Prepare for lift off
来源：未知 作者：凌汽匚 时间：2017-06-20 01:00:06
By Duncan Graham-Rowe THE secret of levitation is to think simple. By going back to basics, a Californian research team has found a simpler, safer way to build magnetic levitation (maglev) systems for trains. It might even be used as a cheap means of launching satellites. Maglev trains are already in use around the world, but existing systems use electromagnets to lift the train off the ground. The “Inductrack” system developed at the Lawrence Livermore National Laboratory near San Francisco uses permanent magnets and a novel linear motor instead, so avoiding the need for expensive superconductors or complicated feedback circuits. When stationary, Inductrack vehicles rest on auxiliary wheels. But once they start moving, permanent magnets mounted on the underside induce currents in closed-circuit coils on the track. These currents generate the magnetic field that lifts the vehicle. This approach contrasts with that of other maglev systems, which rely on an external power source to raise the vehicle. To generate the forward push, Richard Post, who heads the Livermore team, designed a new type of linear motor. Rather than use a separate acceleration coil running the length of the track, he inserted a series of these coils between the levitation coils. These externally powered coils create an electromagnetic pulse which nudges the vehicle along without interfering with the levitation. “The system seems so very simple,” says Ray Whorlow at the University of Sussex, who is working on another maglev system for NASA. Inductrack owes this simplicity to the arrangement of permanent magnets, known as a Halbach array, beneath the train. The array consists of a combination of magnets with vertically aligned magnetic fields and others with horizontally aligned fields. “The magnetic field that does the levitation interacts with the horizontal part of the field, while the acceleration coils are phased so that they interact with the fields that are vertical,” says J. Ray Smith of the Livermore research team. One problem of using electromagnets for maglev is that if the power fails, the train will drop back on the track. At speeds of 500 kilometres per hour or more, this could be dangerous. But Inductrack trains will simply glide along until they come to rest. Although impressed by Inductrack, Robert Budell, an engineer who works on the German Transrapid maglev system, believes that the change from electromagnets could lead to problems. “Permanent magnets provide a static amount of force which doesn’t change with the load. So do you set up the train to be full, or empty?