5 ms·

This is underappreciated. Achieving sufficient confinement pushes toward larger volume/surface area, but getting high power density pushes the other way. In a

This is underappreciated. Achieving sufficient confinement pushes toward larger volume/surface area, but getting high power density pushes the other way. In a way, the fact that 80% of the DT power output is in fast neutrons becomes beneficial -- that fraction is absorbed volumetrically, so only 20% must be conducted through the surface (or convected, if it's a liquid).

pfdietz 6y ago
The neutrons cause their own problems, though, and these problems interact with the purely thermal ones. For example: the plasma facing surfaces on ITER are backed by a CuCrZr alloy with high thermal conductivity. The amount of direct thermal power this innermost wall can withstand, from the plasma and photons, is proportional to this thermal conductivity, since stress there is proportional to the thermal gradient. But in an actual reactor, designed to operate for long periods of time, this alloy cannot be used, as it becomes too activated. Instead, DEMO is going to use a kind of reduced activation steel (Eurofer 97) in which the thermal conductivity is an order of magnitude lower. This is one of the things pushing DEMO far outside the range of potentially competitive cost.
- lambdatronics 6y ago
  Yup. There's also the need to maintain transparency of the first wall to neutrons so the tritium breeding ratio stays above 1. However, a very thin wall is easy to breach in the event of plasma eruptions, or even long-term erosion due to ion bombardment. It seems to me that a thick (~ cm at least) flowing liquid first wall is probably the way to go for achieving high power density. The liquid has its own problems though, such as corrosion compatibility with the underlying structural material, the need to have low vapor pressure at high temperature, the possibility of splashing, and MHD effects if the liquid is conductive.
  - pfdietz 6y ago
    I agree. Also, liquid surfaced walls are probably required for any pulsed fusion approach, which at a given average power/area will have much higher instantaneous power/area. This drives one toward LINUS-like approaches.
    - lambdatronics 6y ago
      Compact Fusion Systems recently got funding to push this approach. On LinkedIn they announced purchase of surplus high-energy capacitors (from the old Shiva Star at AFRL) to build a prototype. They seem to have taken to hear some lessons learned from previous experiments (cannot assume lossless compression, must pay attention to RT instabilities, must think about vaporization of first-wall). One downside to a fully liquid blanket: the peak temperature is limited by the vapor pressure of the first wall, which in turn is limited by the impurity tolerance of the plasma. In a non-imploding device, it would be possible to run the majority of the blanket at a higher temperature & separate that from the cooler first-wall liquid. Of course, this puts some solid material closer to the neutrons, so YMMV. https://arpa-e.energy.gov/sites/default/files/ALPHA_TURCHI.pdf https://arpa-e.energy.gov/sites/default/files/ALPHA_TURCHI.p... https://arpa-e.energy.gov/sites/default/files/06_TURCHI.pdf https://arpa-e.energy.gov/sites/default/files/06_TURCHI.pdf
      - pfdietz 6y ago
        I've seen their slides before, and I was impressed by their engineering-oriented approach. The engineering obstacles to fusion are so great that (IMO) one should use them to drive the design, rather than taking a physics-oriented approach and worry about the engineering later.

pfdietz 6y ago

The neutrons cause their own problems, though, and these problems interact with the purely thermal ones. For example: the plasma facing surfaces on ITER are backed by a CuCrZr alloy with high thermal conductivity. The amount of direct thermal power this innermost wall can withstand, from the plasma and photons, is proportional to this thermal conductivity, since stress there is proportional to the thermal gradient. But in an actual reactor, designed to operate for long periods of time, this alloy cannot be used, as it becomes too activated. Instead, DEMO is going to use a kind of reduced activation steel (Eurofer 97) in which the thermal conductivity is an order of magnitude lower. This is one of the things pushing DEMO far outside the range of potentially competitive cost.

lambdatronics 6y ago
Yup. There's also the need to maintain transparency of the first wall to neutrons so the tritium breeding ratio stays above 1. However, a very thin wall is easy to breach in the event of plasma eruptions, or even long-term erosion due to ion bombardment. It seems to me that a thick (~ cm at least) flowing liquid first wall is probably the way to go for achieving high power density. The liquid has its own problems though, such as corrosion compatibility with the underlying structural material, the need to have low vapor pressure at high temperature, the possibility of splashing, and MHD effects if the liquid is conductive.
- pfdietz 6y ago
  I agree. Also, liquid surfaced walls are probably required for any pulsed fusion approach, which at a given average power/area will have much higher instantaneous power/area. This drives one toward LINUS-like approaches.
  - lambdatronics 6y ago
    Compact Fusion Systems recently got funding to push this approach. On LinkedIn they announced purchase of surplus high-energy capacitors (from the old Shiva Star at AFRL) to build a prototype. They seem to have taken to hear some lessons learned from previous experiments (cannot assume lossless compression, must pay attention to RT instabilities, must think about vaporization of first-wall). One downside to a fully liquid blanket: the peak temperature is limited by the vapor pressure of the first wall, which in turn is limited by the impurity tolerance of the plasma. In a non-imploding device, it would be possible to run the majority of the blanket at a higher temperature & separate that from the cooler first-wall liquid. Of course, this puts some solid material closer to the neutrons, so YMMV. https://arpa-e.energy.gov/sites/default/files/ALPHA_TURCHI.pdf https://arpa-e.energy.gov/sites/default/files/ALPHA_TURCHI.p... https://arpa-e.energy.gov/sites/default/files/06_TURCHI.pdf https://arpa-e.energy.gov/sites/default/files/06_TURCHI.pdf
    - pfdietz 6y ago
      I've seen their slides before, and I was impressed by their engineering-oriented approach. The engineering obstacles to fusion are so great that (IMO) one should use them to drive the design, rather than taking a physics-oriented approach and worry about the engineering later.