The performance of superconducting radio-frequency (SRF) cavities made of bulk Nb at high fields (peak surface magnetic field greater than about 90 mT) is characterized by exponentially increasing rf losses (high-field Q slope), in the absence of field emission, which are often mitigated by low-temperature (100-140 C, 12-48 h) baking. In this contribution, recent experimental results and phenomenological models to explain this effect will be briefly reviewed. New experimental results on the high-field Q slope will be presented for cavities that had been heat treated in a vacuum furnace at high temperature without subsequent chemical etching. These studies are aimed at understanding the role of hydrogen on the highfield Q slope and at the passivation of the Nb surface during heat treatment. Improvement of the cavity performances, particularly of the cavities' quality factor, have been obtained following the hightemperature heat treatments, while secondary ion mass spectroscopy surface analysis measurements on Nb samples treated with the cavities revealed significantly lower hydrogen concentration than for samples that followed standard cavity treatments.

Large-grain Nb has become a viable alternative to fine-grain Nb for the fabrication of superconducting radio-frequency cavities. In this contribution we report the results from a heat treatment study of a large-grain 1.5 GHz single-cell cavity made of "medium purity" Nb. The baseline surface preparation prior to heat treatment consisted of standard buffered chemical polishing. The heat treatment in the range 800 -1400 • C was done in a newly designed vacuum induction furnace. Q0 values of the order of 2 × 10 10 at 2.0 K and peak surface magnetic field (Bp) of 90 mT were achieved reproducibly. A Q0-value of (5 ± 1) × 10 10 at 2.0 K and Bp = 90 mT was obtained after heat treatment at 1400 • C. This is the highest value ever reported at this temperature, frequency and field. Samples heat treated with the cavity at 1400 • C were analyzed by secondary ion mass spectrometry, secondary electron microscopy, energy dispersive X-ray, point contact tunneling and X-ray diffraction and revealed a complex surface composition which includes titanium oxide, increased carbon and nitrogen content but reduced hydrogen concentration compared to a non heat-treated sample.

Superconducting Radio Frequency (SRF) cavities provide enhanced efficiency and reduced energy consumption in present-day particle accelerators. Niobium is the material of choice for SRF cavities due to its high critical temperature and critical magnetic field. In order to understand why certain treatments, especially a low temperature bake, improve performance, it is important to study Nb surface characteristics and identify elemental contaminants which may affect the performance of the cavity. [1] Initial studies using SIMS and Focused Ion Beam (FIB) prepared specimens for Transmission Electron Microscopy (TEM) have helped to characterize the Nb surface and measure the surface oxide layer thickness.[2] C, N and O are of particular interest as interstitial contaminants and earlier studies suggested very high H concentration. In the present study, ion implants of C, N, O and deuterium (D) in Nb and Si were analyzed using SIMS. D was implanted to characterize H while avoiding interference from the high H background. The D implant was easily detectable in Si, but showed a constant value and no implant shape in Nb. This result implies either that D (and by implication, hydrogen) has a high mobility in Nb, or that there is movement of D due to the primary ion beam. Nevertheless, C, N, and O could be quantified using the ion implants. Depth profiles of polycrystalline and single crystal Nb samples were also obtained. Keywords: SIMS; depth profiling; SRF Nb; deuterium; residual gas species Introduction SRF cavities are frequently used to achieve the desired acceleration in particle accelerators. Since high field gradients requiring high power are required, efficiency must be a primary consideration in cavity performance. Superconductors have thus replaced conventional copper cavities due to their ability to provide dramatically low resistance loss even when taking into account the cost of refrigerating at cryogenic temperature. Cavities use a RF standing wave with the frequency chosen to give particles an accelerating push as they pass through. Nb is currently the material of choice for these cavities because of its high critical temperature (9.2• K), high critical thermal-dynamic magnetic field (∼200 mT) and easy formability. Since the penetration range of the electromagnetic fields into Nb is only approximately 60 nm, understanding surface and near-surface region properties of Nb is of great importance. [1] In our initial studies of the Nb surface [2] , transmission electron microscopy (TEM) was used to characterize the oxide and suboxide regions on the Nb surface. Since Nb is a soft material, sample preparation by traditional TEM polishing methods is problematic since polishing can smear the surface and polishing media can become embedded in the material. For our study, [2] TEM liftout sample preparation using a gallium-focused ion beam (FIB) system was used to obtain high-quality TEM samples. [3,4] The cross-sectional TEM images obtained were used to measure oxide thickness on single crystal samples, which showed a ...

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Niobium is used to fabricate superconducting radio frequency accelerator modules because of its high critical temperature, high critical magnetic field, and easy formability. Recent experiments have shown a very significant improvement in performance (over 100%) after a high-temperature bake at 1400°C for 3 h. SIMS analysis of this material showed the oxygen profile was significantly deeper than the native oxide with a shape that is indicative of diffusion. Positive secondary ion mass spectra showed the presence of Ti with a depth profile similar to that of O. It is suspected that Ti is associated with the performance improvement. The source of Ti contamination in the anneal furnace has been identified, and a new furnace was constructed without Ti. Initial results from the new furnace do not show the yield improvement. Further analyses should determine the relationship of Ti to cavity performance.