2
$\begingroup$

Recently I found out that Hydrogen-1 and Boron-11 together are considered a viable nuclear fuel. Fusion of Hydrogen-1 and Boron-11 produces three highly energetic Helium-4 nuclei. Basically the entire amount of energy produced by the reaction is in form of alpha radiation, there is no gamma radiation and no high energy neutron output. The problem with H-B fusion is that the ignition temperature is very high, about 1 billion degrees.

It's unachievable with tokamaks, so researchers in the field develop other methods suitable for fusion of those two elements, most notably the "Dense Plasma Focus" device. This device as well as the other types however, are all basically versions of the hot fusion method. I was thinking isn't there an easier way?

Lets say we have a tubular container full of solid chemically pure Boron-11. Though an opening on one end of the tube the interior boron content is s bombarded by a beam of protons accelerated at about 100keV, so that the beam is directed along the tube's length for maximum impact. A beam of protons accelerated to 100keV can be achieved with a really small cyclotron.

My question is: Would that setup produce a continuous fusion for some period with positive net energy generation?

Here is my argument why I think it would: Since Boron is solid at room temperature, it's density is high, so I think the fusion rate per nucleon would be quite high. As far as I know 100keV is the energy needed for Hydrogen-1 and Boron-11 to fuse, while the resultant three He-4 nuclei should have about 8MeV of energy. So indeed if all accelerated protons fuse then the energy produced should be quite higher than the input. The problem that immediately comes to mind is that as the container starts to rapidly heat up as a result of the reactions the Boron inside would no longer be solid and may even start to leak through the opening. But before that happens, would there be at least a brief period where an efficient fusion can be sustained?

$\endgroup$
  • $\begingroup$ The cross sections for the reaction are well known. The energy conversion part, well, sucks. Your "good part" has a really lousy efficiency, so you will not get our enough energy to actually be net positive energy generation. Sorry. $\endgroup$ – Jon Custer Apr 27 '15 at 15:30
  • 3
    $\begingroup$ "What do you guys think?" or similar questions are both too broad as well as primarily opinion based. $\endgroup$ – ACuriousMind Apr 27 '15 at 15:36
  • $\begingroup$ Your "good part": Yes, I wasn't sure about it too. I've put it in quotes to outline that it is a claim of the promoters of this technology... $\endgroup$ – Ivan Ivanov Apr 27 '15 at 16:57
  • 1
    $\begingroup$ Generally speaking, whenever you see a "new" suggestion with regards to fusion/fission in the media, it's likely to be an old and discarded idea that has been made to look new, again. To your second part: any argument that uses the "density" of the target of a beam to enhance the cross sections is dead in the water. Nuclear reactions are independent of each other. Whether the next nucleus the beam particles may hit is one Angstrom or ten light years away makes no difference whatsoever for the reaction probability at the first nucleus. $\endgroup$ – CuriousOne Apr 27 '15 at 21:09
  • 1
    $\begingroup$ Its worth adding that particle accelerators have traditionally been fiendishly inefficient machines. To the point that including one in your proposal guarantees failure to achieve break over. $\endgroup$ – dmckee May 4 '15 at 5:50
5
$\begingroup$

After doing some more research I found the answer to my question.

The method I proposed was actually one of the first methods for hydrogen-boron fusion that was tested. It's called "fixed/solid target proton-boron-11 fusion". Experimentation very quickly showed that the method could not work because of two big problems:

  1. As #dmckee already commented above, the use of particle accelerator as part of the design was extremely impractical since particle accelerators are very inefficient. But that wasn't the biggest problem;

  2. The electron clouds surrounding the boron nuclei of the solid target, acted as shields, absorbing most of the incoming protons' energy and thus greatly reducing the probability of fusion (1 in 10 000 000 if I remember correctly, and that considering the density of the solid target). The density also proved to be a disadvantage (counter-intuitively), since the 675keV protons could barely penetrate 10 microns into the target, which reduced the number of atoms they could react with.

In the end the number of fusion reactions was so negligible that they could barely measure it.

$\endgroup$
  • 2
    $\begingroup$ The electron cloud does have a role, just as you described, related to penetration depth. However, I would also like to add that the nucleus' own electrostatic repulsion has a large role in poor "efficiency". The straightforward ratio of scattering to fusion events aren't enough to make a profit of energy from the first collision. This is why the DPF attempts to create a hot plasma ball that enables many successive collisions. In accelerator-target designs, you can only hit 1 or 2 nuclei before you've lost most kinetic energy. $\endgroup$ – Alan Rominger May 5 '15 at 11:02
0
$\begingroup$

Usually when you find the term "shielding" as it refers to electrons and fusion, it refers to the electron's aiding fusion, not absorbing energy from an incoming proton.

The electrons screen the nucleus' positive charge until the proton gets much closer to the nucleus than it would if the nucleus were fully or partially ionized as it would be in a very hot plasma.

Although they decay very quickly, there is some research being done on muon assisted fusion as muons have much smaller distances from the nucleus when bound to an atom, more effectively "shielding" the positive charge.

The show stopper with boron-11/proton fusion in the setup you described is ultimately the interaction cross section. The yield is relatively low and the required energy for the proton is relatively high (6ish MeV yield and I believe on the order of 500keV for the proton). The 100keV figure you've found is an "ideal temperature" for a plasma, where the energy distribution will definitely have a lot of protons reaching 500keV.

Feel free to point out any part where I'm wrong.

$\endgroup$
0
$\begingroup$

One way to get around the problem might be to heat the boron so that surface ionization takes place before p ion collision. Running a current through the boron would be an easy way to create surface ionization. With surface ionization established the p ions would not have to tunnel through so many elections resulting in more fusion.

$\endgroup$

protected by rob Dec 15 '17 at 5:29

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.