Permanent magnets of high-T c superconductors

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Abstract
Advances on permanent high‐Tc superconducting magnets are reported. Materials are tested in the form of small tiles. An accurate phenomenological model of the currents in a magnetized tile predicts that the maximum trapped field BT,max∝Jcf(d), where Jc is the critical current density, d is the diameter of the single‐grain tile, and f(d) is a function which increases monotonically with d. Results are reported of increasing Jc via chemical additives, and via bombardment by high‐energy light ions and fission fragments. Increases in d via chemical and temperature gradients are also reported. Methods, data, and most recent results are presented. Present values are d=2 cm, and Jc=85 kA/cm2, at 77 K. A six tile minimagnet, 1.2×1.2×1.5 cm3, fabricated from earlier tiles with d∼1 cm, Jc∼45 kA/cm2, retains 1.52 T at 77 K. It is calculated that the more recent values of Jc and d will result in fields of 3 T at 77 K. BT,max also increases rapidly with T−1, and approximately doubles at 60 K.