Are the claims made about the http://en.wikipedia.org/wiki/High_beta_fusion_reactor realistic? Can such a small fusion reactor really work?
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2$\begingroup$ What claims? Please summarize. $\endgroup$– rob ♦Oct 15, 2014 at 20:23
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1$\begingroup$ @rob - I think asmaier is talking about the proposal here. I don't know if this is accurate, but on another forum some people were saying it seemed to be along the same lines as the magnetic mirror approach to fusion which was apparently worked on in the 80s but mostly stopped due to lack of enough funding. $\endgroup$– HypnosiflOct 16, 2014 at 2:23
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3$\begingroup$ Could you guys try to expand it to a full answer that actually tries to argue that the whole compact design violates some physics argument? I have doubts about these comment-sized answers of yours. It's hard for me to believe that the guy with a fusion-related NASA-led PhD from MIT doesn't know the basic things about the plasma confinement. I guess that he knows what "magnetic mirror" is as well and if the problem was a lack of funding, it may have been overcome in Lockheed Martin, right? $\endgroup$– Luboš MotlOct 16, 2014 at 7:35
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1$\begingroup$ @LubošMotl We don't have any rigorous scientific or technical data regarding the idea to make that kind of assessment. This is why I think it's premature to answer this question. I made a statement based on what I have seen thus far; it doesn't mean it will or won't work. The second point is that the confinement problem is not a "basic thing" - the study of energy and particle transport is a big theoretical and practical problem that some of the best minds in the field (including those at MIT's Plasma Science & Fusion Center) have grappled with for decades. $\endgroup$– user3814483Oct 16, 2014 at 16:58
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1$\begingroup$ Yup, that's exactly what I meant by a "basic thing" – that it's an omnipresent fundamental problem that every researcher in the field has had to deal with much of the time. ... I obviously don't claim that this project has to work, it probably won't. But I think it would be unfortunate to dismiss the work of a trained professional (plus colleagues) in a big corporation tasked with a similar task just because they're not finished yet or because their approach differs from some other, hugely funded approaches, without an actual physics argument why their approach is less promising. $\endgroup$– Luboš MotlOct 17, 2014 at 6:17
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3 Answers
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I personally doubt that the Compact Fusion Reactor as presented by Lockheed Martin last week can work, but I haven't seen enough information to be certain. And to some extent, you never know until you try. (As I understand it, they only have a very early prototype, I mean try as in a full scale prototype.)
What I think I can say with certainty, is that it won't be as small as they claim - "can fit on the back of a truck". Trucks are about the same width as standard containers, so about 2.5m wide. I've had to make quite a few guesses, but I've tried to justify them and choose the smallest size possible.
In the second image here, you can see a grey blanket around the device which absorbs 14MeV neutrons to generate tritium and protect the rest of the plant. The internal coils will also need such a blanket to protect them (it's unclear if the orange skin is this blanket, or just the cryostat). It's also unclear if the outer coils are superconducting or not, but I'll assume they are otherwise the ohmic losses use too much of the power you're supposed to be generating. Superconducting coils need to be cooled with liquid helium and insulated inside a cryostat.
Blankets for a tokamak reactor are estimated at 1m thick. I'm not sure if this is dictated by the tritium breeding or the protection. If it's protection, you might be able to reduce their thickness if you're operating at 100MW instead of 1GW, so let's be optimistic and assume 0.2m thick. I'll assume the same width for the coils and the cryostat (probably optimistic again). I'll neglect any structural elements. So going from the outside of the machine to the centre we have
They don't give any figures for the size of the plasma, but I think it just looks silly if the plasma diameter is less than a third of the coil diameter, so I'll put 0.5m in both of those plasma columns. (Note that this is a very small distance between where the fusion happens at 10^8Kelvin and the wall at 10^3K, and would be extremely good magnetic confinement.)
Totalling up gives 2.6m from the outside to the centre, so the machine is about two trucks wide already. You might give them the benefit of the doubt at this stage, even though all those values were optimistic. But then you need to add peripherals:
heating system (the neutral beam injectors shown in the Lockheed diagram are usually about the size of a truck by themselves)
cryogenic plant for liquid helium (at least half a truck)
power supplies for the coils
vacuum pumping system
steam turbine
bioshield. Even the 1m blanket on a tokamak doesn't block all of the 14MeV neutrons. Safety regulations will require a few metres of concrete shielding in all directions (multiple trucks)
So even if it would work, I don't think anyone will be putting it on a plane.
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$\begingroup$ Could shielding be provided by placing the reactor underground? $\endgroup$– DemiMay 25, 2016 at 15:39
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$\begingroup$ I doubt it. The ground is fundamentally porous and you would almost certainly end up with radioactive material in the groundwater. $\endgroup$– craqMay 29, 2016 at 8:53
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$\begingroup$ There was an update on this in 2017: "After more engineering and scientific research, the new design requires about 2000 ton reactor that is 7 meters in diameter and 18 meters long." nextbigfuture.com/2017/05/… Seems like you were right that it cannot be as small as they claimed first. $\endgroup$– asmaierJan 22, 2018 at 21:59
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$\begingroup$ @asmaier Interesting, thanks for the link. They still have those unsupported internal magnets and the problem that cusps confine high temperature plasma much worse than low temperature plasma. (T4 at 20eV is much colder than 20keV which most magnetic fusion reactors are aiming for.) $\endgroup$– craqJan 22, 2018 at 22:23
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other people far smarter than I have since published more detailed reasons why the concept presented by Lockheed Martin can't work. One example is from two professors of plasma physics on the website of the Max Planck Institute for Plasma Physics. To summarise
- coils inside the plasma need connections for a power supply and coolant. These connections will be in direct contact with the hot core of the plasma.
- size of shielding required for neutron protection is much larger than proposed
- fast particles are not well confined by the magnetic configuration which LM proposes. To understand this you need a good grasp of plasma physics, so I won't try to explain further. The effect has, however, been demonstrated and understood in other plasma physics experiments.
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there are actually at least a dozen high-beta designs that have made it to the "small model" prototype stage, including a few different versions of polywells and field-reversed configurations, none have scaled to anything like a net power reactor though, all have unique issues
some of these designs have been ruled out in theory but then later re-examined (e.g. Rider seemed to have disproven all IEC devices at one point, but later simulations by Luis Chacon and others seemed to suggest the shape of the well was more important than thought)
will any of them ever pan out? maybe... the field seems to have advanced quite a bit in the last ten years
take any criticisms with the same big grain of salt as for proponents' claims... plasma physics is still very much in the learning phase, and people can spend decades on ideas like tokamaks without ever learning much applicable to quasi-neutral or high-beta concepts
