Magnetic Confinement:
Charged particles spiral around magnetic field lines, and if the container closes on itself, like a torus (a donut), the plasma can circulate within the container. Various approaches have been invented for shaping the magnetic fields needed to confine such a toroidal plasma, with different advantages and disadvantages.

Diagram of a coil would around a torus. Note that the wires are closer together on the inside of the torus and farther apart on the outside. When the wires carry electric current, the resulting magnetic field is therefore stronger in the inner part of the torus, so the plasma ions drift towards the walls.

A drawing of the ITER tokamak fusion reactor. The person shown in the foreground gives the scale of this device. Also, note that the torus cross-section is uniform—quite different from the stellarator. (published with permission of ITER)

One wall of the target chamber of the National Ignition Facility (credit: National Ignition Facility)

Stellarator:
The stellarator, the first fusion machine, was designed by Lyman Spitzer in 1951. This device uses complicated coil windings, as shown in the illustration, outside the containment vessel to form the magnetic fields needed to confine the plasma. This approach avoids having to drive a large current in the plasma itself, but fabricating and positioning the magnets is a difficult challenge. The National Compact Stellarator Experiment (NCSX), a collaboration between the Oak Ridge National Laboratory (ORNL) and the Princeton Plasma Physics Laboratory (PPPL), is a proof-of-principle project to evaluate the viability of the compact stellarator. NCSX will cost about $74 million and be completed in 2007.

Tokamak:
Invented in the Soviet Union in the 1960s, this design utilizes a current driven in the plasma itself. This current generates large part of the magnetic field required for confinement. While this internal plasma current is very useful, it can also produce violent instabilities in the plasma. Decades of research have produced much progress in confinement and in the control of these instabilities. A tokamak design will be employed in ITER, an international collaboration that will “demonstrate prolonged fusion power production” and will develop and test key technologies, including the thermal lithium blanket that will breed tritium (see About section). ITER will cost about 10 billion dollars over its 20-year operating life. Construction is about to begin.

Inertial Confinement:
Inertial confinement fusion (ICF) is a method significantly different than that used in the various magnetic confinement schemes described above.  ICF uses high energy beams of laser light or atomic particles to implode a small D-T fuel-filled target shell, greatly increasing the temperature and density of the fuel mixture. For example, the density of the D-T mixture can be raised to 20 times that of lead. The resulting fusion reactions cause the target to explode. Major challenges to ICF include developing high powered sources and beam timing methods that insure simultaneous target impact from dozens of individual beams. One inertial confinement project is the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL). The NIF trains 192 laser beams D-T targets about a millimeter across. NIF lasers will deliver a thousand times the electric power generated in the US, for a few billionths of a second. The NIF is now under construction.

Z Pinch:
A quite different approach to producing fusion reactions in the laboratory from either magnetic or inertial confinement methods is found in the Z Pinch. The version now operating at Sandia National Laboratories was initially devised to model nuclear explosions, and now shows promise for investigating fusion. In a Z pinch machine, a large capacitor bank is rapidly discharged through an array of fine parallel wires in a volume approximately that of a spool of thread. The huge current pulse in the wires heats the metal and vaporizes the wires into a plasma discharge. The magnetic field produced by the current in the wires exerts a radial force (use the right hand rule to visualize this) on the plasma ions, which are driven toward the center axis of the spool, producing a tremendous burst of x-rays. Recent results produced more than 80 times the power output of all the power plants on Earth, but for only a few billionths of a second—like a giant flashbulb that emits x-rays.

The Z Pinch machine at Sandia National Laboratory. In each shot, a large voltage is discharged through an array of tiny wires, imploding them and producing a large amount of x-ray energy. (photo credit: Sandia National Laboratory)

To compare magnetic and inertial confinement, the following table shows the temperatures, densities, and confinement times, along with temperature and density in the interior of the sun.

In fusion research and development, advances in confinement physics have paralleled associated advances in reactor engineering. Beyond the reactor, a heat exchange system employing liquid or gaseous lithium is envisioned that would power conventional steam-driven generators of electricity. Profitable energy production may require significant experience on several generations of energy producing plants.