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It's not hard to imagine geology that didn't give us convenient concentrations of terrestrial helium, or that someday soon we'll have exhausted those natural sources.

How cost-effective is existing fusor technology at producing helium? I'm just looking at a ballpark estimate to produce, say, 1 standard cubic foot of helium from other elements. (Note that presently the market rate for that amount of helium from natural sources is about $.10.)

Update: Since the orders of magnitude are so far off that any net-negative fusor appears to be non-economical, how much usable 3/4helium would (contemplated, efficient) fusion energy reactors producing enough electricity for the U.S. (say, 1TW constant output) produce?

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  • $\begingroup$ Are you wanting to know about fusor technology specifically, or about any current experimental fusion device (tokamaks etc.)? Spoiler: whichever is the case, the answer to your question is the same - no. It just might be good to make it a bit more general, if that is compatible with your intentions. $\endgroup$ – tok3rat0r May 13 '15 at 16:14
  • $\begingroup$ Good point. Just updated to make it more general. $\endgroup$ – feetwet May 13 '15 at 17:22
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Transmuting chemically significant quantities of one element to another using nuclear reactions is not cost effective for any naturally occurring element. Nuclear physics is the end of alchemy.

Two examples I happen have off the top of my head: the "Fat Man" and "Little Boy" nuclear weapons deployed in the second world war each involved about $10^{24}$ fissions, and therefore produced 1–3 grams of free neutrons (depending on whether you count the neutrons that were reabsorbed in the chain reaction). That's roughly the same number of useful neutrons that will be produced at the Spallation Neutron Source (SNS) over its expected lifetime of 30 years.

Wikipedia reports that the highest neutron rate achieved by a fusor is about $10^{11}$ neutrons per second, a factor of a million smaller than the SNS. Alpha production rates would be comparable. That's roughly a microgram of helium per fusor per decade.

To address your update: the reaction $$ \rm {}^2H + {}^3H \to {}^4He + n $$ releases about 17 MeV. A gigawatt power plant would produce such reactions at a rate of about $10^{20}$ per second, or about two moles of alphas and neutrons per hour. I picked a gigawatt because that's a typical size for a large commercial reactor, but this rate would be for a reactor in a fairyland where all of the reaction energy went into power production. This is also the scale of the ITER reactor, but that machine will operate in pulses of less than 1000 seconds each.

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  • $\begingroup$ Notes on calculation of neutron production. For bombs: energy yield was about 50 TJ, generated by fissions each producing 1–3 neutrons and 200 MeV energy. For SNS: proton beam power today is about 1 MW, proton energy 1 GeV, 20–30 neutrons per spallation, wild-guess 50% duty cycle over calendar year, fudged downward because many neutrons are lost rather than cooled to a useful temperature. $\endgroup$ – rob May 13 '15 at 17:12
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    $\begingroup$ So following your observations an efficient 1GW fusion reactor would produce something like 2 moles of helium per hour, which happens to be a little more than 1scf. Therefore if the country's current electric capacity (1TW) were supplied entirely by fusion reactors we would be producing roughly 1000scf of helium every hour. Granted, this is He-4; not sure how much of a difference that makes. But interesting to note: current annual consumption of He-3 is ~$10^{10}$scf, and this would produce less than $10^{7}$scf per year. $\endgroup$ – feetwet May 13 '15 at 19:31

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