Principles of superconducting levitation
Permanent magnet levitating over a high-Tc superconductor
Principles of superconducting levitation
Permanent magnet levitating over a high-Tc superconductor
Superconductors are materials exhibiting two basic features: (i) zero resistivity and
(ii) ideal diamagnetism - i.e. they expel magnetic field from their volume.
The magnet suspension over
the superconductor as shown above resembles interaction of two permanent magnets placed above each other with same poles
oriented to each other. It is, however, a highly unstable configuration.
On the other hand, a permanent magnet together
in combination with a superconductor form a very effective and stable configuration. It is due to the following trick:
Before the "superconductor" is cooled down to the superconducting state, a permanent magnet is placed close (a few
millimeters appart) from it. As the "superconductor" is still in the normal state, magnetic field of the permanent magnet
penetrates the entire "superconductor". After cooling the superconductor below its critical temperature, expelling a
sufficiently high magnetic field (higher than the characteristic value Hc1) from the superconductor volume would
be energetically costly. Instead, tiny channels are formed in the supercoductor, called vortices. The normal cores of
vortices are screened by superconducting screening currents and each vortex carries one quantum of magnetic flux. The
external magnetic field is thus "frozen" in the superconductor in the form of vortices. As vortices are at the superconductor
surface bound to the external magnetic field, any change of the external field is translated to the superconductor interior.
But it is not easy to move vortices. The superconductor opposes to any change of the original configuration of the external
magnetic field, both in magnitude and direction. An effective magnetic trap is thus formed that keeps the permanent magnet
in its original position.
The additional screening currents induced by the external field change force the magnet to return into its original place. This constitutes a very stable and efficient magnetic trap working with the magnet placed either above the superconductor (levitated) or below it (suspended).
When the permanent magnet suspended over
or below the superconductor is brought into rotation, it stays in revolution for a very long time, especially in vacuum,
where any friction is absent. This is the principle of superconducting loss-less bearing, superconducting motor
or superconducting gear. The latter, in a huge dimensions can serve as a energy storage device, an old dream
of scientists. Smaller gears can be used for stabilization of a space craft as their large kinetic energy brakes a change
of the gear orientation. This application is particularly appropriate in the space for the low temperature present there,
which enables a natural and effective cooling of the high-Tc superconductor.
Another attractive application is the train levitating on a magnetic suspension.
Magnetically levitating superexpress
MagLev train on the testing line in Yamanashi
Principles of magnetically levitating train
Probably the most attractive application of the
superconducting levitation at present is the magnetically levitated train, MagLev.
The train consists
of three or five cars, each of them carrying altogether four sets of superconducting coils, two and two on each side. These
coils are at present of conventional low-temperature superconductors and have to be cooled by liquid helium to a temperature
close to absolute zero, -269o C. For a reasonable economical operation the conventional coils need to be replaced
by coils or permanent superconducting magnets of high temperature superconductors that might work at much higher temperatures.
The train runs in a concrete guide way on sides of which there are three systems of copper coils.
One system serves for the train levitation, another one for the train propulsion, and the third one for lateral stability
in the guideway. The upper figure demonstrates the principle of the train levitation. The superconducting coils on the cars
produce high magnetic field of about 5 Tesla. At sufficiently high speed (above 130 km/h) this field induces magnetic field
in the stable copper coils on the bed sides that is high enough to keep the train safely above the bottom. Below the
critical speed the train is driven by a conventional electrical motor and runs on rubber wheels.
Electric current passing through the copper coils on the ground produce alternating magnetic field that attracts the superconducting magnets of the train and propells the train forward.
The levitation and guidance coils on both sides are interconnected so that they keep the vehicle in the
center of the guide way. When the car departs from the center, an attractive force is produced on the further side and a
repulsive one on the nearer side so that the car returns back to the ideal track. Last but not least, the train has several systems of brakes, aerodynamic one - a shield
that throws up from the vehicle at high speed, the electrodynamic one that brakes by means of linear propulsion motor, and
the braking of rubber wheels is available at lower speeds.
Imagine precision of this mechanism:
the horizontal distance between the superconducting and copper coils is only 8 cm and the train has to be safely guided even
in curves at speeds over 500 km/h.
Vehicle with an aerodynamic front
Vehicle with a double-casp front
Aerodynamic brakes |
Interior of the train |
This page was prepared by using information andfigures from the materials of the Railway Technical Research Institute,RTRI, Tokyo, Japan. Further information can be found on the web page of this institute, http://www.rtri.or.jp.
