Precision cryogenic magnetostrictive actuator using a persistent high TC magnet

Cryogenic magnetostrictive materials, such as rare earth zinc crystals, off er high strains and high forces with minimally applied magnetic fields, mak ing the material ideally suited for deformable optics applications. For cry ogenic temperature applications, such as Next Generation Space Telescope, t he use of superconducting magnets offer the possibility of a persistent mod e of operation, i.e., the magnetostrictive material will maintain a strain field without power. High temperature superconductors (HTS) are attractive options if the temperature of operation is higher than 10 degrees Kelvin (K ) and below 77 K. However, HTS wires have constraints that limit the minimu m radius of winding, and even if good wires can be produced, the technology for joining superconducting wires does not exist. In this article, the des ign and capabilities of a rare earth zinc magnetostrictive actuator using b ulk HTS is described. Bulk superconductors can be fabricated in the sizes r equired with excellent superconducting properties. Equivalent permanent mag nets, made with this inexpensive material, are persistent, do not require a persistent switch as in HTS wires, and can be made very small. These devic es are charged using a technique which is similar to the one used for charg ing permanent magnets, e.g., by driving them into saturation. A small norma l conducting coil can be used for charging or discharging. Very fast chargi ng and discharging of HTS tubes, as short as 100 mu s, has been demonstrate d. Because of the magnetic field capability of the superconductor material, a very small amount of superconducting magnet material is needed to actuat e the rare earth zinc. In this article, several designs of actuators using YBCO and BSCCO 2212 superconducting materials are presented. Designs that i nclude magnetic shielding to prevent interaction between adjacent actuators will also be described. Preliminary experimental results and comparison wi th theory for BSSCO 2212 with a magnetostrictive element will be discussed. (C) 2000 American Institute of Physics. [S0021-8979(00)82208-2].