[Borek]

Note: my nuclear knowledge is close to zero.

What about D + D -> He?

[nj_bartel]

 http://science.howstuffworks.com/fusion-reactor2.htm

From that link, my impression is possible, but not with current tech.

[Borek]

That's not what I am asking about. For some reasons some fusion reactions are easier, other are harder. T+D->He+n is the one most commonly listed, I believe it was done in Tokamaks, just the reaction was never selfsustaining. However, most cold fusion experiments were done with heavy water, so the only possible reaction is D+D (or D+H). I wonder if there is any specific reason forbidding D+D->He reaction. I have read a little bit in the meantime and it looks like D+D is possible, but the outcome is not 4He, but either T and proton, or 3He and neutron. I suppose there is a reason for that.

Could be that the final answer is "because that's the way it is"

[Mitch]

D + D -> He-4 is favorable (23.8MeV) from using this:
http://www.chemicalforums.com/index.php?page=scripts#Nuclear%20Reactions%20Calculator

Here is the easiest way to understand nuclear reactions without getting into theory. If the mass of the reactants are larger than the mass of the products than that reaction will be favorable.

[gippgig]

I believe D + D = ⁴He + y is possible but very rare. D + D fusion initially produces an excited ⁴He nucleus which can deexcite either by emitting a proton or neutron (which can happen very quickly) or by emitting a gamma ray (which takes a lot longer). It almost always loses a nucleon before it gets around to emitting the gamma ray. This is analogous to a heavy ion reaction where the compound nucleus deexcites by emitting typically 3-5 neutrons before the excitation energy drops below the binding energy of a neutron at which point the remaining excitation energy is lost by gamma emission (if the nucleus doesn't fission at some point in the process).

[Borek]

I believe D + D = ⁴He + y is possible but very rare. D + D fusion initially produces an excited ⁴He nucleus which can deexcite either by emitting a proton or neutron (which can happen very quickly) or by emitting a gamma ray (which takes a lot longer).

Scooby snack for you, that's the explanation that fits my needs

[vmelkon]

Some people have built Farnsworth Fusors
http://en.wikipedia.org/wiki/Fusor

To my surprise, even amateurs have built it and amateurs tend to use pure deuterium. They buy heavy water and do electrolysis to get deuterium. The reactor emits a lot of neutrons which indicates the reaction is D+D => Helium3 + n

There is even a company that sells the machines to companies that need a neutron source. I guess for measuring metal thickness or something.

Perhaps Futurama's Prof Farnsworth got his name from this since the writers of that show always reference some physics or math.

[Enthalpy]

D+D->He is impossible because it won't conserve energy and momentum.
D-D reactions emit a neutron or a proton.

At present-day technology, and in any future even very vaguely foreseeable, these reactions are very easy to produce (a fusor does it on a table) and completely impossible to use for a net energy production.

A very simple reason is that D-T fusion is still very hard to harness for net energy gain, yet D-D needs horribly more difficult conditions (see Lawson criteria).

Sadly enough, D-T is the only reaction "at grasp", but would be very polluting because it needs tritium. Tritium isn't available from Nature; uranium reactors produce it presently but will always produce more electricity when making tritium than D-T fusion consuming it, so fusion reactors have to regenerate their tritium to be meaningful.

Now, one D-T fusion produces one neutron, but one neutron is needed to regenerate a tritium from lithium, so you need a neutron multiplier to overcome the losses. As it looks, only lead as a multiplier has a chance to achieve regeneration, and lead spallation to obtain the neutrons is about as dirty as uranium fission producing the same energy.

Any fusion reactor using D-T has that fundamental problem: tokamaks, stellarators, and laser inertial fusion as well. Z-striction machines could cope better if some day they can fuse other nuclides, like D-D or p-B.

This pollution issue was known from specialists but not widely enough until I argued about it at Saposjoint.net and Physicsforums.com; now Wikipedia includes this rationale. To my eyes, it makes fusion reactors uninteresting unless someone finds a convincing solution, which hasn't happened in the last two decades.