3 ms·

Tokamaks, while cool (or hot!), are ultimately limited by materials that we know about. There are IAEA reports for fusion calling for materials that survive >1

Tokamaks, while cool (or hot!), are ultimately limited by materials that we know about. There are IAEA reports for fusion calling for materials that survive >100 displacements-per-atom (DPA) and > 1000deg C[1]. Additionally you might also need highly controllable super-conducting magnets as well as advanced sensing of the instabilities of the fusion plasma to ensure they are well confined (a chief requirement for fusion). There are other problems, but these are typical issues with tokamak-based fusion devices. Chiefly most tokamaks around the world do not operate in "continuous mode", ie they operate on "shot mode" or short duration experiments to attempt to ignite the plasma to fusion conditions. Achieving high-duty cycle operation with a ignited plasma is its own hurdle. Making hot plasmas is interesting, but if your technology requires the above, you are looking at around 4-5 once-per-decade material science results. Every national lab in the US is saying we are out of time, we need existing energy solutions to avoid the worst of climate change. Source: I am a nuclear engineer. [1] https://www.sciencedirect.com/science/article/pii/S1369702109702949#fig4 https://www.sciencedirect.com/science/article/pii/S136970210...

deleted 5y ago
[deleted]
DennisP 5y ago
CFS has demonstrated magnets that seem to be up to the job, using REBCO tape. They'll replace the inner wall annually, so it doesn't need to be as durable. MIT already demonstrated practical joints in the tape. We certainly need to decarbonize as fast as we can without waiting for fusion, but if we're lucky then fusion might make things easier down the road.
- omgJustTest 5y ago
  These are certainly superb scientific results. The problem is that even 1 DPA is significant for structural materials, and is basically a nightmare for electrically or mechanically sensitive ones. For example light-water fission reactors achieve about 50 DPA to their reactor pressure vessels over their 40-50 year lifetime. Its hard to imagine that anything called a super-conductor could maintain Cooper-pairs after getting the average atom displaced from its original position. Additionally CFS ran at 20 Kelvin... which again is hard to imagine happening at scales other than CERN-like efforts (ie one-of-a-kind installs): "In September 2021 Commonwealth Fusion Systems (CFS) created a test magnet with ReBCO wires in which flowed a current of 40,000 amperes, with a magnetic field of 20 tesla at 20 K." [1] Edit: Also magnets have 1/R^3 field die-away, in direct competition with 1/R^2 particle dispersion die-away.... meaning your magnets control better as you make cavities smaller, however it also means the magnets get much more DPA :(. [1] https://en.wikipedia.org/wiki/Rare-earth_barium_copper_oxide https://en.wikipedia.org/wiki/Rare-earth_barium_copper_oxide
  - pfdietz 5y ago
    Magnets are highly shielded from neutrons in a fusion reactor. The real problem is the first wall, which necessarily is not shielded. Materials are an ongoing problem, and it's not just dpa. Abdou at UCLA pretty convincingly argues that increasing dpa of wall materials isn't even the most pressing problem; reliability of the wall/blanket is. http://www.fusion.ucla.edu/abdou/abdou%20presentations/2018/Abdou%20CFETR%20Presentation%2011-27-2018%20Final%20V50.pdf http://www.fusion.ucla.edu/abdou/abdou%20presentations/2018/... I should add that fusion neutrons add a problem beyond that experienced in fission reactors: the neutrons are sufficiently energetic that production of helium by (n,alpha) reactions in materials is a serious issue. The helium migrates to small very high pressure bubbles that break materials from the inside.
    - omgJustTest 5y ago
      Temperature & DPA have to be talked about together. I think Abdou's point is that you can replace the blanket more often and target ~20 DPA to reduce the mean-time-between-failures (MTBF). From the PDF: "Results show: anticipated MTBF is hours/days (required is years), and MTTR is 3-4 months (required is days), and availability is very low < 5%" I don't have a problem with talking about MTBF and replacing blankets etc but we are talking about research, not a reactor. Also of note look at the MTBF of magnets and cryogenics in this system! (1.14years and 0.57 years)
      - pfdietz 5y ago
        Abdou's point is that the RAMI problem has to be solved to continue to do research, and solving it has a higher priority than getting higher dpa tolerant materials.
        DennisP 5y ago
        In ARC the blanket is liquid and the wall is replaceable. Seems like that's a practical solution to these issues. What does seem like a real problem for them is the one you've brought up before: the limited supply of beryllium.
pstuart 5y ago
I attended a lecture by a bitter ex-LLL scientist who said they had made great progress with the magnet mirror approach but it was scuttled right before it starting it up. This was so they could do the laser project(s) which was really about learning how to build weapons. Any thoughts on the real focus of the lab?
- omgJustTest 5y ago
  It is an open-secret that NIF is not as much a simulator for fusion as for devices. They have admitted as much on their public site[1]. [1] https://wci.llnl.gov/facilities/nif https://wci.llnl.gov/facilities/nif Edit: However it is good to note that in comparison with other fusion methods, laser based methods relax materials constraints I listed above simply by spreading the energy/particle fluxes over larger surface areas. If you built a giant fusion reactor based on NIF you could reduce DPA and temperature requirements, but it would fundamentally be a "shot" based reactor.
- willis936 5y ago
  Yep. MFTF. Really frustrating stuff. There is a statue of it in front of UW-Madison's Engineering Hall. It's a water display that, funnily, has not worked in many years.
whizzter 5y ago
What's your view on the Deepmind experiments to use NNs to stabilize the plasma fields, won't better control of the plasma ameliorate some of the material issues? (I think a link to that was posted here some weeks ago)
- omgJustTest 5y ago
  The Deepmind control of plasmas is interesting, and I think that some of the more recent successes at NIF have been related to more run-of-the-mill deep-learning based control. The problems above are not just a plasma instability problem, which has impeded progress. The interior walls of the fusion reactors are just pretty extreme environments, and the extremity is made worse by the fact that if you control plasma flow with magnetic fields, you trade the ability to control magnetic fields with dose -to-the-magnets-. Achieving high magnetic fields also require high currents -> super conductors -> low temperatures (0.1-20 Kelvin).

deleted 5y ago

[deleted]

DennisP 5y ago

CFS has demonstrated magnets that seem to be up to the job, using REBCO tape. They'll replace the inner wall annually, so it doesn't need to be as durable. MIT already demonstrated practical joints in the tape. We certainly need to decarbonize as fast as we can without waiting for fusion, but if we're lucky then fusion might make things easier down the road.

omgJustTest 5y ago
These are certainly superb scientific results. The problem is that even 1 DPA is significant for structural materials, and is basically a nightmare for electrically or mechanically sensitive ones. For example light-water fission reactors achieve about 50 DPA to their reactor pressure vessels over their 40-50 year lifetime. Its hard to imagine that anything called a super-conductor could maintain Cooper-pairs after getting the average atom displaced from its original position. Additionally CFS ran at 20 Kelvin... which again is hard to imagine happening at scales other than CERN-like efforts (ie one-of-a-kind installs): "In September 2021 Commonwealth Fusion Systems (CFS) created a test magnet with ReBCO wires in which flowed a current of 40,000 amperes, with a magnetic field of 20 tesla at 20 K." [1] Edit: Also magnets have 1/R^3 field die-away, in direct competition with 1/R^2 particle dispersion die-away.... meaning your magnets control better as you make cavities smaller, however it also means the magnets get much more DPA :(. [1] https://en.wikipedia.org/wiki/Rare-earth_barium_copper_oxide https://en.wikipedia.org/wiki/Rare-earth_barium_copper_oxide
- pfdietz 5y ago
  Magnets are highly shielded from neutrons in a fusion reactor. The real problem is the first wall, which necessarily is not shielded. Materials are an ongoing problem, and it's not just dpa. Abdou at UCLA pretty convincingly argues that increasing dpa of wall materials isn't even the most pressing problem; reliability of the wall/blanket is. http://www.fusion.ucla.edu/abdou/abdou%20presentations/2018/Abdou%20CFETR%20Presentation%2011-27-2018%20Final%20V50.pdf http://www.fusion.ucla.edu/abdou/abdou%20presentations/2018/... I should add that fusion neutrons add a problem beyond that experienced in fission reactors: the neutrons are sufficiently energetic that production of helium by (n,alpha) reactions in materials is a serious issue. The helium migrates to small very high pressure bubbles that break materials from the inside.
  - omgJustTest 5y ago
    Temperature & DPA have to be talked about together. I think Abdou's point is that you can replace the blanket more often and target ~20 DPA to reduce the mean-time-between-failures (MTBF). From the PDF: "Results show: anticipated MTBF is hours/days (required is years), and MTTR is 3-4 months (required is days), and availability is very low < 5%" I don't have a problem with talking about MTBF and replacing blankets etc but we are talking about research, not a reactor. Also of note look at the MTBF of magnets and cryogenics in this system! (1.14years and 0.57 years)
    - pfdietz 5y ago
      Abdou's point is that the RAMI problem has to be solved to continue to do research, and solving it has a higher priority than getting higher dpa tolerant materials.
      - DennisP 5y ago
        In ARC the blanket is liquid and the wall is replaceable. Seems like that's a practical solution to these issues. What does seem like a real problem for them is the one you've brought up before: the limited supply of beryllium.

pstuart 5y ago

I attended a lecture by a bitter ex-LLL scientist who said they had made great progress with the magnet mirror approach but it was scuttled right before it starting it up. This was so they could do the laser project(s) which was really about learning how to build weapons. Any thoughts on the real focus of the lab?

omgJustTest 5y ago
It is an open-secret that NIF is not as much a simulator for fusion as for devices. They have admitted as much on their public site[1]. [1] https://wci.llnl.gov/facilities/nif https://wci.llnl.gov/facilities/nif Edit: However it is good to note that in comparison with other fusion methods, laser based methods relax materials constraints I listed above simply by spreading the energy/particle fluxes over larger surface areas. If you built a giant fusion reactor based on NIF you could reduce DPA and temperature requirements, but it would fundamentally be a "shot" based reactor.
willis936 5y ago
Yep. MFTF. Really frustrating stuff. There is a statue of it in front of UW-Madison's Engineering Hall. It's a water display that, funnily, has not worked in many years.

whizzter 5y ago

What's your view on the Deepmind experiments to use NNs to stabilize the plasma fields, won't better control of the plasma ameliorate some of the material issues? (I think a link to that was posted here some weeks ago)

omgJustTest 5y ago
The Deepmind control of plasmas is interesting, and I think that some of the more recent successes at NIF have been related to more run-of-the-mill deep-learning based control. The problems above are not just a plasma instability problem, which has impeded progress. The interior walls of the fusion reactors are just pretty extreme environments, and the extremity is made worse by the fact that if you control plasma flow with magnetic fields, you trade the ability to control magnetic fields with dose -to-the-magnets-. Achieving high magnetic fields also require high currents -> super conductors -> low temperatures (0.1-20 Kelvin).