Polywell
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Polywell is a gridless inertial electrostatic confinement fusion process 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 positively charged outer grid and a negatively charged inner grid inside a vacuum chamber; essentially a large vacuum tube with spherical grids. Fusable 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, the nuclei will either strike the grid or strike another high-energy nucleus. Most strikes with other nuclei will not result in fusion, but occasionally fusion is the result. On a miss, the nuclei losslessly accelerate 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 lies in the grid itself. Far too often, nuclei tend to 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 to be used as a power plant would almost certainly mean that it would be rapidly vaporized.
The Polywell concept is designed to work around this problem — most notably, it is a gridless inertial electrostatic fusor. Electromagnets in the shape of a truncated regular polyhedron (typically a truncated cube) create a cusped, quasi-spherical magnetic trap, which confines electrons 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] 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] and a seventh power scaling rate that would allow a model only ten times larger to be useful as a fusion power plant.
[edit] History
The fundamental idea of the Polywell device was conceived in 1983.[5] 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.[6] 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 steel cubes of electromagnets. The losses into the metal severely hurt their performance, leading to great electron loss and little centralized core. Later designs began spacing electromagnets, reducing the metal surface area, and most critically, changing from a cubic construction to a polyhedral one. These changes dramatically improved system performance, leading to a great deal of electron recirculation and the confinement of electrons into a progressively tighter core.
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 — very high performance for a small fusor — in conditions that 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, often referred to as, "Should Google Go Nuclear?".[7]
[edit] Future work
Bussard believes that the system has demonstrated itself to the degree that no larger scale models are needed. 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.
[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 2006-12-03.
- ^ SirPhilip (posting an e-mail from "RW Bussard") (2006-06-23). Fusion, eh?. James Randi Educational Foundation forums. Retrieved on 2006-12-03.
- ^ 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 2006-12-03.
- ^ 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 2006-12-03.
- ^ 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 2006-12-03.
