Everything about The Polywell totally explained
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 don't 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, while in the fusor the negative charges reside on a solid-state grid, in the polywell they're 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 produced by a polyhedral arrangement of coils, all pointing toward (or all away from) the center. The magnetic field vanishes at the center, and the magnetic flux that enters the volume through the coils leaves it again through the spaces between the coils. Thus the electrons are confined to the central volume by a
magnetic mirror with a large field ratio, and all the
cusps are points (rather than lines). Ions can be added to produce a
plasma, but there must always be more electrons than ions in order to maintain the potential well. 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 5x10
9 per second at voltages of 120 kV.
He claimed 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 hadn't 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's 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 contrast, the polywell doesn't just have a minimum in the field strength in the center, the field vanishes entirely there. Also the polywell doesn't have a magnetic axis, but rather a polyhedral symmetry.
The most successful plasma concept at this time is the
tokamak. A 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 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.
History
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'd 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 informal overview of the last decade of work was presented at the 57th International Astronautical Congress in October 2006. 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"
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 LANL; Mike Wray, the physicist who ran the key 2005 tests; and Kevin Wray, who is the computer guru 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.
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