Fusor

The Farnsworth-Hirsch 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 basic mechanism consists of two concentric spherical grids in a vacuum chamber with a very dilute fuel plasma. The grids are charged to high voltages.

There are two distinctly different designs. One design accelerates ions to the focus. The inner grid is negatively charged. The other design accelerates electrons, and the ions are attracted to the electron focus.

Most successful research has focused on ion acceleration, because this design focuses the ions at the convergence in the center of the reactor, and can more precisely control the energy of the ions. Both effects make fusion reactions more efficient.

A fusor is far more efficient in energy use than a thermally accelerated Tokamak or a linear accelerator. Since one electron volt equals 11,604 degrees, fusion temperatures and velocities are well within range for practical voltages. The accelerator produces a relatively narrow range of ionic velocities. So unlike a thermal system, in which only a few of the highest velocity ions can react, almost all of the ions are moving at speeds fast enough to react. 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.

Fusors have not achieved break-even because the grids absorb too many ions. Most research has focused on this problem.

Fusors have many theoretical advantages that make them attractive as possible reactors.

Fusors should be very safe. First, there is no possibility of a run-away reaction. A failure quickly discharges the electric field, halting the reaction. Also, only small amounts of fuel and reaction products are ever in the fusor, so even if these are nasty radioactive isotopes, breaching the reactor vessel would not release much. Finally, at high, but still practical voltages, fusors could react 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.

Unlike other fusion reactors, fusors are mostly vacuum. They therefore have low masses. Demonstration fusors have been built in vacuum chambers less than 30cm wide. Fusors therefore might be very suitable for powering vehicles, and possibly even portable equipment.

Since fusors operate at near vacuums, the ionized reaction products could practically be decelerated by electrostatic fields. This would allow the reactor to directly convert nuclear energy to electricity: high-voltage direct current at efficiencies well above 90%. The power would be available at voltages of several million volts. The extremely high voltage could directly power electrostatic motors to run vehicles. The voltages could accelerate intense electron or ion beams for weapons or to heat reaction masses for rocketry. The high voltages could power aircraft or boats directly with ionic wind effects.

The fusor was originally conceived by Philo Farnsworth (the same man who helped develop TV) while he was investigating spherically-concentric multipactor vacuum tubes. He noticed that such tubes had a bright spot near the center, and 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.

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 at right angles (welding wire works). Small spot welders can be constructed from common electronic materials. The fusor's electrode dimensions are not very critical. The outer electrode can range from beach-ball to base-ball size, and the inner from baseball to ping-pong ball size. Of course, the experiementer must calculate the ion acceleration desired, and size and space the electrodes for the desired fusion reaction! Usually such projects use a neon sign transformer (available as scrap from some electrical or demolition contractors), and high voltage rectifiers (purchased or scrounged). 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, which can be salvaged or borrowed. 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

External links:

• American Scientist article
• University of Wisconsin-Madison IEC homepage