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Direct energy conversion (DEC) or simply direct conversion converts a charged particle's kinetic energy into a voltage. It is a scheme for power extraction from nuclear fusion.

A basic direct converter

History and theoretical underpinnings[edit]

Electrostatic direct collectors[edit]

In the middle of the 1960s direct energy conversion was proposed as a method for capturing the energy from the exhaust gas in a fusion reactor. This would generate a direct current of electricity. Richard F. Post at the Lawrence Livermore National Laboratory was an early proponent of the idea.[1] Post reasoned that capturing the energy would require five steps:[2] (1) Ordering the charged particles into linear beam. (2) Separation of positives and negatives. (3) Separating the ions into groups, by their energy. (4) Gathering these ions as they touch collectors. (5) Using these collectors as the positive side in a circuit. Post argued that the efficiency was theoretically determined by the number of collectors.

The Venetian blind[edit]

Designs in the early 1970s by William Barr and Ralph Moir used metal ribbons at an angle to collect these ions. This was called the Venetian Blind design, because the ribbons look like window blinds. Those metal ribbon-like surfaces are more transparent to ions going forward than to ions going backward. Ions pass through surfaces of successively increasing potential until they turn and start back, along a parabolic trajectory. They then see opaque surfaces and are caught. Thus ions are sorted by energy with high-energy ions being caught on high-potential electrodes.[3][4][5]

William Barr and Ralph Moir then ran a group which did a series of direct energy conversion experiments through the late 1970s and early 1980s.[6] The first experiments used beams of positives and negatives as fuel, and demonstrated energy capture at a peak efficiency of 65 percent and a minimum efficiency of 50 percent.[7][8] The following experiments involved a true plasma direct converter that was tested on the Tandem Mirror Experiment (TMX), an operating magnetic mirror fusion reactor. In the experiment, the plasma moved along diverging field lines, spreading it out and converting it into a forward moving beam with a Debye length of a few centimeters.[9] Suppressor grids then reflect the electrons, and collector anodes recovered the ion energy by slowing them down and collecting them at high-potential plates. This machine demonstrated an energy capture efficiency of 48 percent.[10] However, Marshall Rosenbluth argued that keeping the plasma's neutral charge over the very short Debye length distance would be very challenging in practice, though he said that this problem would not occur in every version of this technology.[9]

The Venetian Blind converter can operate with 100 to 150 keV D-T plasma, with an efficiency of about 60% under conditions compatible with economics, and an upper technical conversion efficiency up to 70% ignoring economic limitations.[4]

Periodic electrostatic focusing[edit]

A second type of electrostatic converter initially proposed by Post, then developed by Barr and Moir, is the Periodic Electrostatic Focusing concept.[2][5][11] Like the Venetian Blind concept, it is also a direct collector, but the collector plates are disposed in many stages along the longitudinal axis of an electrostatic focusing channel. As each ion is decelerated along the channel toward zero energy, the particle becomes "over-focused" and is deflected sideways from the beam, then collected. The Periodic Electrostatic Focusing converter typically operates with a 600 keV D-T plasma (as low as 400 keV and up to 800 keV) with efficiency of about 60% under conditions compatible with economics, and an upper technical conversion efficiency up to 90% ignoring economic limitations.[12]

Induction systems[edit]

Conduction systems[edit]

From the 1960s through the 1970s, methods have been developed to extract electrical energy directly from a hot gas (a plasma) in motion within a channel fitted with electromagnets (producing a transverse magnetic field), and electrodes (connected to load resistors). Charge carriers (free electrons and ions) incoming with the flow are then separated by the Lorentz force and an electric potential difference can be retrieved from pairs of connected electrodes. Shock tubes used as pulsed MHD generators were for example able to produce several megawatts of electricity in channels the size of a beverage can.[13]

Induction systems[edit]

In addition to converters using electrodes, pure inductive magnetic converters have also been proposed by Lev Artsimovich in 1963,[14] then Alan Frederic Haught and his team from United Aircraft Research Laboratories in 1970,[15] and Ralph Moir in 1977.[16]

The magnetic compression-expansion direct energy converter is analogous to the internal combustion engine. As the hot plasma expands against a magnetic field, in a manner similar to hot gases expanding against a piston, part of the energy of the internal plasma is inductively converted to an electromagnetic coil, as an EMF (voltage) in the conductor.

This scheme is best used with pulsed devices, because the converter then works like a "magnetic four-stroke engine":

  1. Compression: A column of plasma is compressed by a magnetic field that acts like a piston.
  2. Thermonuclear burn: The compression heats the plasma to the thermonuclear ignition temperature.
  3. Expansion/Power: The expansion of fusion reaction products (charged particles) increases the plasma pressure and pushes the magnetic field outward. A voltage is induced and collected in the electromagnetic coil.
  4. Exhaust/Refuel: After expansion, the partially burned fuel is flushed out, and new fuel in the form of gas is introduced and ionized; and the cycle starts again.

In 1973, a team from Los Alamos and Argonne laboratories stated that the thermodynamic efficiency of the magnetic direct conversion cycle from alpha-particle energy to work is 62%.[17]

Traveling-wave direct energy converter[edit]

In 1992, a Japan–U.S. joint-team proposed a novel direct energy conversion system for 14.7 MeV protons produced by D-3He fusion reactions, whose energy is too high for electrostatic converters.[18]

The conversion is based on a Traveling-Wave Direct Energy Converter (TWDEC). A gyrotron converter first guides fusion product ions as a beam into a 10-meter long microwave cavity filled with a 10-tesla magnetic field, where 155 MHz microwaves are generated and converted to a high voltage DC output through rectennas.

The Field-Reversed Configuration reactor ARTEMIS in this study was designed with an efficiency of 75%. The traveling-wave direct converter has a maximum projected efficiency of 90%.[19]

Inverse cyclotron converter (ICC)[edit]

Original direct converters were designed to extract the energy carried by 100 to 800 keV ions produced by D-T fusion reactions. Those electrostatic converters are not suitable for higher energy product ions above 1 MeV generated by other fusion fuels like the D-3He or the p-11B aneutronic fusion reactions.

A much shorter device than the Traveling-Wave Direct Energy Converter has been proposed in 1997 and patented by Tri Alpha Energy, Inc. as an Inverse Cyclotron Converter (ICC).[20][21]

The ICC is able to decelerate the incoming ions based on experiments made in 1950 by Felix Bloch and Carson D. Jeffries,[22] in order to extract their kinetic energy. The converter operates at 5 MHz and requires a magnetic field of only 0.6 tesla. The linear motion of fusion product ions is converted to circular motion by a magnetic cusp. Energy is collected from the charged particles as they spiral past quadrupole electrodes. More classical electrostatic collectors would also be used for particles with energy less than 1 MeV. The Inverse Cyclotron Converter has a maximum projected efficiency of 90%.[19][20][21][23][24]

X-ray photoelectric converter[edit]

A significant amount of the energy released by fusion reactions is composed of electromagnetic radiations, essentially X-rays due to Bremsstrahlung. Those X-rays can not be converted into electric power with the various electrostatic and magnetic direct energy converters listed above, and their energy is lost.

Whereas more classical thermal conversion has been considered with the use of a radiation/boiler/energy exchanger where the X-ray energy is absorbed by a working fluid at temperatures of several thousand degrees,[25] more recent research done by companies developing nuclear aneutronic fusion reactors, like Lawrenceville Plasma Physics (LPP) with the Dense Plasma Focus, and Tri Alpha Energy, Inc. with the Colliding Beam Fusion Reactor (CBFR), plan to harness the photoelectric and Auger effects to recover energy carried by X-rays and other high-energy photons. Those photoelectric converters are composed of X-ray absorber and electron collector sheets nested concentrically in an onion-like array. Indeed, since X-rays can go through far greater thickness of material than electrons can, many layers are needed to absorb most of the X-rays. LPP announces an overall efficiency of 81% for the photoelectric conversion scheme.[26][27]

Direct energy conversion from fission products[edit]

In the early 2000s, research was undertaken by Sandia National Laboratories, Los Alamos National Laboratory, The University of Florida, Texas A&M University and General Atomics to use direct conversion to extract energy from fission reactions, essentially, attempting to extract energy from the linear motion of charged particles coming off a fission reaction.[28]


  1. ^ Post, Richard F. (November 1969). "Direct Conversion of Thermal Energy of High Temperature Plasma". Bulletin of the American Physical Society. 14 (11): 1052. 
  2. ^ a b Post, Richard F. (September 1969). Mirror Systems: Fuel Cycles, Loss Reduction and Energy Recovery (PDF). BNES Nuclear Fusion Reactor Conference. Culham Centre for Fusion Energy, Oxfordshire, U.K.: British Nuclear Energy Society. pp. 87–111. 
  3. ^ Moir, R. W.; Barr, W. L. (1973). ""Venetian-blind" direct energy converter for fusion reactors" (PDF). Nuclear Fusion. 13: 35. doi:10.1088/0029-5515/13/1/005. 
  4. ^ a b Barr, W. L.; Burleigh, R. J.; Dexter, W. L.; Moir, R. W.; Smith, R. R. (1974). "A preliminary engineering design of a "Venetian blind" direct energy converter for fusion reactors" (PDF). IEEE Transactions on Plasma Science. 2 (2): 71. Bibcode:1974ITPS....2...71B. doi:10.1109/TPS.1974.6593737. 
  5. ^ a b Moir, R. W.; Barr, W. L.; Miley, G. H. (1974). "Surface requirements for electrostatic direct energy converters" (PDF). Journal of Nuclear Materials. 53: 86. Bibcode:1974JNuM...53...86M. doi:10.1016/0022-3115(74)90225-6. 
  6. ^ Morris, Jeff. "In Memoriam." (n.d.): n. pag. Rpt. in Newsline. 19th ed. Vol. 29. Livermore: Lawrence Livermore National Laboratory, 2004. 2. Print.
  7. ^ Barr, William L.; Doggett, James N.; Hamilton, Gordon W.; Kinney, John; Moir, Ralph W. (25–28 October 1977). Engineering of Beam Direct Conversion for a 120kV, 1MW Ion Beam (PDF). 7th Symposium on Engineering Problems of Fusion Research. Knoxville,Tennessee. 
  8. ^ Barr, W. L.; Moir, R. W.; Hamilton, G. W. (1982). "Experimental results from a beam direct converter at 100 kV". Journal of Fusion Energy. 2 (2): 131. Bibcode:1982JFuE....2..131B. doi:10.1007/BF01054580. 
  9. ^ a b Rosenbluth, M. N.; Hinton, F. L. (1994). "Generic issues for direct conversion of fusion energy from alternative fuels". Plasma Physics and Controlled Fusion. 36 (8): 1255. Bibcode:1994PPCF...36.1255R. doi:10.1088/0741-3335/36/8/003. 
  10. ^ Barr, William L.; Moir, Ralph W. (January 1983). "Test results on plasma direct converters". 3 (1). American Nuclear Society: 98–111. ISSN 0272-3921. 
  11. ^ Barr, W. L.; Howard, B. C.; Moir, R. W. (1977). "Computer Simulation of the Periodic Electrostatic Focusing Converter" (PDF). IEEE Transactions on Plasma Science. 5 (4): 248. Bibcode:1977ITPS....5..248B. doi:10.1109/TPS.1977.4317060. 
  12. ^ Smith, Bobby H.; Burleigh, Richard; Dexter, Warren L.; Reginato, Lewis L. (20–22 November 1972). An Engineering Study of the Electrical Design of a 1000-Megawatt Direct Converter for Mirror Reactors. Texas Symposium on Technology of Controlled Thermonuclear Fusion Experiments and the Engineering Aspects of Fusion Reactors. Austin, Texas: U.S. Atomic Energy Commission. 
  13. ^ Sutton, George W.; Sherman, Arthur (July 2006). Engineering Magnetohydrodynamics. Dover Civil and Mechanical Engineering. Dover Publications. ISBN 978-0486450322. 
  14. ^ Artsimovich, L. A. (1963). Управляемые термоядерные реакции [Controlled Thermonuclear Reactions] (in Russian) (2nd ed.). Moscow: Fizmatgiz. 
  15. ^ Haught, A. F. (1970). "Magnetic Field Confinement of Laser Irradiated Solid Particle Plasmas". Physics of Fluids. 13 (11): 2842. Bibcode:1970PhFl...13.2842H. doi:10.1063/1.1692870. 
  16. ^ Moir, Ralph W. (April 1977). "Chapter 5: Direct Energy Conversion in Fusion Reactors". In Considine, Douglas M. Energy Technology Handbook (PDF). NY: McGraw-Hill. pp. 150–154. ISBN 978-0070124301. 
  17. ^ Oliphant, T. A.; Ribe, F. L.; Coultas, T. A. (1973). "Direct conversion of thermonuclear plasma energy by high magnetic compression and expansion". Nuclear Fusion. 13 (4): 529. doi:10.1088/0029-5515/13/4/006. 
  18. ^ Momota, Hiromu; Ishida, Akio; Kohzaki, Yasuji; Miley, George H.; Ohi, Shoichi; Ohnishi, Masami; Sato, Kunihiro; Steinhauer, Loren C.; Tomita, Yukihiro; Tuszewski, Michel (July 1992). "Conceptual Design of the D-3He Reactor Artemis" (PDF). Fusion Science and Technology. 21 (4): 2307–2323. 
  19. ^ a b Rostoker, N.; Binderbauer, M. W.; Monkhorst, H. J. (1997). "Colliding Beam Fusion Reactor" (PDF). Science. 278 (5342): 1419–22. Bibcode:1997Sci...278.1419R. doi:10.1126/science.278.5342.1419. PMID 9367946. Archived from the original (PDF) on December 20, 2005. 
  20. ^ a b US patent 6850011, Monkhorst, Hendrik J. & Rostoker, Norman, "Controlled fusion in a field reversed configuration and direct energy conversion", issued 2005-02-01, assigned to The Regents Of The University Of California and University Of Florida Research Foundation 
  21. ^ a b WO application 2006096772, Binderbauer, Michl; Bystritskii, Vitaly & Rostoker, Norman et al., "Plasma electric generation system", published 2006-12-28, assigned to Binderbauer, Michl and Bystritskii, Vitaly 
  22. ^ Bloch, F.; Jeffries, C. (1950). "A Direct Determination of the Magnetic Moment of the Proton in Nuclear Magnetons". Physical Review. 80 (2): 305. Bibcode:1950PhRv...80..305B. doi:10.1103/PhysRev.80.305. 
  23. ^ Yoshikawa, K.; Noma, T.; Yamamoto, Y. (May 1991). "Direct-Energy Conversion from High-Energy Ions Through Interaction with Electromagnetic Fields". Fusion Science and Technology. American Nuclear Society. 19 (3P2A): 870–875. 
  24. ^ Rostoker, N.; Binderbauer, M.; Monkhorst, H. J. (1997). Office of Naval Research Reports (Technical report). 
  25. ^ Taussig, Robert T. (April 1977). High thermal efficiency, radiation-based advanced fusion reactors. Palo Alto, CA: Electric Power Research Institute. 
  26. ^ US patent 7482607, Lerner, Eric J. & Blake, Aaron, "Method and apparatus for producing X-rays, ion beams and nuclear fusion energy", issued 2009-01-27, assigned to Lawrenceville Plasma Physics, Inc. 
  27. ^ US application 2013125963, Binderbauer, Michl & Tajima, Toshiki, "Conversion of high-energy photons into electricity", published 2013-05-23, assigned to Tri Alpha Energy, Inc. 
  28. ^ l.c. Brown (2002). "Direct Energy Conversion Fission Reactor Annual Report for the Period August 15,2000 Through September 30,2001". doi:10.2172/805252. 


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