102 Ë 4 Superconducting magnetic levitation
is smaller than lower critical fieldHc1. Interaction between the magnetic field and
the type II superconductor in the Meissner-Ochsenfeld state produces a magne-
tic levitation force belowHc1, but this force is smaller than that of the type II
superconductor in the mixed state. It is a very important matter and also is easily
confused.
If a superconductor in the normal state is cooled belowTcby the zero field
cooled (ZFC) method, and the normal state transitions to the superconducting state
without any magnetic field present, when the applied magnetic field is gradual-
ly increased with the superconductor in the superconducting state, there will be
two different situations: one is for type I superconductors in the superconducting
state which completely repel the magnetic field below critical fieldsHc, the other
is the type II superconductors in the superconducting state which also complete-
ly repel the magnetic field below lower critical fieldsHc1. When the applied ma-
gnetic field reachesHc, the superconducting state of type I superconductors is
destroyed. When the applied magnetic field reaches the lower critical fieldHc1,
the superconducting state of type II superconductors is not destroyed. Instead, it
enters the mixed state. When the applied magnetic field reachesHc2, the super-
conducting state of type II superconductors is destroyed. In practical application,
HTS are limited by the irreversibility fieldHc3, above which magnetic vortices melt
or decouple. The irreversibility fieldHc3is betweenHc1andHc2(Hc1<Hc3<Hc2),
andHc3is much lower thanHc 2 (see Section 1.11). The mixed state of the type II
superconductors atHc1<H<Hc3is a necessary condition to produce large levitation
forces.
As the applied magnet field is increased toHc1, the flux lines of the magnetic
field are gradually compressed. When the applied magnetic field increases to a
certain value, a part of the magnetic flux penetrates the superconductor edge through
the pinning centers (vortex), forms a lossless superconducting shield current. The
magnetic field direction of the shield current is the reverse of the external magnetic
field; thus, there is a levitation force in the HTS Maglev system. This levitation force
between the shield current in the HTS bulk and the applied magnetic field is much
larger than that in the type I superconductors. The physical nature of the process is due
to flux pinning in type II superconductors; otherwise, it is impossible for the shielding
current to continually exist in the HTS bulk.
When a superconductor in the normal state is cooled belowTcby the field
cooled (FC) method, the normal-state transitions to the superconducting state with a
magnetic field present. Thereafter, the field will be expelled from the superconductor
at applied magnetic field belowHc1for type II superconductors. When the applied
magnetic field isHc1<H<Hc3, the magnetic flux penetrates the HTS bulk through
the pinning centers (vortex) and forms trapped flux. The trapped flux provides the
force of lateral stability, and the force is called the guidance force.
The total Maglev energy between the HTS bulks and the PMs is restricted by
the sum of magnetic energy of the HTS bulks and the PMs. The magnitudes of the