88 Ë 4 Superconducting magnetic levitation
characteristics of self-stabilization of HTS bulk Maglev is a unique phenomenon in
nature.
HTSCs form a large family of superconducting ceramic materials. They have a
higher critical temperature, critical current density and critical field than LTSCs. The
ability to use relatively inexpensive and easily handled liquid nitrogen as a coolant
has increased the range of practical applications of superconductivity.
HTS bulk materials are especially attractive for practical applications because of
their excellent current transport capacity in the presence of strong magnetic fields and
extremely high trapped field ability in a compact space, particularly unique features
for self-stable levitation without active control systems.
Since the discovery of the HTS Maglev phenomenon, much progress has been
made in theory and application of HTS Maglev, especially in the fields of energy
storage [12] and transportation [13]. There are some comprehensive review papers
on HTS Maglev which can be found elsewhere [14–18] Special comments on the HTS
Maglev vehicle can be found in Refs. [19–26].
4.2 LTS Maglev
It is generally known that a superconductor is not resistive. Strictly speaking, the
ratio of resistance of the normal conductor to the superconductor has been shown
to exceed 10 [14]. Not only do superconductors (LTSCs and HTSCs) have high critical
current densities and high critical fields, but also they exhibit perfect diamagnetism
(Meissner effect). The magnetic levitation between superconducting bulks and PMs
is a unique and inherently self-stable levitation system. These special properties of
superconductors are especially suitable for applications.
Up to now, the Nb-Ti has achieved a critical current density in excess of 10^6 A/cm^2
at 4.2 K and 5 T [27, 28]. The maximum critical fieldsHc2are 14.5 T (Nb-Ti) and 30 T
(Nb 3 Sn) [29, 30]. These superconducting wires can be widely used in a variety of needs
for strong magnetic field, of course, including the Maglev.
Normal conductive Maglev is usually the EMS described in Section 3.4. The
PM Maglev in the section actually belongs to the EMS type, because it is unstable
without feedback control (by Earnshaw’s theorem). Full-scale EMS Maglev systems
of the Transrapid in Germany, the HSST in Japan, and the UTM-02 in Korea have
entered into the commercial stage, because active feedback control technology is quite
mature.
At present, LTS Maglev is the EDS version. In the EDS Maglev, the changing
magnetic field of the moving coil induces eddy currents in the conducting plate.
By Lenz’s law, the induced eddy currents generate a magnetic field which opposes
the field of the coil, thereby providing a levitation force between the coil and the
conducting plate. It will have enough levitation force to lift the entire train if the