Terrestrial Approaches to Fusion Energy - Nanophysics of Solar and Renewable Energy - page 85

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Terrestrial Approaches to Fusion Energy

We regard deuteron fusion as a renewable energy process given the large supply of
deuterons in the sea. In this chapter, we build upon what we learned about fusion in
the sun with the hope of applying the same process on earth.
A summary of fusion reactions of technological interest is given in Table 4.1. We
have seen in Chapter 2 that the proton - proton p - p fusion reaction that powers the
sun is very dif cult to achieve. The reactions of technological interest in Table 4.1
havemuch higher cross sections than the p - p reaction. The reactions involvingDand
T (tritium) that have been observed in laboratories andmay power a Tokamak reactor
to produce controlled fusion energy on earth are the main subject here.
In Table 4.1, Q is the kinetic energy release in the reaction, which is shared among
the products depending on their masses. The cross section in units of cm 2 is that at
the most favorable energy for the reaction to occur, which is given in the last column,
in keV. (The typical variation of cross section with energy is sketched in Figure 4.4.)
Fusion energy can be considered renewable if based on deuterium, since the ocean
contains a huge dilute reservoir of deuterons in the form of heavy water, HDO and
DDO. Furthermore, the ocean contains large amounts of lithium, which can also be
used as a fusion reactant [44]. The reactions shown in Table 4.1 have cross sections or
probabilities about 25 orders of magnitude larger than the p - p reaction described in
Chapter 2. These reactions take deuterons as starting material. An interesting
question posed by the set of reactions shown is why should the D-T

T reaction have
a higher cross section than the DD reactions? It appears that this difference is related
to an aspect of the quantum mechanical process of tunneling.
The primary thrust toward producing energy from fusion is the ITER Tokamak
reactor, planned with a toroidal DT plasma. ITER (http://www.iter.org/) is a 840m 3
torus reactor being built in Cadamarche, France, by an international consortium. In a
sense this is an attempt to scale the successful fusion conditions on the sun to
parameters, notably the lower pressure, that can be attained on earth. The pressure
has to be much lower, so scaling leads to a lower density of reactants, and the
temperature can be raised to compensate. The temperature is easier to control in a
laboratory plasma, and can exceed the temperature in the suns core. The ITER
approach is based upon magnetic con nement, to keep the 100 million K plasma
from being cooled by, and damaging, the containing walls. Before we discuss the

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