Polywell

Polywell WB-6 model assembled

A polywell device is a type of fusion reactor that traps electrons in a magnetic confinement inside its hollow center. The negatively charged electrons then attract positively charged ions creating a plasma which reaches sufficient density to produce inertial electrostatic confinement fusion. The polywell consists of positively charged electromagnet coils that are arranged in a polyhedron called a MaGrid. Electrons are introduced from outside and are accelerated into the MaGrid due to the electric field. Within the MaGrid, magnetic fields confine most of the electrons. Those that escape are retained by the electric field. This configuration traps the electrons in the middle of the device. The electrons act as a virtual cathode (negative electric potential).

Ions are injected into the device. The virtual cathode attracts and electrostatically confines them so densely that they fuse, releasing energy. The energy required to confine the electrons is far smaller than that required to directly confine ions, as is done in other fusion projects such as ITER.

Bussard developed it as an improvement of the Elmore-Tuck-Watson fusor, which was in turn based on the Farnsworth-Hirsch fusor.

Robert Bussard theorized that a polywell device could potentially generate net energy production and thus become a source for electric power. His company developed the initial devices for the U.S. Navy.

The name polywell is a portmanteau of "polyhedron" and "potential well."

Design

Farnsworth-Hirsch

A Farnsworth-Hirsch fusor consists of a vacuum chamber containing a positively charged outer grid and a negatively charged inner grid; essentially a large vacuum tube with spherical grids. Fusible atomic nuclei are injected as ions into the system and accelerated toward the inner grid. Most of the time, the ions pass through the grid and pass through the core. As the ions move outward, they pass through the inner grid which decelerates them and then reaccelerates them inward wherein they return through the core. They continue to pass back and forth through the core until they strike either the grid or another nucleus. Most strikes with other nuclei do not result in fusion, but occasionally the strikes are sufficiently energetic that fusion results. Grid strikes raise the temperature of the grid and lower the energy of the plasma.

A benefit with the Farnsworth-Hirsch fusor is that without the motion of electrons and magnetic fields, there are no synchrotron losses and (if the grid is kept cool to reduce thermionic emission) low levels of bremsstrahlung.^[1]

The fundamental problem with this system is with the grid itself. Far too often, nuclei strike the grid. This wastes the energy that went into ionizing and accelerating the particle thus cooling the plasma, and most critically, heats and damages the grid. Even if the cooling problem was not critical, in a reactor emitting enough power to use in a power plant, a fine mesh grid would almost certainly be rapidly vaporized.

Elmore-Tuck-Watson

An Elmore-Tuck-Watson (ETW) fusor inverts the charge on the grids. It consists of a vacuum chamber containing a negatively charged outer grid (which may be the chamber) and a positively charged inner grid. Electrons (instead of ions) are injected into the system and accelerated toward the inner grid. As with the Farnsworth fusor, the particles move back and forth through the inner grid and core. As they pass repeatedly through the core, they generate a negatively charged zone, a potential well, which is called a virtual cathode. Fusible atomic nuclei are then introduced inside the inner (positive) grid where they are ionized. The virtual cathode accelerates the ions toward the center where they oscillate within the potential well. Since the ions never (in theory) reach the grid, they never lose their energy to impacts and continue to oscillate through the core. Given enough oscillations, the ions strike other high-energy ions and fuse.

The fundamental problem with this variation system is still the grid itself. Far too often the electrons strike the grid.

Polywell

Like the Farnsworth-Hirsch fusor, the polywell confines positive ions through their attraction to the negative potential well at the center. However in the Polywell this negative well is created by electrons that are held inside a magnetic mirror, creating a virtual negative cathode. This configuration avoids the Farnsworth losses related to ions striking the central negative cathode.

Ions are added at a density nearly equal to that of the electrons to produce a quasi-neutral plasma, but a slight excess of electrons maintains the negative potential well.^[2] Polywell differs from traditional magnetic confinement because the fields confine only electrons, which is much easier than containing ions.^[3][4][5]

In most experiments to date, the MaGrid arrangement has been approximated 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 MaGrid arrangement of the magnetic field has only point cusps but acknowledged that the circular coils produce line-like cusps at the closest approaches of the coils.^[6]

The basic concepts and principles of Polywell operation have been visualized in a web-based 3D simulator by John Coady (requires a web browser supporting WebGL).^[7]

Results

Despite initial difficulties in spherical electron confinement, at the time of the 2005 research project's termination, Bussard reported a fusion rate of 10⁹ per second running D-D fusion reactions at only 12.5 kV (based on detecting a total of nine neutrons in five tests,^[8][9] giving a wide confidence interval). He stated, that the fusion rate achieved by WB-6 was roughly 100,000 times greater than what Farnsworth achieved at similar well depth and drive conditions.^[10][11] By comparison, researchers at the University of Wisconsin–Madison reported a neutron rate of up to 5×10⁹ per second at voltages of 120 kV with an electrostatic fusor without magnetic fields.^[12]

Bussard asserted, by using superconductor coils, the only significant energy loss channel is through electron losses proportional to the surface area. He also stated 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 conditions, 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 did not publicly document the physical reasoning underlying this estimate,^[13] if true, it would enable a model only ten times larger to be useful as a fusion power plant.^[8]

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.^[4] 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, the concept behind ITER. A net power fusion reactor based on the tokamak concept, if possible, would certainly be a large and complex machine. Bussard predicted, in contrast, that a net power fusion reactor of similar power based on the polywell concept, would be a much smaller, simpler, and cost effective machine.^[14] 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

All of Bussard's Polywell experiments were based on deuterium-deuterium (²H+²H ) reactions. This was because it has a high reaction cross section at a low temperature relative to other fuels. Deuterium-tritium (²H+³H ) reactions have a higher cross section at a lower temperature, but since tritium is only naturally occurring in trace amounts on Earth it would have been too costly to use for polywell experiments.^[14] However, tritium is produced in ²H+²H reactions and could be fed back into the polywell as fuel, giving the highest possibility of net power.^{[citation needed]} Bussard calculated that a Polywell reactor with a radius of 1.5 meters would produce economically viable net power using (²H+²H ) reactions.

The downside to both ²H+²H and ²H+³H reactions is that they produce fast neutron radiation, which did not meet Bussard's goal for radiation free fusion power.^[14] Therefore, Bussard proposed fueling the reactor with boron-11 and proton fuel. However, there is some controversy about the viability of p-¹¹B reactors. In his thesis, MIT doctoral student Todd Rider had calculated that bremsstrahlung losses with this fuel will exceed fusion power production by at least 20%.^[15] In contrast, Bussard's calculations indicate that the bremsstrahlung losses would be as little as one twelfth of the fusion power production.^[16] 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.^[8]

History

WB-2

WB-3

WB-6 during assembly with coils showing

In the late 1960s there were several investigations of polyhedral magnetic fields as a possibility to confine a fusion plasma.^[17][18] The first proposal to combine this magnetic configuration with an electrostatic potential well in order to improve electron confinement was made by Lavrentiev in 1975.^[19]

The idea was picked up by Robert Bussard in 1983, a link acknowledged in the references cited by his 1989 patent application,^[6] though in 2006 he appears to claim to have re-discovered the idea independently.^[20] Research was funded by the Department of Defense beginning in 1987, and the United States Navy began providing low-level funding to the project in 1992.^[21] Bussard, who had formerly been an advocate for Tokamak research, became the premiere advocate for and researcher on the concept, so that the idea is now indelibly associated with his name. In 1995 he 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-8 (see FY2009-11 sections below for details on current WB-7 and WB-8 models). 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). Bussard's WB-8 was planned to be a higher-order polyhedron, with 12 electromagnets (this design was not used in the actual WB-8 machine).

Funding became tighter and tighter. According to Bussard, "The funds were clearly needed for the more important War in Iraq."^[11] An extra $900k of Office of Naval Research funding allowed the program to continue long enough to reach WB-6 testing in 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.^[11]

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?"^[14] An informal overview of the last decade of work was presented at the 57th International Astronautical Congress in October 2006.^[8]

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

Recent US Navy funded work

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"^[10] 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 had 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.” ^[8]

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."^[10]

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."^[22]

Fiscal year (FY) 2008 work

In August 2007, EMC2 received a $1.8M U.S. Navy research contract to continue the reactor development.^[23] Before Bussard's death in October, 2007,^[24] 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 led 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 prior 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.^[25][26]

No specific information has been published as of 2010, due to a publishing embargo on research data maintained by US Navy.^[27] The prior 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.^[28]

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.^[29] the pre-solicitation was targeted toward EMC2 as preferred supplier.

In October 2008 the US Navy publicly pre-solicited two more contracts^[30][31] also targeted toward 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."^[32] Other than Dr. Nebel's comments, there is no direct evidence that these pre-solicitations ever went to award.

In December 2008, following many months of review by the expert review panel of the submission of the final WB-7 results, 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."^[33]

In January 2009 the Naval Air Warfare Center pre-solicited another contract for "modification and testing of plasma wiffleball 7"^[34] 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 WB-7.1. This pre-solicitation started as a $200k contract but the final award was for $300k, which suggests that the earlier pre-solicitations were included in this one.

In April 2009, the DoD published a plan to provide Polywell a further $2 million in funding as part of the American Recovery and Reinvestment Act of 2009. The citation in the legislation was labelled as Plasma Fusion (Polywell) - Demonstrate fusion plasma confinement system for shore and shipboard applications; Joint OSD/USN project.^[35] The citation occurs 166 pages into the document, and suggests development of the device for 'Domestic Energy Supply / Distribution'.

In May 2009, Richard Nebel was interviewed in a popular science/futurism blog. He stated: "We are hoping to have a net energy production product within six years. It could take longer, but this definitely won't be a 50 year development project. [...] So if the concept works we could have a commercial plant operating as early as 2020."^[36]

In September 2009, the FBO (Federal Business Opportunities web site) confirmed the award of Recovery Act funding under Navy contract in the amount of $7.86M to construct and test a WB-8, the next Polywell prototype. This device will have an eightfold increase in magnetic field strength compared to prior WB series devices, with the expectation of higher performance. Of particular importance within the Navy contract is the option for an additional $4.46M for "...based on the results of WB8 testing, and the availability of government funds the contractor shall develop a WB machine (WB8.1) which incorporates the knowledge and improvements gained in WB8. It is expected that higher ion drive capabilities will be added, and that a “PB11” reaction will be demonstrated".^[37]

In September 2009, the US Department of Defense announced this award as required by law. The announcement stated that the funding was provided for "research, analysis, development, and testing in support of the Plan Plasma Fusion (Polywell) Project. Efforts under this Recovery Act award will validate the basic physics of the Plasma Fusion (Polywell) concept, as well as provide the Navy with data for potential applications of polywell fusion." ^[38] The basic contract for WB-8 is expected to be completed by April 2011. The optional contract for WB-8.1 has a completion date of 31-Oct-2012.

FY 2010 work

Other than the Recovery Act Tracking site,^[39] there has been no indication to date of the progress being made on this contract.

The contract^[37] has these delivery dates for the Contract Line Item Numbers (CLINs).

• CLIN 0001 - 30 Apr 2010 (= plasma wiffleball 8) - Completion of device build.
• CLIN 0002 - 30 Apr 2011 (= Data) - Completion of WB8 testing
• CLIN 0003 - 31 Oct 2011 (= Optional WB 8.1) - Completion of optional device build
• CLIN 0004 - 31 Oct 2012 (= Optional Data) - Completion of optional device testing

The first quarterly report on the Recovery Act site stated: The main focus of this quarter was the design, procurement and construction of equipment for the new WB-8 Polywell device. Theoretical work was also initiated to build the computational tools required to analyze and understand the data from WB-8.^[39]

The second quarterly report on the Recovery Act site stated: on budget, on schedule for new lab test facility. Primary focus has been construction, procurement and relocation of personnel and chamber. (Slightly different format to award number so on a different page)

As of 1 Nov 2010 a third quarterly report on the Recovery Act site has not been published.

The fourth quarterly report on the Recovery Act site stated: WB8 is fully under construction, progress made on Theoretical modeling of the Polywell. 2 full-time physicists hired. (On the original page).^[39] The location of work was also updated to San Diego. Confirmation of a lab move to San Diego was provided by an on-site visit.^[40]

FY 2011 Work

The 1Q FY11 report stated: "WB-8 device construction is completed. The first plasma was generated successfully on Nov. 1, 2010." ^[39] The report listed Dr. Jaeyoung Park as the Company Officer.

The 2Q FY11 report stated: "As of 1Q/2011, the WB-8 device operates as designed and it is generating positive results. EMC2 is planning to conduct comprehensive experiments on WB-8 in the next 9-12 months based on the current contract funding schedule." ^[39]

As of 2011 Nebel's LinkedIn profile stated he is an "Independent Research Professional" in Santa Fe. Dr. Jaeyoung Park's LinkedIn profile listed him as the acting CEO/President of Energy/Matter Conversion Corp. (EMC2).^[41]

In a May 2011 interview, when referring to WB-8, Dr. Park commented that "This machine should be able to generate 1,000 times more nuclear activity than WB-7, with about eight times more magnetic field.... We'll call that a good success. That means we're on track with the scaling law." Dr. Park also made reference to the Navy's position on the possible future usage of successful polywell technology, saying that "currently all our funding comes from the Navy... that's our customer. Our customer desired that we keep most of our progress confidential. ... They're somewhat concerned about making too much hype without delivering an actual product.... Our understanding is they want us to be successful.... They want us to provide something for our sponsors. They also want us to do well commercially as well, as long as we remain US-owned and control the technology."^[42]

The 3Q FY11 report stated: "As of 2Q/2011, the WB-8 device has demonstrated excellent plasma confinement properties. EMC2 is conducting high power pulsed experiments on WB-8 to test the Wiffle-Ball plasma scaling law on plasma energy and confinement." ^[39]

As of 3Q/2011, the WB-8 device had generated over 500 high power plasma shots. EMC2 is conducting tests on Wiffle-Ball plasma scaling law on plasma heating and confinement.^[43]-

During 4Q of 2011, EMC2 modified the electron injectors to increase the plasma heating. The higher plasma density in WB-8 prompted the need for higher heating power. They planned to operate WB-8 in high beta regime with the modified electron injectors during 1Q of 2012.^[44]

Other projects

University of Sydney

The University of Sydney in Australia have been conducting studies and experiments with Polywell devices. To date, they have published two papers in Physics of Plasmas on this topic, one in 2010^[45], and one in late 2011^[46]. The May 2010 paper discussed experimental work, testing a small device for its ability to capture electrons. The paper posited that the machine had an ideal magnetic field strength which maximized its ability to catch electrons. The December 2011 paper was the 8th most downloaded paper on the physics of plasma website for the month of December^{[citation needed]}. The paper analyzed magnetic confinement in the polywell using analytical solutions as well as simulations. The work linked the magnetic confinement in the polywell to magnetic mirror theory. This research was presented at the 12th US-Japan Workshop on Inertial Electrostatic Confinement Fusion ^[47], and summarized by John Santarius of the University of Wisconsin ^[48]

Nuclear Science and Technology Research Institute, AEOI, Tehran, Iran

A research report discussed Polywell study progress at the Plasma Physics and Nuclear Fusion Research School, Nuclear Science and Technology Research Institute, AEOI, Tehran, Iran. The report stated that OOPIC simulations as well as physical testing had been conducted. The study suggested that well depths and ion focus control can be achieved by variations of field strength. The report referenced other research with physical test articles. The group ran an IECF machine in continuous mode at -140KV with 70mA of current, with D-D fuel, producing 2x10^7 neutrons per second. The machine was not identified as a Polywell configuration. Other referenced studies indicate 10 fold improvements in fusion yields using ion sources.^[49]

See also

• Aneutronic fusion, including pB11 (proton and Boron-11) fusion reaction
• Tokamak
• List of plasma (physics) articles
• Magnetic mirror
• Dense plasma focus
• Magnetized target fusion

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.
• 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.
• List of 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
• Film about the Polywell with Thomas Ligon
• Prometheus Fusion - A blog describing amateur experiments aimed at creating a polywell.
• Simulation videos of a Polywell reactor by happyjack27