tailieunhanh - Applications of High Tc Superconductivity Part 7

Tham khảo tài liệu 'applications of high tc superconductivity part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Magnetic Relaxation - Methods for Stabilization of Magnetization and Levitation Force 109 P u . 21 M0 U0 01 where Mo JoR 3 q 1 - Ổ R 3 is the factor of creep retardation. Assuming the function q in Eq. 7 varies slightly with the sample displacement we have F t io Fo p t . The logarithmic relaxation rate s qkT U0. Depending on the rigidity of mechanical constraint the factor q can range from unity fixed superconductor to zero levitation . Using Eqs. 11 and 21 and relations S ABzl 1 -a t ABz KBAz Az APm km APm AF 1 - a t where Az is the suspension displacement ABz is the field variation on the boundary r R KB dBz dz is the field gradient on the boundary AF and APm are the variations of magnetic force and elastic mechanical force km is the rigidity of mechanical constraint the retardation factor q may be estimated from equation Cq q1 3 - 1 0 where C FoKs jiokmRJo. Using F0 N Kb T m Jo 2X107 A m2 R 5x10-3 m and rigidity km X 500 N m and 15 N m the calculations yield the corresponding values of q 1 and respectively. From the experiment the values of q will be found by using the dependences 1-4 Fig. 4 . The slope of the dependence 1 determines the logarithmic relaxation rate in the absence of sample displacement . factor q 1. Using this condition we obtain the kT-normalized activation energy Uo kT 29. The dependences 2-4 show retarded relaxation with the rate s qkT U0 which yields q the suspension under the magnet with km 500 N m q and the suspension under and above the magnet respectively with km 15 N m . The qualitative agreement between experimental and calculated results for the factor q is quite acceptable. The magnetic relaxation slows down when the suspension system is close to the true levitation . when the magnetic rigidity dF dz is much greater than the rigidity of mechanical constraint magnetic rigidity of the magnetsuperconductor system was 100 N m . The different values s when the suspension is under dependence