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Nuclear fusion: What of the future? 205

increase. Gaining access to the interior of the vessel for maintenance and component
replacement is complicated.
For a magnetized plasma, we need a figure of merit indicating how effective the
confinement is. Usually the ratio of plasma pressure to magnetic field pressure,
P hi
β ¼ . , is used. Here <P> is the volume-averaged plasma pressure, B is the
B 2
2μ 0
magnetic field in the plasma, and μ 0 is the vacuum permeability. Although the trans-
port processes in a magnetically confined plasma are complex, the simplest scaling of
1
2 2
1
the confinement time is roughly a B T 2n , where aB is the number of diffusive
1
steps to the edge of the plasma and T 2n 1 is the collision rate. Due to the tokamak
geometry, however, the magnetic field varies strongly within the plasma leading to
additional classes of orbits much larger than the step size assumed here, and turbulent
processes increase the transport further. Nevertheless, the easiest ways of achieving
good energy confinement and hence high fusion power are to increase the machine
size and magnetic field.
A tokamak is an axisymmetric torus in which the toroidal field (supplied by the
magnets) is much larger than the poloidal field (supplied by a toroidal current flowing
in the plasma). A tokamak geometry is defined by its major radius R 0 , minor radius a
(or aspect ratio A ¼ ), the toroidal field in the plasma B, and the plasma current I p .
R 0
a
The plasma may also have elongation κ and triangularity δ, which change its cross
section from a simple circle to a D-shape.
aB
A useful additional figure of merit for a tokamak is the normalized beta β ¼ β ,
N
I p
which indicates how close the plasma is to destabilizing MHD (magnetohydrody-
namic) activity for a particular geometry and operating regime.
Spherical tokamaks are more like a cored apple than a doughnut shape, with the
consequence that the toroidal magnetic field varies much more across the plasma
than in a conventional tokamak. This leads to a number of potential improvements
in plasma performance. In particular, a higher pressure can be achieved for a given
magnetic field, which, combined with the compactness of the device, could lead to
more cost-effective approaches to fusion. The limited space on the inboard plasma
side can make it hard to shield the center column and magnets from neutron radiation.
Stellarators attempt to avoid some of the issues of tokamaks by externally impos-
ing the magnetic field twist through the use of distorted TF coils. This is an elegant
physics solution but a complex engineering one (Fig. 5.3). The resulting plasma
usually has an oval cross section, which twists as it moves around the device, meaning
that the plasma chamber—whose walls must “hug” the plasma—also has a very var-
iable cross section, making the robotic handling of components inside this volume
extremely challenging. However, stellarators do not require a current to be driven
in the plasma (although they do require auxiliary heating to achieve fusion burn)
and the plasmas tend to be much more quiescent than tokamak plasmas, resulting
in a number of promising simplifications.
A simple estimate of the size and requirements for performance of a magnetic-
confinement-based fusion power plant can be made. If the plasma-facing components