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Register a new userSurface Superconductivity in Niobium for Superconducting RF Cavities
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S. Casalbuoni
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A systematic study is presented on the superconductivity (sc) parameters of the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz cavities for the linear collider project TESLA. Cylindrical Nb samples have been subjected to the same surface treatments that are applied to the TESLA cavities: buffered chemical polishing (BCP), electrolytic polishing (EP), low-temperature bakeout (LTB). The magnetization curves and the complex magnetic susceptibility have been measured over a wide range of temperatures and dc magnetic fields, and also for di erent frequencies of the applied ac magnetic field. The bulk superconductivity parameters such as the critical temperature Tc = 9.26 K and the upper critical field Bc2(0) = 410 mT are found to be in good agreement with previous data. Evidence for surface superconductivity at fields above Bc2 is found in all samples. The critical surface field exceeds the Ginzburg-Landau field Bc3 = 1.695Bc2 by about 10% in BCP-treated samples and increases even further if EP or LTB are applied. From the field dependence of the susceptibility and a power-law analysis of the complex ac conductivity and resistivity the existence of two different phases of surface superconductivity can be established which resemble the Meissner and Abrikosov phases in the bulk: (1) coherent surface superconductivity, allowing sc shielding currents flowing around the entire cylindrical sample, for external fields B in the range between Bc2 and Bcohc3, and (2) incoherent surface superconductivity with disconnected sc domains between Bcohc3 and Bc3. The coherent critical surface field separating the two phases is found to be Bcoh c3 = 0.81Bc3 for all samples. The exponents in the power law analysis are different for BCP and EP samples, pointing to different surface topologies.
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In this paper we present the discovery of a new surface treatment applied to superconducting radio frequency (SRF) niobium cavities, leading to unprecedented accelerating fields of 49 MV/m in TESLA-shaped cavities, in continuous wave (CW); the corresponding peak magnetic fields are the highest ever measured in CW, about 210 mT. For TESLA-shape cavities the maximum quench field ever achieved was ~45 MV/m - reached very rarely- with most typical values being below 40 MV/m. These values are reached for niobium surfaces treated with electropolishing followed by the so called mild bake, a 120C vacuum bake (for 48 hours for fine grain and 24 hours for large grain surfaces). We discover that the addition during the mild bake of a step at 75C for few hours, before the 120C, increases systematically the quench fields up to unprecedented values of 49 MV/m. The significance of the result lays not only in the relative improvement, but in the proof that niobium surfaces can sustain and exceed CW radio frequency magnetic fields much larger than Hc1, pointing to an extrinsic nature of the current field limitations, and therefore to the potential to reach accelerating fields well beyond the current state of the art.
Point contact tunneling (PCT) spectroscopy measurements are reported over wide areas of cm-sized cut outs from niobium superconducting RF cavities. A comparison is made between a high-quality, conventionally processed (CP) cavity with a high field Q drop for acceleration field E $>$ 20 MV/m and a nitrogen doped (N-doped) cavity that exhibits an increasing Q up to fields approaching 15 MV/m. The CP cavity displays hot spot regions at high RF fields where Q-drop occurs as well as unaffected regions (cold spots). PCT data on cold spots reveals a near ideal BCS density of states (DOS) with gap parameters, $Delta$ as high as 1.62 meV, that are among the highest values ever reported for Nb. Hot spot regions exhibit a wide distribution of gap values down to $Delta sim$ 1.0 meV and DOS broadening characterized by a relatively large value of pair-breaking rate, $Gamma$, indicating surface regions of significantly reduced superconductivity. In addition, hot spots commonly exhibit Kondo tunneling peaks indicative of surface magnetic moments attributed to a defective oxide. N-doped cavities reveal a more homoegeneous gap distribution centered at $Delta sim$ 1.5 meV and relatively small values of $Gamma/Delta$. The absence of regions of significantly reduced superconductivity indicates that the N interstitials are playing an important role in preventing the formation of hydride phases and other macroscopic defects which might otherwise severely affect the local, surface superconductivity that lead to hot spot formation. The N-doped cavities also display a significantly improved surface oxide, i.e., increased thickness and tunnel barrier height, compared to CP cavities. These results help explain the improved performance of N-doped cavities and give insights into the origin of the initial increasing Q with RF amplitude.
As a result of a collaboration between Jefferson Lab and niobium manufacturer CBMM, ingot niobium was explored as a possible material for superconducting radiofrequency (SRF) cavity fabrication. The first single cell cavity from large grain high purity niobium was fabricated and successfully tested at Jefferson Lab in 2004. This pioneering work triggered research activities in other SRF laboratories around the world. Large grain niobium became not only an interesting alternative material for cavity builders, but also material scientists and surface scientists were eager to participate in the development of this material. Most of the original expectations for this material of being less costly and allowing less expensive fabrication and treatment procedures at the same performance levels in cavities have been met. Many single cell cavities made from material of different suppliers have been tested successfully and several multi-cell cavities have shown the performances comparable to the best cavities made from standard poly-crystalline niobium. Several 9-cell cavities fabricated by Research Instruments and tested at DESY exceeded the best performing fine grain cavities with a record accelerating gradient of Eacc = 45.6 MV/m. Recently- at JLab- by using a new furnace treatment procedure a single cell cavity made of ingot niobium performed at a remarkably high Q0-value (~5x10^10) at an accelerating gradient of ~20 MV/m, at 2K. Such performance levels push the state-of-the art of SRF technology to new limits and are of great interest for future accelerators. This contribution reviews the development of ingot niobium technology and attempts to make a case for this material being the choice for future accelerators.
Even when cooled through its transition temperature in the presence of an external magnetic field, a superconductor can expel nearly all external magnetic flux. This Letter presents an experimental study to identify the parameters that most strongly influence flux trapping in high purity niobium during cooldown. This is critical to the operation of superconducting radiofrequency cavities, in which trapped flux degrades the quality factor and therefore cryogenic efficiency. Flux expulsion was measured on a large survey of 1.3 GHz cavities prepared in various ways. It is shown that both spatial thermal gradient and high temperature treatment are critical to expelling external magnetic fields, while surface treatment has minimal effect. For the first time, it is shown that a cavity can be converted from poor expulsion behavior to strong expulsion behavior after furnace treatment, resulting in a substantial improvement in quality factor. Future plans are described to build on this result in order to optimize treatment for future cavities.
In a recent comment [arXiv:1405.2978v1 (2014)] Romanenko and Grassellino made unsubstantiated statements about our work [Appl. Phys. Lett. 104, 092601 (2014)] and ascribed to us wrong points which we had not made. Here we show that the claims of Romanenko and Grassellino are based on misinterpretation of our Letter and inadequate data analysis in their earlier work [*]. [*] A. Romanenko and A. Grassellino, Appl. Phys. Lett. 102, 252603 (2013)
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