B. Hillenbrand

Superconducting Nb 3 Sn cavities with high microwave qualities

As the lower critical flux density B c1 apparently presents no limit for microwave superconductivity, Type II superconductors with a high transition temperature offer advantages over niobium, the material normally used for superconducting cavities. We have developed Nb 3 Sn cavities by exposing a Nb structure to a saturated tin vapour atmosphere at about 1000 °C. A report is given on the preparation of smooth and homogeneous Nb 3 Sn layers and on measurements on X-band cavities. At 4.2 K, Q-values up to 2.7 × 109have been measured which are higher by a factor of seventy than for Nb resonators at the same temperature. Critical flux densities of 89 mT at 4.2 K and 106 mT at 1.5 K have been reached. With Nb 3 Sn cavities of this kind it would for the first time appear to be possible to operate superconducting cavities for technical applications at a temperature of 4.2 K. This would offer Considerable advantages.

TE011 X-band niobium cavity with critical magnetic flux density higher than Bc1☆

Abstract Microwave critical magnetc flux densities B ac c of nearly 160 mT were obtained with a TE 011 X-band niobium cavity, these values being higher than the lower critical field B c1 of the cavity material.

High critical magnetic flux densities in single piece TM010-X-band cavities of niobium☆

Abstract In single piece TM 010 -X-band niobium cavities critical magnetic fields of 90 mT with Q o s of 5 sx 10 9 have been obtained. For a cold-prepared cavity, filled with helium gas, the corresponding values were 130 mT and 2.1 × 10 9 .

Superconducting Nb 3 Sn-cavities

Microwave critical magnetic flux densities B c acup to 160 mT and higher than B c1 were obtained with niobium-cavities by special surface treatments. Breakdown in this flux density region occurs by a thermal breakdown mechanism in which no superconducting critical flux density is involved. Considerable increase in B c acmay be expected with high-T c -superconductors such as Nb 3 Sn. Nb 3 Sn-layers were produced on niobium structures by exposing them to a saturated Sn-vapour at 1000 °C. B c ac-values up to 40 mT and surface resistances smaller than 10-6Ω were measured with TE 011 -cavities at 9.5 GHz.

Optimized preparation conditions of RF co-sputtered TbFeCo magneto-optical storage films

Amorphous TeFeCo films have been prepared by RF co-sputtering from two composite Tb/sub 26/Fe/sub 74/ and Tb/sub 26/Co/sub 74/ (area %) targets. Chemical, structural, magnetic and magnetooptical properties of these films have been investigated systematically as functions of argon pressure and applied bias voltage during sputtering. For this particular preparation system, optimum sputtering parameters with respect to a high carrier-to-noise ratio have been found to be a 20-40 V bias at 8*10/sup -3/ mbar argon pressure. Dynamic write-read tests on 5-1/4-in polycarbonate disks give a carrier-to-noise ratio of 55 dB at 6-MHz carrier frequency and 30-kHz bandwidth, at a bit size of 1.4 mu m. >

Gas exposure tests of high field niobium X-band cavities

After exposing two high field TE011 niobium X-band cavities to different gases including carbon dioxide, and one TM010 cavity to air, no degradation was observed. The cavities were not protected with an anodic oxide layer.

On the preparation of Nb3Sn-layers on monocrystalline Nb-substrates

Superconducting Nb3Sn cavities are prepared, using the standard technique, by exposing polycrystalline Nb cavities to Sn vapour at 1050°C for some hours. The influence of the niobium crystal orientation on the quality of the Nb3Sn layers is studied by microscope (optical and SEM) and AES. Without preanodization of the niobium singlecrystal substrates all layers are patchy, especially for (111)-and (531)-planes. With preanodization homogeneous dense layers are obtained independent of the niobium orientation. All layers are essentially free from C and O contaminations.

Nb- and Nb 3 Sn-cavities produced by sheet material

X-band TM 010 cavities have been fabricated from Nb sheets by spinning and welding together. Studies of different surface preparations have shown that smooth surfaces and relatively good RF properties can be obtained by a simple chemical polishing treatment. Also the degradation caused by welding can be overcome by chemical polishing without any heat treatment. On welded TM 010 cavities critical flux densities of more than 70 mT and high-field Q-values of 5.109have been measured. Up to this field level no electron loading was observed. This procedure is very promising for the fabrication of Nb cavities for technical applications as well as of substrates for Nb 3 Sn cavities.

Superconducting properties and limitations of thin-wall niobium and niobium tin cavities

Thin-wall TM010-resonators of cylindrical and spherical shape were made from niobium sheets and investigated experimentally in the frequency range around 10 GHz. In many cases critical magnetic flux-densities up to 90mT — corresponding to accelerating fields of nearly 23 MV/m — and unloaded Qs up to 1010 were reached at a temperature of 1.3K. Measuring the thermal conductivity we found that the originally cold worked niobium is annealed by electron beam welding, and we could calculate thermal limits of flux density. Comparing experimental and calculated critical flux densities we conclude that we are near the thermal limit and no essential improvement is possible. The resonators were also coated with Nb3Sn. In this case critical flux densities of up to 70 mT and unloaded Qs of about 109 were measured at 4.2K and 1.5K.