Does fusion always have to occur in a plasma?

[Salgado]

Hi guys,

I am a close follower of the Focus Fusion initiative and also other approaches working to unleash the tremendous energies entrapped in the nuclei of atoms.

Not being a particle physicist, I would like to throw a question to the forum: how come all fusion experiments and ideas revolve around high energy plasma?
Obviously, you need energy to overcome the electrostatic repulsion of the nuclei for them to fuse, but why does that mean having both reagents at high energy?

As an example of what I am asking, consider this: why would not just shooting high energy protons down a crystalline boron shaft release enough energy (mainly thermal) to sustain the original proton accelerator?
Would the crystalline boron shaft need to be as long as the Earth? Or be as sphere as big as the Earth? Would the proton accelerator waste too much energy?

I would appreciate your thoughts, please don’t be too nasty. 🙂

[Tulse]

Salgado wrote: how come all fusion experiments and ideas revolve around high energy plasma?

Historically, there have been claims of fusion in non-plasmas — that’s pretty much what the “cold fusion” research claims (fusion in solids), and is also the case in alleged “sonofusion” (where collapsing bubbles in a liquid produce the necessary pressure and temperature for fusion events). Both of these approaches are very controversial, and none has unequivocal experimental support.

[Salgado]

I am aware of the “cold fusion” claims. Very interesting stuff, as much as it is controversial.

My proposal was to have one part hot fuel (high speed protons) and one part cold fuel (solid crystalline boron).
If fusion yield is an issue, maybe the protons can be put in a circular path (like a circular accelerator), going round and round until it hits something (hopefully boron!).
Would the protons just scatter in the crystal lattice, eventually running out of the necessary energy for fusion?

[KeithPickering]

Salgado wrote: Hi guys,

I am a close follower of the Focus Fusion initiative and also other approaches working to unleash the tremendous energies entrapped in the nuclei of atoms.

Not being a particle physicist, I would like to throw a question to the forum: how come all fusion experiments and ideas revolve around high energy plasma?
Obviously, you need energy to overcome the electrostatic repulsion of the nuclei for them to fuse, but why does that mean having both reagents at high energy?

As an example of what I am asking, consider this: why would not just shooting high energy protons down a crystalline boron shaft release enough energy (mainly thermal) to sustain the original proton accelerator?
Would the crystalline boron shaft need to be as long as the Earth? Or be as sphere as big as the Earth? Would the proton accelerator waste too much energy?

I would appreciate your thoughts, please don’t be too nasty. 🙂

It’s not easy to fuse nuclei. First, because normal atoms are protected by electron shells, and in order to get at the nucleus, you need to strip away the electrons. When you do that, for any gas light enough to fuse, you have a plasma by definition. Even your protons are a plasma, as they are simply ionized hydrogen gas, i.e., another plasma.

So, once you’ve got your plasma — i.e., your bare nuclei — then with no electrons in the way, they can fuse, right?

Wrong.

Nuclei are all positively charged, which means the repel each other by electrical force — and the closer they get, the stronger the repulsion. You need to get those nuclei close enough so that the strong nuclear force overcomes the electrical force, which means they need to get very, very close indeed. And generally the way to do that is to get them very hot, which is to say, get them moving very fast. If their kinetic energy is great enough, occasionally some nuclei will run into other nuclei fast enough to overcome the electrical repulsion, and they fuse.

The crystalline boron shaft (or any shaft made of neutral materials) won’t work, because those positively charged protons will be attracted to the electrons in the shaft material and won’t go down the middle. The proton will simply collide with the wall, pick up an electron, and revert to normal hydrogen again. If the plan is to collide protons together, magnetic confinement is the way to go, like a linear accelerator. And yes, some of them will fuse, but almost certainly not enough to reach break-even.

[jamesr]

Salgado wrote: I am aware of the “cold fusion” claims. Very interesting stuff, as much as it is controversial.

My proposal was to have one part hot fuel (high speed protons) and one part cold fuel (solid crystalline boron).
If fusion yield is an issue, maybe the protons can be put in a circular path (like a circular accelerator), going round and round until it hits something (hopefully boron!).
Would the protons just scatter in the crystal lattice, eventually running out of the necessary energy for fusion?

You will get some fusion if the protons have enough energy for head on collisions to overcome the Coulomb repulsion barrier. Indeed this is the method Cockroft & Walton used to achieve the first fusion reactions in 1932 (they used deuterium ion beam electrostatically accelerated into a deuterated metal hydride target)

However this would be an incredibly small proportion, and the energy needed to accelerate the ion beam would be orders of magnitude higher than that released by the reaction.

The majority of small angle scatters will just deposit the energy of the protons in the crystal, ending up as heat. If the beam was intense enough to produce a significant level of fusion reactions the heat generated by all the ones that don’t fuse would instantly vaporise it and turn the target area into plasma anyway.

[Salgado]

KeithPickering wrote:
It’s not easy to fuse nuclei. First, because normal atoms are protected by electron shells, and in order to get at the nucleus, you need to strip away the electrons. When you do that, for any gas light enough to fuse, you have a plasma by definition. Even your protons are a plasma, as they are simply ionized hydrogen gas, i.e., another plasma.

Yes, that I know. 😉

KeithPickering wrote:
So, once you’ve got your plasma — i.e., your bare nuclei — then with no electrons in the way, they can fuse, right?

Wrong.

Nuclei are all positively charged, which means the repel each other by electrical force — and the closer they get, the stronger the repulsion. You need to get those nuclei close enough so that the strong nuclear force overcomes the electrical force, which means they need to get very, very close indeed. And generally the way to do that is to get them very hot, which is to say, get them moving very fast. If their kinetic energy is great enough, occasionally some nuclei will run into other nuclei fast enough to overcome the electrical repulsion, and they fuse.

Well, my reasoning in this case is that the targets would be still – well relatively speaking, they’re not at 0K – and the proton would carry all the kinetic energy required to overcome the strong nuclear force.

KeithPickering wrote:

The crystalline boron shaft (or any shaft made of neutral materials) won’t work, because those positively charged protons will be attracted to the electrons in the shaft material and won’t go down the middle. The proton will simply collide with the wall, pick up an electron, and revert to normal hydrogen again. If the plan is to collide protons together, magnetic confinement is the way to go, like a linear accelerator. And yes, some of them will fuse, but almost certainly not enough to reach break-even.

So the crystal would just get “doped” with hydrogen for the vast majority of the proton shots, with a minuscule number of hits, just as Rutherford got a century ago…
Can anyone make the actual calculation, even if very rough?

It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center, carrying a charge.[2]
—Ernest Rutherford

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