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Miroirs supraconducteurs et assemblage
The very long cavity lifetimes reached recently rely on a new technology for the cavity mirrors, based on Niobium sputtering on a high quality copper surface.
Obtaining a high-quality niobium mirror is not an easy task. Niobium polishing is extremely difficult, since it has at the same time a large Young modulus and a very low plastic deformation limit. Our early mirrors made up of massive niobium always exhibit large-scale surface defects, wich severly limited the field lifetime. The recirculating ring technology only partially solved this problem.
We have considerably improved the quality [1] by using an optically polished copper mirror onto which a niobium layer is then deposited by cathode sputtering. These mirrors combine a very good surface accuracy with a good quality of the superconducting material.
The copper mirrors are diamond-machined (Kugler company, see photograph above). The reflecting surface has a 10 nm residual roughness and an overall 300 nm surface accuracy over the whole 50 mm diameter. We have chosen a toroidal surface, with two different radii of curvature (39.4 and 40.6 mm) in two orthogonal planes. This geometry lifts by 1.2 MHz the degeneracy between the two TEM
modes with orthogonal linear polarizations. It is then easy to ensure that the atoms are efficiently coupled to a single mode only (with spherical mirrrors, the degeneracy was randomly lifted by imperfections, from a few kHz up to 100 kHz only).
After machining of a gross shape, the mirrrors are annealed to 400°C in vacuum and cooled down to liquid nitrogen to remove stresses. The final surface is then machined and measured by optical interferometry. The thickness of the mirrrors (30 mm) avoids surface deformation at mounting time. Moreover, the back of the mirrors is polished to a 1 
m accuracy and rests on a polished holder.
These mirrors have no coupling holes. For low 
cavities, it is convenient to couple microwave in and out of the mode through irises pierced at the mirror apexes (typical diameter 0.4 mm, thickness 0.4 mm). The microwave cavity transmission can thus be directly measured and the factor precisely determined. However, these irises strongly affect the mirror surface quality at the critical apex point. We thus have realized a cavity without coupling holes. Microwave is fed in the mode trough the residual diffraction losses.
After a very careful cleaning of the mirror surface, the niobium layer is deposited by cathode sputtering. We use a sputtering facility at CEA, Saclay (in collaboration with E. Jacques, P. Bosland and B. Visentin), optimized for the realization of niobium layers for radiofrequency cavities used in particle accelerators. We deposit 12 m of niobium. The mirrors are then immediately mounted in the evacuated cryogenic apparatus.
This photograph shows the mirror in front of the high-purity niobium cathode, surrounded by a radio-frequency argon plasma.
This photograph presents the assembed cavity block. The side of the two superconducting mirrors are visible. The atoms enter and exity the assembly though the 1x2 cm gap between the mirrors. They fly far enough from the metallic surfaces to avoid the obnoxious effect of stray electric fields.
This photograph presents the assembly with the top mirror removed. The high quality surface of the bottom mirror is clearly apparent. The gold guard rings in the gap between the two mirrors improve the static field homogeneity across the cavity. The four posts are used for assembly. They are surrounded by four piezo elements (centered by the white teflon cylinders) used for cavity tuning.
[1] S. Kuhr, S. Gleyzes, C. Guerlin, J. Bernu, U. B. Hoff, S. Deléglise, S. Osnaghi, M. Brune, J.M. Raimond, S. Haroche, E. Jacques, P. Bosland, B. Visentin, Appl. Phys. Lett. 90, 164101 (2007) : « Ultrahigh finesse Fabry-Pérot superconducting resonator »
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