Maglev 's New Promise

__"Maglev: A New Approach" by Richard F. Post, in Scientific American (Jan. 2000), 415 Madison Ave., New York, N.Y. 10017–1111.__

For decades, it’s been said that the maglev, or magnetically levitated train, would soon be arriving to whisk people off on silkysmooth rides at 300 miles per hour or more. It hasn’t happened. The maglevs demonstrated in Germany and Japan have been too complicated and expensive--and not fail-safe.

No full-scale commercially operating maglev system has been built. But now from Lawrence Livermore National Laboratory in California comes a new approach that Post, a senior scientist there, says may finally bring the maglev into the station. In a maglev system, magnetic fields levitate the train while electricity or some other sort of power drives it forward. The Japanese system used superconducting coils to produce the magnetic fields (as two American scientists first proposed in the late 1960s). But because such coils must be kept very cool, costly cryogenic equipment is required on the train cars. "The German maglev uses conventional electromagnets rather than superconducting ones, but the system is inherently unstable because it is based on magnetic attraction rather than repulsion," Post says. In both systems, a malfunction "could lead to a sudden loss of levitation while the train is moving." Minimizing that hazard means increased "cost and complexity."

The Livermore approach uses permanent room-temperature magnets, powerful kin to the familiar refrigerator magnet and once thought inadequate to the levitational task. "On the underside of each train car," explains Post, "is a flat, rectangular array of magnetic bars called a Halbach array" (after its inventor). With the bars in that special pattern, the magnetic-field lines combine to produce a very strong field below them.

The other critical element in the "Inductrack" (as the new maglev system is called) is track "embedded with closely packed coils of insulated wire." When the train cars move forward, the magnets arrayed beneath them induce currents in the track’s coils, which in turn generate an electromagnetic field that repels the Halbach arrays, lifting the train. "As long as the train is moving...a bit faster than walking speed," the arrays "will be levitated a few centimeters above the track’s surface." Side-mounted Halbach arrays provide lateral stability. Because the levitating force increases as the magnets get closer to the coils (if the train is carrying a heavier load, for instance, or rounding a bend), this maglev system is "inherently stable," Post says.

What would happen if the drive power suddenly failed? "The train cars would remain levitated," Post says, "while slowing down to a very low speed, at which point the cars would come to rest on their auxiliary wheels."

A 1997 study concluded that an Inductrack system would be cheaper than the German maglev, and "proved that the concept is workable," Post says. And it may work for more than high-speed rail: The National Aeronautics and Space Administration thinks the Inductrack approach could prove helpful in getting rockets off the ground.