Fusion Reactors, Field Magnets

Also in 1950 Sakliarov and Tamm proposed an idea for a controlled thermonuclear fusion reactor, the TOKAMAK (acronym for the Russian phrase for toroidal chamber with magnetic coiF ), which achieved the highest ratio of output power to input power of any fusion device of the twentieth centuiy. This reactor grew out of interest in a controlled nuclear fusion reaction, since 1950. Sakharov first considered electrostatic confinement, but soon came to the idea of magnetic confinement. Tamm joined the effort with his work on particle motion in a magnetic field, including cyclotron motion, drifts, and magnetic surfaces. Sakharov and Tamm realized that... 

One possible way to achieve nuclear fusion is to use magnetic fields to confine the reactant nuclei and prevent them from touching the walls of the container, where they would quickly slow down below the velocity required for fusion. Using 400-ton magnets, it is possible to sustain the reaction for a fraction of a second. To achieve a net evolution of energy, this time must be extended to about one second. A practical fusion reactor would have to produce 20 times as much energy as it consumes. Optimists predict that this goal may be reached in 50 years. 

Atoms are first stripped of their electrons at very high temperatures this creates a plasma (ionized gas) of positive ions. Then the positive ions must be brought into close enough proximity, so that the strong attractive force between nucleons can overwhelm the Coulomb repulsion between them. Magnetic fields can confine hot plasmas of ions, provided that collective instabilities of these plasmas can be controlled. For a successful nuclear fusion reactor, three requirements must be met (1) The density of the plasma must exceed some critical value p. (2) The plasma confinement time must exceed some critical value t. (3) The temperature of the plasma must exceed some critical value 9... 

In the Tokamak fusion reactor depicted in Fig. 21.9, electric current to the poloidal coils on the primary magnetic transformer generates the axial current in the secondary plasma composed of deuterium and tritium ions. These ions are heated to ignition temperature and then the reaction becomes self-sustaining. The toroidal field coil suspends the plasma away from the metal conducting walls. Contact with the wall would both cool the plasma below ignition temperature and contaminate the plasma with heavy ions. The relevant reactions are given below. 

Major components of a tokamak fusion reactor are shown here. Giant coils of superconducting metal create powerful magnetic fields that confine the superhot plasma—a hot gas of charged particles—in a magnetic bottle. ... 

The neutron carries much of the energy released by this reaction. The neutrons have no electric charge so they are not contained by the magnetic field. They must be captured in a blanket material. When the blanket material absorbs them, their energy is transformed into heat. The heat from the neutrons and the radiant energy emitted from the hot plasma and directly adsorbed by the walls of the reactor are the heat output of the fusion reactor. This heat can be used to produce steam for the generation of electric power by conventional steam turbines. 

One 45-kA conductor model has been constructed and its use for large energy storage magnets and poloidal field coils in fusion reactors is under investigation. 

Existing tokamak fusion reactor designs with magnetic divertors have therefore exclusively poloidal field or multipole divertors. But using versions with multipoles installed inside, as in ASDEX, it has not yet been possible to find a feasible solution compatible with reactor access, manufacture and service requirements. 

The plasma in a fusion reactor must not touch the walls of its vacuum vessel, which would he vaporized. In the Tokamak fusion test reactor, the plasma is contained within a magnetic field shaped like a doughnut. The magnetic field is generated hy D-shaped coils around the vacuum vessel. 

A FIGURE 19.14 Tokamak Fusion Reactor A tokamak uses powerful magnetic fields to confine nuclear fuel at the enormous temperatures needed for fusion. The high temperatures produce a plasma, a state of matter in which some fraction of the atoms are ionized. 

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