A model to explain the Q-increase by moderate-temperature treatment in Nb SRF cavities

It is well known that moderate temperature baking of niobium cavities can improve the surface resistance. Presently, it is believed that the diffusion of oxygen into the bulk, resulting in interstitial defects, is responsible for the change. In this note we propose that the damaged surface layer remaining after dissolution of the thin niobium pentoxide may in fact be the dominant contributor to the improved the cavity quality factor by strongly pinning trapped flux lines. We propose some of experiments to test this theory.Comment: 3 pages, 2 figure

Moderate temperature heat treatment of a niobium superconducting cavity without oven

We have demonstrated a possibility to perform a successful moderate-temperature (mid-T) heat treatment of a superconducting cavity directly in a cryostat. This method significantly reduces the number of operations required for Mid-T baking effect research. The data obtained indicate the possibility of baking the cavity directly in the cryomodule, allowing full use of the Mid-T baking effect in a real accelerator.Comment: 4 pages, 5 figure

Growth of few-wall carbon nanotubes with narrow diameter distribution over Fe-Mo-MgO catalyst by methane/acetylene catalytic decomposition

Few-wall carbon nanotubes were synthesized by methane/acetylene decomposition over bimetallic Fe-Mo catalyst with MgO (1:8:40) support at the temperature of 900°C. No calcinations and reduction pretreatments were applied to the catalytic powder. The transmission electron microscopy investigation showed that the synthesized carbon nanotubes [CNTs] have high purity and narrow diameter distribution. Raman spectrum showed that the ratio of G to D band line intensities of IG/ID is approximately 10, and the peaks in the low frequency range were attributed to the radial breathing mode corresponding to the nanotubes of small diameters. Thermogravimetric analysis data indicated no amorphous carbon phases. Experiments conducted at higher gas pressures showed the increase of CNT yield up to 83%. Mössbauer spectroscopy, magnetization measurements, X-ray diffraction, high-resolution transmission electron microscopy, and electron diffraction were employed to evaluate the nature of catalyst particles

Nitrogen Infusion R&D on Single Cells at DESY

A first series of single cell cavities underwent the "Nitro-gen Infusion" treatment at DESY. Samples, which were inthe furnace together with the cavities, underwent a seriesof SEM/EDX measurements and showed some unexpectedstructures. In parallel, the cavity performance deterioratedafter the treatment. The furnace pressure and temperatureand the residual gases during the treatment were analyzed tofind the possible cause for the deterioration and next stepsto prevent this deterioration in following treatments are dis-cusse

Surface analyses and optimization of centrifugal barrel polishing of Nb cavities

Detailed microscopy investigations of the niobiumsurface quality after centrifugal barrel polishing (CBP)have been performed applying metallographic techniques.The results imply the need for further optimisation of thepolishing procedure, mainly to reduce the thickness of thelayer that is damaged at the surface as well as pollution bythe polishing media. The most realistic application ofCBP is a combination using CBP initially to removesurface defects followed by chemical polishing to obtain achemically clean niobium surface

Surface study of centrifugal barrel polished 1.3 GHz Nb cavities

Superconducting radio-frequency (SRF) cavities are the key components of accelerators such as the European X-ray Free Electron Laser and the planned International Linear Collider (ILC). Centrifugal barrel polishing (CBP) is a promising technique both for repairing and improving the performance of such cavities due to ability to remove large defects effectively and obtaining mirror-like surface without chemistry. In spite of numerous attempts, the CBP-treated cavities show yet worse SRF performance than expected. The present study explores the multi-step recipe last developed at FNAL/JLab by using a coupon cavity with removable samples. It allows investigation of the interior surface after each polishing step by microscopic techniques such as laser profilometry, SEM/EDX, AFM etc. and measurement of the roughness and material removal rates at the most relevant areas. The study reveals some polishing media (e.g. Al2O3) to be embedded into the surface which causes new surface scratches in the final polishing step and * being normal conducting * most probably the worsening of the SRF performance. Additionally, a possibly detrimental shearing and deformation of the upper surface layer is observed. An improvement of the recipe is under study

Toward Optimization of Centrifugal Barrel Polishing P rocedure for Treatment of Niobium Cavities

Centrifugal barrel polishing (CBP) is a simple and environmentally friendly method that can be applied for mechanical abrasion of the cavity interior in order to remove the mechanically damaged surface after its production. The CBP recipes described in the literature, however, require CBP to be performed in many stages, require long processing times and nevertheless are unable to provide good cavity RF performance without additional chemical processing. Here, we report new results on characterization of cavity surfaces treated with a typical CBP recipe, including the contamination with abrasive particles, plastic deformation and hydrogen contamination, and critically evaluate it. Methods to reduce the depth of significant plastic deformation as well as the modified commercially viable CBP procedure followed by final electropolishing are proposed and tested on samples