Inductrack Passive Magnetic Levitation
PERMANENT MAGNETS under an Inductrack train car are arranged in a Halbach array (above) so that the magnetic-field lines reinforce one another below the array but cancel one another out above it. When moving, the magnets induce currents in the track's circuits, which produce an electromagnetic field that repels the array, thus levitating the train car. Halbach arrays can also provide lateral stability if they are deployed alongside the track's circuits (below).
Called the Inductrack, the new system is passive in that it uses no superconducting magnets or powered electromagnets. Instead it uses permanent room-temperature magnets, similar to the familiar bar magnet, only more powerful. On the underside of each train car is a flat, rectangular array of magnetic bars called a Halbach array. (It is named after its inventor, Klaus Halbach, a retired Lawrence Berkeley National Laboratory physicist.) The bars are arranged in a special pattern, so that the magnetic orientation of each bar is at right angles to the orientations of the adjacent bars [see top illustration on this page]. When the bars are placed in this configuration, the magnetic-field lines combine to produce a very strong field below the array. Above the array, the field lines cancel one another out.
The second critical element is the track, which is embedded with closely packed coils of insulated wire. Each coil is a closed circuit, resembling a rectangular window frame. The Inductrack, as its name suggests, produces levitating force by inducing electric currents in the track. Moving a permanent magnet near a loop of wire will cause a current to flow in the wire, as English physicist Michael Faraday discovered in 1831. When the Inductrack's train cars move forward, the magnets in the Halbach arrays induce currents in the track's coils, which in turn generate an electromagnetic field that repels the arrays. As long as the train is moving above a low critical speed of a few kilometers per hour-a bit faster than walking speed-the Halbach arrays will be levitated a few centimeters above the track's surface.
The magnetic field acts much like a compressed spring: the levitating force increases exponentially as the separation between the track and the train car decreases. This property makes the Inductrack inherently stable-it can easily adjust to an increasing load or to acceleration forces from rounding a bend in the track. Thus, the system would not require control circuits to maintain the levitation of the train cars. All the train would need is some source of drive power to accelerate it.
In the past, engineers believed permanent magnets could not be used in maglev systems, because they would yield too little levitating force relative to their weight. The Inductrack's combination of Halbach arrays and closely packed track coils, however, results in levitation forces approaching the theoretical maximum force per unit area that can be exerted by permanent magnets. Calculations show that by using high-field alloys-neodymium-iron-boron, for example-it is possible to achieve levitating forces on the order of 40 metric tons per square meter with magnet arrays that weigh as little as 800 kilograms per square meter, or one fiftieth of the weight levitated.
In a full-scale Inductrack system, the track would consist of two rows of tightly packed rectangular coils, each corresponding to one of the steel rails in a conventional track. The main levitating Halbach arrays would be placed on the underside of the train car so that they would run just above the rows of coils [see the second illustration on this page]. Smaller Halbach arrays could be deployed alongside the rows of coils to provide lateral stability for the train car. Such a configuration would somewhat resemble its counterpart in an ordinary train-namely, a flanged wheel rolling on a steel rail. In the Inductrack the role of the "flanges" is played by the small side-mounted Halbach arrays, whereas the role of the "wheel" is fulfilled by the main levitating arrays.
