Novel magnetic applications of high‐T_c bulk superconductors: Lenses for electron beams

This paper presents a review on the latest state of the novel bulk devices, with an emphasis on lenses for electron beams. As one application of high critical‐temperature (T_c) bulk superconductors, the author proposed the lenses (Supertrons) and his group has developed them. The principle of the lenses is simple: When one injects electron beams into narrow superconducting tubes, the tubes confine the self‐magnetic field of the beams to the bore of the tubes because of the Meissner effect. The enhanced field accordingly focuses the beams into thinner ones. The first two sections outline high‐T_c superconducting materials and fundamental characteristics of electron beams. In the body of this article, the lenses are evaluated according to the focusing of intense electron beams (∼340 keV, ∼1 kA, ∼10 ns) and assigned a figure of merit. Lenses were made from powder‐pressed yttrium‐ (T_c=∼90 K), bismuth‐ (∼105 K), thallium‐ (∼120 K), and melt‐processed yttrium‐based superconductors. From the experimental results, one can say that powder‐pressed lenses are suitable for short pulsed, low repetition‐rate electron beams (a ferrite‐core model) and that melt‐processed lenses are, on the other hand, probably appropriate for continuous or slowly time‐varying electron beams. In addition to the temperature‐dependent focusing of the electron beams, a criterion for design of the lenses is discussed. To show the potential of the lenses, two applications of the lenses are described: a wiggler for free‐electron lasers and an electron‐beam guide for induction linear accelerators. Finally, other novel applications (current leads, magnets, and bearings) indicate the interest in bulk materials which are rapidly being developed. The review will provide fundamentals for potential applications of bulk superconductors.