google TechTalks, 9 November 2006.
- "The Advent of Clean Nuclear Fusion: Super-performance Space Power and Propulsion", Robert W. Bussard, Ph.D., 57th International Astronautical Congress, 2–6 October 2006.
- David, Schneider, "Fusion from Television?". American Scientist, July–August
- University of Wisconsin-Madison IEC homepage
- RTFTechnologies.org IEC Fusion Reactor Detailed IEC reactor construction information
- Mr. Fusion — Blog of an experimenter
- Neutrons for sale — New Scientist article
- Fusion Experiments Show Nuclear Power's Softer Side — Wired article
- Various Patents and Articles Related to Fusion, IEC, ICC and Plasma Physics
- How a Small Vacuum System and a Bit of Basketweaving Will Get You a Working Inertial-Electrostatic Confinement Neutron Source
- Description of Bussard's "aneutronic" boron version
- Fusor.net Forum for hobbyist fusor builders
- Should Google Go Nuclear? Clean, cheap, nuclear power (no really) Google Video of Dr. Bussard explaining his work