5 ms·

Having read about tokamaks for thirty years, I'd be curious what specific breakthroughs and innovations have occurred since the 1980s, which lead to the optimis

Having read about tokamaks for thirty years, I'd be curious what specific breakthroughs and innovations have occurred since the 1980s, which lead to the optimism described in the article (which is otherwise frustratingly devoid of detail). It's great there are seven-peer reviewed articles about SPARC, but plasma was not my specialty in physics -- would any specialists care to comment on whether there is anything particularly exciting here?

jjoonathan 6y ago
Superconductors recently got much better in a way that has cubic effect on reactor size.
- cultus 6y ago
  I know essentially nothing about superconductors, but if they all have essentially zero resistance, what makes one better for this application? I thought the main advantage of high-temp superconductors was using liquid nitrogen instead of helium.
  - jjoonathan 6y ago
    They still saturate. High-temperature is nice, but a separate consideration.
  - willis936 6y ago
    HTS coils would still be run with liquid helium for higher critical current and higher magnetic field.
  - smaddox 6y ago
    High temperature superconductors provide more stable resistance-free conduction than traditional superconductors. If both are cooled to liquid helium temperatures, that increased stability translates to being able to sustain a higher magnetic field. Higher magnetic fields provide tighter plasma confinement and higher density, which allows for much smaller fusion reactors.
  - mensetmanusman 6y ago
    Think of it as the amount of energy you can store in a superconductor. This is talked about in terms of the ‘phase diagram for superconductors’ http://www.supraconductivite.fr/en/index.php?p=supra-levitation-phase-more http://www.supraconductivite.fr/en/index.php?p=supra-levitat... As you put more current in the supercondutor, eventually you break it and it leaves the mode, as it gets hotter, likewise, as the magnetic field gets stronger, likewise. You want the largest ‘area under the curve(s)’ to handle more and more ‘superconductivity’ New materials are enabling this which has the cubic effect mentioned for reducing confinement volume and making things more affordable and increasing energy densities that can be contained, etc.
  - sbierwagen 6y ago
    All superconductors superconduct, but there's wide variation in how strong of a magnetic field they can endure before leaving the superconducting state: https://www.researchgate.net/figure/Upper-critical-fields-and-coherence-lengths-for-sev-eral-type-II-superconductors-The_tbl1_273297345 https://www.researchgate.net/figure/Upper-critical-fields-an... ITER (and the LHC) used NbSn magnets, since it was designed in the 90s. Newer designs will use newer superconductors.
    - saalweachter 6y ago
      I want to emphasize this beyond upvoting. Superconductors are like magic, but they aren't actually magic. They have limits.
aaronblohowiak 6y ago
so you get the plasma spinning and you have two problems: containment and instability. better magnets and faster control systems are the two big things that (in my layman's-bad-at-physics understanding) have really advanced in the last decade.
- Florin_Andrei 6y ago
  The intense non-linearity of magnetic phenomena is a source of major issues here. Why can't natural laws be more simple and linear? (joking, sure, but it does feel that way sometimes)
  - willis936 6y ago
    It often times boils down to turbulence.
    - hackeraccount 6y ago
      "When I meet God, I am going to ask him two questions: why relativity? And why turbulence? I really believe he will have an answer for the first."
wcarss 6y ago
My layman's understanding is that recently available (~last 5 years) industrial scale processes to produce rebco tape[1] is the specific advance in superconductor tech that's specifically enabling higher tesla magnetic field strengths from significantly smaller and lighter and easier to manage magnets, so you can get a system that's "powerful enough" to run net posititve at a size that's small enough to be built by a few universities rather than a few dozen nations (i.e. ITER), so the complexity of the engineering project drops from "unimaginable" to "just very high". 1 - https://www.fusionenergybase.com/concept/rebco-high-temperature-superconducting-tape https://www.fusionenergybase.com/concept/rebco-high-temperat... 2 - really great talk from a few years ago about this from MIT's Plasma Science Fusion Centre: https://www.youtube.com/watch?v=L0KuAx1COEk https://www.youtube.com/watch?v=L0KuAx1COEk (really, if you like this stuff, give the talk a watch. It's great.)
- rkaplan 6y ago
  This is the correct answer. A key reason Rebco is so much better than older alternatives is that it can be cooled to superconductivity using only liquid nitrogen (77K), as opposed to liquid helium, which is much harder to work with.
  - foobarian 6y ago
    Nit: ultimately the new magnets need to be stronger for the same volume no matter how easy the cooling is to work with. That part is just a nice bonus.
- gorkish 6y ago
  Yep; the key technology that took magnetic confinement fusion from 'ITER will probably work' to 'Maybe we should just go ahead and skip ITER' is rebco tape. The problem is that all known superconducting materials known will lose their ability to superconduct when exposed to a sufficiently strong magnetic field. The large field strengths induce eddy currents in the material which disrupt the propagation of the cooper pairs in its superconducting mode. The current sufficient to self-induce this field is called the critical current. Layered rebco tape appears to shield against this effect or otherwise trap the eddy currents in a way that superconductivity is preserved even in the presence of extremely strong fields. The critical current in REBCO tape is enormously higher than in previously known materials or winding configurations. Obviously the bigger the reactor volume is, the smaller the magnetic field needed to steer and confine the plasma within it can be. So back when they were designing ITER, engineers figured out the strongest magnet they could make, then they designed the reactor to be small enough (lol) that said magnet could still sustain confinement. However now that we have stronger magnets we can make reactors smaller. This is even something of an gross understatement as the relationship is cubic. For a doubling in field strength, the reactor can be 8 times smaller. That effect is very meaningful when considering that you are essentially talking about shrinking something the size of ITER's 28m main reaction vessel to something that might fit into a garage. I'm not really any kind of expert on this, so please treat this explanation as very simplistic. I have not yet personally seen anything about this new lattice confinement modality that seems to give me anywhere near the same level of confidence that they will see viable applications compared to the magnetic confinement approach. (NIF already stood down their tries at laser inertial confinement) Maybe someone has some good insight on whether or not this is all still speculative fanfare or if researchers are finding real meat.

jjoonathan 6y ago

Superconductors recently got much better in a way that has cubic effect on reactor size.

cultus 6y ago
I know essentially nothing about superconductors, but if they all have essentially zero resistance, what makes one better for this application? I thought the main advantage of high-temp superconductors was using liquid nitrogen instead of helium.
- jjoonathan 6y ago
  They still saturate. High-temperature is nice, but a separate consideration.
- willis936 6y ago
  HTS coils would still be run with liquid helium for higher critical current and higher magnetic field.
- smaddox 6y ago
  High temperature superconductors provide more stable resistance-free conduction than traditional superconductors. If both are cooled to liquid helium temperatures, that increased stability translates to being able to sustain a higher magnetic field. Higher magnetic fields provide tighter plasma confinement and higher density, which allows for much smaller fusion reactors.
- mensetmanusman 6y ago
  Think of it as the amount of energy you can store in a superconductor. This is talked about in terms of the ‘phase diagram for superconductors’ http://www.supraconductivite.fr/en/index.php?p=supra-levitation-phase-more http://www.supraconductivite.fr/en/index.php?p=supra-levitat... As you put more current in the supercondutor, eventually you break it and it leaves the mode, as it gets hotter, likewise, as the magnetic field gets stronger, likewise. You want the largest ‘area under the curve(s)’ to handle more and more ‘superconductivity’ New materials are enabling this which has the cubic effect mentioned for reducing confinement volume and making things more affordable and increasing energy densities that can be contained, etc.
- sbierwagen 6y ago
  All superconductors superconduct, but there's wide variation in how strong of a magnetic field they can endure before leaving the superconducting state: https://www.researchgate.net/figure/Upper-critical-fields-and-coherence-lengths-for-sev-eral-type-II-superconductors-The_tbl1_273297345 https://www.researchgate.net/figure/Upper-critical-fields-an... ITER (and the LHC) used NbSn magnets, since it was designed in the 90s. Newer designs will use newer superconductors.
  - saalweachter 6y ago
    I want to emphasize this beyond upvoting. Superconductors are like magic, but they aren't actually magic. They have limits.

aaronblohowiak 6y ago

so you get the plasma spinning and you have two problems: containment and instability. better magnets and faster control systems are the two big things that (in my layman's-bad-at-physics understanding) have really advanced in the last decade.

Florin_Andrei 6y ago
The intense non-linearity of magnetic phenomena is a source of major issues here. Why can't natural laws be more simple and linear? (joking, sure, but it does feel that way sometimes)
- willis936 6y ago
  It often times boils down to turbulence.
  - hackeraccount 6y ago
    "When I meet God, I am going to ask him two questions: why relativity? And why turbulence? I really believe he will have an answer for the first."

wcarss 6y ago

My layman's understanding is that recently available (~last 5 years) industrial scale processes to produce rebco tape[1] is the specific advance in superconductor tech that's specifically enabling higher tesla magnetic field strengths from significantly smaller and lighter and easier to manage magnets, so you can get a system that's "powerful enough" to run net posititve at a size that's small enough to be built by a few universities rather than a few dozen nations (i.e. ITER), so the complexity of the engineering project drops from "unimaginable" to "just very high". 1 - https://www.fusionenergybase.com/concept/rebco-high-temperature-superconducting-tape https://www.fusionenergybase.com/concept/rebco-high-temperat... 2 - really great talk from a few years ago about this from MIT's Plasma Science Fusion Centre: https://www.youtube.com/watch?v=L0KuAx1COEk https://www.youtube.com/watch?v=L0KuAx1COEk (really, if you like this stuff, give the talk a watch. It's great.)

rkaplan 6y ago
This is the correct answer. A key reason Rebco is so much better than older alternatives is that it can be cooled to superconductivity using only liquid nitrogen (77K), as opposed to liquid helium, which is much harder to work with.
- foobarian 6y ago
  Nit: ultimately the new magnets need to be stronger for the same volume no matter how easy the cooling is to work with. That part is just a nice bonus.
gorkish 6y ago
Yep; the key technology that took magnetic confinement fusion from 'ITER will probably work' to 'Maybe we should just go ahead and skip ITER' is rebco tape. The problem is that all known superconducting materials known will lose their ability to superconduct when exposed to a sufficiently strong magnetic field. The large field strengths induce eddy currents in the material which disrupt the propagation of the cooper pairs in its superconducting mode. The current sufficient to self-induce this field is called the critical current. Layered rebco tape appears to shield against this effect or otherwise trap the eddy currents in a way that superconductivity is preserved even in the presence of extremely strong fields. The critical current in REBCO tape is enormously higher than in previously known materials or winding configurations. Obviously the bigger the reactor volume is, the smaller the magnetic field needed to steer and confine the plasma within it can be. So back when they were designing ITER, engineers figured out the strongest magnet they could make, then they designed the reactor to be small enough (lol) that said magnet could still sustain confinement. However now that we have stronger magnets we can make reactors smaller. This is even something of an gross understatement as the relationship is cubic. For a doubling in field strength, the reactor can be 8 times smaller. That effect is very meaningful when considering that you are essentially talking about shrinking something the size of ITER's 28m main reaction vessel to something that might fit into a garage. I'm not really any kind of expert on this, so please treat this explanation as very simplistic. I have not yet personally seen anything about this new lattice confinement modality that seems to give me anywhere near the same level of confidence that they will see viable applications compared to the magnetic confinement approach. (NIF already stood down their tries at laser inertial confinement) Maybe someone has some good insight on whether or not this is all still speculative fanfare or if researchers are finding real meat.