8.3 Methods to improve Maglev performances Ë 295Thus, from these Maglev experiments of individual and double-layer HTS bulk above
the PMG, it was verified that the levitation forces and guidance forces can be indeed
improved by adding the upper-layer HTS bulk, but the final Maglev performance
of the double-layer HTS bulk cannot be calculated by simple addition. Due to the
shielding and interaction of the two adjacent HTS bulk layers, only part of the Maglev
performance of the upper-layer HTS bulk had been excited. In the typical FCH, an
improvement of 16.9% in levitation forces and 8.8% in guidance forces was obtained.
The added upper-layer HTS bulk was more effective to improve the Maglev perfor-
mance in the ZFC case than the FC case and more effective to improve the levitation
forces than the guidance forces. The increase of a layer of HTS bulk meant a double
increase in the cost of superconductor. However, compared with the huge PMG cost,
the cost of the upper-layer HTS bulk was not so significant. Thus, this optimization
method was able to enhance the load capability and stability of the levitation system
simultaneously without high cost.
8.3.4A laying mode using thec-axis orientation of bulk HTSC
In this section, a laying mode of the HTS bulk above the PMG will be introduced based
on the anisotropy properties of HTS bulk materials and the magnetic field feature of
the applied double-pole Halbach PMG so that they can match to each other efficiently
as shown in Fig. 8.36.
Due to the layered structure of the Cu-O plane in the YBCO crystal, the HTS bulk
exhibits a high anisotropy between thec-axis and thea-bplane. The typical critical
current density,Jc, in thea-bplane is found to be about three times that along the
c-axis [28]. This property indicates that it will be more efficient to excite the induced
current in thea-bplane compared with along thec-axis. At the field-pole position,
it reaches the maximum vertical component of magnetic field, while, at the middle
of the Halbach PMG, it was the maximum horizontal component of the magnetic
field, where most of the magnetic flux lines are parallel to the horizontal direction.
In an HTS levitation system, the levitation or guidance force (electromagnetic force)
Fig. 8.36:Schematic illustrations of the different laying modes of a HTS bulk working in the
horizontal-applied magnetic field, where the bulk keeps itsc-axis vertical to the applied field
(a) or parallel to the field (b).