8.2 Dynamic characteristics Ë 279when the ring-shaped YBCO sample was aligned with the peak line ofBz,x= −25 mm,
of the Halbach PMG.
Figure 8.21 shows the experimental curves. It is interesting to note that the
levitation forces decreased visibly from the first movement to the third movement at
the relative higher temperature of 77 or 73 K atx= −25 mm. However, the decrease rates
gradually reduced although levitation forces continuously decreased when increasing
the round-trip movement times. Moreover, the influence of the round-trip movement
times of the bulk on levitation forces was not obvious at lower temperatures, for
example, at 63 or 68 K, three groups of curves were almost the same during different
cycling movements atx= −25 mm. This indicated that the influence of YBCO bulk’s
round-trip movement times on the levitation forces was larger at higher temperature
and the influence became smaller at lower temperature. This was because the strength
ofBzwas extremely small atx=0 mm, 20 times smaller than at the−25 mm position.
The influence of the bulk’s round-trip movement times on the levitation force was
very small atx=0 mm position at any temperature. Based on Maxwell’s equations,
the induced current was related to the magnetic field difference between the applied
field and the trapped magnetic field in the bulk HTSC. In addition, according to the
critical state model [27], it was well known that lower hysteresis of the levitation
forces happens with decreasing temperature. Therefore, the influence of the YBCO
Fig. 8.21:Levitation forces curves under three different round-trip movements at 35-mm FCH and
different positions ofx=−25 mm andx=0 mm.