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From Wikipedia, the free encyclopedia

The polywell is a plasma confinement concept that combines elements of inertial electrostatic confinement and magnetic confinement fusion, intended ultimately to produce fusion power. The name polywell is a portmanteau of "polyhedron" and "potential well."

The polywell consists of electromagnet coils arranged in a polyhedral configuration and positively charged to between several 10s and low 100s of kilovolts. This charged magnetic polyhedron is called a MaGrid. Electrons are introduced outside the "quasi-spherical" MaGrid and are accelerated into the MaGrid due to the electric field. Within the MaGrid, magnetic fields confine most of the electrons and those that escape are retained by the electric field. This configuration traps the electrons in the middle of the device focusing them near the center which produces a virtual cathode (negative electric potential). The virtual cathode is used to accelerate and confine the ions to be fused which, except for minimal losses, never reach the physical structure of the MaGrid. It was developed by Robert Bussard under a US Navy research contract as an improvement of the Farnsworth-Hirsch fusor.

Contents 1 Design 1.1 Problems with Farnsworth-Hirsch fusors 1.2 The polywell approach 2 Comparison to conventional confinement concepts 3 Possibility of net power 4 History 5 Future of Polywell 5.1 FY 2008 Work 5.2 FY 2009 Work 6 References 7 External links

[edit] Design

[edit] Problems with Farnsworth-Hirsch fusors

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.^{[citation needed]}

The fundamental problem with this traditional 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.

[edit] The polywell approach

Like the fusor, the polywell confines positive ions through their attraction to negatively charged electrons. The difference is that in the fusor, the negative charges reside on a solid-state grid, while in the polywell, they are confined to the inner region of the reactor by magnetic fields. The reactor volume is defined by the coils producing the magnetic field. The coils are held at a positive potential relative to the surrounding walls of the vacuum chamber, the idea being to reflect any electrons which escape back into the region inside the coils. The charged coils are known as MaGrids, from "Magnetic Grids". Although the polywell, like the fusor, has material electrodes, the ions are not required to pass through them, and in addition the magnetic fields will tend to protect them from the impact of energetic electrons and ions.

On the other hand, the polywell has electrons and ions existing in the same volume. While this avoids the limit on the fusor power density due to space charge effects, it also introduces bremsstrahlung, which can be an important energy loss channel.

The magnetic field is usually produced by a symmetrical arrangement of discrete, circular coils, all pointing toward (or all away from) the center. The magnetic field vanishes at the center by symmetry, and the magnetic flux that enters the volume through the coils leaves it again through the spaces between the coils. This configuration confines electrons to the central volume by a magnetic mirror with a large field ratio or, under some conditions, a magnetic cusp. Bussard claimed^[1] that the magnetic field has only point cusps (as opposed to line cusps), but this has been disputed. Ions are added at a density nearly equal to that of the electrons to produce a quasineutral plasma, but a slight excess of electrons maintains a negative potential well.^[2] While this concept differs from the original fusor in that it uses magnetic fields, it also differs from traditional magnetic confinement because the fields do not need to confine ions — only electrons, which is much easier.^[3][4][5]

If the configuration is looked at as solenoids on the faces of a polyhedron, then the polyhedron chosen must have an even number of faces at each vertex, so that the polarity of the solenoids can alternate. Infinitely many polyhedra satisfy this property, for instance all antiprisms, 2n-agonal bipyramids, and all rectified (fully truncated) polyhedra. As can be seen in the picture, WB-6 is a truncated cube. Bussard's planned WB-8 would be a truncated dodecahedron. These geometries have several interesting properties. The shape of the magnetic potential well is the dual polyhedron of the machine. Each polyhedron could be constructed two different ways from circular coils. The edges of the polyhedron could also be wired directly as with Bussard's NPG polyhedral grid (there is a eulerian path because all vertices are even), but this is probably undesirable because every line passing through the center and a vertex will by symmetry have zero magnetic field.

Despite initial difficulties in spherical electron confinement, at the time of the 2005 research project's termination, Bussard had reported a fusion rate of 10⁹ per second running D-D fusion reactions at only 12.5 kV (based on detection of a total of nine neutrons in five tests,^[6] giving a wide confidence interval). He claimed that the fusion rate achieved by WB-6 is roughly 100,000 times greater than that Farnsworth managed to achieve at similar well depth and drive conditions.^[7][8] Researchers at the University of Wisconsin-Madison have also claimed a neutron rate of up to 5×10⁹ per second at voltages of 120 kV with a electrostatic fusor without magnetic fields.^[9]

Bussard claimed that, assuming superconductors are used for the coils, the only significant energy loss channel is through electron losses proportional to the surface area. He also claimed that the density would scale with the square of the field (constant beta conditions), and the maximum attainable magnetic field would scale with the radius (technological constraints). Under those assumptions, the fusion power produced would scale with the seventh power of the radius, and the energy gain would scale with the fifth power. While Bussard had not publicly documented the physical reasoning underlying this estimate,^[10] if true, it would enable a model only ten times larger to be useful as a fusion power plant. ^[11]

[edit] Comparison to conventional confinement concepts

The polywell is related to various other plasma confinement concepts, but differs markedly from all of them. It is most closely related to the fusor, which, like the polywell, confines ions by an inwardly directed electric field and requires a grid of solid-state electrodes within the plasma vessel. Both concepts intend to operate with a highly non-thermal, ideally mono-energetic, distribution of ion energies. If the ion energies can be held near the optimum value, the fusion rate for a given plasma pressure can be a few times higher than the maximum rate possible for ions with a thermal distribution. On the other hand, collisions and collective instabilities have a tendency to restore a thermal distribution, so that it generally costs power to maintain a mono-energetic distribution.

The polywell differs from the fusor in that the electrons are magnetically confined, so that it is also related to magnetic confinement fusion, most closely to magnetic mirrors. In common with magnetic mirrors is the field minimum in the central region, the confinement (in part) by the mirror effect, and (at least to some extent) a non-thermal distribution of the electron energies. In some mirror configurations, the field in the center is a minimum in every direction, as it is in the central region of a polywell. The magnetic field in such a case is said to have "good curvature" because a certain class of fluctuations are stable in a plasma contained by such a field. In contrast to mirror machines, the polywell does not just have a minimum in the field strength in the center, the field vanishes entirely there. Also the polywell does not have a magnetic axis, but rather a polyhedral symmetry.

The most actively developed plasma confinement concept at this time is the tokamak. A net power fusion reactor based on the tokamak concept, if possible, would certainly be a large and complex machine. The advocates of the polywell predict that a polywell reactor of similar power would be much smaller and simpler. The tokamak has a toroidal geometry with nested flux surfaces, so that both ions and electrons can only be lost by transport across magnetic field lines (primarily as a result of instabilities with very short wavelengths). The confinement of particles in a polywell is more complex, involving both magnetic and electric fields, transport of particles both across and along magnetic field lines, and different processes for the ions than for the electrons.

[edit] Possibility of net power

Bussard believed that this device can run with net energy production on boron-11 and proton fuel. Controversies exist over whether the ions and electrons will thermalise and whether bremsstrahlung losses will emit more energy in an unrecoverable form than can be produced by the fusion reaction.^{[citation needed]}

Todd Rider calculates that bremsstrahlung losses with this fuel relative to the fusion production will be 1.20:1.00.^[12] Bussard said that his calculation of the losses are about 5% of this, and therefore, greater gains than unity are possible.^[13]

According to Bussard the high speed and therefore low cross section for Coulomb collisions of the ions in the core makes thermalizing collisions very unlikely, while the low speed at the rim means that thermalization there has almost no impact on ion velocity in the core.^[11]

Another paper on the feasibility of IEC fusion, using the full bounce-averaged Fokker-Planck equation operator, concluded that IEC systems could produce large fusion energy gain factors (Q values). However, a deuterium-tritium reaction was necessary to minimize operating potential and Bremsstrahlung losses in order to reach large Q.^[14]

[edit] History

The fundamental idea of the polywell device was conceived in 1983.^[15] 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.^[16] 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 believed that there are better alternatives to Tokamaks.

Polywell models were produced through an iterative process, ranging from WB-1 through WB-6 (with WB-7 and 8 planned, but not yet constructed). 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. Until 2005 all of the reactors 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."^[8] An extra $900k of Office of Naval Research funding allowed the program to continue long enough to reach WB-6 testing on November 2005. 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 felt 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, costing approximately $150-200M (depending on the fuel), should be an effective power plant, producing notably more energy than it consumes. A last-minute test of WB-6 ended prematurely when the insulation on one of the hand-wound electromagnets burned through, destroying the device. With no more funding during 2006 and partly 2007, the project's military-owned equipment was transferred across town to SpaceDev, which also hired three of the team's researchers.^[8]

After the transfer, Bussard tried to attract new investors, giving talks trying to raise interest in his design. A talk at Google headquarters had the title, "Should Google Go Nuclear?"^[17] An informal overview of the last decade of work was presented at the 57th International Astronautical Congress in October 2006.^[11]

Dr. Bussard formed EMC2 Fusion Development Corporation, [1] a non-profit organization, to seek funding for continuation of the project.

[edit] Future of Polywell

With the success of WB-6, Bussard believed that the system had demonstrated itself to the degree that no intermediate-scale models would be needed, and noted, "We are probably the only people on the planet who know how to make a real net power clean fusion system"^[7] He proposed to rebuild WB-6 more robustly to verify its performance. After conducting and publishing the results of dozens of repeatable tests, he planned to convene a conference of experts in the field in an attempt to get them behind his design. Assuming his design would have been backed, the project would have immediately moved toward a full-scale demo plant. The first step in that plan was to design and build two more small scale designs (WB-7 and WB-8) to determine which full scale polyhedral potential well would be best. He wrote “The only small scale machine work remaining, which can yet give further improvements in performance, is test of one or two WB-6-scale devices but with “square“ or polygonal coils aligned approximately (but slightly offset on the main faces) along the edges of the vertices of the polyhedron. If this is built around a truncated dodecahedron, near-optimum performance is expected; about 3-5 times better than WB-6.” ^[11]

Bussard noted that, "Thus, we have the ability to do away with oil (and other fossil fuels) but it will take 4-6 years and ca. $100-200M to build the full-scale plant and demonstrate it."^[7]

Bussard said "Somebody will build it; and when it's built, it will work; and when it works people will begin to use it, and it will begin to displace all other forms of energy."^[18]

[edit] FY 2008 Work

In August 2007, EMC2 received a $2M U.S. Navy research contract to continue the reactor development.^[19] Prior to Bussard's death in October, 2007,^[20] Dolly Gray, who co-founded EMC2 with Bussard in 1985, and served as its president and CEO, helped assemble the small team of scientists in Santa Fe to carry on his work. The group is lead by Richard (Rick) Nebel and includes Jaeyoung Park; (both Nebel and Park are physicists on leave from the Los Alamos National Laboratory (LANL)); Mike Wray, the physicist who ran the key 2005 tests; and Kevin Wray, who is the computer specialist for the operation.

What is now called WB-7, the more robust version of the WB-6 fusion device, was constructed at a machine shop in San Diego and shipped to Santa Fe to the EMC2 testing facility. The device, like previous ones, was designed by engineer Mike Skillicorn. This WB-7 however was not the “square” coil suggested by Dr. Bussard.

WB-7, achieved "1st plasma" in early January, 2008^[21][22].

No specific information, has been published at this moment due to a publishing embargo on research data maintained by US Navy.^{[citation needed]} It's unclear what the terms of the embargo are, but the previous project, led by the late Dr. Bussard, had been under an embargo for 11 years between 1994 and 2005 when that series of contracts with the US Navy ended.

In August 2008, the team finished the first phase of their experiment and were waiting for the peer review of their results and a verdict from their federal funders on whether the experiment should proceed to the next phase. Dr. Nebel has said "we have had some success", referring to the team's effort to reproduce the promising results obtained by Dr. Bussard. "It's kind of a mix", Dr. Nebel reported. But he stated that the team has "a plan to go forward.". "We're generally happy with what we've been getting out of it, and we've learned a tremendous amount" he also said. At the moment this article has been written, the results of the verdict from the funders were not known^[23].

[edit] FY 2009 Work

In September 2008 The Naval Air Warfare Center, Weapons Division, China Lake, CA publicly pre-solicited a contract for research on an Electrostatic "Wiffle Ball" Fusion Device^[24] which was awarded to EMC2 as preferred supplier in the absence of other bids.

In October 2008 the Navy publicly pre-solicited two more contracts^[25][26] which were also awarded to EMC2 as preferred supplier. These two tasks were to develop better instrumentation and to develop an ion injection gun. Rick Nebel commented "This isn't a big deal. This is small, interim funding. It's called staying alive until they make a decision^[27]."

Following submission of the final WB-7 results in December 2008, Dr Richard Nebel commented that "There's nothing in there [the research] that suggests this will not work," but that "That's a very different statement from saying that it will work."

Stephen Chu, Nobel laureate and as of 2009^[update] United States Secretary of Energy, answered a question about Polywell at a talk at Google in 2007, saying "So far, there's not enough information so [that] I can give an evaluation of the probability that it might work or not...But I'm trying to get more information"^[28].

In January 2009 The Naval Air Warfare Center pre-solicited another contract for "modification and testing of plasma wiffleball 7"^[29] which appears to be funding to install the instrumentation developed in a prior contract, install a new design for the connector (joint) between coils, and operate the WB-7 with the modifications. The modified unit is now called WB7.1.

[edit] References

Polywell cartoons http://scratch.mit.edu/projects/GeneMachine/34906 http://scratch.mit.edu/projects/GeneMachine/77493 http://scratch.mit.edu/projects/GeneMachine/258412

[edit] External links

• Should Google Go Nuclear? Video of Dr. Bussard's presentation to Google.
• Should Google Go Nuclear?(transcript) Illustrated transcript of Bussard's Google presentation.
• Economic Impact of Cheap Fusion Three minutes of excerpts from Bussard's Google presentation.
• EMC2 Fusion Development Corporation Official website of Dr. Bussard's non-profit with links to papers.
• Presentation at International Space Development Conference (ISDC). Dallas, May 2007.
• Links Compendium of informative links related to polywell fusion.
• Listof technical papers and references.
• IEC Fusion for Dummies Youtube video with a graphical explanation of a polywell.
• Talk-Polywell.org BBS for discussing polywell.
• University of Wisconsin-Madison Introduction to IEC including the polywell.
• Obituary for Dr. Bussard.
• Latest Fusion developments (WB-7 - June 2008) based on the work of Dr. Robert Bussard
• Polywell Nuclear Fusion website.

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