A Magnetic Bumper Tether System Using ZFC Y123

We consider the use of magnetic forces in a bumper system, to soften docking procedures. We investigate a system which exhibits no magnetic field except during the docking process, which, if desired, can automatically tether two craft together, and which provides lateral stability during docking. A system composed of zero field cooled Y1.7Ba2Cu307-6 (Y123) tiles and electromagnets is proposed. The Y 123 high temperature superconductor (HTS) is mounted on one craft, and the electromagnet on the other. Results of small prototype laboratory experiments are reported. The electromagnet has, for convenience, been replaced by a permanent SmCo ferromagnet in these measurements. When the two craft approach, a mirror image of the ferromagnet is induced in the Y123, and a repulsive bumper force, FB, results. FB is velocity dependent, and increases with v. For presently available HTS materials, bumper pressure of -3.7Nkm2 is achieved using SmCo. This extrapolates to -18N/cm2 for an electromagnet, or a force of up to 20 tons for a lm2 system. After reaching a minimum distance of approach, the two colliding craft begin to separate. However, the consequent change of SmC-o magnetic field at the Y 123 results in a reversal of current in the Y 123 so that the Y 123 is attractive to the SmCo. The attractive (tether) force, FT, is a function of R=BFE/Btmax, where BFe is the field at the surface of the ferromagnet, and Bt,max is the maximum trapped field of the Y123, i.e., the trapped field in the so-called critical state. For R 2 2, FT saturates at a value comparable to FB. For a range of initial approach velocities the two craft are tethered following the bumper sequence. Most of the kinetic energy of the collision is first converted to magnetic field energy in the Y 123, and then into heat via the creep mechanism. About 15% of the work done against magnetic forces during collision remains stored as magnetic energy after 1 hour. Experiments have also been conducted on the spatial range of the bumper force for arrays of HTS tiles. For a single HTS tile - 2cm in diameter, the range of FB is - lcm. For a lm2 array the range of FB will be circa 50 cm.