Building a small, transportable fusion power
plant has long been a dream of fusion researchers. In the course of their
research, however, it became clear that a functioning power plant has to be of
a certain minimum size. Nevertheless, there are occasionally renewed attempts
(see “The fusion upstarts”, in Nature, Vol. 511, 14/7/2014, p. 398 ff.). IPP
scientists Professor Sibylle Günter and Professor Karl Lackner explain why also
the latest version proposed by US technology concern Lockheed Martin might well
remain a dream: The patent applications for the device
proposed by Lockheed Martin do not involve a really new concept, but combine
the known concepts of a magnetic cusp and a magnetic mirror. Both are impaired
by the fact that charged particles can escape along the magnetic field lines
out of the confinement region. This leads to an intolerable energy loss,
because it is primarily the fast, hot particles that get lost first. Nor does
it help here, as proposed, to link several cusps behind one another or combine
them with magnetic mirrors. Photo: Lockheed Martin What is envisaged is incorporating coils in
the vessel, i.e. inside the plasma. This needs connections to the outside and
fixtures in the plasma vessel. Hot plasma particles from the core of the device
would thus come into direct contact with these fixtures. The fundamental idea
of magnetic confinement, however, is precisely to keep the high-energy plasma
particles in the core moving along the magnetic field lines at always the same
volume without impinging on material walls. Otherwise the plasma cools down
very fast. One solution here would be superconducting coils levitating in the
vessel without support, this leaving, however, the above energy loss problem:
The configuration proposed is not suitable for confining hot plasmas. Furthermore, the coils inside the plasma vessel
have to be shielded not only from the surrounding hot plasma, but also from the
neutrons produced in the fusion process. With superconducting coils, at least
80 centimetres of shielding around each coil is needed. This does not accord
with the power plant size envisaged. All of these problems have been resolved by
the tokamak and stellarator concepts pursued today. Nevertheless, it is not
possible to build small, transportable power plants. This is because attaining
a positive energy balance, i.e. producing more fusion power than needed for
heating the plasma, calls for extremely good thermal insulation of the plasma,
viz. about 50 times better than styropor. In a power plant a temperature in the
plasma core of 100 to 200 million degrees is needed, while at the walls no more
than 1,000 degrees is tolerable. Such large temperature differences in the
plasma drive turbulent flows that mix hot and cold regions with one another,
i.e. impair the thermally insulating effect of the magnetic field. This has to
be compensated with a larger volume. Here it is the size of the temperature
gradient that determines the turbulent flows and hence the minimum size of a
power plant. How a positive energy balance is to be achieved with the compact
version propagated by Lockheed Martin is not even remotely mentioned in the
patent applications. Original Title: Are mini fusion power plants
possible? Lockheed Martin’s compact reactor concept / fusion drives for
aircraft and trucks? |
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