Surface analysis of Nb materials for SRF cavities

Abstract

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 ...

High fieldQslope and the baking effect: Review of recent experimental results and new data on Nb heat treatments

“…The implanted samples showed typical Gaussian shaped curves for all four elements in silicon, but only for C, N, and O in niobium. The D profile in niobium showed no implant peak, indicating that the D is very mobile and can diffuse through the metal either without assistance or with the influence of the charged analysis beam [49]. Analysis of the SIMS standard provided relative sensitivity factors from which quantitative results for C, O, and N could be obtained.…”

“…Depth resolution for SIMS is typically improved by analysis at lower impact energy. Some of the initial analyses were made at Cs þ impact energy of 6 keV and show improved depth resolution at the surface compared with 14.5 keV bombardment [49]. This condition was primarily used as a check for N at the surface after the initial nitrogen exposure experiments.…”

Effect of high temperature heat treatments on the quality factor of a large-grain superconducting radio-frequency niobium cavity

“…The analysis chamber was also kept under UHV conditions ($ 10 À10 Torr) to minimize contamination. Ion implantation was used to obtain standards of C, O, and N in Nb [18]. Since N does not have a significant negative secondary ion yield in SIMS, NbN À ions were used to monitor the N À signal.…”

SIMS analysis of high‐performance accelerator niobium

“…Quantification of O was established by ion-implanted standards from an earlier study. [1] Analyses for C and N showed no significant contribution in the samples. Analysis for H is complicated by the extremely high mobility of this element in Nb.…”

“…The results were quantified with the use of ion-implanted standards. [1,2] Performance of SRF Nb was shown to improve after a lowtemperature bake, and it was found that reduction of H plays an important role. No significant changes were observed for C and N. Oxygen was associated primarily with the 5-7 nm surface oxide, and only minor differences in the bulk oxygen concentration were noted.…”