FOR the past two decades, prototype magnetically levitated (maglev) trains cruising at up to 400 kilometers per hour have pointed the way to the future in rail transport. Their compelling advantages include high speeds, little friction except aerodynamic drag, low energy consumption, and negligible air and noise pollution.
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.

External Power Needed
Post notes that a power source is needed to accelerate the cart to its operating speed of 10 to 12 meters per second. The first section of the test track uses a set of electrically energized track coils--aided by a stretched bungee cord--to reach this speed. A full-scale train might use an electronic drive system, as found on experimental German trains, or even a jet turbine, as proposed by Inductrack engineer Smith. "Inductrack allows you the possibility of carrying all the power with you," emphasizes Post.
Even though the electromagnetic drag associated with Inductrack becomes small at high speeds, an auxiliary power source would also be needed to maintain the train's high speed against aerodynamic drag. The amount of power needed depends on the weight of the vehicle and its maximum speed. If the external drive power ever fails, or when the train arrives at a station, the train cars would simply coast to a stop, easing down on their auxiliary wheels. In this sense, Inductrack is a true fail-safe system.
Livermore is one of the few institutions to explore the uses of the Halbach array. Indeed, the Inductrack concept arose from Post's research on an electromechanical battery designed for superefficient cars and trucks (See April 1996 S&TR, "A New Look at an Old Idea."). The Livermore battery uses circular Halbach arrays both to generate power and to achieve nearly frictionless magnetic bearings that minimize the loss of stored energy.
"We just unrolled the circular magnetic arrays from the electromechanical battery into a linear array on the car that seemed ideal for trains and other vehicles," he explains.
The Halbach array offers other benefits besides levitation. Because its magnetic fields cancel out above the magnets, there is no worry about magnetic fields affecting passengers' heart pacemakers. In contrast, passengers must be magnetically shielded on maglev trains employing superconducting coils.
The consulting company of Booz-Allen & Hamilton conducted a preliminary feasibility study of Inductrack and compared it to other maglev technologies. The study found that while an Inductrack system would cost more to build than conventional rail systems, it should be less expensive than maglev trains using superconducting coils. The study also found that Inductrack should be able to achieve speeds of 350 kilometers per hour and up and demonstrate lower energy costs, wheel and rail wear, propulsion maintenance, and noise levels.

Launching Rockets
Last October, negotiations were completed on a three-year contract with NASA to build a new Inductrack model at Lawrence Livermore to demonstrate the concept at speeds up to Mach 0.5 (170 meters per second). NASA is interested in maglev technology to help launch rockets at sharply reduced costs. As conceived, a track would use a reusable launcher to propel a rocket up a ramp to almost Mach 1 speeds before the rocket's main engines fire. According to Smith, the technology should be able to save about 30% of the weight of the launch vehicle. "Rocket engines are not fuel-efficient at low speed," he points out.
The Livermore team is designing a 150-meter-long track, to be built at the Laboratory site, on which a scaled launch cradle and rocket will be accelerated. Unlike the present track, the one for NASA will interleave powered drive coils with passive levitation coils to reach the required speeds. The team is partnered with computer scientists at Pennsylvania State University, who are developing an integrated design code that includes magnetics, aerodynamics, stresses, and control stability to assess full-scale systems.
Post believes Inductrack offers NASA the potential for a far less expensive technology for magnetic levitation launchers than approaches using superconducting coils. He and Smith note, however, that while the existing Inductrack model has demonstrated the principle of the concept, there are new issues to be addressed in launching rockets. Among these are high g forces, sustained speeds of Mach 0.5 or higher, the effects of fluctuating aerodynamic forces on the launching cradle and its payload, and aerodynamic and other issues associated with detachment and flight of rockets.
--Arnie Heller

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