4 ms·

You don't need a wakefield accelerator to get protons up to the energy needed for p-B11 reactions, since the peak reaction cross section is only around 600,000

You don't need a wakefield accelerator to get protons up to the energy needed for p-B11 reactions, since the peak reaction cross section is only around 600,000 eV (as opposed to the 5,000,000,000 eV in the article). You can already do this with an accelerator which would fit in a room. More important parameters for an accelerator of that purpose are efficiency (wakefield accelerators have incredibly poor efficiency) and brightness (how close you can get the particles together, both in space and in velocity). The real challenge for beam-based fusion isn't getting an accelerator for your beam, it's making the reaction happen where there aren't a bunch of cold electrons present to absorb your beam's energy before the fusion happens. This usually turns the situation back into a plasma containment problem.

ars 4y ago
A proton is around 2000 times as heavy as an electron, so your energy numbers are actually semi equivalent. (At least that's how it seems to me - I don't know if this would even work on protons.) I hear your point about efficiency though. Why do you need similar velocity for the particles? It seems to me you just need a LOT of particles, but beam quality doesn't matter much. The cold electrons you mention are the ones bound to the Boron? Can we give the Boron a positive charge? All the charge will live on the surface, which is exactly where we want it. Make the box metal to the height of the boron, then an insulator after that, and charge the box. Put the output of the accelerator directly above the boron (the output is facing downwards), and keep pumping out helium. Cool the box from the outside to capture energy, and also capture electrical current from neutralizing the charged helium. Am I missing something major?
- ben_w 4y ago
  > Am I missing something major? Multiple things, I'm afraid. I'm answering this as more of an amateur fusion enthusiast than a physicist, so I expect this list is a rough guide not a complete and precise one: • A wakefield accelerator has a plasma region significantly more massive (in aggregate) than the particles being accelerated, and the acceleration comes from the electrons in the plasma being easier to move than the ions, which creates a high charge gradient, and that charge gradient accelerates the particle beam. The plasma is "cold" compared to the beam. • You give boron a positive charge by removing electrons, but the more you remove, the harder it is for them to fuse (they mutually repel before they get close) • Even when the nuclei hit perfectly and in optimal condition, they mostly just bounce off instead of fusing. This releases photons (because all acceleration of charges always does that), which saps energy away. Stars only get around this by the photons being reabsorbed almost immediately and re-heating the plasma in the surrounding volume. • While the setup you describe will do fusion if you pick the right energy level, the energy level of this accelerator will instead mostly produce heat, tau-antitau particle pairs, and possibly some proton-antiproton (and neutron-antineutron?) pairs. • For stuff like this, the interesting thing is the energy not the speed, so while (in Newtonian physics) 5 GeV electrons go at the same speed as 2.5 MeV protons or 227 KeV B-11, that's wildly into the relativistic range for electrons so they don't do that anyway.
noobermin 4y ago
You wouldn't use a beam for fusion anyway, somewhat because of the "cold electrons" but you want it to be in a randomized soup that you can then compress, which is why ICF is a better scheme for fusion.

ars 4y ago

A proton is around 2000 times as heavy as an electron, so your energy numbers are actually semi equivalent. (At least that's how it seems to me - I don't know if this would even work on protons.) I hear your point about efficiency though. Why do you need similar velocity for the particles? It seems to me you just need a LOT of particles, but beam quality doesn't matter much. The cold electrons you mention are the ones bound to the Boron? Can we give the Boron a positive charge? All the charge will live on the surface, which is exactly where we want it. Make the box metal to the height of the boron, then an insulator after that, and charge the box. Put the output of the accelerator directly above the boron (the output is facing downwards), and keep pumping out helium. Cool the box from the outside to capture energy, and also capture electrical current from neutralizing the charged helium. Am I missing something major?

ben_w 4y ago
> Am I missing something major? Multiple things, I'm afraid. I'm answering this as more of an amateur fusion enthusiast than a physicist, so I expect this list is a rough guide not a complete and precise one: • A wakefield accelerator has a plasma region significantly more massive (in aggregate) than the particles being accelerated, and the acceleration comes from the electrons in the plasma being easier to move than the ions, which creates a high charge gradient, and that charge gradient accelerates the particle beam. The plasma is "cold" compared to the beam. • You give boron a positive charge by removing electrons, but the more you remove, the harder it is for them to fuse (they mutually repel before they get close) • Even when the nuclei hit perfectly and in optimal condition, they mostly just bounce off instead of fusing. This releases photons (because all acceleration of charges always does that), which saps energy away. Stars only get around this by the photons being reabsorbed almost immediately and re-heating the plasma in the surrounding volume. • While the setup you describe will do fusion if you pick the right energy level, the energy level of this accelerator will instead mostly produce heat, tau-antitau particle pairs, and possibly some proton-antiproton (and neutron-antineutron?) pairs. • For stuff like this, the interesting thing is the energy not the speed, so while (in Newtonian physics) 5 GeV electrons go at the same speed as 2.5 MeV protons or 227 KeV B-11, that's wildly into the relativistic range for electrons so they don't do that anyway.

noobermin 4y ago

You wouldn't use a beam for fusion anyway, somewhat because of the "cold electrons" but you want it to be in a randomized soup that you can then compress, which is why ICF is a better scheme for fusion.