In reply to Jed Rothwell's message of Mon, 13 Nov 2017 14:09:40 -0500:
>said, that total is always more than the energy added to the plasma, up to
>the iron limit.
Actually, iron is not the limit. It depends on which nuclei you are trying to
fuse. A lone proton has zero net binding energy, so would fuse with any element
in the periodic table, if you could get it close enough to the nucleus.
The "iron limit" applies only to elements fusing with themselves, i.e. one iron
nucleus fusing with another iron nucleus.
The limit applies to fusion in stars because the hydrogen in the core gets used
up creating helium. Then the helium gets used up creating carbon etc. etc. at
each stage of the process you are mostly left with a single element, so that
when you get to iron, there are no lighter nuclei left to fuse with, and iron
can't fuse with itself.
Robin van Spaandonk