Polywell
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Polywell is a gridless inertial electrostatic confinement fusion process utilizing magnetic mirrors designed by Robert Bussard under a Navy research contract, designed to overcome the losses in the Farnsworth-Hirsch fusor and create a breakeven fusion reactor.
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[edit] Design
A traditional Farnsworth-Hirsch fusor consists of a vacuum chamber containing a positively charged outer grid and a negatively charged inner grid within; essentially a large vacuum tube with spherical grids. Fusible atomic nuclei are injected as ions into the system, repelled by the outer grid, and accelerated toward the inner grid. Most of the time, the ions miss the grid, but occasionally, given long enough, nuclei strike either the grid or another high-energy nucleus. Most strikes with other nuclei do not result in fusion, but occasionally fusion results. On a miss, the nuclei move outwards, are repelled by the outer grid again, and return through the core. Without the motion of electrons and magnetic fields, there are no synchrotron losses and low levels of bremsstrahlung radiation.
The fundamental problem with the system is with the grid itself. Far too often, nuclei strike the grid. This damages the grid, wastes the energy that went into ionizing and accelerating the particle, and most critically, heats the grid. Even if the former problems were not critical, having a fine mesh grid in a reactor producing enough power to be used as a power plant would almost certainly mean that it would be rapidly vaporized.
The Polywell concept is designed to avoid this problem. Most notably, it is a gridless inertial electrostatic fusor. Using a cusped, quasi-spherical magnetic mirror, electrons can be confined in a slightly electron-rich plasma.[1] The electrostatic potential from the trapped excess electrons, not a negatively charged grid, confine the fusion fuel ions toward a dense focus in the center. The odds of collision with an electron are vanishingly small, and there is no grid to overheat. While this concept uses magnetic fields, which the original fusor managed to avoid, the fields do not need to confine nuclei — only electrons, which are orders of magnitude simpler to confine.[2] The shape of the magnetic mirror must be a zonohedron by construction, with cusps at each vertex. In Bussard's polywell devices, not all vertices were driven by solenoids to minimize electron losses. This allowed a rhombic dodecahedral field to be constructed from 6 solenoids. Bussard's unbuilt PW-7 would use twice as many solenoids to create a more spherical rhombic triacontahedral field.
Despite initial difficulties in spherical electron confinement, at the time of the research project's termination, Bussard had reported a neutron rate of 109 per second (based on detection of three neutrons, giving a wide confidence interval). He claims that this is roughly 100,000 times greater fusion rate than Farnsworth managed to achieve at similar well depth and drive conditions.[3][4]
He claims that, assuming superconductors for the coils, the only significant losses are electron losses, meaning that the fusion power output of the device scales as the seventh power of the radius, and the energy gain scales as the fifth power. While Bussard has not publicly documented the physical reasoning underlying this estimate, if true, it would enable a model only ten times larger to be useful as a fusion power plant.[5]
[edit] History
The fundamental idea of the Polywell device was conceived in 1983.[6] Research was funded by the Department of Defense since 1987, and the United States Navy began providing low-level funding to the project in 1992.[7] Bussard, who had formerly been an advocate for Tokamak research, in 1995 sent a letter to the United States Congress stating that he had only supported Tokamaks in order to get fusion research sponsored by the government, but he now believes that there are better alternatives.
Polywell models were produced through an iterative process, ranging from WB-1 through WB-6 (with WB-7 and 8 planned, but not produced due to a lack of funding). Early designs consisted of tightly welded stainless steel cubes of electromagnets, wound on square-cross section spools. These designs suffered from "funny cusp" losses at the joints between magnets, and from the magnetic field clipping the corners of the spools. The losses into the metal severely hurt their performance, leading to lower electron trapping performance than predicted. Later designs (starting with WB-6) began spacing electromagnets apart instead of touching, and changed to circular cross sections instead of square, reducing the metal surface area unprotected by magnetic fields. These changes dramatically improved system performance, leading to a great deal of electron recirculation and the confinement of electrons into a progressively tighter core. All of the designs built to date (January 2007) have been 6-magnet designs built as a cube (or more specifically as a truncated cube). WB-8 is planned to be a higher-order polyhedron, with 12 electromagnets.
Funding became tighter and tighter. According to Bussard, "The funds were clearly needed for the more important War in Iraq." An extra 900k of CNR funding allowed the program to continue long enough to reach WB-6 testing. The last-produced model, WB-6 produced a fusion rate of 109 per second. Drive voltage on the WB-6 tests was about 12.5 kV, with a resulting potential well depth of about 10 kV, thus deuterons arriving in the center of the machine will have a kinetic energy of 10 keV. By comparison, a Fusor running deuterium fusion at 10 kV would produce a fusion rate difficult to detect at all. Hirsch reported a fusion rate this high only by driving his machine to 150 kV and by using deuterium-tritium fusion (a much easier reaction). While the pulses of operation in WB-6 were sub-milliseconds, Bussard feels the conditions should represent steady state as far as the physics are concerned. Most critically, the models of the system indicate that a full-sized model, approximately 150-200M$, should be an effective power plant, producing notably more energy than it consumes. Another last minute attempt on WB-6 burned through the insulation on one of the hand-wound electromagnets, effectively destroying the device. With no more funding and no more time, the project's equipment was moved across town to SpaceDev, who hired three of the team's researchers.
Since then, Bussard has been travelling the country, giving talks trying to raise interest in his design, most famously a talk at Google headquarters, referred to with the title, "Should Google Go Nuclear?".[8] An informal overview of the last decade of work was presented at the 57th International Astronautical Congress in October 2006.[5]
[edit] Future work
Bussard believes that the system has demonstrated itself to the degree that no intermediate-scale models are needed, and notes, "We are probably the only people on the planet who know how to make a real net power clean fusion system, and we are out of support!"[1] Instead, should funding be revived, he intends to build two more designs to determine what full scale model would be best (WB-7 and WB-8), and with them, conduct and publish the results of dozens of repeatable tests. He then plans to convene a conference of experts in the field in an attempt to get them behind his design. Assuming his design is backed, the project would immediately move to a full-scale demo plant construction.
Bussard notes that, "Thus, we have the ability to do away with oil (and other fossil fuels) but it will take 4-6 years and ca. 100-200 M$ to build the full-scale plant and demonstrate it. Anyone care?"[2]
[edit] Accolades
In 2006, Dr Bussard and the Polywell device were awarded the Outstanding Technology of the Year Award by the International Academy of Science (Missouri). [9]
[edit] References
- ^ US4,826,646 (1989-05-02) Robert W. Bussard Method and apparatus for controlling charged particles
- ^ Krall, Nicholas A.; Bussard, Robert W. (1995). "Forming and maintaining a potential well in a quasispherical magnetic trap". Physics of Plasmas 2 (1): 146. DOI:10.1063/1.871103. ISSN 1070664x.
- ^ Robert W. Bussard (2006-03-29). Inertial Electrostatic Fusion systems can now be built. fusor.net forums. Retrieved on December 3, 2006.
- ^ SirPhilip (posting an e-mail from "RW Bussard") (2006-06-23). Fusion, eh?. James Randi Educational Foundation forums. Retrieved on December 3, 2006.
- ^ a b "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
- ^ Posted to the web by Robert W. Bussard (February 2006). A quick history of the EMC2 Polywell IEF concept (Microsoft Word document). Energy/Matter Conversion Corporation. Retrieved on December 3, 2006.
- ^ Posted to the web by Robert W. Bussard. Inertial electrostatic fusion (IEF): A clean energy future (Microsoft Word document). Energy/Matter Conversion Corporation. Retrieved on December 3, 2006.
- ^ Dr. Robert Bussard (lecturer) (2006-11-09). Should Google Go Nuclear? Clean, cheap, nuclear power (no, really) (Flash video). Google Tech Talks. Google. Retrieved on December 3, 2006.
- ^ International Academy of Science (2006). Outstanding Technology of the Year Award - 2006 (Webpage). Retrieved on March 5, 2007.
[edit] External links
- Should Google go Nuclear? Summary of Bussard's lecture and work
