Nitrogen infusion R&D at DESY a case study on cavity cut-outs

A first series of nitrogen infusion runs of 1.3 GHz single-cell cavities at DESY resulted in an unexpected and severe deterioration observed during the vertical cold test. To investigate the origin of the deterioration, one of the cavities underwent extensive radio-frequency measurements and a temperature- and magnetic field-mapping was performed in collaboration with the Helmholtz-Zentrum Berlin. After combining all results, regions of interests were identified and cut-out of the cavity. Subsequent surface analysis techniques (EBSD, PALS, PIXE, SEM/EDX, SIMS, XPS) were applied in order to identify the microscopic origin of the deterioration and especially the differences between hot and cold spots as well as quench spots. An excess of niobium carbides, reducing the thermal conductivity, was identified as the probable cause for the deterioration, and the size- and density-distributions were observed to be crucial for the resulting performance reduction. The origin for the local differences in the niobium carbide formation between hot and cold spots is an effect of preexisting variations of the crystal structure.BMBF, 05H18GURB1, Verbundprojekt 05H2018 - R&D BESCHLEUNIGER (STenCiL): Optimierung der Hochfrequenzeigenschaften supraleitender Resonatoren durch Oberflächenbehandlun

Nitric Acid Soaking after Imperfect Furnace Treatments

Annealings of niobium cavities in UHV or nitrogen atmospheres are crucial for the performance in the later cryogenic tests and operation. Recovery methods for imperfect annealing conditions have been discussed, and a more recent proposal, the so-called "nitric acid soak" has been studied here in detail. It shows surprising recov-ery potential, albeit the unclear origin of this improve-ment. We present our investigation on the several poten-tial origins. For this, we used SEM, SIMS and XPS meas-urements of niobium samples to study the surface mor-phology and contaminations. We can reject the favored hypothesis on the origin of the improvement, and pro-pose an alternative origin

Influence of High-Pressure Rinsing on the Oxide-Layer Thickness and Oxygen-Concentration of Niobium Samples

This study is devoted to investigate the effect of High-Pressure Rinsing (HPR) on the outcome of annealing procedures of Niobium (Nb) superconducting radio-frequency cavities. Recently, a so-called “mid-T bake” treatment has exhibited very high-quality factors for Nb cavities. The complementary developed models assume that the quality factor severely depends on the oxygen concentration in the near-surface of niobium. On the other hand, based on our observation, we realize that HPR affects the thickness of oxide layers on the surface of niobium cavities, which is the dominant source of the oxygen diffusion during annealing. Thus, we have measured the oxide thicknesses after various HPR durations of Nb samples before and after applying mid-T bake treatment via X-ray Photoelectron Spectroscopy (XPS) and used Secondary Ion Mass Spectrometry (SIMS) to obtain the interstitial oxygen concentration after the annealing. The results of this investigation will be presented and discussed in the context of theoretical models

Electron spin resonance in a proximity-coupled MoS 2 /graphene van der Waals heterostructure

Coupling graphene’s excellent electron and spin transport properties with a higher spin–orbit coupling (SOC) material allows tackling the hurdle of spin manipulation in graphene due to the proximity to van der Waals layers. Here, we use magneto-transport measurements to study the electron spin resonance on a combined system of graphene and MoS$_2$ at 1.5 K. The electron spin resonance measurements are performed in the frequency range of 18–33 GHz, which allows us to determine the g-factor in the system. We measure the average g-factor of 1.91 for our hybrid system, which is a considerable shift compared to that observed in graphene on SiO$_2$. This is a clear indication of proximity induced SOC in graphene in accordance with theoretical predictions

N infusion study at DESY

The current status of the Nitrogen Infusion R&D at DESY is presented.Current results on cavity studies and material analyses on samples arediscussed. Consequences for the infusion procedures and the furnace usedare shown

Temperature-dependent near-surface interstitial segregation in niobium

Niobium's superconducting properties are affected by the presence and precipitation of impurities in the near-surface region. A systematic wide-temperature range x-ray diffraction study is presented addressing the effect of low temperatures (108 K–130 K) and annealing treatments (523 K in nitrogen atmosphere, 400 K in UHV) on the near-surface region of a hydrogen-loaded Nb(100) single-crystal. Under these conditions, the response of the natural surface oxides (Nb$_2$O$_5$, NbO$_2$, and NbO) and the changes in the subsurface concentration of interstitial species in Nb are explored, thereby including the cryogenic temperature regime relevant for device operation. The formation and suppression of niobium hydrides in such conditions are also investigated. These treatments are shown to result in: (i) an increase in the concentration of interstitial species (oxygen and nitrogen) occupying the octahedral sites of the Nb bcc lattice at room temperature, both in the near-surface region and in the bulk. (ii) A decrease in the concentration of interstitials within the first 10 nm from the surface at 130 K. (iii) Hydride formation suppression at temperatures as low as 130 K. These results show that mild annealing in nitrogen atmosphere can suppress the formation of superconducting-detrimental niobium hydrides, while subsurface interstitial atoms tend to segregate towards the surface at 130 K, therefore altering the local concentration of impurities within the RF penetration depth of Nb. These processes are discussed in the context of the improvement of niobium superconducting radio-frequency cavities for next-generation particle accelerators

Size-Controlled Synthesis and Microstructure Investigation of Co₃O₄ Nanoparticles for Low-Temperature CO Oxidation

Noble-metal-free functional oxides are active catalysts for CO oxidation at low temperatures. Spinel-type cobalt oxide (Co₃O₄) nanoparticles can be easily synthesized by impregnation of activated carbon with concentrated cobalt nitrate and successive carbon burn off. Mean size and particle size distribution can be tuned by adding small amounts of silica to the carbon precursor, as witnessed by whole powder pattern modeling of the X-ray powder diffraction data. The catalytic tests performed after silica removal show a significant influence of the mean domain size and of size distribution on the CO oxidation activity of the individual Co₃O₄ specimens, whereas defects play a less important role in the present case

Surface characterization of nitrogen-doped Nb (100) large-grain superconducting RF cavity material

(100) Oriented niobium (Nb) crystals annealed in the vacuum conditions close to that used in mass production of 1.3 GHz superconducting radio frequency cavities for linear accelerators and treated in nitrogen at a partial pressure of 0.04 mbar at temperatures of 800 and 900 °C have been studied. The surfaces of the nitrogen-treated samples were investigated by means of various surface-sensitive techniques, including grazing-incidence X-ray diffraction, X-ray photoemission spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy in planar view and on cross-sections prepared by a focused ion beam. The appearance of a dense layer of epitaxial rectangular precipitates has been observed for the Niobium nitrided at 900 °C. Increased nitrogen concentration in the near-surface region was detected by glow-discharge optical-emission spectroscopy, focused ion-beam cross-sectional images and X-ray photoelectron spectroscopy. Crystalline phases of NbO and β-Nb2N were identified by X-ray diffraction. This information was confirmed by X-ray photoelectron measurements, which in addition revealed the presence of Nb2O5, NbON, NbN, and NbN x O y components on the surface. These results establish the near-surface Nb phase composition after high-temperature nitrogen treatment, which is important for obtaining a better understanding of the improved RF cavity performance