Why do they use hydrogen as the fusionable material in almost all fusion speculations? In all the ideas I’ve read, it has been that.
The main difficulty in fusion is getting the nuclei to bang into each other. They’re all positively charged, so they repel each other. Hydrogen has the least number of protons, and thus the lowest nuclear charge of any element.
Because anything else is too difficult to get to fuse. In fact, even ordinary hydrogen can’t be used. You need deuterium and tritium, two heavy isotopes of hydrogen that fuse much more easily than ordinary (protium) hydrogen.
And note that we still can’t fuse heavy hydrogen in a controlled fusion reactor.
Nitpick:
Not exactly true that we can’t get controlled fusion. We can. It’s just that it takes more power to produce the fusion than you get out of it. There are lots of experimental fusion reactors that work, except they don’t do anything worthwhile.
Also note that 2 Protium ([sup]1[/sup]H) atoms’ nucleii would make [sup]2[/sup]He, which isn’t a very stable configuration. Two Deuteriums ([sup]2[/sup]H) will make[sup]4[/sup]He, which is very stable. [sup]3[/sup]He is also stable, made from a Protium and Deuterium.
All the elements can be fused under the right conditions, but only those whose end product is atoms of elements lighter than iron produce energy. Bigger atoms require energy to stay together, and release it when fissioned, a la uranium.
And all of these fusion processes take more and more pressure and temperatures to begin. It’s only when a large star has used up a good deal of its hydrogen that it starts to collapse and increase its core pressure. This increased pressure allows the helium “waste” from the hydrogen to fuse into heavier elements. Once the helium is used up, its “waste” can similarly be fused.
Mea Culpa
The hydrogen fusion reactions are:
[sup]2[/sup]H + [sup]2[/sup]H = [sup]3[/sup]He + n (neutron)
[sup]3[/sup]H + [sup]2[/sup]H = [sup]4[/sup]He + n (neutron)
Hydrogen also has the greatest mass-defect when fused: The Helium atom is more tightly bound than the Hydrogen atom, due to the strong binding force associated with atomic nucleii. The Binding Force has a specific radius (about 63 protons worth, as I recall). This force binds all the protons and nuetrons, :. the more particles in the nucleus, the more tightly bound the nucleus becomes. Beyond a nuclear mass of 126 (the most tightly bound nucleus), some particles are no longer within range of all other particles, and so are not as tightly bound, as the repulsive effect of electromagnetic forces fight the binding of the nucleus.
The binding is expressed in a reduction of potential energy of the nucleus, by way of a minute reduction in mass. This mass-defect is released as energy, and is the source of enregy sought from nuclear reactions. Graphing binding force vs. atomic weight gives a sharply rising curve which peaks at about 126, then slowly the curve lowers as atomic number increases and more particles fall outside the completly mutually-bound radius. Large atomic weights are good candidates for fission, as splitting them results in two or more much more tightly-bound fragments, releasing their binding energy as electromagnetic radiation and kintetic energy. Low atomic weights are suitable for fusion, as they form a more tightly bound atom, and release their energy as electromagnetic radiation.
It is theoretically possible to fuse elements other than Hydrogen, but Hydrogen produces the most bang for the buck, and Hydrogen Isotopes such as Dueterium and Tritium produce better effects still, as you wind up with a greater mass-defect, but only have to spend the enegry to overcome the repulsive electromagnetic forces of two protons.
Of course, it’s been about 15 years since I went to NNPS, so my theory is a bit rusty, and my explanation is less succinct that it could be.
Also, one other big reason…Hydrogen is cheap and plentiful.
Lithium Deuteride (LiD) fuel has been used in some nuclear weapons, but the lithium itself doesn’t actually fuse. The lithium-7 can be made to fission (though this absorbs a fair amount of energy) to make hydrogen-3 (tritium) and helium-4. The tritium then fuses with deuterium to form helium-4 and a neutron.
Another possible reaction is the fusion of helium-3 and deuterium. This reaction is supposed to be easier to initiate than deuterium-tritium, but helium-3 is extremely rare on the earth. It is more plentiful on the surface of the moon, and in the atmospheres of the gas giants.
Really, fusion of any similar atoms will release energy up until iron(IIRC). But, as was mentioned, the energy we need to perform such a feat is larger than what we get out of it.
After iron it actually takes energy to get them to fuse. This is why the heavier elements can only be formed in nova and supernova.