3 ms·

>”Break-even" is nowhere near enough to get practical power out, because the energy extraction process must be so inefficient, with two stages of heat exchange

>”Break-even" is nowhere near enough to get practical power out, because the energy extraction process must be so inefficient, with two stages of heat exchange before you get a working fluid you can run through your turbine. Correction: break-even is most of the way to a working plant. A “burning fusion plasma” is one that self heats. The plant requires very little power when the plasma is supplying most of its own heating. Keep the confining coils cool, keep the control systems (correction coils and heaters) running, and watch as the heat comes in. Q = 1 is the balance point. The distance (in terms of lawson criterion) between Q = 0.5 and 1 is much larger than Q = 1 and infinity, Also, two stages of heat exchange are not needed and not a killer for power generation regardless. A liquid lithium wall blanket operates as tritium breeder and coolant. Run the lithium through a heat exchanger to boil water. Watch as the turbine spins. Very similar to PWRs.

joncrane 6y ago
Liquid lithium wall blanket? I've heard the use of Lithium as a tritium breeder (which is important, assuming we're committed to duterium-tritium fusion), but never as a liquid. it makes sense though because in that case you don't have to completely disassemble the tokamak and replace the neutron blanket every few years(?). But again the technology to completely coat the inside of the tokamak in an even layer of liquid lithium has got to be daunting. What goes between the lithium and the plasma?
- willis936 6y ago
  Beryllium coated tungsten and a copper-tungsten weave. This is still a hot area of research. It will be part of the research done at ITER and DTT (Divertor Tokamak Test facility).
- pfdietz 6y ago
  The Achilles Heel of fusion is low volumetric power density. This is because all the energy has to go through the first wall, which has limited area compared to the surfaces of fuel pins in a fission reactor. So, there's been effort to try to maximize the power/area at the first wall. One part of that is the power that directly strikes the surface (ions, electrons, photons). The capacity to carry away this heat is limited by the strength of the wall material and its thermal conductivity, a fact that was pointed out by Pfirsch and Schmitter in the 1980s. https://pure.mpg.de/rest/items/item_2131865_1/component/file_2131864/content https://pure.mpg.de/rest/items/item_2131865_1/component/file... (see section 4.1.1) One could raise this limit by doing away with a solid wall (with its limits on thermal conductivity and stress) and instead using a liquid wall. This would also address erosion of the wall by sputtering, and help reduce damage from plasma disruptions (which may well otherwise be a showstopper for tokamaks.)

joncrane 6y ago

Liquid lithium wall blanket? I've heard the use of Lithium as a tritium breeder (which is important, assuming we're committed to duterium-tritium fusion), but never as a liquid. it makes sense though because in that case you don't have to completely disassemble the tokamak and replace the neutron blanket every few years(?). But again the technology to completely coat the inside of the tokamak in an even layer of liquid lithium has got to be daunting. What goes between the lithium and the plasma?

willis936 6y ago
Beryllium coated tungsten and a copper-tungsten weave. This is still a hot area of research. It will be part of the research done at ITER and DTT (Divertor Tokamak Test facility).
pfdietz 6y ago
The Achilles Heel of fusion is low volumetric power density. This is because all the energy has to go through the first wall, which has limited area compared to the surfaces of fuel pins in a fission reactor. So, there's been effort to try to maximize the power/area at the first wall. One part of that is the power that directly strikes the surface (ions, electrons, photons). The capacity to carry away this heat is limited by the strength of the wall material and its thermal conductivity, a fact that was pointed out by Pfirsch and Schmitter in the 1980s. https://pure.mpg.de/rest/items/item_2131865_1/component/file_2131864/content https://pure.mpg.de/rest/items/item_2131865_1/component/file... (see section 4.1.1) One could raise this limit by doing away with a solid wall (with its limits on thermal conductivity and stress) and instead using a liquid wall. This would also address erosion of the wall by sputtering, and help reduce damage from plasma disruptions (which may well otherwise be a showstopper for tokamaks.)