Quantum Physics

30.3 Nuclear Fusion 983VacuumCurrentBPlasma(a)Courtesy of Princeton Plasma Physics Laboratory(b)Figure 30.5 (a) Diagram of a tokamak used in the magnetic confinement scheme. The plasma istrapped within the spiraling magnetic field lines as shown. (b) Interior view of the Tokamak Fusion TestReactor (TFTR) vacuum vessel located at the Princeton Plasma Physics Laboratory, PrincetonUniversity, Princeton, New Jersey. (c) The National Spherical Torus Experiment (NSTX) that beganoperation in March 1999.Courtesy of Princeton University(c)combination of two magnetic fields to confine the plasma inside the doughnut. Astrong magnetic field is produced by the current in the windings, and a weakermagnetic field is produced by the current in the toroid. The resulting magneticfield lines are helical, as shown in the figure. In this configuration, the field linesspiral around the plasma and prevent it from touching the walls of the vacuumchamber.In order for the plasma to reach ignition temperature, some form of auxiliaryheating is necessary. A successful and efficient auxiliary heating technique that hasbeen used recently is the injection of a beam of energetic neutral particles into theplasma.When it was in operation, the Tokamak Fusion Test Reactor (TFTR) at Princetonreported central ion temperatures of 510 million degrees Celsius, more than30 times hotter than the center of the Sun. TFTR n values for the D–T reactionwere well above 10 13 s/cm 3 and close to the value required by Lawson’s criterion.In 1991, reaction rates of 6 10 17 D–T fusions per second were reached in theJET tokamak at Abington, England.One of the new generations of fusion experiments is the National SphericalTorus Experiment (NSTX) shown in Figure 30.5c. Rather than generating thedonut-shaped plasma of a tokamak, the NSTX produces a spherical plasma thathas a hole through its center. The major advantage of the spherical configurationis its ability to confine the plasma at a higher pressure in a given magnetic field.This approach could lead to the development of smaller and more economicalfusion reactors.There are a number of other methods of creating fusion events. In inertial laserconfinement fusion, the fuel is put into the form of a small pellet and then collapsedby ultrahigh-power lasers. Fusion can also take place in a device the size ofa TV set, and in fact was invented by Philo Farnsworth, one of the fathers of elec-