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It would be really cool if you could build cybernetic arms with incredible strength, power coil guns with a man portable unit, or use power armor for extended combat. One of the ways this is justified in video games like Fallout is the use of microfusion power cells. The idea is that you could generate power with a small portable fusion device.

Recent advances hint that man made fusion could be a viable energy source (https://pubs.aip.org/phyfsicstoday/Online/41898/National-Ignition-Facility-surpasses-long-awaited), but the equipment needed must be gargantuan. If fusion were viable as a power source could you fit the equipment in your house? Your car? Your backpack?

Is there a lower limit on the size of the equipment needed to achieve a fusion power cell that could power man portable devices?

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  • $\begingroup$ I'm using the power-sources tag despite this question addressing man portable devices. If you would like I can remove it. $\endgroup$ Commented Nov 11, 2023 at 20:15
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    $\begingroup$ Any particular reason for insisting on fusion rather than fission? Fusion is not 'clean energy'; both fusion and fission give off ionising radiation. But fission requires getting two very small nuclei with a tiny radius of each other, which is a lot harder than encouraging a larger and unstable nucleus to break up. There may be advances that overcome this, but they haven't happened yet. Meanwhile, Grandfather's watch with the luminous dial is an atomic powered light source. $\endgroup$ Commented Nov 12, 2023 at 10:43
  • $\begingroup$ @RichardKirk i guess any method of power generation or storiage system would work. At the time i wrote this question fusion seemed the most sci fi. It's significantly easier to focus on asingle type of power yielding mechanism for this QA board. Do you believe that fission or some other device, such as an atomic battery, could produce / store enough energy to power the examples in the original post? $\endgroup$ Commented Nov 12, 2023 at 13:35
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    $\begingroup$ @EnglishmanBob pure fission doesn't scale down very well, due to concerns about critical mass and weapons proliferation. It may be possible to make an antimatter-triggered microfission pile though, requiring production and storage of nanograms of antimatter. You'd still have energy extraction issues which limits the ultimate size of the thing. Why not ask a separate question, focussing on nuclear power of some specific item, and see what suggestions you get? $\endgroup$ Commented Nov 12, 2023 at 14:47
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    $\begingroup$ Neither fission nor fusion scale down to human level. They involve extreme levels of neutron radiation. Even if you solved the containment problem and put a fusion (or fission) reactor in a backpack, anyone wearing it would die within seconds of starting it up. It even damages steel. You can't reduce this radiation, because it's what makes fission and fusion work. You can shield against it. $\endgroup$
    – Therac
    Commented Nov 13, 2023 at 19:43

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So lets cheat.. https://en.wikipedia.org/wiki/Diamond_anvil_cell but its miniaturized to dust-grain size- and its combined with other diamond anvil cells. All get a intake valve, a wall that can be temporary charged so that it pushes away the atom to fuse. And once fusion happens, the energy is extracted by extracting the heated up gas- and by using one anvil to push back upon another anvil. Put all that in a ring, falling through the medium, gradually creating heat you extract. Voila.

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From a worldbuilding perspective, as small as you want

Before 1925 newly hired engineers at General Electric would be told, as a joke, to develop a frosted lightbulb. The experienced engineers believed this to be impossible. In 1925, newly hired Marvin Pipkin got the assignment, not realizing it was a joke, and succeeded.

Back when I was designing semiconductors (pre 1997) we knew... we knew... that a FET gate couldn't be built smaller than about 30nm. The math proved it. Tests proved it. We knew it as an absolute truth of science. (You're about to discover why I'm contemptuous of anyone who thinks science closes any interpretation.) Then, in 2016, Berkley Labs built the first 1nm gate.

In other words, anybody who tells you, "yes, there's a limit, and here's the science to prove it" doesn't actually know. Isaac Asimov toyed with the idea of limitless miniaturization in his Foundation series, which had fusion generators in necklace pendants.

In fact, in your previous question, you list references that blithely discuss what ten years ago would have caused serious presenters to be laughed off the stage. You presented papers about synthetic nano-dust sensors. We can't even build something thousands of times bigger... but we're so sure of its potential today, we're seriously presenting papers about it.

So, how small could I go and still be believable?

Let's throw all caution to the wind and talk about molecular fusion generators. In a practical sense, what will limit the size of the generator is the amount of power you need... not circa-2023 science. Need a gigawatt to power your super cool Delorean time machine city? You might need something the size of a house. Maybe. Want to power a house? Let's declare the generator to be the size of a shoe box buried in your front yard with a sign that reads something like "Don't dig up the big box of plutonium, Mark."

All you need to do is power your party lights? Molecular fusion generator! Let's be unreasonably pessimistic and say it's the size of an Energizer 303 battery and that it'll happily run up to 1,000 party lights for the remainder of human sovereignty.

In other words, believability isn't based on size, it's based on power output

And when it comes to worldbuilding, believability is the name of the game. The only people I've met who thought Asimov's pendant personal shields with integrated fusion generator were "unrealistic" have been people who think the Science Of Today is the end-all God-wrote-it-in-stone interpretation of science.

No it ain't.

I frankly don't know how small a fusion generator can be built. But I do know they can and will be built smaller than whatever the size is of the first fusion generator. And I can believe a lot smaller.

As long as we're talking about party lights. :-)

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    $\begingroup$ See also, Clarke's first law. $\endgroup$ Commented Nov 12, 2023 at 11:18
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    $\begingroup$ @StarfishPrime I need to repeat that for the viewing audience because it's so terribly true. "When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong." $\endgroup$
    – JBH
    Commented Nov 12, 2023 at 18:54
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    $\begingroup$ I am tickled by the idea of a future world where we have micro fusion reactors but quirks of product development history means they're still all in standard-from-today battery forms. As in: this home replicator takes 2 standard AA fusion reactors. $\endgroup$
    – Ben
    Commented Nov 12, 2023 at 23:54
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    $\begingroup$ (The reader should note that "molecular fusion reactor" is a collection of words put together for the purposes of this answer and not an actual thing. This doesn't invalidate the answer in any way, I'm just saving people the time taken to google it.) $\endgroup$
    – N. Virgo
    Commented Nov 13, 2023 at 5:17
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    $\begingroup$ @N.Virgo isn't wrong, I used the words to create a concept. However, now that N. mentions it... DIBS! "Molecular Fusion Reactor"(TM) JBH, 2023. Talk to me and I'll tell you where to send the licensing checks. $\endgroup$
    – JBH
    Commented Nov 13, 2023 at 6:47
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To my mind, the major problems you have to overcome are these:

  • make the energy recovery equipment as small as possible
  • make the radiation shielding as small as possible

These things are in conflict.

There are three ways you can extract power from a fusion reaction:

  • Heat. Just let some massive object absorb all the energy, thermalize, and use it to run some kind of heat engine like a classic Brayton cycle power plant or something more unusual like a Stirling cycle engine.
  • Ions. A load of those fusion byproducts are are charged and have lots of kinetic energy. Aim them through some conducting loops, extract that energy as electricity.
  • Electromagnetic radiation. Fusion plasma is pretty hot, and so tends to radiate in the x-ray spectrum. You can build x-ray photoelectric converters as a sort of analog to photovoltaic cells for ionizing radiation.

Heat engines are obviously simplest, because we can make them now. Unfortunately, you need some kind of working fluid, and a heat sink, and some mechanical stuff to make your generator work. These things tend to be quite large, quite heavy, and require various kinds of ongoing maintenance in a way that solid-state direct conversion systems do not. The existence of compact fossil-fuelled generators shows that they can be made relatively compact, though you probably wouldn't want to carry one around all day, and it could be pretty noisy.

Neutral particle radiation, in the form of fast neutrons, isn't suitable for direct energy conversion like ions or EM radiation, and so can only really contribute to the energy output of your system via thermal effects (neutrons heat the reactor shield, cooling loop pulls the heat out) or using their energy to do nuclear chemistry, like tritium breeding or actinide burning. Those activities are potentially quite large and industrial-scale things, and in any case not necessarily very useful to miniaturize.

More importantly, neutron radiation is very highly penetrating. This means you need massive shielding around any highly neutronic fusion reaction. The 14 MeV fast neutrons trucking out of a nice easy-to-ignite D-T reaction are particularly problematic (eg. shield using 5-6ft of water or 10s of centimeters of iron, lead and plastic), and as such neutronic fusion reactions may be impractical to miniaturize too far... maybe train size, but much smaller starts requiring implausible shielding that can't really be made out of matter, only magic.

Aneutronic fusion is better, but the problem there is that the plasma is very hot and as such loses a lot of energy through Bremsstrahlung radiation. High energy x-rays can be captured by photoelectric converters, but again, they're highly penetrating so you need quite a thickness of converters around the fusion source. Because the converters will end up looking a bit like layers of metal foils with gaps between them, they're quite low density compared to regular shielding and therefore quite bulky. If you could make a perfect athermal fusor, where you can extract the fusion energy from the ions before it gets transferred to the electrons in the fusion plasma and released as Bremsstrahlung x-rays, you could make something much more compact... the Focus Fusion project tried to do this with a dense plasma focus, but they've only managed to reduce the issue, not eliminate it.

And aneutronic fusion is often more like "its basically aneutronic don't make such a fuss" like p-11B, which can produce 3 MeV fast neutrons from a 11B + p → 11C + n side reaction, which happens about 0.1% of the time but that's still often enough to require hefty shielding. Maybe, 3He + 3He fusion in an athermal fusor might give you the no-neutron, minimal-x-ray reaction you need, but that reaction apparently has a wide range of reaction product energies, meaning it isn't very useful for direct energy conversion from ion kinetic energy for various engineering reasons, because physics hates you and your ideas.

could you fit the equipment in your house? Your car? Your backpack?

In order, I would guess

  • Yes, you could probably have something in your basement, especially if it was underneath a nice thick concrete slab.
  • Probably. I'm pretty certain you could fit one in a big truck, but a reasonable-sized car doesn't seem beyond the realms of possibility.
  • No, I don't think you could have a backpack-mounted fusion reactor, because you'll either have too much radiation, too bulky a generator, or too much power conditioning equipment to make something that small.
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    $\begingroup$ +1 for the radiation shielding. Sci-fi wise you could ignite with a futuristic gamma ray laser a microscopic fuel pellet that would generate a moderate amount of heat, but the radiations would still be there. $\endgroup$
    – FluidCode
    Commented Nov 13, 2023 at 12:04
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The normal route with fission would be to convert the radiation energy into heat, and then convert that into electricity. This is not very efficient. In theory, the nuclear energy has very little entropy, so thermodynamics lets you convert a lot of that energy directly into other forms, but to do that we would have to control the geometry of the nuclear collision, or do some Maxwell's Demon magic to extract the energy from the energetic particles in whatever direction they fly off in. I have always imagined that is what 'dilithium crystals' did. That is way beyond anything we can do at present, even if thermodynamics allows it: we can align nuclear spins at millikelvin temperatures; controlling the geometry of how two tiny balls hit each other is going to be very hard.

If you convert your nuclear energy into heat, you have to get rid of a lot of heat. Not only will you have to get rid of the kilowatts of heat that your apparatus is consuming, the power plant will have to get rid of many times that. It won't be like a battery - it will have to have a lot of heat sinking.

However, suppose that all you want is a burst of radiation. This gets around the efficiency question because the radiation is the end product. As a lightbulb burns out, it will momentarily generate enormous temperatures, as the inductance of the apparatus wants to keep the current flowing, but the last bit of conductor turns straight from solid tungsten to plasma at metallic densities, emitting hard thermal x-rays from temperatures millions of degrees. Suppose your wire had a deuterium-tritium filled void in the centre. If you could control the implosion geometry, you could get fusion. You now have to generate a pulsed current of millions of amps for a few tens of picoseconds to drive the thing. This could be done using a Marx generator, and those can be huge. If you want a one-off device, it could be a slab of piezo crystal and C6 explosive sandwich. The whole gadget could be hand-sized. It might be possible to make a device that worked more than once if you could store the energy from one implosion and use it to drive the next.

That was the answer to the first half of the question. The actual reactor is sub-millimetre scale but the energy to drive the implosion takes some organising.

Next: the radiation. D-T fusion generates a high energy gamma and neutron. Those are very penetrating radiations. It would go through 30 cm of aluminium, or similar alloy. However, it would be stopped by a few millimetres of depleted uranium, which would also release more energy. This is good design from a radiation shielding point of view, but it also means you have in effect made a fast breeder reactor, which is where we are now. Fusion is not 'unlimited clean energy'.

If I more the goalposts a bit, I can get you in a much more feasible place. Suppose you have a Thorium reactor, that does not have by-products that can be used for weapons. It does not need the uranium processing cycle that uranium uses, so you could seal the reactor with its fuel, and when it's power has dropped too far to be useable, you dump it or recycle it. If you are using Uranium, you can seal it if you have a wave reactor - one half conventional and one half fast breeder. But that's another story.

If you are using this to drive a car, you only need the peak power for acceleration. It is possible to have a sporty car with a 300cc engine if you put the energy into a flywheel or an ultra capacitor, and get it out for the bursts when it is necessary. This would not work for a helicopter, which is fighting like mad all the time not to fall, but it would work for many other things. If you have a Thorium reactor providing steady power, and put smarts into your powertrain, you should get a lot more out of a smaller power source. This is the way electric cars are going. If you have a powered exoskeleton, it will have a springy step that recycles the energy from one pace to make the next. It can use technology from the near future.

But probably not fusion IMHO.

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    $\begingroup$ I've just been reading up on the power-to-weight ratios of small fission reactors, and honestly they aren't great. Not that I'm expecting fusion plants to be much better, mind you. Its just really remarkably hard to beat combustion engines and fuel cells when you need something small and light and convenient and portable that can deliver high power levels. Might not actually be worth using dinky reactors for something smaller than a ferry, unless its a space probe. $\endgroup$ Commented Nov 12, 2023 at 19:50
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    $\begingroup$ @StarfishPrime No-one has really tried to make a mobile power source for use on earth, apart from some hairy experiments in the fifties and early sixties when they tried all sorts of mad things. If we could make fusion in a backpack, I feel fission would get there first. But internal combustion is a tough act to follow. $\endgroup$ Commented Nov 13, 2023 at 11:02
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If you want to work with Deuterium-Tritium fuels, then the fusion reaction will generate 14MeV neutrons. To breed Tritium you need about 1m of blanket, with neutron multipliers and Lithium. Beyond that, to shield the rest of the world from the ionising effects of the neutrons that make it past the blanket, then you need at least another metre or so of concrete. These numbers are based on nuclear cross sections, so they are unlikely to change even if technology improves. (The only alternative I am aware of would be aneutronic fusion, which has other complications.) So if the part where the fusion reaction occurs is infinitessimaly small, then the reactor is at least a sphere 4m in diameter.

All currently theorised fusion reactor designs need a lot of infrastructure such as heat sources, fuel storage, equipment to measure and control your reactor, and some form of energy extraction (e.g. heat exchanger and steam turbine). Most need powerful magnets and an accompanying cryoplant. The size of the infrastructure depends more on engineering that physics, so it's harder to fix a lower limit. With current technology, 1000sq.m. is a very optimistic estimate of how small it could be built.

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