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Nuclear fusion: What of the future? 211

5.3.6 Heating and current drive

Considerations from plasma power balances tell us that a magnetic confinement
fusion reactor should operate at an average plasma temperature of around 15keV
and aim to achieve an energy confinement of nTτ>8.3 atm s (that is, a plasma pres-
sure of several atmospheres for several seconds). A fusion plasma requires energy
input to heat it to the point where significant self-heating from fusion alpha particles
occurs and, for a tokamak, to provide a steady-state plasma current to maintain
magnetic stability. A range of technologies exist, from radiofrequency schemes like
electron- or ion-cyclotron resonances to the injection of high-energy neutralized par-
ticle beams. However, these methods are significantly inefficient, requiring at least
double and often three times the coupled power in electrical power. They result in high
recirculating power within the plant, meaning that a considerable proportion of the
electricity generated is used to run the plasma control and heating systems. Some
advanced tokamak concepts rely on plasma regimes with a high bootstrap current,
a self-driven current in the plasma generated by pressure gradients within a toroidal
geometry [19]. Conversely, modern stellarator designs are optimized to minimize this
current to increase the plasma controllability.

5.3.7 Other plant issues

As well as the considerations outlined here, a fusion power plant requires other
supporting systems. In particular, an isotope separation system/tritium plant to clean
exhaust gases for reinjection and extract new tritium from the blanket coolant is
needed, as is a cryogenic plant for maintaining the low temperatures of the magnets.
Hot cells for storage and recycling of removed blanket and divertor components are
required, as is space for the preparation of new components to be inserted. A river or
ocean to provide water for the removal of waste heat from systems is also needed.
The plant power flow, to illustrate the demands imposed by plant systems, is shown
in Fig. 5.5. The overall plant efficiency, given by the net electrical power produced
divided by the fusion power, is strongly dependent on the effectiveness of the
plant subsystems, particularly the heating and current-drive systems, and the primary
coolant pumps used to drive the heat extraction from the in-vessel components.
The energy conversion system efficiency is also affected by the temperatures achiev-
able in those components, which is limited by the materials available for use in this
high-neutron-irradiation environment.
Nuclear fusion has chiefly been a plasma physics experiment for much of the
last 70years. However, with renewed interest in alternative forms of energy, the devel-
opment of ITER, and particularly the EUROfusion and CFETR programs (see
Section 5.5), there is now a shift toward engineering the systems required for efficient
power plant operation. However, as soon as significant neutron radiation—required to
qualify the systems under plant conditions—is invoked specialist materials and
remote handling is required; as the reactor is burning tritium to produce those neu-
trons, a breeder blanket is also required; to achieve significant radiation doses long
pulses are required and so superconducting magnets are required: the best machine