Magnetic levitation

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Magnetic levitation is the process by which an object is suspended above another object with no other support but magnetic fields. The electromagnetic force is used to counteract the effects of the gravitational force.

Earnshaw's theorem proved conclusively that it is not possible to levitate using static, macroscopic, "classical" electromagnetic fields. The forces acting on an object in any combination of gravitational, electrostatic, and magnetostatic fields will make the object's position unstable. However, several possibilities exist to make levitation viable, by violating the assumptions of the theorem.

Table of contents

1 Methods

1.1 Diamagnetism
1.2 Superconductivity
1.3 Feedback control systems
1.4 Oscillating fields
1.5 Gyroscopic motion

2 Trains

2.6 Technology
2.7 Maglev systems

3 See also
4 External links

Methods

Diamagnetism

A substance which is diamagnetic repels a magnetic field. Earnshaw's theorem does not apply to diamagnets since they behave in the opposite manner of a typical magnet (relative permeability μ_r < 1). Many materials have diamagnetic properties, but the effect is very weak, and usually overcome by the object's paramagnetic or ferromagnetic properties. A material which is predominantly diamagnetic will be repelled by a magnet, although typical objects only feel a very small force. This can be used to levitate light pieces of pyrolytic graphite or bismuth above a moderately strong permanent magnet. As water is predominantly diamagnetic, this property has been used to levitate water droplets and even live animals, such as a grasshopper and a frog. The magnetic fields required for this are very high, however; in the range of 16 Teslas, and create significant problems if ferromagnetic materials are nearby.

Superconductivity

Due to the Meissner effect, a superconductor also expels magnetic fields (μ_r = 0), much better than a diamagnet. Due to this (and flux pinning) the magnet is held at a fixed distance from the superconductor or vice versa.

This is the principle in place behind EDS (electrodynamic suspension) maglev trains.

Feedback control systems

If the position and trajectory of the object to be levitated can be measured, the field of nearby electromagnets (or even the position of permanent magnets) can be continuously adjusted via feedback control systems to keep the levitated object in the desired position.

This is the principle in place behind common tabletop levitation demonstrations, which use a beam of light to measure the position of an object. The electromagnet (arranged to pull the ferromagnetic object upwards) is turned off whenever the beam of light is broken by the object, and turned back on when it falls beyond the beam. This is a very simple example, and not very robust. Much more complicated and effective measurement, magnetic, and control systems are possible.

This is also the principle upon which EMS (electromagnetic suspension) maglev trains are based. The train wraps around the track, and is pulled upwards from below.

Oscillating fields

A conductor can be levitated above an electromagnet with a high frequency alternating current flowing through it. This causes any regular conductor to behave like a diamagnet, due to the eddy currents generated in the conductor. Since the eddy currents create their own fields which oppose the magnetic field, the conductive object is repelled from the electromagnet.

This effect requires high frequencies and non-ferromagnetic materials, as the ferromagnetic ones are attracted to the electromagnet.

This is the principle in place behind the Inductrack maglev train system, which avoids the problems inherent in both the EMS and EDS systems, in that it uses only permanent magnets (in a Halbach array) and unpowered conductors to provide levitation. The only restriction is that the train must already be moving at a few km/h (about human walking speed) to levitate.

Gyroscopic motion

The reason a permanent magnet suspended above another magnet is unstable is because the levitated magnet will easily overturn and the force will become attractive. If the levitated magnet is rotated, the gyroscopic forces can prevent the magnet from overturning. This is the principle behind the Levitron toy.

Trains

A magnetic levitation train or maglev is a train-like vehicle that is suspended in the air above the track, and propelled forward using the repulsive and attractive forces of magnetism. Because of the lack of physical contact between the track and the vehicle, the only friction is that between the carriages and the air. Consequently maglev trains can travel at very high speeds with reasonable energy consumption and noise levels (systems have been proposed that operate at up to 650 km/h (400 mph), which is far faster than is practical with conventional rail transport). Whilst the very high maximum speeds make maglev trains potential competitors to airliners on many routes, the cost of constructing the tracks are still high.

Technology

There are three primary types of maglev technology: One that relies on superconducting magnets (electrodynamic suspension), one that relies on feedback controlled electromagnets (electromagnetic suspension), and a newer, potentially more economical system that uses permanent magnets (Inductrack).

Japan and Germany are active in maglev research producing several different approaches and designs. In one design, the train can be levitated by the repulsive force of like poles or the attractive force of opposite poles of magnets. The train can be propelled by a linear motor on the track or on the locomotive or both. Massive electrical induction coils are placed along the track in order to produce the magnetic field necessary to propel the train, leading some to speculate that the cost of constructing such tracks would be enormous.

Magnetic bearings are unstable because of the Earnshaw theorem. Conventional maglev systems are stabilized with electromagnets that have electronic stabilization. The electromagnets and electronics tend to be large, power-hungry, and expensive.

The weight of the large electromagnet is a major design issue. A very strong magnetic field is required to levitate a massive train, so conventional maglev research uses superconductor research for an efficient electromagnet.

The effect of a powerful magnetic field on the human body is largely unknown. For the safety of the passengers, shielding might be needed, which would add additional weight to the train. The concept is simple, but the engineering and design aspects are complex.

A newer, perhaps less-expensive system is called "Inductrack." The technique has a load-carrying ability related to the speed of the vehicle, because it depends on currents induced in a passive electromagnetic array by permanent magnets. In the prototype, the permanent magnets are in a cart; horizontally to provide lift, and vertically to provide stability. The array of wire loops is in the track. The magnets and cart are unpowered, except for the speed of the cart. Inductrack was originally developed as a magnetic motor and bearing for a flywheel to store power. With only slight design changes, the bearings were unrolled into a linear track. Inductrack was developed by physicist William Post at Lawrence Livermore National Laboratory.

Inductrack uses Halbach arrays for stabilization. Halbach arrays are arrangements of permanent magnets that stabilize moving loops of wire without electronic stabilization. Halback arrays were originally developed for beam guidance of particle accelerators.

Currently, some space agencies, such as NASA, are researching the use of maglev systems to launch spacecraft. In order to do so, the space agency would have to get a maglev-launched spacecraft up to escape velocity, a task which would otherwise require elaborate timing of magnetic pulses (see coilgun) or a very fast, very powerful electric current (see railgun).

Maglev systems

In Berlin, the M-Bahn was built in the 1980s: a driverless maglev system with a 1.6 km track connecting 3 metro stations. Testing with passenger traffic started in August 1989, and regular operation started in July 1991. Because of traffic changes after the fall of the wall, deconstruction of the line began only 2 months later and was completed in February 1992. The line was replaced with a regular metro line.

A maglev service ran from the airport terminal of Birmingham International Airport (UK) to the nearby railway station from 1984 till 1995. The length of the track was 600 metres, and trains "flew" at an altitude of 1.5 cm. It was in operation for nearly eleven years, but it was unreliable and was replaced by a bus.

Transrapid (a German maglev company, which has a test track in Emsland, Germany), constructed the first operational maglev railway in the world, from Shanghai, China to the new Shanghai airport in Pudong. It was inaugurated in 2002. It has a peak speed of 430 km/h (269 mph) and a track length of 30 km.

Japan has a test track in Yamanashi prefecture where test trains have reached 581 km/h (363 mph), much faster than wheeled trains.

Unimodal is a proposed personal rapid transit system using Inductrack suspension to achieve average commute speeds of 160kph (100mph) in the city.

On December 31, 2000, the first manned high-temperature superconducting maglev was successfully tested in Southwest Jiaotong University, Chengdu, China. This system is based on the principle that bulk high-temperature superconductors can be levitated or suspended stably above or below a permanent magnet. The load is over 530 kg and the levitation gap is over 20 mm. The system uses liquid nitrogen, which is very cheap, to cool the superconductor.