Boozer, Mauel, and Navratil Featured in the New York Times
Systems Designed to Hold a Homemade Sun
By Malcome W. Browne
New York Times, Science: June 8, 1999
Scientists have developed a variety of devices and systems in which they hope to be able to compress hydrogen to the densities and temperatures needed to sustain thermonuclear fusion reactions. These are among them.
Tokamaks, reactors that are shaped like doughnuts and occupy large buildings, are chambers from which air has been removed and into which is injected a diffuse plasma of hydrogen isotopes, especially mixtures of deuterium and tritium. The plasma, a hot, electrically charged gas, is compressed within the doughnut by two sets of magnetic fields, one in which the magnetic field circles the doughnut the long way around and the other in which the magnetic field lines run the short way around the doughnut tube.
By varying the geometry and strength of the fields, they can be adjusted to produce a uniform compressive force on the plasma. But even with uniform compression, turbulence and instabilities develop in a tokamak's plasma, cooling it and enormously complicating the goal of achieving sustained fusion.
To ignite fusion, the compressed plasma in a tokamak must be raised to the temperature of the Sun, and various strategies have been developed for this. Part of the necessary heating is achieved simply by compressing the plasma. Another technique involves passing an electric current through the plasma, thus heating it in the same way electricity heats a toaster wire.
Heating can also be achieved by ducting microwave energy into the plasma, and another technique heats the plasma by shooting beams of neutral particles into it from an accelerator. Once fusion is initiated, helium ions (also called alpha particles) produced by the fusion of hydrogen nuclei transfer their energy to the plasma and help to heat it.
Dozens of tokamaks have been built and tested in many countries. Although the United States withdrew from a project called the International Thermonuclear Experimental Reactor -- a huge tokamak the planning for which has already consumed $1 billion -- Russia, Europe and Japan are continuing some work on the machine. But prospects for financing the project to completion seem bleak.
Meanwhile, the largest existing tokamaks continue to produce impressive fusion pulses. The Joint European Torus in England achieved a record for fusion power in a burst of 20 million watts.
Spherical Torus Machines
These are similar to tokamaks, in that they compress plasma fuel within toruses using interlocking magnetic fields. But they have much thinner cores than ordinary tokamaks, giving them a shape more similar to that of an apple than a doughnut. Their more or less spherical shape permits more efficient compression of plasma than can be achieved in a conventional tokamak.
A small spherical torus built and tested in Britain has already proved to have a very high ratio of plasma compression to magnetic field strength. Improving this ratio is a major goal of all magnetic confinement fusion devices.
The Princeton Plasma Physics Laboratory has built a somewhat larger spherical torus reactor that is scheduled to begin an experimental program in August. Scientists believe that this type of reactor, with a tight magnetic field passing through its core, may be able to compress plasma and reach fusion conditions more cheaply than is possible with conventional tokamaks. Princeton's National Spherical Torus Experiment will not achieve fusion, but it will allow operators to experiment with a wide variety of conditions.
Like tokamaks, these are more or less doughnut shaped, but their magnetic fields are designed to shape the compressed plasma into a warped helix with many kinks. Unlike a tokamak, a stellarator need not be perfectly symmetrical. Its configuration is believed to control plasma turbulence more efficiently than does a tokamak. Oak Ridge National Laboratory and the Princeton laboratory hope to get Federal financing next year to build a small stellarator.
Levitated Dipole Experiment
This scheme, a project of the Massachusetts Institute of Technology and Columbia University, is one of several based on highly speculative ideas that scientists have identified as worth a small exploratory investment of several million dollars.
The levitated dipole idea came in 1987 to Dr. Akira Hasegawa, then working at Bell Laboratories and analyzing data from the Voyager flybys of Jupiter and its moons.
He was inspired by the discovery that Jupiter's magnetic field seemed to be able to capture and confine plasma reaching it from the solar wind and the moon Io, and he theorized that similar plasma confinement might be achieved in the laboratory with a magnetically levitated superconducting coil playing Jupiter's role.
The scientists in charge of the present attempt to put Dr. Hasegawa's idea into practice are Dr. Michael Mauel of Columbia and Dr. Jay Kesner of M.I.T.
"The Department of Energy held a competition two years ago for the funding of new experiments, and out of 42 proposals two were chosen, including ours," Dr. Kesner said.
"You'd think that this model for plasma confinement would have occurred to someone a long time ago," he said, "but plasma researchers didn't know there were plasmas captured by magnetospheres surrounding planets."
Dr. Kesner and Dr. Mauel are building a doughnut-shaped superconducting magnet that will weigh about half a ton. When finished, the magnet will be suspended in a vacuum chamber supported solely by a feedback-controlled magnetic field. The two scientists expect to float their heavy magnet in the empty space within their test chamber for at least eight hours at a time.
An electric current introduced into the superconducting magnet will continue to flow indefinitely, because a superconductor offers no resistance to electricity. This could create a magnetic field capable of drawing in charged plasma.
Whether such a device could ever be used to raise plasma to fusion temperature remains to be seen, and it is probable that even if it did, Dr. Kesner said, the fuel would have to be an exotic mixture of deuterium and helium-3 -- a fuel that would not heat up the ultracold magnet and destroy its superconductivity by emitting swarms of high-energy neutrons. The trouble is that helium-3 is very rare on Earth, and the necessary quantities might be available only on the Moon, where it is created by the impact of cosmic rays on the lunar surface.
Still, a floating dipole reactor could efficiently power a long-endurance rocket that might one day be sent to a nearby star, the scientists believe.
Some scientists and engineers have argued that fusion energy is inherently impractical because a fusion reactor costing billions would quickly wear out. The flood of very high-energy neutrons produced by hydrogen fusion penetrates metal and disrupts its crystalline structure, causing it to expand and become dangerously brittle. If the neutron bombardment were to reach the metal wall of a reactor's vacuum chamber, the entire chamber could collapse, destroying the reactor.
Most of the latest reactor plans call for capturing the neutrons in a metal "blanket" (probably made of lithium) surrounding the hot plasma fuel. The neutrons would heat the blanket, and this heat would be transferred to a steam boiler to power a conventional turbine. The lithium blanket and some other internal components of the reactor would wear out in a few years, but could be periodically removed and replaced at relatively modest cost. The main part of the reactor would be spared.
Besides compressing and igniting fusion fuel magnetically, the fusion reactions could also be ignited by an implosion as in hydrogen bombs.
In an inertial confinement reactor, the mixture of deuterium and tritium fuel is encapsulated as a thin frozen coating on the inside of tiny plastic hollow spheres. For each shot, a sphere is encased in a metal "hohlraum," a chamber about the size of a pencil eraser placed at the focus point of 192 powerful lasers.
When the lasers hit the hohlraum, it emits a blast of X-rays that vaporize and compress the fuel sphere it encloses, imploding the sphere and raising the temperature of the fuel inside it to the ignition point, 100 million degrees Celsius or so. During the brief fusion reaction that ensues, the immense energy it produces is absorbed by a surrounding metal "blanket," which transfers heat to a steam-powered generating plant.
This will be the basis of the $1.2 billion National Ignition Facility under construction at Lawrence Livermore National Laboratory in California. Because experiments using the machine will help to monitor the combat readiness of America's arsenal of nuclear weapons, part of its financing comes from the Department of Energy weapons program.
A variant of inertial confinement fusion called a "Z pinch" is under development at Sandia National Laboratories in New Mexico.
This device would implode little thermonuclear fuel capsules like those the Livermore laboratory is developing, but without lasers. Instead, scientists have invented a cylindrical array of very fine tungsten wires and a gigantic capacitor, a device that can store and instantaneously discharge huge electric charges.
When the Sandia capacitor is fired, it sends a current of 20 million amperes through the fine wires in the Z pinch, and the wires are converted into super-hot plasma. The plasma is driven inward upon itself by the huge magnetic field produced by the sudden current. The collision of the imploding plasma generates a gigantic X-ray pulse and this, it is hoped, can implode fuel capsules to reach ignition.
The Z pinch and a more powerful successor planned by Sandia, the X-1, will not ignite a sustained reaction but should produce a pulse of fusion.