Is Positron-Catalyzed Fusion Possible?

[rootone]

So far we have made anti hydrogen in small quantities, for experiments such as LHC, but no other anti matter that I know of.
It is possible to imagine a rocket propelled by antimatter, but making antimatter in sufficient quantity is, well let's say not economically feasible

 

[Vanadium 50]

You're moving in the wrong direction -antiprotons are much more difficult/expensive to produce than positrons. Maybe by a factor of millions.

This is like saying "is it possible to make a matchstick powered by a nuclear reaction?" If you have nuclear power, you don't need matches.

 

[Vardaan Bhat]

I'm a bit confused by that analogy; if we produce antimatter, we can never have energy positive annihilation because producing the antimatter will take as much energy or more than that which is released by annihilation. On the other hand, if we spend a little energy producing a tiny amount of antimatter, and use that to catalyze fusion ignition, we can get tons of energy out.

 

[Vanadium 50]

1. Positrons don't work.
2. Everything that makes positrons not work is worse for antiprotons.
3. This thread will never end. After we deal with antiprotons, what's to stop you from coming back with anti-deuterons, or anti-carbon, or anti-water, or anti-gasoline, or anti-elephants?

 

[mheslep]

I don't fully understand why these rays can't be used to heat up nuclei.

A fundamental problem with what you propose are limits imposed by thermodynamics. In your description of the energy released by some (annihilation) event, you imagine there's some method that might focus, beam, direct etc all the energy from that event onto a nucleus, as if a ball on a pool table were shot into one other ball in isolation. But there is no such mechanism that you can control on a per event basis; the target instead is a rack of *all* the other balls so that the striking energy is distributed, wasted. Some nuclei may be hit, but much of the energy *must* go elsewhere. The initial energy is thermalized, i.e. it will "heat up" all the plasma around it, which includes the like of electrons which don't contribute to fusion power. It's been shown fairly conclusively that any attempt to beam or direct energy onto fusion targets, as opposed to heating up the plasma as a whole, will always produce less fusion energy that which is put into the process.

"Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium"
https://dspace.mit.edu/handle/1721.1/11412

 

[Vardaan Bhat]

How can one "heat up plasma as a whole" in general?

 

[mfb]

With electromagnetic radiation, compression, or by shooting fast particles into it.

 

[Vardaan Bhat]

... gamma rays are electromagnetic radiation, no?

 

[mheslep]

Yes, and why are they better than any other EM?

 

[Vardaan Bhat]

If they're higher energy, I'd imagine you'd need less and the setup would be more compact.

 

[Vardaan Bhat]

What other kind of em is used for fusion? With icf, why can't we just shoot X-rays with soft X-ray light at the fuel?

 

[mfb]

If they're higher energy, I'd imagine you'd need less and the setup would be more compact.

That argument doesn't work. They have more energy per photon. So what? It is much harder to produce those photons, and their interactions with matter are less helpful.

What other kind of em is used for fusion? With icf, why can't we just shoot X-rays with soft X-ray light at the fuel?

Tokamaks and stellarators use microwaves. Simply because it is easy to produce MW of microwave power, and they couple efficiently to the electrons in the plasma, heating it. ICF also needs something interacting strongly with the material. Gamma rays don't work. There is also no suitable conventional x-ray source strong enough, hence the detour with lasers heating some material that emits x-rays.

 

[e.bar.goum]

If they're higher energy, I'd imagine you'd need less and the setup would be more compact.

This has been said a bunch of times in this thread, but I think a plot might make this clearer. To heat up the plasma, you need the gamma rays to deposit energy in the plasma. The problem with using higher energy photons is that their probability of depositing this energy goes down. Have a plot:

On the y-axis is the attenuation coefficient for photons in different materials, and on the x-axis is the photon energy. Note the log-log scale. If you want to deposit a lot of energy in a material, you're better off using low energy photons.

 

[Vardaan Bhat]

Have a plot:

On the y-axis is the attenuation coefficient for photons in different materials, and on the x-axis is the photon energy. Note the log-log scale. If you want to deposit a lot of energy in a material, you're better off using low energy photons.

Wow! Thank you for the graph. That helps me visualize the problems with my idea.

With electromagnetic radiation, compression, or by shooting fast particles into it.

What kinds of fast particles?

 

[mfb]

What kinds of fast particles?

Deuterium, sometimes deuterium with tritium. The neutral beams heat the plasma and become part of it, so you don't want other isotopes in the plasma.

 

[Vardaan Bhat]

How do you make controlled beams of a neutral particle or atom?

 

[mfb]

You make an ionized beam and then add electrons to make it neutral.

 

[Vardaan Bhat]

So, to be clear, you take some deuterium, ionize it, accelerate it, and add electrons at the end? What defines the end? When it's in the plasma?

Why does this work, and can it be applied to other fuels? Can it be used to ignite fusion alone (even just theoretically) or does it need to be coupled with something else (electric fields or magnetic fields or something like that)?

 

[mfb]

So, to be clear, you take some deuterium, ionize it, accelerate it, and add electrons at the end? What defines the end? When it's in the plasma?

No, before it enters the plasma. Before it enters the plasma chamber. As usual, Wikipedia has an article.

Why does this work, and can it be applied to other fuels?

The atoms are fast, they get slowed down in the plasma, heating it. It can be applied to other fuels.

The plasma still needs containment: strong magnetic fields.

Please start a new threads if you have follow-up questions because this is not about positrons any more.

 

[nikkkom]

A video of neutral beam injector in action

http://www.ccfe.ac.uk/news_detail.aspx?id=117

 

[rootone]

Why does it look bright pink, in the visible range.
Lots of blue and red there, almost no yellow/green.

 

[ChrisVer]

I don't want to sound evil but read the text and not only the video:

its visibility caused by the excitation of the background hydrogen gas

 

[rootone]

I don't want to sound evil but read the text and not only the video:

Ah, OK thanks.

 

[Pooua]

I've toyed with this subject for several years, but, as a non-specialist with very little University Physics behind him, I'm severely limited in what I've been able to do. Antimatter-catalyzed fusion has gotten a bit of interest, and even some government research grants over the years.

http://www.theverge.com/2013/8/28/4659834/unlocking-the-positron-fusion-annihilation-laser
http://large.stanford.edu/courses/2011/ph241/palke1/
http://ffden-2.phys.uaf.edu/213.web.stuff/Scott Kircher/fissionfusion.html

However, I think I concluded a few years ago that antimatter-catalyzed fission is more practical and potentially achievable in the near term than is AC fission.

https://en.wikipedia.org/wiki/Antimatter-catalyzed_nuclear_pulse_propulsion
http://www.universetoday.com/131494...rt-antimatter-propulsion-system-another-star/