Principles of Superconducting Levitation
Principles of Superconducting Levitation
Principles of Superconducting Levitation
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).
At the root of Fuji, close to Tokyo, a 43 km long track of the new testing line
was constructed for testing components, functionality, and principles of the
levitating trains. Thanks to the low friction, this train is able to reach extreme
speeds. The present record, reached in December 2003 by a train composed of
5 wagons, with 12-member crew, is 581 km/h. Two trains running in opposite
directions ran at the maximum relative speed over 1050 km/h. The vehicles
may not deviate from their axis by more than about 2 cm. This poses extreme
needs on quality of the components and all systems and requires development
of completely new technologies.
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.
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.
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.
Technologies of MagLev