Effective medium temperature baking of 1.3 GHz single cell SRF cavities

Self Cite

Three 1.3 GHz 9-cell large grain superconducting niobium cavities were experimented with the proposal of medium temperature baking, using buffered chemical polishing to remove the previous heat treatment impurity profiles. 2 K vertical tests of the cavities gave the average intrinsic quality factor of 2.7×1010 at 16 MV/m, as well as the maximum accelerating gradients of 20-22 MV/m. These promising values confirmed the effectiveness of an improved medium temperature baking recipe for niobium cavities which have been benefitting the superconducting radiofrequency community. Furthermore, the resistance analysis demonstrated that medium temperature baking reduced both the BCS resistance and the residual resistance of the cavities. Impurity analysis on niobium samples provided some proof that the former reduction was due to the shortened electron mean free path, while the later reduction was probably correlated with the mitigation of the increased interstitial impurity atoms.

Section: Major Impuritiesmentioning

confidence: 99%

“…Interstitial nitrogen was proposed to benefit the superconducting performance of med-T baked niobium cavities due to the non-doping increase in nitrogen [12,39], similar to the mechanism of nitrogen doping and nitrogen infusion. As shown in figure 9, on a shallow surface (<5 nm) all the samples had comparable signals.…”

Section: Major Impuritiesmentioning

confidence: 99%

Surface resistance effects of medium temperature baking of buffered chemical polished 1.3 GHz nine-cell large-grain cavities

Yang¹,

Hao²,

Quan³

et al. 2022

Self Cite

“…With claims of new recipes and new associated physics emerging, there is a need to provide an accurate picture of the processed Nb surface to build the foundation used for relevant theoretical modeling and process development. Secondary ion mass spectroscopy results have been widely reported [11,17,42,44,46]. Confusion abounds concerning the extremely low nitrogen concentration (∼0.01 at.%) in 'N 2 -infused' Nb, in contrast to 1-3 orders of magnitude higher oxygen and carbon concentrations (figure S1) [46].…”

Section: Introductionmentioning

confidence: 96%

“…Impurity processing has recently become a key element of Nb SRF cavity treatments, producing record accelerating gradients and quality factors approaching the theoretical limit [11,12]. These treatments involve N 2 processing (socalled 'N 2 doping' [12] and 'N 2 infusion' [11,41,42]) at 20-45 mTorr N 2 pressures at low (120 • C-160 • C) [11,41,42], medium (300 • C-400 • C) [43,44], and high (800 • C-1000 • C) [12] temperatures, or UHV baking similarly at low [10] and medium [45] temperatures. With claims of new recipes and new associated physics emerging, there is a need to provide an accurate picture of the processed Nb surface to build the foundation used for relevant theoretical modeling and process development.…”

Section: Introductionmentioning

confidence: 99%

Surface oxides, carbides, and impurities on RF superconducting Nb and Nb₃Sn: a comprehensive analysis

Sun,

Baraissov,

Dukes

et al. 2023

Surface structures on radio-frequency (RF) superconductors are crucially important in determining their interaction with the RF field. Here we investigate the surface compositions, structural profiles, and valence distributions of oxides, carbides, and impurities on niobium (Nb) and niobium–tin (Nb3Sn) in situ under different processing conditions. We establish the underlying mechanisms of vacuum baking and nitrogen processing in Nb and demonstrate that carbide formation induced during high-temperature baking, regardless of gas environment, determines subsequent oxide formation upon air exposure or low-temperature baking, leading to modifications of the electron population profile. Our findings support the combined contribution of surface oxides and second-phase formation to the outcome of ultra-high vacuum baking (oxygen processing) and nitrogen processing. Also, we observe that vapor-diffused Nb3Sn contains thick metastable oxides, while electrochemically synthesized Nb3Sn only has a thin oxide layer. Our findings reveal fundamental mechanisms of baking and processing Nb and Nb3Sn surface structures for high-performance superconducting RF and quantum applications.

“…The mid-T baking was also tested on cavities at higher temperatures and different thermal pre-treatments on large-grain niobium cavities [23], with nitrogen-gas addition [24] and also on 650 MHz cavities [25]. The surface of the Nb samples treated together with cavities was studied primarily by SIMS that allowed determination of the concentration profile in the surface layer characterized mainly by the increase in oxygen-, carbon-and nitrogen-containing species.…”

Section: Introductionmentioning

confidence: 99%

In-situ synchrotron x-ray photoelectron spectroscopy study of medium-temperature baking of niobium for SRF application

Prudnikava,

Tamashevich,

Makarova

et al. 2024

In order to determine optimal parameters of vacuum thermal processing of superconducting radiofrequency niobium cavities exhaustive information on the initial chemical state of niobium and its modification upon a vacuum heat treatment is required. In the present work the chemical composition of the niobium surface upon ultra-high vacuum baking at 200–400 °C similar to “medium-temperature baking” and “furnace baking” of cavities is explored in-situ by synchrotron X-ray photoelectron spectroscopy (XPS). Our findings imply that below the critical thickness of the Nb2O5 layer (≈ 1 nm) niobium starts to interact actively with surface impurities, such as carbon and phosphorus. By studying the kinetics of the native oxide reduction, the activation energy and the rate-constant relation have been determined and used for the calculation of the oxygen-concentration depth profiles. It has been established that the controlled diffusion of oxygen is realized at temperatures 200–300 °C, and the native-oxide layer represents an oxygen source, while at 400 °C the pentoxide is completely reduced and the doping level is determined by an ambient oxygen partial pressure. Fluorine (F to Nb atomic ratio is 0.2) after the buffered chemical polishing was found to be incorporated into the surface layer probed by XPS (≈ 4.6 nm), and its concentration increased during the low-temperature baking (F/Nb=0.35 at 230 °C) and depleted at higher temperatures (F/Nb=0.11 at 400 °C). Thus, the influence of fluorine on the performance of mid-T baked, nitrogen-doped and particularly mild-baked (120 °C/48 h) cavities must be considered. The possible role of fluorine in the educed Nb+5 → Nb+4 reaction under the impact of an X-ray beam at room temperature and during the thermal treatment is also discussed. The range of temperature and duration parameters of the thermal treatment at which the niobium surface would not be contaminated with impurities is determined.