Micro-fusion reactors

[MJ12 Commando]

A more science/engineering than SF question, but tiny reactors is a pretty common thing in a lot of sci-fi (Battletech has 'em, Fallout, and some others), so here's a question that I want to know the answer for but don't really have:

What are the limits to making a small fusion reactor? What design problems become more difficult as you miniaturize them? Etc. I'm sure that having a tokamak in your backpack is pretty silly but I'm not sure what the practical lower limit for size is.

Thanks.

[Junghalli]

Well, a fusion reactor is going to produce a lot of heat and energy, as it's basically got a miniature sun burning in the middle of it. You'll definitely need quite a bit of equipment around it to deal with all the heat, which suggests to me it's probably not going to work very well for, say, a backpack power source on power armor, even with very advanced (not physics raping) technology.

[General Zod]

A more science/engineering than SF question, but tiny reactors is a pretty common thing in a lot of sci-fi (Battletech has 'em, Fallout, and some others), so here's a question that I want to know the answer for but don't really have:

What are the limits to making a small fusion reactor? What design problems become more difficult as you miniaturize them? Etc. I'm sure that having a tokamak in your backpack is pretty silly but I'm not sure what the practical lower limit for size is.

Thanks.

Depends on the type of science fiction involved and the level of unobtanium you're willing to use. Are you talking "realistic" limitations or what?

[Cykeisme]

Regardless of the fictional tech to create and sustain the fusion reaction, having a man-portable fusion reaction requires shielding to prevent it from frying to a crisp the aforementioned man who's porting it around, and everyone around him.
To have the said shielding also fit into a man-portable device, you need unobtanium or totally fictional superscience.

There's also the question of the mechanism used to convert the heat to electrical energy.. I don't think you can scale down a remotely efficient functional turbine and generator assembly in a backpack-sized device.
Of course, there's always fictional superscience, again.

So, yeah, pretty low on the realism scale.

[Batman]

I suspect the technology needed to make manportable fusion reactors feasible to begin with would render them obsolete as a power source at the same time.
Screw the fusion reaction, use whatever allows you to maintain the magnetic bottle and radiation shielding instead. Because you're NOT using simple matter shielding in something manportable.

[Sky Captain]

What about aneutronic fusion which don`t give off tons of deadly neutrons thus requiring far less shielding and has the added bonus of being able to produce electricity directly from charged fusion products. Still reactor probably not going to be backpack size but possibly smaller than smallest feasible tokamak designs. Perhaps small enough to put in a car sized vehicle.

[Ford Prefect]

It's unlikely that any fusion reaction will ever be totally clean, and the problem of heat still remains.

[Stark]

I think it's best to consider that any game or scifi with 'reactors' of this type is not referring to nuclear fusion, but some other form of powerplant. The Fallout 'fusion batteries' are just fancy magazines; it could be any kind of 'joining' reaction.

[McC]

This might be deviating from the OP a bit, but how realistic would it be to have high-capacity, high-yield batteries charged by more fixed fusion reactors (or other electrical power generator)? For instance, take the first-generation "arc reactor" in the Iron Man film, which for all intents and purposes is pretty clearly meant to be a miniaturized tokamak, capable of a 3 GW output for 15 minutes (2.7 TJ total discharge). Now replace it with a battery that has a similar peak draw capacity (3 GW) and a similar total charge storage (2.7 TJ).

How crazy a notion is that, and how much would you need to scale it back to make it not-as-crazy?

[MJ12 Commando]

You could do it with antimatter, which is a pretty good substance if you need a battery.

As a bonus, it explodes when breached for EXTRA fuck-you factor (FYF).

[Cykeisme]

Having a small matter-antimatter annihilation reactor doesn't solve many of the problems we're discussing, namely containing the heat and radiation from the energetic reaction, and generating electrical power from the thermal power output.

And, antimatter has the added problem of fuel containment.
I remember a thread where Wong mentioned that a vacuum chamber containing a magnetically suspended anti-iron flask filled with anti-hydrogen fuel would logically be the best solution, but this does not sound like an arrangement that can be miniaturized, especially when a man-portable device is going to undergo rapid acceleration and deceleration (shock, impacts). A significant safety space between the antimatter flask and the walls of the outer chamber need to be maintained; failure to do so results in immediate catastrophic kaboom.


Edit - IIRC, I'm fairly certain Fallout's Micro-Fusion Cells are described as miniature nuclear fusion reactors. Crazy, right?

[Stark]

Yeah, and some of the guns are described as 'accurate'. Funny, huh? 15 'scorch a drywall' shots from a tiny tiny fusion reactor light enough to conveniently carry around?

[Sky Captain]

Radiation protection is going to be biggest problem for small reactors, waste heat could be dealt with some heavy duty fans and radiators, but radiation needs tons of shielding. If aneutronic fusion still produces unacceptable doses of neutron radiation then I really can`t see practical fusion reactor being small and lightweight.

[Bilbo]

I think it's best to consider that any game or scifi with 'reactors' of this type is not referring to nuclear fusion, but some other form of powerplant. The Fallout 'fusion batteries' are just fancy magazines; it could be any kind of 'joining' reaction.

I was thinking the same thing. Turbolasers are not realy lasers so why do we assume the portable fusion generator Luke uses on Degobah is what we would consider a fusion reactor.

[Darth Wong]

You could do it with antimatter, which is a pretty good substance if you need a battery.

As a bonus, it explodes when breached for EXTRA fuck-you factor (FYF).

Umm, that's more of a "fuck ourselves" factor, since everyone in the vicinity of the person carrying this device would be killed. Most likely, that would be his own comrades. Not a bad deal for the enemy, being able to take out large groups of enemy troops by hitting a single man with a sniper rifle.

[Count Chocula]

Another approach would be to have a receiver for beamed energy that could handle high throughput. That way, you'd only have to provide for power distribution rather than generation and distribution. Admittedly OT as it's not a "micro-fusion" reactor.

IIRC, the purpose of Nikola Tesla's Wardenclyffe tower was to distribute electricity without wires, and there's an account in one of his biographies (Prodigal Genius I think) of him driving a 200HP car that, he said, was powered by energy received through an aerial. There was nothing but an electric motor under the hood.

The technique is being investigated now.

[Starglider]

I have often wondered if you could get net power by fusing individual atoms with tiny particle accelerators. For example, using a bunch of looped nanotubes wrapped in superconducting nanofilaments as a kind of cyclotron. Current particle accelerators are horribly inefficient as fusion machines because most of the particles in each bunch pass each other without colliding, but if you could make the alignment much tighter (by having massively sharper field gradients) it might be practical. Unfortunately I don't have the expertise to know if this is possible even in theory.

[Admiral Valdemar]

Umm, that's more of a "fuck ourselves" factor, since everyone in the vicinity of the person carrying this device would be killed. Most likely, that would be his own comrades. Not a bad deal for the enemy, being able to take out large groups of enemy troops by hitting a single man with a sniper rifle.

That would depend on the mission this AM cell was being used for. For instance, to power a basic exoskeleton, you could have an amount equivalent to several gallons of gasoline, rather than fit a small nuclear device to your backpack. Any detonation would still be harmful, but no more so than if a back-pack of explosives was taken out. Clearly you wouldn't be using such power in squads of dozens of soldiers working together in close proximity. For exoskeletons that have to do far more in sci-fi, they tend to have a pinhead worth of AM in a heavy carapace and operate as one-man armies, which given their power available, is much more possible.

I'm sure some faction would consider cheap, effective AM cells for certain purposes if they had access to it. A micro-fusion reactor or some other reactor would be preferable though.

[MJ12 Commando]

Umm, that's more of a "fuck ourselves" factor, since everyone in the vicinity of the person carrying this device would be killed. Most likely, that would be his own comrades. Not a bad deal for the enemy, being able to take out large groups of enemy troops by hitting a single man with a sniper rifle.

Realistically an AM cell is going to be carrying a fairly small amount of AM and if you're using soldiers like Book MI with wide dispersion, or aerospace assets (similarly, wide dispersion is common) or perhaps even armored vehicles, it shouldn't work too badly. A small nuclear device has a hard-kill radius of a few hundred meters, and aerospace would have kilometer+ dispersion, while Book MI also had hundred meter + dispersion in "close formation" and the like. As long as you're using units that are going to be widely dispersed an AM battery isn't an entirely horrible idea, assuming you have the technological know-how to make it effective for the unit's size.

Unrealistically it's going to be working via Hollywood physics so the fact that an AM cell chock-full of antimatter is going to be a suicide bomb won't matter until it's dramatically important.

And Starglider, your idea is awesome, no matter how much impracticality is in it.

[Ford Prefect]

It's like you missed the initial problem of all your dead infantry potentially turning into bombs with a hard kill radius measured in a couple of hundred metres. Do you honestly think that every battle can be fought with your infantry spread half a kilometre apart? Laughably simplistic views of how a battlefield works do not justify strapping small, volatile nuclear weapons to your soldiers.

Valdemar, is, however, correct. If you are powering some sort of infantry power suit or armoured vehicle with antimatter, it's going to be a very small amount, simply because you do not need terajoules of power. Even so, a small amount of antimatter is still an awful lot of 'explode' if the containment is breached. You just wouldn't use antimatter for this sort of purpose, assuming you were using it at all.

[MJ12 Commando]

It's like you missed the initial problem of all your dead infantry potentially turning into bombs with a hard kill radius measured in a couple of hundred metres. Do you honestly think that every battle can be fought with your infantry spread half a kilometre apart? Laughably simplistic views of how a battlefield works do not justify strapping small, volatile nuclear weapons to your soldiers.

A hard kill radius of a couple of hundred meters requires them to be equipped with weaponry and other systems that are likely to draw gigajoules to terajoules of energy over the expected combat time, because you won't issue them with more power than they conceivably need for a prolonged combat situation. In which case the actual kill radius is more likely to be within the dozens of meters, making this no more deadly than carrying nitroglycerin. (and also coincidentally making the idea of "infantry" pretty obsolete). Assuming a ~1 terajoule detonation, a back-of-napkin calc of energy density (using a rough sphere) makes the energy density at 400 meters ~1.4 MJ/m^3, definitely lethal to unarmored or lightly armored infantry but survivable for a lot of sci-fi power armors. And having 1 terajoule of energy stored in your backpack means you probably have need of it, which generally means you're in the realm of sci-fi where modern materials science isn't a good guide to the maximum durability of the alloys and whatnot you're building.

It's best off for aerospace assets in a more realistic mileu, which conceivably can make good use of having the high TWR that an antimatter-based engine can give, as well as being more useful on missiles.

[Cykeisme]

What?

Why would you build a man-portable anti-matter reactor (along with all the probably bulky systems needed to safely store it, and convert the output of an annihilation reaction into electrical energy), and then only carry a pinhead of the stuff?
Why not just carry some battery or fuel of a more conventional sort instead?

You'll need fictional superscience to build that man-portable reactor anyway; might as well have fictional superscience batteries or something of a more believable sort.
Otherwise it sounds like you want them to be carrying fusion or antimatter power sources just so you can say you have a nuke plant strapped to your back.
Okay, Rule of Cool and all, but still, the OP was asking what a "practical lower limit for size is". I'm not qualified to answer that precisely, but it's basically "not small".

[Admiral Valdemar]

What?

Why would you build a man-portable anti-matter reactor (along with all the probably bulky systems needed to safely store it, and convert the output of an annihilation reaction into electrical energy), and then only carry a pinhead of the stuff?
Why not just carry some battery or fuel of a more conventional sort instead?

You'll need fictional superscience to build that man-portable reactor anyway; might as well have fictional superscience batteries or something of a more believable sort.
Otherwise it sounds like you want them to be carrying fusion or antimatter power sources just so you can say you have a nuke plant strapped to your back.
Okay, Rule of Cool and all, but still, the OP was asking what a "practical lower limit for size is". I'm not qualified to answer that precisely, but it's basically "not small".

There's no physical way to get the energy density of AM any other way. The concept relies on a small, powerful power source that doesn't have the limitations of batteries, which look like never going anywhere near hydrocarbon density to compactness ratio, nevermind nuclear. If you use some future supercapacitor instead, you're still carrying giga- or terajoules of energy that can go off and be released instantly. For this kind of energy output you are, by definition, not going to be using traditional infantry like today. It would be like expecting fighter jets today to play Battle of Britain when they have far superior energy outputs which make such lower powered dogfights of old redundant.

If you're still worried about blue on blue, then use there power sources in drones sent into enemy territory alone or in sparse groups. If the enemy takes one out, they deal with the consequences on their own land still. If AM is cheap and abundant from Asimov arrays or the like, then I don't see some power utilising the energy advantage given reliable, compact storage systems. Like nuclear was once considered the weapon to end all wars but then became advanced enough to power ships or produce tactical scale bombs, AM could be used the same way.

[Sky Captain]

Closest real life equivalent to sci-fi backpack size nuclear reactor is radioisotopic generator. Current RTG`s are extremely inefficient but given some advancements in energy conversion technology it might be possible to build one which can steadily produce few kw of electrical power, last for years and still be relatively portable.