I'm going to start by throwing out two names here: Tom Ligon (picture here) and Dr. Robert W. Bussard. Back in 1995 Bussard hired Ligon to work on a project he'd started at the Energy/Matter Conversion Corporation. In 1998 Ligon wrote up some of the developments that followed under the title "The World's Simplest Fusion Reactor And How to Make It Work" which appeared in the December 98 edition of Analog Science Fiction and Fact as a science fact article. It's been reprinted in Infinite Energy and made available on-line. Ligon followed it up in the January/February 2008 Analog with another science fact article: The World's Simplest Fusion Reactor Revisited. (Sorry, can't find it on line anywhere.)
I've been working up all the background here the better to make clear with this installment what Bussard has achieved and how significant it is. Without that background, it's hard to appreciate the conceptual breakthrough that's taken place. I'm going to be drawing heavily from those two articles and several other sources to explain why everything people think about the way to achieve fusion is wrong!
Hot & Heavy
Part Three yesterday looked at the two mainstream approaches to fusion: laser fusion and magnetic fusion. Both approaches are thermonuclear: the atoms involved are heated to millions of degrees so that they are moving fast enough to overcome the mutual repulsion from the positive electric charge on their nuclei (the Coulomb barrier in Ligon's phrase) and fuse together. The challenge is heating them to those temperatures while keeping them confined long enough to collide. It's a brute force approach, but conceptually easy to understand because it is an attempt to mimic what the sun does. It's not the only approach though.
The high temperatures are only to get the atoms moving fast; it's that speed, not the temperature, that matters, and there are other ways to do that. Particle accelerators use magnetic fields to push charged particles up to near light speed before slamming them into a target - and the result is fusion. It's not a way to generate energy, but it's how scientists have been studying what happens when atoms collide for a long time, and how they already know how different fusion reactions work. Electric fields can also accelerate particles.
A better way of thinking about this is by describing the energy in terms of electron volts, rather than by temperature when talking about particles moving in a plasma. The conversion factor is each electron volt is equivalent to 11,604 degrees Kelvin. Electrons hit the inside of a television screen CRT at about 200 million degrees by this measure, and 50 million degrees is "a paltry 4300 electron volts." (Ligon) Speaking of CRTs (cathode ray tubes), there's a connection between them and another way to do fusion. It's a development of vacuum tube technology.
It's All About The Tubes!
Back in the 1950's, Philo T. Farnsworth developed a vacuum tube called the Fusor, a very simple device. (Later improved by Dr. Robert Hirsch) An outer vacuum chamber contains an inner spherical grid. Add a trace of deuterium gas, put a high negative voltage to the inner grid at the right combination of pressure and voltage and you can get fusion. The grid creates a spherical region of high negative electrostatic charge; the positive deuterium nuclei are drawn into the center of that charged region from all directions. The electric field accelerates them, and they tend to collide head on and fuse. Those that miss shoot out the other side, and fall back for another pass.
It doesn't take millions of degrees - the high voltage does it all. Later versions switch the arrangement around. The inner grid in an Elmore-Tuck-Watson accelerator is positively charged; it draws a cloud of electrons moving back and forth through the center of the vacuum chamber, which also creates a highly negative charged region which in turn attracts the positive deuterium ions. They collide, and again fusion can result.
Ligon's 1998 article about this got attention from a lot of hobbyists who like to try these things for themselves. There's a dedicated group who can be found at fusor.net with a lot of information. Ligon explained the principles, the equipment needed, and they did the rest. In 2003 a high school student Michael Li won second place in the Intel Science Talent Search with a fusor he built. Others have followed his example since then.
The two different variants of the vacuum tube approach to fusion are calledInertial Electrostatic Confinement devices. Although they can produce nuclear fusion on a table top, they can not reach break-even and yield more energy than it takes to get fusion going in the first place. The reason is the presence of the grid inside the vacuum chamber. While the particles may make a number of passes back and forth through the central region before fusing, a certain percentage end up hitting the grid on each pass, and are lost. Those losses add up enough to keep the process from becoming self-sustaining.
Enter Dr. Robert W. Bussard. His first degree was in engineering, with a Ph.D. in Physics from Princeton. With a long interest in spaceflight, he turned to fusion research after developing a working fission rocket engine. He developed a variation on the standard tokamak design called a Riggatron, which was intended to address some of the inherent problems with the design, based on the state of the art at the time, the 1970s. As it turned out, it probably would not have worked based on what's been learned since about the behavior of plasmas. Eventually Bussard turned to Inertial Electrostatic Confinement, and came up with several insights which led to a new approach to fusion.
The Conceptual Breakthrough
Bussard had grown disenchanted with the tokamak approach to fusion (and the funding politics), and was well acquainted with IEC fusion reactors like the Hirsch-Farnsworth machine. Using a spherical electric field which would both accelerate the plasma particles to speeds high enough to crash the Coulomb barrier, and keep them zipping in and out through the central region until they did collide was a much easier job than trying to use giant superconducting magnets to contain the plasma while simultaneously heating it to millions of degrees. If not for the problem with particle losses to collisions with the grids inside IEC machines, they'd be a far more effective means of achieving fusion. And then, Bussard came up with the critical insight.
An Elmore-Tuck-Watson fusor uses a positively charged grid to confine a cloud of electrons in the center of the vacuum chamber. That creates the negatively charged region/electric field which accelerates the positive fuel nuclei ions for fusion. An electron and a proton each have the same amount of charge, the first negative, the second positive, but very different masses. (Remember the comparison between gnats and bowling balls?)
A tokamak uses magnetic fields to confine the positive ions in the reaction chamber and keep them away from the walls. The same strength magnetic field could contain vastly greater numbers of electrons because they have far less mass. So, what happens if you take the positively charged grid of an Elmore-Tuck-Watson fusor and use magnetic fields to insulate it so that the electrons never collide with it?
Bussard set up a corporation with a small highly motivated working group and got a grant from the U.S. Navy to find out. Tom Ligon's second Analog article (January/February 2008) tells the story of how the research went, culminating with the last tests in 2005.
Magrids and Wiffle Balls
Bussard's insight is the key to building what is known as a Polywell reactor. Take a geometric solid like a equilateral pyramid, cube or dodecahedron, put a circular solenoid magnet on each face of the solid with the same magnetic pole facing inward, and the result is a electron containment device which will keep a cloud of electrons circulating through the center of the device without hitting the grid. A computer model of the magnetic fields of a Polywell reactor looks very much like a plastic toy wiffle ball - hence the concept test models all had the prefix WB. The electrons are in constant motion around the magnetically insulated grids (magrids), so instead of "Electrostatic" confinement, it's more properly Electrodynamic. (Pictures of some of the test magrids here.)
It turned out that the magrids not only worked to keep electrons from hitting the grid, the field also had an additional effect. The population of electrons that ends up inside - and the positively charged ions of fuel they attract - is much higher inside the magrid than outside.
Addendum 4-26-08 Roger Fox, who has posted about about this months ago, put up a Youtube video in the coments that shows very nicely how the magrids work to trap electrons and create the potential well that pulls in the fuel ions for fusion.
Further, they found they could get D-D fusion (deuterium - deuterium), with the electron potential provided by maintaining a million more electrons per cubic centimeter than deuterium ions. If there were say 5 million deuterium ions per cc, 6 million electrons per cc would be enough. As long as they kept that differential, the electrical potential was sufficient over a range of electron/ion concentrations to get some fusion reactions.
To make a long story short, Bussard's team ended up building 5 test machines, testing them to destruction in several cases, and finally working up to WB6. That last machine (built in a race against the end of funding, the limitations of the facility there were using, and several other constraints) proved that they finally had figured out the critical factors needed to design a working magrid. They ran four tests in November 2005 using D-D fuel and got measurable neutron counts showing they were actually getting fusion. A fifth and final test deliberately pushed WB6 to its limits, blew out a coil, but gave them one last batch of data. (You can find pictures of some of the test magrids here.)
Analysis of the data showed WB6, operating in a pulsed mode because of power limitations, was still able to produce about half a billion fusion reactions per second. This was not even close to break-even, BUT it showed the models they had developed were working, the problems with earlier designs had been overcome, and they now knew how much the magrid would have to be scaled up to reach break-even - that is, produce as much power as was going into it.
Estimates are, a magrid with a 1.5 meter radius would be large enough to use cooled copper magnets in continuous operation at field strengths able to reach break-even while burning deuterium fuel. (It would do even better with a deuterium-tritium fuel mix.) That's a size large enough to think about using superconducting magnets which would make even greater fields strengths attainable; the models predict that would really improve performance. Scale the machine up to a 2 meter radius, and it should be able to burn p-B11 fuel. (More on that below.)
Dr. Bussard estimated a 1.5 meter Polywell reactor could be built for $150 million that would be able to hit break even burning just deuterium; $200 million would get a reactor able to burn p-B11 fuel successfully while reaching break even. If not, it would still be able to run successfully on deuterium. How long would it take to build a demonstration machine? A decade or far less, depending on what kind of resources are made available.
The Potential
It's difficult to avoid getting excited over this news. In the space of just over ten years, Dr. Bussard and his team look like they've come up with an approach that could finally make fusion power a reality. They've solved the theoretical problems - the rest is engineering. A comparison between the Polywell reactor and the ITER reactor is illustrative.
Ligon's first article zeroes in on things the ITER article in Physics World omits. Quoting from Ligon,back in 1998:
The focus of most of the present Department of Energy (DOE) research is large tokamaks. How large? The ITER machine was planned with the supporting equipment and structure, to be about the volume and mass of an aircraft carrier. If it is ever built, it will use giant toroidal superconducting magnets, storing magnetic energy equivalent to 1/40 of a Hiroshima bomb, which would be released suddenly if the liquid helium cooling system were ever breached and any one of the magnets warmed above the critical superconducting temperature. Surrounding the machine is a blanket of molten lithium one to two meters thick, not nice to spread around the countryside if the magnets blow. The core of the machine is a torus (donut) sixteen meters high and twenty two meters across with a crossection diameter of 5 meters,whose structural material will become radioactive as the machine runs. This beast might actually hit break even occasionally (ie: produce as much power as it consumes), with a little luck. Presuming working power plants would be even larger and heavier, the system does not look promising for strapping on the back of a rocket.
A few more comments from the vantage of 2008. The ITER system is only the next to last step before using the lessons that will be learned from it to build a DEMO power reactor. ITER won't even begin operating until 2016 if it stays on schedule and within its budget of billions; a proof of concept 2 meter Polywell reactor of far higher performance could conceivably already be operating by then at a cost orders of magnitude lower. The shortcomings of the tokamak design are further indicated by plans to run ITER on deuterium-tritium. It's the fusion fuel easiest to ignite, but tritium is radioactive and has to be produced in a reactor to begin with. Burning D-T fuel produces neutrons which, as noted above, will turn the reactor structure radioactive as it operates.
In contrast, the Polywell design looks efficient enough to burn using deuterium alone, avoiding the complications from tritium. Better yet, it looks like it could use an ever better fuel mix: p-B11.
The "p" is a proton - which is nothing more than a hydrogen atom with the electron stripped away. The "B11" is not a vitamin; it's an isotope of boron with an atomic mass of 11. Around 80% of boron as found in nature is in the form of this isotope. Neither of them are radioactive. When they fuse, the end product is 3 alpha particles, helium nuclei missing 2 electrons. They are easily blocked by shielding, unlike neutrons, and they can be captured by electromagnetic fields.
In fact, that's potentially the most exciting part. Brake the alpha particles with a 1.88 million volt field so they gently hit a metal plate - and it'll cause a net current flow of 2 electrons at that voltage. This is nuclear power being converted almost directly into energy. Ligon thinks it might be possible to do this with 85% efficiency.
In contrast, the ITER style tokamak will use the heat produced by the neutron radiation to turn a circulating liquid into steam to run turbines to make electricity. About 2/3 of the energy will be lost while doing this, plus adding the expense of pumps, turbines, cooling towers, etc. Polywell definitely looks like the better way to go.
If a Polywell reactor can be built running p-B11 fuel, and if the power conversion works as well as hoped, then something even more amazing will be possible. You'll be looking at an energy system that would have a power to mass ratio suitable for use as a rocket engine. From Ligon's first article:
Dr. Bussard has done some preliminary design studies on spacecraft that could realistically be built around p-B11 reactors. Most use a large and very powerful reactor of close to 10 billion watts capacity, typically with two reactors for redundancy. While fairly bulky, with a diameter of around 5 meters, the reactor is mostly empty vacuum, with only the MaGrid™ and a few electron and ion guns in it. It is thus exceptionally light for the power produced. Supporting cryogenic and power conversion equipment should also be practical space hardware, and not especially massive.
Because the reactor produces no radioactive waste and only a trace of radiation, it will be safe to operate in the atmosphere. Using high-voltage electron beams to superheat gas, one could build either an air-breathing jet or a rocket (relying on on- board reaction mass). In space, the rocket configuration will be used. Because the reactor can work only if there are far more electrons in it than fuel ions, it is also “intrinsically safe”: if you feed it too much fuel, it just chokes off.
So, considering all the potential here - not just unlimited electrical power in a package with no greenhouse gases or radiation problems, but practical spacecraft - where are things now? Dr. Bussard passed away in October of 2007 - but he left a team behind that is still working on the concept (WB7 is running and producing data.) News is slowly getting out; the Navy funding he got to do his work kept him from publishing papers on a lot of the work; some of it is still embargoed if I understand correctly. However if you Google Polywell, you can get a lot of links right off the top.
Ligon and Bussard are interviewed on The Space Show. (mp3 file) and Google has a video lecture Bussard gave in 2006. (pdf notes)
Here's a couple of sites with many links:
http://www.askmar.com/...
http://www.strout.net/...
There are a lot of people who are interested in following this up - almost an underground movement. There's also quite a few people who want the Polywell concept to just go away. More on that in the next installment of this series, Part 5: Werewolves, Vampires, and Zombies versus the Silver Bullet.
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