Re: Proton-Proton Fusion- the easy way

Logan Kearsley wrote: 
So, I've been pondering proton-proton fusion, and wondering about ways to
make it easier to harness for small-scale (that is, smaller than a star or
large planet) energy production.

Really, since you have to end up converting two protons into neutrons
anyway, I figure it might be easier to think of ways of producing deuterium,
which is in fact what the sun does anyway. At that point, we can either go
with direct deuterium-deuterium fusion, or use the deuterium to produce
helium-3. Either way, the first and probably hardest step is producing
neutrons.

So, some questions: once I have a neutron source, what's required to turn a
neutron and a protium atom into a deuterium atom? Can one just dump
low-energy neutrons into the protium fuel and expect then to pair up? If so,
one might use a fission reactor or a spallation apparatus (proton beam into
molten lead?) to release the required neutrons.

Second question, what are some methods that could be used to transform a
protium atom into a neutron?


Turning the neutron into a proton is tha hard part.  The problem is
that a free proton won't turn into a neutron on its own (energetically
disallowed; in fact the neutron decays into a proton + electron + antineutrino).

So, the proton has to be a nucleus where the neutron is energetically
favored.  In the sun, the p + p --> d + e+ + nu reaction occurs when
the two protons have (temporarily) tunneled together, and the decay
just happens then.  This is the rate-limiting step in the solar fusion
cycle, and is extremely slow.

At higher temperature, you can do something like:

   12 C (p,gamma) 13 N (,e+nu) 13 C

but the first reaction is very slow, since 12C has a large coulomb
barrier.  So, in a reactor smaller than a star, the plasma will
radiate orders of magnitude faster than fusion energy will be
generated.

	Paul
.

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