However, maglev trains also pose significant drawbacks in maintenance costs, mechanical and electronic complexity, and operational stability. Some maglev train cars, for example, employ superconducting coils to generate their magnetic field. These coils require expensive, cryogenic cooling systems. These maglev systems also require complicated feedback circuits to prevent disastrous instabilities in their high-speed operation.Lawrence Livermore scientists have recently developed a new approach to magnetically levitating high-speed trains that is fundamentally much simpler in design and operation (requiring no superconducting coils or stability control circuits), potentially much less expensive, and more widely adaptable than other maglev systems. The new technology, called Inductrack, employs special arrays of permanent magnets that induce strong repulsive currents in a "track" made up of coils, pushing up on the cars and levitating them.
Totally Passive Technology
During the past two years, a Livermore team, headed by physicist Richard Post, has successfully demonstrated the Inductrack concept in test trials. The test runs demonstrated the system's totally passive nature, meaning that achieving levitation requires no control currents to maintain stability, and no externally supplied currents flowing in the tracks. Instead, only the motion of train cars above the track is needed to achieve stable levitation. The results have been so promising that NASA has awarded a three-year contract to the team to explore the concept as a way to more efficiently launch satellites into orbit.
Inductrack involves two main components: a special array of permanent, room-temperature magnets mounted on the vehicle and a track embedded with close-packed coils of insulated copper wire. The permanent magnets are arranged in configurations called Halbach arrays, named after Klaus Halbach, retired Lawrence Berkeley National Laboratory physicist. Originally conceived for particle accelerators, Halbach arrays concentrate the magnetic field on one side, while canceling it on the opposite side. When mounted on the bottom of a rail car, the arrays generate a magnetic field that induces currents in the track coils below the moving car, lifting the car by several centimeters and stably centering it.
When the train car is at rest (in a station), no levitation occurs, and the car is supported by auxiliary wheels. However, as soon as the train exceeds a transitional speed of 1 to 2 kilometers an hour (a slow walking speed), which is achieved by means of a low-energy auxiliary power source, the arrays induce sufficient currents in the track's inductive coils to levitate the train.
To test the Inductrack concept, Post, project engineer J. Ray Smith, and mechanical technician Bill Kent assembled a one-twentieth-scale model of linear track 20 meters long (Figure 1). The track contained some 1,000 rectangular inductive wire coils, each about 15 centimeters wide. Each coil was shorted at its ends to form a closed circuit but not otherwise connected to any electrical source. Along the sides of the track, they attached aluminum rails on which a 22-kilogram test cart could ride until the levitation transition velocity was exceeded (Figure 2). Finally, the team secured Halbach arrays of permanent magnet bars to the test cart's underside for levitation and on the cart's sides for lateral stability.
The cart was then launched mechanically at the beginning of the track at speeds exceeding 10 meters per second. High-speed still and video cameras revealed that the cart was consistently stable while levitated, flying over nearly the entire track length before settling to rest on its wheels near the end of the track.
Post says the test results are consistent with a complete theoretical analysis of the Inductrack concept he performed with Livermore physicist Dmitri Ryutov. The theory predicts levitation forces of up to 50 metric tons per square meter of magnet array using modern permanent magnet materials such as neodymium-iron-boron. The theory also shows levitation of loads approaching 50 times the weight of the magnets, important for reducing the cost relative to maglev vehicles.
