8.2 Dynamic characteristics Ë 277“force-displacement” experiments were performed with a levitation gap range from
80 to 10 mm.
Figure 8.19 shows the levitation forces of YBCO bulk at 77, 73, 68, 63, and 60 K in
the 20-, 25-, 30-, and 35-mm FCHs while Fig. 8.20 summarizes the maximum levitation
forces of YBCO bulk as a function of the temperature at 10 mm levitation gap under
the different FCHs. It is interesting to note that at 35, 30, and 25 mm FCHs, the forces
drastically increased with temperature from 77 to 60 K. However, a much different
phenomenon was observed for 20-mm FCH, The temperature effect on the levitation
forces was negligibly small at the 20-mm FCH.
That was because the variation of the penetration depth,훿 0 [see Eq. (8.5)],
was different when the FCH of YBCO bulk was different. For the 20-mm FCH, the
vertical component of magnetic field at FCH was strong. Thus, when YBCO was
moved fromBFCtoBz,훿 0 of YBCO at the 20-mm FCH was small. The variation of
훿 0 at the 20-mm FCH was also small when the temperature of YBCO was decre-
ased from 77 to 60 K. Thus, according to Eq. (8.4), the variation of the levitation
forces was negative with decreasing temperature. That was, the levitation forces
Fig. 8.19:Levitation forces between YBCO bulk and the Halbach PMG at 60–77 K under different
FCHs are shown in (a), (b), (c), and (d).