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Thanks for this insightful comment. I know very little about this field - but thankfully I know just enough to understand everything you said above. > there i

Thanks for this insightful comment. I know very little about this field - but thankfully I know just enough to understand everything you said above. > there is no clear path towards developing some kind of fusion reactor that could use this better fusion reaction If you have the time to spare, could you unpack this a little bit? I'm sure many of us would appreciate the added insight. Thanks!

adrian_b 5y ago
I have already mentioned the problem in another reply above. People try mostly the fusion between hydrogen isotopes, or at most between hydrogen and lithium, because these nuclei have a low electric charge, 1 for hydrogen and 3 for lithium, so that the repulsion forces between them are relatively small. When nuclei with greater electric charge are involved, e.g. boron, the repulsion forces increase. To overcome them, the nuclei must have a higher relative velocity before collision. There are 2 ways to obtain high-speed nuclei, either by heating some plasma until the nuclei have a random thermal motion with high-enough typical velocities, or instead of heating everything and hoping for random collisions, you accelerate somehow some nuclei towards others, so that their movement is directed, not random. It is already very difficult to contain plasma hot enough for deuterium fusion. There are very small chances for being able to contain and control the much hotter plasma that would be needed for fusion reactions with heavier nuclei. So attempting to do hydrogen-boron fusion with the methods tried for deuterium-tritium seems doomed to fail. The workaround is to abandon the thermal way and to search for a way to cause the fusion of accelerated nuclei, using some combination of electric fields, magnetic fields and lasers. To produce more energy than consumed, it is required to have a very high probability that the accelerated nuclei will collide with their target (which might be fixed or also accelerated). For this either the ion beams would have to be very dense, or their positions would have to be controlled with nanometer precision or a single primary collision should cause multiple secondary collisions, or some other means would have to be discovered to ensure that the accelerated ions collide with the target instead of missing it. All of the methods that have been imagined yet have a collision probability lower than needed by a few orders of magnitude. So several real breakthroughs would be needed to be able to produce energy in this way. The only hope for this is based on the fact that are no known reasons that would make this target impossible, while for the deuterium based fusion reactions there are a few serious disadvantages that are impossible to overcome, mainly caused by the facts that the energy is produced as neutrons and that deuterium and tritium have low availability.
- pfdietz 5y ago
  Helion's DD (+ D3He) approach produces much fewer (and lower energy) neutrons than DT reactors, to the extent that the first wall could become a lifetime component of the reactor. If you want extremely low neutron production, Princeton Satellite Systems has a scheme using a field reversed configuration (like Helion) but driven by an "odd parity" rotating magnetic field. This scheme causes 3He ions to have high energy, but the D ions to not, so DD fusion is suppressed. Neutron output is claimed to be just 0.1% of the fusion output, similar to p-11B. But you'd need a source of 3He. I have a question about recovery of energy from scattered 3He ions that didn't fuse in this scheme.
  - adrian_b 5y ago
    Yes, as you say, the main problem with this is that 3-helium has a much lower availability than even tritium, which has to be produced from deuterium, consuming a part of the energy output of a deuterium-tritium fusion reactor. It seems that there is some 3-helium stored in the core of the Earth, but only a small part of it trickles continuously to the surface. When it reaches the atmosphere without being captured, it is lost quickly to the outer space. Because of that, 3-helium is very expensive, but more importantly, the quantity that could be extracted each year is limited. Even if the entire flow of 3-helium from the core could be captured, the energy that could be produced would be much less than what can be produced from uranium, not even counting the much larger reserves of thorium. There have been some speculations that there might be larger reserves of 3-helium on the Moon, so that the only possible use of deuteron-helion fusion reactors in a not too distant future might be in some Moon stations.
    - pfdietz 5y ago
      3He is very sparse in the lunar regolith, maybe 10 ppb. So that's not a long term source, and extracting it would be very difficult. I have hoped there's a "Planet X" in the outer solar system that's sufficiently small to easily land on and return to space, but large enough that it could retain some primordial helium in its atmosphere. If so, it might be the best place in the solar system to obtain the isotope, assuming a D3He fusion rocket could get out there and back quickly enough.

adrian_b 5y ago

I have already mentioned the problem in another reply above. People try mostly the fusion between hydrogen isotopes, or at most between hydrogen and lithium, because these nuclei have a low electric charge, 1 for hydrogen and 3 for lithium, so that the repulsion forces between them are relatively small. When nuclei with greater electric charge are involved, e.g. boron, the repulsion forces increase. To overcome them, the nuclei must have a higher relative velocity before collision. There are 2 ways to obtain high-speed nuclei, either by heating some plasma until the nuclei have a random thermal motion with high-enough typical velocities, or instead of heating everything and hoping for random collisions, you accelerate somehow some nuclei towards others, so that their movement is directed, not random. It is already very difficult to contain plasma hot enough for deuterium fusion. There are very small chances for being able to contain and control the much hotter plasma that would be needed for fusion reactions with heavier nuclei. So attempting to do hydrogen-boron fusion with the methods tried for deuterium-tritium seems doomed to fail. The workaround is to abandon the thermal way and to search for a way to cause the fusion of accelerated nuclei, using some combination of electric fields, magnetic fields and lasers. To produce more energy than consumed, it is required to have a very high probability that the accelerated nuclei will collide with their target (which might be fixed or also accelerated). For this either the ion beams would have to be very dense, or their positions would have to be controlled with nanometer precision or a single primary collision should cause multiple secondary collisions, or some other means would have to be discovered to ensure that the accelerated ions collide with the target instead of missing it. All of the methods that have been imagined yet have a collision probability lower than needed by a few orders of magnitude. So several real breakthroughs would be needed to be able to produce energy in this way. The only hope for this is based on the fact that are no known reasons that would make this target impossible, while for the deuterium based fusion reactions there are a few serious disadvantages that are impossible to overcome, mainly caused by the facts that the energy is produced as neutrons and that deuterium and tritium have low availability.

pfdietz 5y ago
Helion's DD (+ D3He) approach produces much fewer (and lower energy) neutrons than DT reactors, to the extent that the first wall could become a lifetime component of the reactor. If you want extremely low neutron production, Princeton Satellite Systems has a scheme using a field reversed configuration (like Helion) but driven by an "odd parity" rotating magnetic field. This scheme causes 3He ions to have high energy, but the D ions to not, so DD fusion is suppressed. Neutron output is claimed to be just 0.1% of the fusion output, similar to p-11B. But you'd need a source of 3He. I have a question about recovery of energy from scattered 3He ions that didn't fuse in this scheme.
- adrian_b 5y ago
  Yes, as you say, the main problem with this is that 3-helium has a much lower availability than even tritium, which has to be produced from deuterium, consuming a part of the energy output of a deuterium-tritium fusion reactor. It seems that there is some 3-helium stored in the core of the Earth, but only a small part of it trickles continuously to the surface. When it reaches the atmosphere without being captured, it is lost quickly to the outer space. Because of that, 3-helium is very expensive, but more importantly, the quantity that could be extracted each year is limited. Even if the entire flow of 3-helium from the core could be captured, the energy that could be produced would be much less than what can be produced from uranium, not even counting the much larger reserves of thorium. There have been some speculations that there might be larger reserves of 3-helium on the Moon, so that the only possible use of deuteron-helion fusion reactors in a not too distant future might be in some Moon stations.
  - pfdietz 5y ago
    3He is very sparse in the lunar regolith, maybe 10 ppb. So that's not a long term source, and extracting it would be very difficult. I have hoped there's a "Planet X" in the outer solar system that's sufficiently small to easily land on and return to space, but large enough that it could retain some primordial helium in its atmosphere. If so, it might be the best place in the solar system to obtain the isotope, assuming a D3He fusion rocket could get out there and back quickly enough.