Dr Nebel said: The one you have to worry about is the input power scaling, because that one is related to the plasma losses (or transport). This one answers the question of “How much power do I need to supply to the device to maintain constant Beta”. Theoretical modeling of transport has a much poorer track record than plasma equilibrium has. These scaling laws are where the major risks for the larger device reside. The major saving grace is that for the Polywell is that the projected average densities are ~ 2 orders of magnitude higher than they are in Tokamaks so the energy confinement times don’t have to be all that good. (It’s the product of the density and the confinement time that’s important.) Our contention is that since our projections for a power producing device only require a machine [1.5 Meters in diameter would in theory be able to produce something around 100MW of net power] we might as well build the next one in that size range and accept the risk. The machines just aren’t all that expensive. Also, there are a multitude of things that can be done to improve confinement (such as pulse discharge cleaning, pellet injection, etc.) that have been successful in the magnetic confinement program that can be instituted if our projections fall short. This approach will minimize the development time and lead to a lower costs for the overall program.
The peak fields for the reactor designs (at least for our reactor designs) are in the 5-10 T range. however, these are work in progress.
We have run Gauss meters all over the face of the cubes and through the corners and we don't see any low field regions. The fields peak near the conductors and fall off near the coil centers, as you would expect.
Other Dr Nebel comments of interest:
1. The theory says that you can beat Bremstrahlung, but it's a challenge. The key is to keep the Boron concentration low compared the proton concentration so Z isn’t too bad. You pay for it in power density, but there is an optimum which works. You also gain because the electron energies are low in the high density regions.
2. The size arguments apply for machines where confinement is limited by cross-field diffusion like Tokamaks. They don't apply for electrostatic machines.
3. The Polywell doesn't have any lines of zero field. Take a look at the original papers on the configuration. See :
Bussard R.W., FusionTechnology, Vol. 19, 273, (1991) .
Krall N.A., Fusion Technology. Vol. 22, 42 (1992).
Furthermore, one expects adiabatic behavior along the field lines external to the device. Thus, what goes out comes back in. Phase space scattering is small because the density is small external to the device.
4. The machine does not use a bi-modal velocity distribution. We have looked at two-stream in detail, and it is not an issue for this machine. The most definitive treatise on the ions is : L. Chacon, G. H. Miley, D. C. Barnes, D. A. Knoll, Phys. Plasmas 7, 4547 (2000) which concluded partially relaxed ion distributions work just fine. Furthermore, the Polywell doesn’t even require ion convergence to work (unlike most other electrostatic devices). It helps, but it isn’t a requirement.
5. The system doesn’t have grids. It has magnetically insulated coil cases to provide the electrostatic acceleration. That’s what keeps the losses tolerable.
6. The electrostatic potential well is an issue. Maintaining it depends on the detailed particle balance. The “knobs” that affect it are the electron confinement time, the ion confinement time, and the electron injection current. There are methods of controlling all of these knobs.
Bussard thought the truncated dodecahedron might be better than the truncated cube of WB-6. Reason, the cusps are smaller, the triangular corners of the "cube". The electrons would have a tougher time escaping.
Where Dr Nebel originally posted his comments about making a reactor of "that size"
Polywell forum discussion speculating on the cost to make just one 100MW prototype system
$20-50 million depending upon the magnets and power supplies. If you run D-D and don't care about coil longevity (1 hr estimated) we can make do with some specially constructed MRI magnets (water jackets for alpha/neutron cooling). That might be acceptable for initial experimental purposes. (360 - 10 second runs). Thus $20 million for big test machine and $50 million for a better big test machine.
More questions from Art Carlson, the critique who was having a productive exchange with Dr Nebel
M Simon notes some problems and challenges for a 100MW version of an IEC fusion reactor.
The "first wall" problem is the hardest of the "we have very little idea" problems. A B11 coating has been tried for ITER. That would be ideal if it works. However, ITER is now looking into diamond coating. No mention of Boron these days.
Cooling the coils from alpha impingement is hard. But we do know where to start and we do have some tricks.
Some other lesser problems: design for compactness and energy extraction. Power converter designs. Control of reactant flows. Superconducting magnets. Integrated reactor controls. POPS enhancement.
Roger Fox has written a diary on Dr Nebel's work and Dr Nebels comments and adds his own speculation
Currently the fuel is "puffed" in gaseous form, there is no carburetor. The fuel ions are puffed in, the plasma lights up, some fusion occurs, and the magnets get very hot. All this occurs in under a second. It takes hours for the magnets to cool down for the next run. Superconducting magnets would solve this problem, but at a much higher cost.
Theory says if you scale up the 35 cm magnets to 2 meters, you will have a 500 mw net power reactor. This scaling theory is unproven. A carburetor also needs to be built and there is a possibility that slightly different designs can be more efficient.
An introduction to inertial confinement fusion
IEC fusion uses magnets to contain an electron cloud in the center. It is a variation on the electron gun and vacuum tube in television technology. Then they inject the fuel (deuterium or lithium, boron) as positive ions. The positive ions get attracted to the high negative charge at a speed sufficient for fusion. Speed and electron volt charge can be converted over to temperature. The electrons hitting the TV screen can be converted from electron volts to 200 million degrees. The old problem was that if you had a physical grid in the center then you could not get higher than 98% efficiency because ions would collide with the grid. Bussard's innovation was to use magnets in a configuration that the electrons and ions never hit and keep losses 100,000 times lower. 99.999+% efficiency.
Previous update on the inertial confinement fusion demonstration project
A review of new funded approaches to nuclear fusion
If IEC nuclear fusion works as well as hoped then not only does it solve energy issues but also provides super space capabilities with launch costs reduced 1000 times
Even expensive net power generation means that one fusion reactor can burn the fuel of ten regular fission reactors to make all nuclear power cleaner.