Fusor

The Farnsworth-Hirsch Fusor, or simply fusor, is an apparatus designed by Philo T. Farnsworth to create nuclear fusion by focusing highly accelerated ions in inertial electrostatic confinement of a plasma.

The fusor was originally conceived by Philo Farnsworth, the man who is largely responsible for television, while he was investigating spherically-concentric multipactor vacuum tubes. He noticed that such tubes had a bright spot near the center, explained it, and then conceived the notion of using it for fusion reactions. Later, under Farnsworth's administration at the ITT laboratories, Robert Hirsch further developed the fusor. Hirsch published neutron production rates of up a billion per second, and has been reported to have observed rates of up to a trillion per second.

The basic mechanism consists of two concentric spherical grid electrodes in a vacuum chamber with a very dilute fuel plasma. Depending on the design, the inner electrode is negative and thus accelerates ions toward the center of the chamber, or alternately the inner electrode is positive and accelerates electrons towards the center. Most research has focused on ion acceleration, the ions, being heavier, are much easier to focus and give a consistant energy.

In theory the fusor is perhaps the most promising form of fusion reactor studied. Energy is added to the fuel directly through acceleration, as opposed to various indirect means required in a Tokamak or similar magnetically confined systems. One electron volt equals 11,604 degrees, so fusion temperatures are well within the range of voltages that can be generated with standard lab high-voltage equipment. Better yet, since the fusor is accelerating the ions (or electrons) directly, the range of velocities (or temperatures) is quite narrow. This means that most of the ions have enough energy to undergo fusion, whereas in a magnetically confined system it is typically only the "hotest" ions that have anywhere near the required energy. Finally, failed collisions scatter the ions elastically, so no energy is lost from failed collisions. Since a fusor's accelerating field is spherical, the lost ions are returned without energy or ionic losses to the focus for more chances to react.

There is, of course, a downside. No matter how thin the inner electrodes are, the ions being accelerated past them have a finite chance of hitting them. In addition to removing energy from the system, this also releases a cascade of heavy ions into the center of the chamber, which are far too slow to fuse. This effectively "quenches" the reaction, a problem that is endemic to magnetically confined systems as well, where it is known as sputtering. The electric force between the electrodes means that there is a very real lower limit to their size, so this problem is not easy to avoid. Fusors have not achieved break-even for these reasons, and in fact have generally been limited to power ranges far less than the "high-end" Tokamaks.

Fusors have many theoretical advantages that make them attractive as possible reactors. The nature of their acceleration means that much higher average ionic energies are possible than in "conventional" designs, allowing the fusor to run on fuels that are impractical for thermal reactors. One of the most attractive such combinations is the proton - boron-11 reaction, which uses cheap natural isotopes, produces only helium, and produces neither neutrons nor gamma rays. This is a very clean reaction that would dramatically reduce waste when decommisioning a plant.

Small demonstration fusors that achieve fusion (but not break-even!) can and have been constructed by amateurs, including high-school students for science projects. Each electrode is spot-welded from three hoops of stainless-steel wire (often welding rod) at right angles. The fusor's electrode dimensions are not very critical. The outer electrode can range from beach-ball to baseball size, and the inner from baseball to ping-pong ball size. Usually such projects use the high-voltage transformer form a neon sign, and high voltage rectifier from a hobby shop. Spark plug wires carry the voltages, with spark plugs to pass the voltages into the vacuum chamber. Deuterium is available in lecturer bottles and is not a controlled nuclear material. Neutrons can be sensed by measuring induced radioactivity in aluminium foil after moderating the neutrons with wax or plastic, or a plastic neutron luminescent material can be used with a photodetector. The major expense is the vacuum pump. Note that the voltages are dangerous (though less dangerous than a TV), and neutron emissions do present some hazard. The x-ray emissions are less than a color TV (the voltages are less).

References

"The World's Simplest Fusion Reactor, and How to Make It Work", Tom Ligon, Analog, December 1998. This an amusing reference for laymen. Analog is a science fiction magazine that publishes one fact article each month; this is the fact article. The article describes homebrewed fusors, as well as applications of fusors to spacecraft.
P.T. Farnsworth, Patent #3,258,402, issued 28 june 1966
"Inertial-Electrostatic Confinement of Ionized Fusion Gases" Robert L. Hirsch, Journal of Applied Physics, v. 38, no. 7, October 1967
G.L. Kulcinski, Progress in Steady State Fusion of Advanced Fuels in the University of Wisconsin IEC Device, March 2001
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).
D-3He Fusion in an Inertial Electrostatic Confinement Device; 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.
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
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, August 27-29, 1997, Aspen CO; Journal of Fusion Energy, Vol. 17, No. 1, 1998, p. 33].
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, "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 posited in science-fiction for interstellar rocketry- possibly to his embarrassment)
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. Snappy title, eh?
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
R.W. Bussard, "Method and apparatus for controlling charged particles", Patent 4,826,626, Issued 2 May 1989 About avoiding the grid losses by using magnetic fields.
Irving Langmuir, Katherine B. Blodgett, "Currents limited by space charge between concentric spheres" Physics Review, 23, pp49-59, 1924
"On the Intertial-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

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