Polywell

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, within which the magnetic fields confine a cloud of electrons. This configuration traps electrons in the middle of the device which produces a "quasi-spherical" negative electric potential and is used to accelerate and confine the ions to be fused. It was developed by Robert Bussard under a US Navy research contract as an improvement of the Farnsworth-Hirsch fusor.

Design

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.

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.

The polywell approach

Like the fusor, the polywell confines positive ions through their attraction to negatively charged electrons. The difference being that in the fusor, the negative charges reside on a solid-state grid. 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, rather than by electrically charged grids. The advantage of coils over grids is that the magnetic fields produced by the coils also help protect them from the energetic electrons and ions. On the other hand, the polywell has electrons and ions existing in the same volume, reintroducing the Bremsstrahlung that the fusor can avoid.

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 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. 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.

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 cuboctahedron. Bussard's planned WB-8 would be an icosidodecahedron. 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 neutron 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, 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. 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.

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, if true, it would enable a model only ten times larger to be useful as a fusion power plant.

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 successfully developed plasma confinement concept at this time is the tokamak. A net power fusion reactor based on the tokamak concept 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.

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.

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

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.

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.

History

The fundamental idea of the polywell device was conceived in 1983. 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. 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." 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.

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?" An informal overview of the last decade of work was presented at the 57th International Astronautical Congress in October 2006. Bussard's polywell work won an "Outstanding Technology of the Year" award from the International Academy of Science in 2006

In August 2007, EMC2 (a non-profit organization founded by Bussard to seek funding for continuation of the project) received a $2M U.S. Navy research contract to continue the reactor development. Following Bussard's death in October, 2007, Richard Nebel took the helm on the polywell design team at EMC2, and the latest experimental device, WB-7, achieved "1st plasma" in early January, 2008. Depending on the results of ongoing experiments, the research could continue in pursuit of the final full-sized model.

Current/future work

Bussard believed that the system had demonstrated itself to the degree that no intermediate-scale models will 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" Since August 2007 with a new U.S Navy research contract, he intended 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 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 to a full-scale demo plant construction.

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." A web site registered to Robert Bussard and EMC2 Fusion Development Corporation, "a charitable research and development organization", was created to solicit donations to enhance further research.

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.

Dr. Bussard passed away on October 6, 2007. His work is being continued by the staff of physicists he was able to assemble at EMC2. Dolly Gray, who co-founded EMC2 with Bussard in 1985, and served as its president and CEO, has helped assemble the small team of scientists in Santa Fe. The group includes Rick Nebel, Jaeyoung Park, both 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. The latest device, WB-7, was constructed at a machine shop in San Diego and shipped to Santa Fe, where a small group of scientists have set up a testing facility and are currently running experiments. The device, like previous ones, was designed by engineer Mike Skillicorn.

Suggestions have been made to have a multi-agency review of the results and schematics to encourage timely public release of all findings and documentation. No specific information, however, can be published at this moment due to a publishing embargo on research data maintained by US Navy. 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 (since 1994) until the funding contract with US Navy has ended.

In August 2008, researchers had 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.

References

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