3.4 Levitation phenomenon
Levitation phenomenon not only attracted wide attention of the public, but also attracted the interest of scientists and engineers. In fact, levitation and suspension can be achieved by some physical methods.
Maglev technologies have a series of advantages（see Table 3. 3.2）.Maglev is currently attracting worldwide interest for both high speed intercity vehicles and low speed urban transit vehicles, especially high or ultra-high speed intercity traffic.
In 1842，Earnshaw15 theoretically studied the classical inverse-square law forces for gravity, electrostatics, and magnetism field, and proved mathematically that stable levitation is impossible for an object placed in a repulsive or attractive static force field. Earnshaw wrote it before Maxwell's work and the discovery of the elec tron.Earnshaw's theorem forbids magnetic levitation in many common situations.However, this is not generally true.The theorem applies only to individual parti cles.Braunbeck16went further to show that the instability is due to the relative permeability of the materials involved being greater than unity.
In the 19th century, buoyancy was considered and studied to support objects without contact to the ground. Scientists have proposed several suspension princi-ples, for example, levitation using water pressure, levitation based on the aircushion principle, levitation and propulsion with a blower, etc.A model train levitated by water pressure based on an idea of Girard was demonstrated at the Paris Exposition of 188917.
On March 19，1912，Bachelet18was awarded a patent，"levitating transmittingapparatus"，for an electromagnetic suspension system. His invention was first intended to be applied to smaller mail carrying systems, but its potential application to larger train-like vehicles is certainly clear.Bachelet's invention opened the door to human Maglev application.
In 1934，Kemper19was awarded a patent，"monorail vehicle with no wheels attached". These inventions eventually led to the development of the Transrapid Maglev train program begun in 1969 in Germany.This magnetic levitation was called the electromagnetic suspension（EMS），EMS was achieved by the attraction forces between an electromagnet and the iron rail.Stable levitation was produced by electromagnets using an active feedback loop.
Powell and Danby20，21are the inventors of the superconducting Maglev trans portation systems. They filed a patent application（No.684775）for an electromag netic inductive suspension and stabilization system for a ground vehicle on Nov.21，1967，and the patent was granted（No.3470828）on Oct.7，1969.This mag netic levitation is called the electrodynamic suspension（EDS）.The time-varying magnetic fields induce eddy currents in the conductors which create a mutually re pulsive magnetic field.The time-varying magnetic fields can be produced by motion of a magnet.Their inventions, included inductive levitation and stable guidance, null flux geometry, and the linear synchronous motor for vehicle propulsion.This invention of the EDS using LTS magnets is the basis for the 500 km/h LTS Maglev train route from Tokyo to Osaka now under construction.
In 1933，Meissner and Ochsenfeld22found that when a sphere is cooled below its transition temperature in a magnetic field, it expels the magnetic flux. In 1945，the stable levitation of a permanent magnet over a superconductor was first discovered for a magnet above a concave lead（low Tc superconductor）disk by Arkadiev23，24，at Moscow State University, USSR.
For both diamagnetic materials16，25and high Tc superconductors26，27，stable levitation is possible in static magnetic fields. The levitation of high Tc supercon-ductors is a most conspicuous phenomenon.It is really static（requires no energy input），quiet, and exceedingly stable.It is called the unique self-stable levitation.A high Tc superconductor can be levitated above or below a PM.
HTS bulk could realize very stable levitation without any active control. Thephysical characteristics of self-stabilizing of a HTS bulk Maglev is a unique phenom-ena in nature.A lot of progresses have been made in the theory and application of HTS Maglev, especially in the fields of energy storage and transportation.In 2001，Jia-Su Wang and Su-Yu Wang were awarded a patent，"a superconducting Maglev system"28，for a manned HTS Maglev vehicle.
Other types of stable levitation such as aerodynamic, acoustic, optical, electric, and radio-frequency（RF）levitation, etc.，can be found elsewhere29.
The invitation keynote speeches from Japanese, Chinese, German, American and Korea experts at the 2011 Conference（21st International Conference on Magnetically Levitated Systems and Linear Drives, October 10 to 13，2011，Daejeon, Korea.）reported several main Maglev systems using EMS, EDS, HTS, etc.
In order to compare and understand the unique advantage of HTS Maglev, traditional magnetic levitation will be reviewed first.