I’ve just read a thisMSNBC science post about Dr Richard Nebel and IEC fusion and it seems rather interesting. It also seems rather reminiscent of Cold Fusion.
So what’s the Straight Dope? Interesting concept? Potentially useful? Or hyped-up quackery?
Well, as far as I understand it, Bussard’s polywell concept is similar to the Farnsworth-Hirsch fusor in so far as it uses a potential well to confine the ions supposed to fuse in a small enough space with a high enough energy to do so (which is the concept behind IEC – inertial electrostatic confinement – fusion); the main difference between the two concepts being that the polywell uses magnetic confinement, essentially trapping electrons to supply the potential well, where the fusor accomplishes that with solid-state grid electrodes. This circumvents the problem of grid losses the Fusor suffers – accelerated nuclei strike the grid, damaging it, wasting the energy that went into their acceleration, and heating it up; in general, having fine mesh electrodes in the presence of plasma just doesn’t bode very well for the electrodes, in other words. Thus, while the fusor can be used as a neutron source, it’s not feasible for power generation.
By eliminating the need for these electrodes, using magnetic confinement instead, it’s hoped that the polywell might provide a feasible power source, however, this has yet to be shown as far as I know. Additionally, the polywell’s design opens up the possibility of bremsstrahlung losses – bremsstrahlung is produced by the deceleration of charged particles, and carries energy away from the setup in an unrecoverable way. Whether these losses will offset the power production is controversial at the moment, as far as I know.
However, compared to other controlled fusion approaches, such as the currently most popular tokamak design, the polywell is relatively easily scalable, and comparatively cheap. An additional reason for following this design route is that it can conceivably utilise aneutronic fusion reactions, which (nearly) eliminates the problems of radioactive activation in surrounding materials and ionizing damage to anything in the vicinity (including people).
So, to sum up: the polywell is a device capable of fusion, only if it is possible to build one feasible for power generation remains to be seen, though by all accounts things look promising.
[THREAD=420033]Here[/THREAD] is a thread on the topic from last year. This is basically a variation on the Farnsworth-Hirsch fusor (which itself is a practical nuclear fusion device, albeit not for net power production) that eliminates the central electrode.
It is an interesting approach and potentially fruitful, but I don’t think they’ve demonstrated feasibility, much less scalability into a system practical for commerical energy production, and issue with overwhelming Bremsstrahlung losses may not make the concept as scalable as suggested; the detection of only three neutrons in one reactor cycle and the lack of peer review don’t argue much in favor of the process, and while I don’t believe that Dr. Bussard was trying to flim-flam anyone, I think he was near the end of his life and hopeful of producing something viable after spending mostly fruitless decades on fusion research and administration.
On the other hand, research on this scale is peanuts compared to the cost of conventional magnetic and inertial confinement fusion, and the possible benefits are enormous if a means could be found to make it workable for net energy production, if even but marginally. The timeline in the EMC2 page is laughable in the extreme, though, at least for commercial development. I would double those estimates, even with a near-heroic estimate, and the claim that “Successful Phase 2 marks the end of fossil fuels,” is presumptive at best; even if we had a non-polluting, scalable power supply today it wouldn’t result in the end of fossil fuel use in five years; too much of our economy and infrastructure is based on fossil fuels, and transitioning to another source would take considerable time and incur a massive upfront cost, regardless of the eventual savings.
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I don’t really understand those Bremsstrahlung losses. Surely the energy lost manifests as heat somewhere and can thus be extracted?
Bremsstrahlung losses are self-interaction losses between charged particles in a plasma which cause losses due via electromagnetic radiation. Because the loses increase with charged particle density to a square (with some other correcting factors), getting a plasma dense enough and with high enough temperature to overcome the nuclear binding energies takes a heck of a lot of power, and Bremsstrahlung losses “cool” the energetic core of the plasma, keeping it from reaching a sufficient combination of temperature, density, and confinement time (the “triple product” criteria for fusion) for fusion ignition.
We can do this, of course, in nuclear weapons by using a fission Primary which releases an enormous amount of energy which triggers (momentarily, for a few dozen shakes) a fusion reaction in some combination of deuterium, tritium, and lithium, but doing so in a controlled plasma is very, very difficult; this is the basic stumbling point of magnetic confinement fusion.
Of course, the PolyWell doesn’t have a running plasma current, so the effect of Bremsstrahlung losses might be significantly reduced over a tokamak-type reactor, but you still have a plasma inflow that seems (qualitatively) like it should have some pretty substantial Bremsstrahlung cooling, and the larger you make the apparatus and the denser you get the plasma (for higher efficiency) the worse it will get. p-[sup]11[/sup]B Bremsstrahlung losses are much greater than with D-T fusion at ignition triple point (because temperature and pressure much be much higher) and the effective power density is about three and a half magnitudes lower, so I tend to question the viability despite the practical value of a relatively neutron-free fusion reaction. I’m not saying it doesn’t work–I have neither the detail nor expertise to make that claim–but that Bussard seemed quite blase about how easy it would all be without having an actual demonstratively working apparatus or peer-reviewed results and analysis.
In short, the losses from B-rad are losses of energy you put in, not heat from net energy production out.
Again, it would be a real boon if it works even a fraction as well as claimed, and probably worth the seed money it would cost just to get the system running sufficiently to try to repeat the results which can then be reviewed by independent parties. Contrary to what Bussard claims in his Google presentation, there are in fact experts who are competent to peer review the work; even setting aside high energy plasma physicists currently working in conventional magnetic confinement fusion, the field of solar and astrophysics is full of people who know all about energetic magnetohydrodynamics, and while localized confinement fusion may not quite be their game the fundamentals are not that terribly different from “gravitational confinement” fusion, i.e. stellar fusion.
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