Qucik Fusion Question

[Kitsune]

I am discussing with a person in a friendly discussion
The person is telling me that helium-3 fusion is harder that hydrogen fusion. The reason why I am asking is because I planned to use hydrogen fusion in my starships in my stories and it is far more common.

[Admiral Valdemar]

It is, given it is a larger atom, but it is cleaner and gives better yields. You can still use hydrogen like D + T reactions, but they're not optimal.

[Kitsune]

I meant 'easier' not 'harder' but I think you still answered my question.
I think I will stay with hydrogen fusion for my sf universe.
Is modern fusion research ebing done with hydrogen or helium-3?

[Admiral Valdemar]

Hydrogen, because He3 only exists in any real amounts on the Moon it is thought and the upper atmosphere of our local gas giants.

[tharkûn]

Yes and no. Yes the coulombic barrier is higher for helium - you do have more charge in the nuclei; likewise because the particles mass you have to go to higher temperatures to get the right velocity distribution. Note that any low Z fusion cycle is going to have to have to get to helium-4 to get out useful energy (in the D+D case below most egineers would further react the T and 3 He).

Your best reactions are going to be:
D + T -> 4He + n
D + D -> (1/2) T + (1/2) p + (1/2) 3 He + (1/2) n
3He + 3He -> 4He + p

The first is my bet and for starships has several key advantages. First and foremost you can store your fuel a DLi, if you isotopicly enrich the Li you can then breed T as you go and have a nice solid fuel that stores without pressurization or cryogens.

[tharkûn]

Is modern fusion research ebing done with hydrogen or helium-3?

Most research is looking into deuterium and tritium, rather than generic hydrogen. Proton fusion has several major problems that make it undesirable.

Hydrogen, because He3 only exists in any real amounts on the Moon it is thought and the upper atmosphere of our local gas giants.

Really then what does tritium transmute into when it decays? Research grade 3He is produced by beta decay from T. T can be mass produced from Li which can be mined by the ton. Given the physics interest in superfluidity and other fun things one can now buy the stuff by the liter. It is expensive as all get out, but that hasn't stopped research using it in other fields.

The real reason 3He isn't all that popular is that working with one proton in your nuclei is a good thing. D+D has the advantage of being only one species and being readily availible. D+T seems to be the best candidate from a physics standpoint. In most regards 3He fusion is better only in that it produces protons instead of neutrons; however running a full fusion cycle will ultimately give those anyways.

[Admiral Valdemar]

Really then what does tritium transmute into when it decays? Research grade 3He is produced by beta decay from T. T can be mass produced from Li which can be mined by the ton. Given the physics interest in superfluidity and other fun things one can now buy the stuff by the liter. It is expensive as all get out, but that hasn't stopped research using it in other fields.

That's great, really. But guess what? We want to mine the stuff in bulk, not have to wait to make the stuff or expend other such resources when the stuff is abundant in Lunar soil. And this is for power, not physics research.

The real reason 3He isn't all that popular is that working with one proton in your nuclei is a good thing. D+D has the advantage of being only one species and being readily availible. D+T seems to be the best candidate from a physics standpoint. In most regards 3He fusion is better only in that it produces protons instead of neutrons; however running a full fusion cycle will ultimately give those anyways.

Yes, but it gives off less neutrons, or had you forgotten that shielding such reactions is a major problem with the neutrons being less than healthy? The other drawback is the ignition temperature, but other than that, D + He3 would be a good one to use if you could get it.

[tharkûn]

That's great, really. But guess what? We want to mine the stuff in bulk, not have to wait to make the stuff or expend other such resources when the stuff is abundant in Lunar soil. And this is for power, not physics research.

Currently it costs around 150 dollars a liter to buy 3He, I can place the order today, pay some ugly shipping charges, and have it here next week. Of course if I didn't want spectroscopic grade and went outside of our normal isotope supplier I could get a better price, but what the hell.

Mining regolith on the lunar surface means going through cubic kilometers of dirt to recover the 20 ppb or so helium 3 to be had.

Lithium is relatively cheap, and some 190,000 metric tons are produced per annum at relatively low prices.

Less than 1% of the worlds Lithium production would need to be diverted to 3He production to mass produce it for fuel. Nobody is researching 3He fusion with an eye towards power generation because it isn't a more cost effective method than either D or T fusion. If you want to research 3He fusion you can do it cost effectively today, it just isn't all that wonderful.

Yes, but it gives off less neutrons, or had you forgotten that shielding such reactions is a major problem with the neutrons being less than healthy?

Coat the chamber with boron, and you care why? Being exposed to ANY MeV particle is less than healthy. The only real problem with neutrons is that they transmute the materials used in construction, but that can be dealt with every few years. I've been over an running neutron breeder reactor (not to be confused with a Pu breeder reactor), the sum total shielding between me and it was a few meters of water and my badge registered nothing. Neutrons are workable, we've doing it with fission for decades.