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Comment by barbegal | Hacker News Reader

I have yet to read an article or paper which has convinced me that energy generating fusion is possible at low costs and with materials that we currently know of. I don't see why anyone (including governments) should invest in these types of projects. Not only is the science extremely complex, the implementation is a huge engineering challenge. We already have a huge fusion power plant at the heart of our solar system and we can harness its power with some pretty cheap solar panels.

If you want to read a better article explaining the science then try http://fusionandthings.eu/2019/06/05/new-calculations-show-p...

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There are a few reasons why the investment is flowing.

1) New enabling technologies, including high temperature superconducting tape, algorithms for plasma control and diagnostics which take advantage of new hardware (GPUs), and advanced manufacturing techniques are now available.

2) Optimism that private companies can synthesize the past 70 years of plasma physics research with these enabling technologies to develop transformative approaches to fusion.

If you're interested I wrote a short article about this topic a few months ago,

https://www.fusionenergybase.com/article/the-number-of-fusio...

I remember reading recently that a Nobel Prize was won a few years ago for work with chirped pulse laser amplification using titanium sapphire lasers that can apparently achieve nano or microsecond energy pulses in the terawatt range. A potential contender for a non-fission fusion spark, but still does not solve the containment problem. The article says the laser could generate a magnetic field somehow?

https://newatlas.com/energy/hb11-hydrogen-boron-fusion-clean...

Here's the last of a great series of articles on that idea (with links to the earlier articles): https://asiatimes.com/2020/05/meet-the-father-of-the-hydroge...

And a startup hoping to try it, run by the guy who came up with the idea decades ago: https://www.hb11.energy/

There are several groups doing experiments with it, and it seems to be going really well.

There are two lasers. One hits a target that generates a magnetic field; it'd be hard to describe without a picture but see the articles at the first link. Basically the laser blasts electrons off a metal surface, they hit another surface and flow through a coil. For a nanosecond there's a 4000 tesla field. (An MRI machine generates around 3 tesla.)

The second laser is faster and more powerful: 10 petawatts or more, for only a picosecond. That hits the fuel. It's enough to kick off fusion by itself, but the magnetic containment creates an avalanche effect that multiplies output. Then it all blows up, you harvest the energy and cycle in another target.

Thanks for the link. It is a much more thorough explanation.

How fast is the fuel used up within the field? Would there be a way to inject the actively fusing reaction with a steady fuel input rate for long term generation (neutron bombardment embrittles superconducting metal containment with the D/T reaction, unlike boron encased in supposed laser induced magnetic field?)

I'd imagine this would occur in a sphere (closed and contained). Tokamak designs aren't spheres, but also closed relying on magnetism to push back against a reaction that is pushing out as fusion occurs:

  To produce thrust - what if it was a half sphere somehow? Propellant implies ejection of something, and a fusion reaction ball is magnetically interactive, with no radioactive material byproduct? What if a fusion thruster harvested some energy from the reaction to  "push" back against an actively fusing pellet feed rate? Could this propel a craft or am I missing something fundamental here?

The magnetic field disappears in a nanosecond, the fuel pellet gets used up, and it explodes, destroying the coil that generated the field. So you just send in another target assembly and fire the laser again, every second or two.

There's nothing wrong with a pulsed system like that. Lots of fusion designs are pulsed. A gasoline generator with an internal combustion engine is a pulsed system too.

Add a magnetic nozzle and you could definitely turn this into a rocket. Thrust would be low but efficiency very high, so it'd be useless for launch but great for long-distance travel.

There is a very general problem with pulsed systems for fusion. The issue is that plasma-facing surfaces are confronted with extreme instantaneous power levels. The depth to which heat can diffuse is proportional to (pulse length)^(-1/2). A nanosecond pulse will deposit heat in a tenth of a micron thickness, or less.

This forces any fusion reactor that uses pulses to have a sacrificial ablative layer on these surfaces that must be renewed (and to deal with the forces from the explosive vaporization of this thin layer). This is problematic if the reactor also requires high vacuum. The scheme for p-11B fusion that this subthread was talking about, for example, has been presented with a direct conversion scheme that uses a megavolt level vacuum capacity. Imagine what happens to such a capacitor when its surfaces flash superheated vapor.

Interesting. But if the direct conversion works, then the magnetic field removes most of their kinetic energy from the charged particles before they get to the walls. All it gets is the x-rays. There has to be some radius where that's no longer a problem. Each pulse in this design would be about 300 kWh; offhand I don't know what percentage is x-rays.

If it's too hard to maintain vacuum, then reverting to a plain ol' thermal cooling could be a backup plan.

A magnetic field leaves the kinetic energy of a charged particle unchanged.

The electric field is supposed to reduce the energy of the alpha particles, but (1) the alphas from p-11B are not monoenergetic, and (2) what is keeping the electrons (that are inevitably liberated in the extremely energetic explosion of the target, the impact of the alphas with the collecting electrode, and photoelectric emission from all surfaces exposed to photons from the plasma) from shorting the whole thing out?

The scheme does not make any sense.

"capacitor"

Formatting error, last paragraph:

To produce thrust - what if it was a half sphere somehow? Propellant implies ejection of something, and a fusion reaction ball is magnetically interactive, with no radioactive material byproduct? What if a fusion thruster harvested some energy from the reaction to "push" back against an actively fusing pellet feed rate? Could this propel a craft or am I missing something fundamental here?

> algorithms for plasma control and diagnostics which take advantage of new hardware (GPUs)

I don't know about that. Here's a few bullet points from [1] (which someone else linked to in this thread; it describes the approach taken by these guys at TAE) that don't inspire a whole lot of confidence:

"Our prior is “reasonable”, but is it really the marginal distribution over all possible plasmas? hahahahhahahaha. We model many effects, but plasmas are complex beasts and we do not model all. We only have one measurement, of much smaller dimension than our unknowns. We never sample from the tails. takes too long to get samples. by definition you can’t really validate them. Will we ever know we’re right about anything? we have zero golden data"

[1] http://hyperion.usc.edu/UQ-SummerSchool/pres/Langmore.pdf

Sadly, that huge fusion plant is offline half the time for any given location on the Earth, and is severely degraded by cloudy weather. Also using it to power the world will require large areas of land to be dedicated to collecting its output.

So what?

I give you Space Grid ©.

[1] https://en.wikipedia.org/wiki/Space-based_solar_power

edit: I don't know why that symbol doesn't show as copyleft as intended.

That seems like it could have more difficult engineering problems than a fusion reactor! Seems like a quick way to accidentally microwave a desert into a sea of glass. Hope you don't put anything remotely near the power receiver on earth.

Power density on the ground would be way lower than you're thinking.

The leading proposals, like SPS-Alpha, put a gigawatt-scale power station in geostationary orbit. Power is beamed to ground via microwave. The receiver station has to be kilometers wide, which puts the power density low enough so birds wouldn't be harmed; even that amount of focus is only doable by sending a pilot signal from the receiver, so the beam can't trivially be redirected to other targets. Despite its size, the receiver would be inexpensive, since it's mostly wire antennae.

Hey, think positive! If microwaves prove too dangerous, and they finally do manage to build a cable from the orbital power station down to earth, they have a space elevator as well!

Space advocates seem to have been going off the idea of that over the past few years. I'm not sure how much of that is due to a better appreciation of the downsides, and how much is due to the very rapid improvements we've been seeing in ground-based solar.

A couple of recent (late 2019) articles from Casey Handmer:

https://caseyhandmer.wordpress.com/2019/08/20/space-based-so...

https://caseyhandmer.wordpress.com/2019/09/20/no-really-spac...

It's worth reading the comments too; there are some interesting points there, although I think the game-changing ones rely on new tech and/or well-developed lunar industry.

Yes. I'm aware of that. But while economically usable fusion remains SciFi so far, space based solar just lacks some bootstrapping. Which seems to be more attainable as time passes.

If "we" really wanted this, we could do it now because the technology is there. There is no what if like in any path to fusion.

edit: especially considering all the hype about colonizing Mars, or a Moonbase.

Space solar is a way to get the most out of expensive PV cells. But PV is no longer expensive. It's cheaper to just overbuild on Earth and transmit energy through time (storage) rather than through space (microwaves).

High-altitude solar platforms:

http://www.stratosolar.com/blog

There are other options for using the sun's energy. We can grow trees to burn, we can optimise windows and window coverings to use solar energy to heat and cool buildings, we can put solar panels in the dessert, or in the ocean, or above roads or in space (but that's not very cost effective).

My go to book on energy is this one http://www.withouthotair.com/ It gives good numbers for all the energy that we could generate from renewables and all the energy we currently use.

There is a lot of land in the world. And definitely have the capacity to build the supply chains. It's just that it's slow and expensive while oil, gas and nuclear is fast and cheap.

We need policy changes. Or in a very ideal world, fusion.

New solar is much cheaper than new nuclear, per kWh.

I did not know that. That's great!

It's startling how quickly this happened. Even ten years ago it wasn't true. Many people have not updated their priors yet and still think nuclear is a realistic option. At this point, nuclear is a long shot, which is why I refer to SMRs as HMRs, "Hail Mary Reactors".

> I don't see why anyone (including governments) should invest in these types of projects.

- the basic research has a benefit, and may lead to other discoveries

- if they happen to achieve fusion, the benefit to humanity will be enormous

There must be "moonshot" projects, like when we originally went to the moon, an unpractical journey.

Have you seen General Fusion?

They spin a large ball of molten lead using pumps and shoot hot plasma into the vortex that forms in the middle, then strike the walls of the lead chamber with carefully timed steam pistons to make an implosion pressure wave not entirely unlike Fat Man’s design, except reusable and for fusion.

The fusion heat and radiation gets absorbed by the lead and they run a heat exchanger on the pump loop for power generation. They also think they can breed fuel with some lithium in the lead.

I love their plan aesthetically but I’m not qualified to judge how feasible it is. It’s very satisfyingly physical though.

Do you know if they've achieved fusion yet? I remember reading about them in PopSci way back in the day, and wondered how they were doing.

Do you end up with a huge ball of radioactive lead at the end of the day? From what little I know of fusion, it seems you still end up with irradiated lead or other shielding.

(And yes, I know that coal releases radiation, and that the installation of solar panels and wind turbines is, currently, a dangerous job, etc.)

Huh, I suppose you do! I had a naive short-circuit in my head that "neutron flux is bad because it destroys your your materials. If your material is molten lead anyway who cares?"

I also didn't know lead has such a low neutron absorption cross section.

For what it's worth lead with extra neutrons seems a lot less scary than eg uranium fission waste. Stable isotopes 206, 207, and 208 represent 98% by abundance, 208 has the lowest cross section, and 209 has a 3 hour half-life into Bismuth-209 which is nearly stable (2e19 year half-life). So it seems almost all of your neutron captures just make other stable lead isotopes or briefly-terrifying 209 that's totally safe after a couple days. You only get real scary stuff if the trace amounts of undecayed 209 manage a second capture.

Edit: I should add they want to mix lithium into the lead to absorb neutrons and regenerate fuel

What about 204? If that produces 205 you'd have some long-lived waste.

10 million year half life is long enough that it's not very dangerous anymore.

Lead has a low neutron absorption cross section, but it has a fairly hefty inelastic scattering cross section for fusion neutrons (for fission neutrons, which are less energetic, it just elastically scatters them, with very little moderation). It also causes some (n,2n) reactions with fusion neutrons, which would be nice to make up for tritium losses.

I was withholding judgment on General Fusion. They had a concept that, at least in principle, would allow engineering limits on the power density at the first wall to be evaded (since the first wall would be thick liquid metal, not a solid that could sustain damage.)

But they gave up on their original "acoustic" compression scheme, and now will compress the plasma via subsonic motion of the liquid metal. This scheme involves a solid conductor going down the middle of the chamber. It will be exposed to orders of magnitude higher radiation flux than the first wall of mainstream fusion concepts, as well as pulsed loads from magnetic fields up to 100T (which correspond to pressures far higher than the chamber of a gun, and higher than the deepest point in the oceans.) Getting this conductor to survive even one shot would require heroic engineering; keeping it cooled and together as its material properties rapidly degraded would require superheroic engineering.

Can you point me to where they say the central cones are a change in their hypothetical production design?

I was under the impression it's just there for diagnostics in the sub-scale prototype.

The abandonment of the acoustic compression scheme is explained in this poster.

https://generalfusion.com/wp-content/uploads/2018/07/ICPP_20...

The move to a spherical tokamak is explained in page 9 of these slides.

https://arpa-e.energy.gov/sites/default/files/11_LABERGE.pdf

The slower compression of the non-acoustic scheme requires better confinement than the spheromak provided.

Hmm yeah that does sound like a permanent change in the second link doesn't it? Too bad, I was really rooting for their plan.

The Turchi mentioned there was associated with Los Alamos, where the LINUS concept was investigated. General Fusion seems to be moving in that direction, so this slide from Turchi at Compact Fusion Systems could be of interest. It has a focus on practical engineering and cost issues that I find encouraging.

https://nucleus.iaea.org/sites/fusionportal/Shared%20Documen...

I heard that the shock wave was spalling off the front few cm of the liquid metal wall as a spray of jets/droplets -- the shock reflects from the vacuum interface & where the wave interfered destructively with itself, the pressure dropped to zero, and cavitation occurred. This happens in a random & uneven fashion.

That sounds very plausible. Do you have a reference I could cite?

Here it is: https://generalfusion.com/wp-content/uploads/2016/08/Suponit...

Edit: Here's a second one that's similar (but behind a paywall). http://www.sciencedirect.com/science/article/pii/S0045793013...

The preprint is here: https://arxiv.org/abs/1310.6010v2

Edit 2: Here's a better one with video frames from the experiment showing the jets. https://generalfusion.com/wp-content/uploads/2016/08/Richard...

Cool, thanks!

The one that gave me most hope from this point of view was the Polywell design. It made some waves a few years ago then got classified and sputtered out, then this happened: https://en.wikipedia.org/wiki/Polywell#%22Final_nail%22?

I'm entirely with you as far as tokamaks go - they're relying on unobtainium for a significant component - but I think other designs are possible, just under-researched.

Are you aware of https://en.wikipedia.org/wiki/Wendelstein_7-X? It is well on its way to produce sustained plasma discharges of multiple minutes. The stellarator design has some decisive advantages over Tokamaks.

It does not avoid the generic problem of DT fusion reactors, which is lousy power density. The magnets are also famously complex and expensive.

That sounds like engineering and scaling challenges, not inherent limitations... I mean, they clearly have the magnets figured out and so far there doesn't seem to have been a lot in the way of unforeseen roadblocks.

That sounds like the dismissive attitude a plasma physicist, for whom mere engineering issues are trivial and of lesser importance. But engineering issues are perfectly capable of ruining the prospects for a technology. Fission, for example, faces nothing BUT engineering issues (where cost is considered a part of engineering.) The equivalent of breakeven and ignition was achieved for fission in 1942.

DT fusion faces fundamental engineering obstacles, obstacles that are not solvable by tweaking the confinement scheme. Scaling up makes the main problem, power density, worse, not better (see a later comment by me on this issue.)

I like how the chamber and magnet design looks like something made by Cthulhu in cooperation with Salvador Dali himself. Even just due to that it should work. :)

Polywell was 100% BS, I'm afraid. The whole concept of forming a "virtual cathode" made no sense, due to plasma conductivity shorting it out. The group at Sydney who did experiments with similar devices concluded that maintaining the virtual cathode in a reactor-scale Polywell would require a 20 gigawatt electron beam.

That's what the link I gave explains.

Thanks.

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