a helical path around its field lines will drift across the field and escape. To prevent
this, a tokamak uses a poloidal field whose field lines are circles around the toroid
axis. The poloidal field is produced by a current set up in the plasma itself by the
changing field of an electromagnet in the center of the toroid. This current also heats
the plasma; once the plasma is sufficiently hot, the current needs little help to
continue.466 Chapter Twelve
ITER
T
he planned International Thermonuclear Experimental Reactor (ITER) represents what is
hoped to be the final step before practical fusion energy becomes a reality. ITER is currently
sponsored by Japan and several European countries; the United States pulled out of the project
because of concerns about its original design and cost, and Russia withdrew (except for pro-
viding some staff ) because it cannot afford to participate. The redesigned ITER is expected to
generate 400 MW from deuterium-tritium reactions, to weigh 32,000 tons, to cost $3 billion,
and to take 10 to 15 years to build. Superconducting magnets (a large part of the cost) will keep
the reacting ions in a doughnut-shaped region whose volume is that of a large house. About
80 percent of the energy released will be carried off by the neutrons that are produced, and these
neutrons will be absorbed by lithium pellets in tubes that surround the reaction chamber. Cir-
culating water will carry away the resulting heat; this is the heat that could be used in a commercial
reactor to power turbines connected to electric generators.
Even if ITER works as planned, though, not every pessimistic observer of the fusion program
is likely to become a convert to the cause. Fusion reactors will certainly be enormously com-
plex and expensive and not wholly safe: lithium is an extremely reactive metal that burns or ex-
plodes on contact with water. Also, when lithium absorbs neutrons in the reactions of Eqs.
(12.31) and (12.32), radioactive tritium is produced. Hence an accident could be catastrophic.
Of course, the optimists could turn out to be correct, and fusion will become the preferred en-
ergy source of the future. But even if this happens, many decades lie ahead in which energy
problems will remain. Fission reactors employ an established technology and ways exist to make
them very safe, but memories of Three Mile Island and Chernobyl, plus continuing questions
about the disposal of radioactive wastes, continue to affect their public image. Meanwhile fossil
fuels are being used up and burning them produces enough CO 2 to affect weather and climate.
Such “green” energy sources as solar cells and wind turbines are unlikely to provide more than
a small (though welcome) fraction of energy needs. An energy strategy for the world that is both
sensible and widely acceptable is not obvious.Poloidal Toroidal magnetic field
magnetic fieldPath of plasma ionFigure 12.30In a tokamak, combined toroidal and poloidal magnetic fields confine a plasma.bei48482_ch12.qxd 1/23/02 12:08 AM Page 466 RKAUL-9 RKAUL-9:Desktop Folder: