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A multilayered structure with a single superconductor layer and a single insulator layer formed on a bulk superconductor is studied. General formulae for the vortex-penetration field of the superconductor layer and the magnetic field on the bulk superconductor, which is shielded by the superconductor and insulator layers, are derived with a rigorous calculation of the magnetic field attenuation in the multilayered structure. The achievable peak surface field depends on the thickness and its material of the superconductor layer, the thickness of the insulator layer and material of the bulk superconductor. The calculation shows a good agreement with an experimental result. A combination of the thicknesses of superconductor and insulator layers to enhance the field limit can be given by the formulae for any given materials.Technologies to fabricate the superconducting RF cavities made of Nb have been advanced. The maximum accelerating gradient E acc of the TESLA type 1.3 GHz 9cell cavities during performance tests in vertical cryostats regularly exceed 35 MV/m at several laboratories. The gradient record had been increasing and recently two 9cell cavities made from large grain Nb reached 45 MV/m at DESY 1 . Further high gradients, however, would not be expected because their gradients are thought to be close to the empirical limit imposed by the thermodynamic critical field ≃ 200 mT of Nb 2 . A. Gurevich suggested 3,4 that a multilayered nanoscale coating on Nb cavity may push up the RF breakdown field to the level of the vortex-penetration field of the coating materials at which the Bean-Livingston surface barrier 5 disappears. While some experimental studies have been conducted on the subject based on the idea 6,7 , not much theoretical progress followed on it. In fact, the best parameter set for the multilayer coating model such as thicknesses of layers and choices of materials are not clear from a theoretical point of view. In this letter, the multilayered structure is carefully evaluated with a rigorous calculation on the electromagnetic field distribution to keep its self-consistency. The resultant vortex-penetration field, the best combination of parameters, and materials are described.The multilayer coating model 3 consists of alternating layers of superconductor layers (S) and insulator layers (I). The simplest configuration with a single superconductor layer and a single insulator layer is seen in Fig. 1. Each S layer is expected to withstand higher field than bulk Nb, and to shield the bulk Nb from the applied RF surface magnetic field B 0 , because B i (the RF surface field on the bulk Nb) is smaller than B 0 . Then the multilayered structure is thought to withstand a higher field than the bulk Nb if B 0 is smaller than the vortexpenetration fields of the top S layer and B i is smaller than that of the bulk Nb. The vortex-penetration field of the S layer was given by B v = φ 0 /4πλξ in the original a) kubotaka@post.kek.jp paper 3 , where φ 0 = 2.07 × 10 −15 Wb is the flux quantum 8 , and λ and ξ are a Lo...

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A system for inspecting the inner surface of superconducting rf cavities is developed in order to study the relation between the achievable field gradient and the defects in the inner surface. The inspection system consists of a high resolution complementary metal-oxide-semiconductor camera and a special illumination system built in a cylinder that has a diameter of 50 mm. The camera cylinder can be inserted into the L-band 9 cell superconducting cavity. The system provides a resolution of about 7:5 m=pixel. Thus far, there have been good correlations between locations identified by thermometry measurements and positions of defects found by this system. The heights or depths of the defects can also be estimated by measuring wall gradients using the reflection angle relation between the camera position and the strip illumination position. This paper presents a detailed description of the system and the data obtained from it.