05 What are the basic fusion reactions?
Description
This article is from the Fusion FAQ, by Robert F. Heeter heeter1@llnl.gov with numerous contributions by others.
05 What are the basic fusion reactions?
While it is possible to take any two nuclei and get them to fuse,
it is easiest to get lighter nuclei to fuse, because they are
less highly charged, and therefore easier to squeeze together.
There are complicated quantum-mechanics rules which determine which
products you will get from a given reaction, and in what amounts
("branching ratios"). The probability that two nuclei fuse is
determined by the physics of the collsion, and a property called
the "cross section" (see glossary) which (roughly speaking)
measures the likelihood of a fusion reaction. (A simple analogy
for cross-section is to consider a blindfolded person throwing
a dart randomly towards a dartboard on a wall. The likelihood
that the dart hits the target depends on the *cross-sectional*
area of the target facing the dart-thrower. (Thanks to Rich
Schroeppel for this analogy.))
Below is an annotated list of many fusion reactions discussed
on the newsgroup. Note: D = deuterium, T = tritium, p = proton,
n = neutron; these and the other elements involved are discussed
in the glossary/FUT. (FUT = list of Frequently Used Terms; section
10 of the FAQ.) The numbers in parentheses are the energies
of the reaction products (in Millions of electron-Volts, see
glossary for details). The percentages indicate the branching
ratios. More information on each of the elements is given below.
Table I: Fusion Reactions Among Various Light Elements
D+D -> T (1.01 MeV) + p (3.02 MeV) (50%)
-> He3 (0.82 MeV) + n (2.45 MeV) (50%) <- most abundant fuel
-> He4 + about 20 MeV of gamma rays (about 0.0001%; depends
somewhat on temperature.)
(most other low-probability branches are omitted below)
D+T -> He4 (3.5 MeV) + n (14.1 MeV) <-easiest to achieve
D+He3 -> He4 (3.6 MeV) + p (14.7 MeV) <-easiest aneutronic reaction
"aneutronic" is explained below.
T+T -> He4 + 2n + 11.3 MeV
He3+T -> He4 + p + n + 12.1 MeV (51%)
-> He4 (4.8) + D (9.5) (43%)
-> He4 (0.5) + n (1.9) + p (11.9) (6%) <- via He5 decay
p+Li6 -> He4 (1.7) + He3 (2.3) <- another aneutronic reaction
p+Li7 -> 2 He4 + 17.3 MeV (20%)
-> Be7 + n -1.6 MeV (80%) <- endothermic, not good.
D+Li6 -> 2He4 + 22.4 MeV <- also aneutronic, but you
get D-D reactions too.
p+B11 -> 3 He4 + 8.7 MeV <- harder to do, but more energy than p+Li6
n+Li6 -> He4 (2.1) + T (2.7) <- this can convert n's to T's
n+Li7 -> He4 + T + n - some energy
From the list, you can see that some reactions release neutrons,
many release helium, and different reactions release different
amounts of energy (some even absorb energy, rather than releasing
it). He-4 is a common product because the nucleus of He-4 is
especially stable, so lots of energy is released in creating it.
(A chemical analogy is the burning of gasoline, which is relatively
unstable, to form water and carbon dioxide, which are more stable.
The energy liberated in this combustion is what powers automobiles.)
The reasons for the stability of He4 involve more physics than I
want to go into here.
Some of the more important fusion reactions will be described below.
These reactions are also described in Section 2 in the context of
their usefulness for energy-producing fusion reactors.
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