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204 Managing Global Warming

5.2.1 Magnetic confinement

A generic toroidal fusion reactor is shown in Fig. 5.2. The toroidal geometry is
required due to the properties of magnetic fields (the so-called hairy-ball theorem
[10]), although it makes the engineering more complex. Usually, in considering a
power plant, steady-state operation is targeted to provide continuous power output
and reduce cyclic loads on components, but this is not a fundamental requirement
and some concepts are intrinsically pulsed.
The role of the magnetic field is to confine the plasma and hold it away from the
reactor first wall, which provides protection to the structures behind it from direct radi-
ation and particle loads. Behind the first wall is the breeder blanket, which removes
the heat from the reactor for electricity generation and also creates new fuel. The
blanket also provides shielding to the vacuum vessel, which seals the reactor against
the outside world. Wrapped around the vacuum vessel are the toroidal field (TF) coils,
the magnets that provide the main confinement field, and outside these may be
poloidal field (PF) coils that provide plasma shaping and control capabilities. Finally,
there is a set of plant infrastructure, which provides fuel extraction and injection,
plasma heating, and thermal power extraction as well as maintenance for the reactor.
Further considerations of the requirements on these systems are discussed in
Section 5.3.
A purely toroidal magnetic field does not provide stable plasma confinement, so it
is necessary to generate a poloidal component to make it “twist” around the plasma. In
a tokamak, this poloidal field is generated by a current flowing in the plasma, and
maintaining this current is one of the biggest challenges in creating a steady-state toka-
mak power plant. In a stellarator, the twist in the magnetic field is supplied by dis-
torting the TF coils. Engineering such coil shapes is difficult and as the coils are
moved further from the plasma to include blanket and shielding, the distortion must

Toroidal direction
Magnets

Poloidal direction
Heat exchanger
Tritium extraction
I mag
Turbines
a
R 0

Ash/particle extraction
Plasma
Blanket and shield
Fig. 5.2 A generic toroidal fusion reactor showing the plasma, blanket and shield, magnets, and
power extraction.