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For a cylindrical sample of an imperfect type II superconductor in an external field Hext parallel to its axis, it is found that in the mixed state no flux enters or leaves the specimen within an interval Hext±ΔH; the positive or negative sign applies when Hext is reached in a decreasing or increasing field, respectively. Similarly, when the sample is cooled in a field Hext, an increase of the external field less than ΔH does not change the trapped flux. ΔH depends only on Hext and not on B, the bulk flux density, which itself depends on the magnetic history of the specimen and its defect structure (flux pinning, bulk critical current). ΔH is affected by various surface treatments, such as machining and chemical polishing, but not significantly by metal plating. The results can be explained in terms of an intrinsic surface critical current, and comparison with existing theoretical calculations yields reasonable agreement.

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This paper reviews the present state of development of high-field superconductivity with particular regard to the prospective use of that phenomenon in magnet coils for thermonuclear research. It is concluded that although magnets for thermonuclear research are not yet practical, their development is advancing rapidly to the stage of feasibility for that purpose. Progress at present is dependent upon the achievement of a greater understanding of the basic and applied science of superconductivity and the development of materials less susceptible to a phenomenon known as "flux jumping." A review of the present state of scientific knowledge pertinent to large-magnet technology is provided.

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