I got a tour of the Laboratory for Laser Energetics shortly after it opened, sometime back in the early 1970s. Everybody there and everybody in our tour group were positive that this was the future. Positive, I tell ya.
Forty years later somebody announces that they’d “like to get to a prototype in five generations”?!?!?! If they get it working in the next decade, I’ll eat Stranger’s hat.
It’s a big hat; we can share.
The problem with nuclear fusion is that it seems very simple in concept, but the more we learn about plasma dynamics at high energy and magnetic field levels, the more complex the problem becomes. Conditions that seemed to be almost trivial–except for achieving threshold energy levels–circa 1955 is still not achievable today except in very transient, unstable, and unmaintainable scenarios like a Z-pinch generator. And all of the easy ways (electrostatic fusor, Polywell, microwave-heated torroidal chamber, muon-catalyzed fusion, ‘cold’ palladium-catalyzed fusion) to reduce the thresholds all have fundamental physical limitations or just lack a complete physical basis to even begin to rigorously develop a technology. If I had to pick one that is most likely to achieve some useful result it would probably be the Polywell electrostatic confinement, but that has been in development for almost thirty years (and is based on the Farnsworth fusor concept developed prior to that by Philo Farnsworth, inventor of the modern television) and has yet to demonstrate over-unity conditions, much less being commercially viable (e.g. producing significantly more energy than the entire input energy cycle). However, we do know that this type of system can achieve fusion conditions, albeit at an power-negative production rate.
The only known, controllable way to achieve sustained nuclear fusion conditions with net energetic yield is to take a large mass of hydrogen and push it all together into a gravitationally-bound reactor. This isn’t terribly efficient and requires the ability to move material and construct structures on astronomical scales, but at least it works. Every other proposed method is still speculative at this point.
Stranger
E.g., the Sun? What’s the smallest possible gravity-bound fusion reactor, assuming we could come up with enough mass of the optimal material? (Not to mention the difficulty of harvesting the energy.) Is it the same as the smallest possible natural star? Seems to me it would be, unless maybe there’s a big advantage to having all the right isotope of the optimal element(s).
The smallest celestial objects capable of maintaining nuclear fusion conditions (low-mass red dwarfs) are <0.1 solar masses. Of course, their surface temperature is significantly lower so the energetic yield is less, they’ll also last a lot longer instead of burning out in a quick, 10 billion year flash. On the other hand, the largest mass stars will burn very, very brightly, but only for a few million years. Take your pick.
The optimal material for a long-lived star is mostly hydrogen with a small amount of helium and a low concentration of higher metallic elements, e.g. our sun. Of course, I state that with prejudice. I’m sure some Silver Ghost will think highly of a near-brown dwarf start that is fusing just enough to maintain temperature, while a world-eating Celestials probably prefer hot burning stars to spice up their diet.
Harvesting the energy produced is a trivial exercise provided you don’t do something stupid like put a rigid ring spinning around your star, requiring additional attitude control systems and a string of sequels to explain away all of the problems you failed to consider on your first attempt. A large shell which which is radiatively stabilized, or a vast series of small solar collectors orbiting at various distances so as to almost completely collect the radiated energy is quite feasible even using only the mass of a gas giant. Provided, of course, that you have the technology to transmute elements and create physically unrealistic substances with ridiculously high thermal capacities and mechanical properties. It is left as an exercise to the reader to flesh out the details and submit a proposal to your nearest college grant board to support the necessary research and infrastructure developments to make this happen.
Stranger
You know, it occurs to me that the dyson sphere concept may not be more viable than a ringworld. Solar radiation is not very uniform, your shell would have to have constant dynamic adaptability to wind and EM pressure, even with a red dwarf. If you built a significant enclosure, you would probably have to address venting the solar wind, but more than that, you have to consider drag: the sun we have here pushes out on the interstellar medium, if you constrict that, the shell will be subject to aerodynamic forces that will want to push it off-center to the star.
Then there are the cosmic rays, I suspect capturing too much solar wind could leave the shell rather vulnerable to erosion.
The real trick, though, is transporting the harvested energy. You probably do not want to live on a collector array, nor on either surface of your shell, how do you get this abundant energy into people’s homes?
This is a bit of a hijack… but have you seen the General Fusion design? I know just the basics when it comes to fusion, but on principle it looks like a good idea, for example it doesn’t even bother to maintain a constant containment of the reaction so that should make things tremendously easier to handle.
What do you think of it?
It sounds like something out of a J.J. Abrams movie. I’m surprised there is no mention of the injection of “red matter” to kick it off.
Seriously, the complexity of this mechanical device sounds so ridiculously over the top–practically Rube Goldberg-esque–that the engineering to make it function reliability is is probably the work of a decade if it is possible at all.
Stranger
Actually, if you can customize your isotope mix, you can make a functional star smaller than any of the natural ones. Brown dwarfs aren’t big enough for fusion of normal hydrogen, but they can still fuse deuterium (this is in fact the definition of a brown dwarf). Natural hydrogen has only a very small amount of deuterium, so brown dwarfs don’t produce much energy, but a pure-deuterium star presumably would.
In the OP link, one of the comments has a link to a spheromak project at the U of Washington that is burbling with optimism. Unlike Skunkworks, though, they are talking about scaling up very large (if fusion were to succeed, I think a lot of smaller units would be preferable to a few larger ones, if the former were doable).
Both the stability and thermodynamic efficiency of charged plasma-based processes tend to improve with scaling.
Stranger
We *are *living today on the surface of a collector array. Works OK. In the US our collector surface absorbs 4-6 KWh/m2 per day and that’s at the bottom of an atmosphere. Sure, we ought to build some concentrators here and there, and we’re just now in the early 21st Century getting started on the effort. But not all of the surface is ideal living land anyway. Spending some effort and land to hold technological collector arrays more efficient than the extant natural arrays is a reasonable tradeoff.
A Dysonesque sphere doesn’t work by gathering more energy per unit area; it gathers more energy by the simple approach of “build it bigger” IOW provide more area. So if we imagine a future Earth civilization with the population density of Earth today but living on the inner surface of such a sphere in our present orbit, the insolation *per capita *is no more than we enjoy today.
If conversely the e.g. Koch brothers have their way and the poor are stopped from breeding so the Overlords can have the whole sphere to themselves and their robot minions, then the energy budget *per capita biotica *would be enormous.
Skunkworks experiences material leakage across their information confinement field. They describe the device as a cusp/mirror hybrid, with leaked plasma recirculation, and estimate 80~150cm of shielding will be needed for the magnets.