⇦Chinese Physics Letters, 2021, Vol. 38, No. 9, Article code 092901 Low-Temperature Baking Effect of the Radio-Frequency Nb$_{3}$Sn Thin Film Superconducting Cavity Ziqin Yang (杨自钦)1*, Shichun Huang (皇世春)1, Yuan He (何源)1*, Xiangyang Lu (鲁向阳)2, Hao Guo (郭浩)1, Chunlong Li (李春龙)1, Xiaofei Niu (牛小飞)1, Pingran Xiong (熊平然)1, Yukun Song (宋玉堃)1, Andong Wu (吴安东)1, Bin Xie (谢斌)1, Zhiming You (游志明)1, Qingwei Chu (初青伟)1, Teng Tan (谭腾)1, Feng Pan (潘峰)1, Ming Lu (路明)1,3, Didi Luo (罗迪迪)1, Junhui Zhang (张军辉)1, Shenghu Zhang (张生虎)1, and Wenlong Zhan (詹文龙)1 Affiliations 1Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China 2State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China 3University of Chinese Academy of Sciences, Beijing 100049, China Received 4 July 2021; accepted 28 July 2021; published online 2 September 2021 Supported by the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2020410), the Major Research Plan of National Natural Science Foundation of China (Grant No. 91426303), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB25000000), and the National Postdoctoral Program for Innovative Talents (Grant No. BX201700257).
*Corresponding authors. Email: yzq@impcas.ac.cn; hey@impcas.ac.cn Citation Text: Yang Z Q, Huang S C, He Y, Lu X Y, and Guo H et al. 2021 Chin. Phys. Lett. 38 092901    Abstract The Nb$_{3}$Sn thin film cavity, having the potential to be operated at a higher temperature and higher gradient compared to the cavity made from bulk niobium, is one of the most promising key technologies for the next-generation radio-frequency superconducting accelerators. In our work, several 1.3 GHz single-cell TESLA-shaped Nb$_{3}$Sn thin film cavities, coated by the vapor diffusion method, were tested at Peking University and Institute of Modern Physics, Chinese Academy of Sciences. It was observed that the performance of the Nb$_{3}$Sn thin film cavities in the tests without the slow cooling down procedure and the effective magnetic field shielding was significantly improved by using a low temperature baking at 100 ℃ for 48 hours. Although the peak electric field of the cavity remained unchanged, the rapid drop of the unloaded $Q$ value ($Q_{0}$) with the increasing accelerating field ($Q$-slope) was effectively eliminated, resulting in an improvement of the $Q_{0}$ in the intermediate field region by $\sim $8 times. Furthermore, under better test conditions with the shielded magnetic field less than 5 mG and the slow cooling down procedure in the temperature range of 25–15 K, the $Q_{0}$ was still improved by about 20%. Our study shows that the low temperature baking can be an effective supplement to the effective post-treatment for the Nb$_{3}$Sn thin film cavity. DOI:10.1088/0256-307X/38/9/092901 © 2021 Chinese Physics Society Article Text Radio-frequency superconducting (SRF) accelerators can be operated in long-pulse mode or continuous wave (CW) mode due to the extremely low surface resistance of the SRF cavities. Many accelerator projects under construction and planned, such as a synchrotron radiation source,[1] high energy experimental project,[2] free electron laser,[3] proton and heavy ion accelerator,[4] have chosen to use SRF technology. The SRF cavities for the acceleration of the charged particle are the key components of the SRF accelerators, which are routinely fabricated by high purity niobium (Nb) with a residual resistivity ratio (RRR) larger than 300. After more than half century of development, the performance of the niobium cavities is now approaching their fundamental limits, in terms of both the maximum field and the surface resistance at typical operating temperatures.[5–7] To further reduce the construction and operation cost of the large-scale SRF accelerators, it is of utmost importance to look into alternative materials offering SRF performances beyond Nb. Compared to the high-purity Nb used for accelerator applications, Nb$_{3}$Sn has a theoretical superconducting transition temperature ($T_{\rm C}$) of about 18.3 K ($\sim $9.25 K for Nb), an energy gap $\varDelta$ of about 340 mV ($\sim $140 mV for Nb), and a superheating magnetic field ($H_{\rm sh}$) of about 425 mT ($\sim $240 mT for Nb).[8] Therefore, the Nb$_{3}$Sn thin film SRF cavity can be operated at 4.2 K or even higher temperature, having a theoretical accelerating field two times higher than the Nb SRF cavity. This will not only have a huge impact on the large-scale facilities such as International Linear Collider (ILC), but also make the SRF technology have bright application prospects in small scientific research platforms such as compact light sources[9] and photo-neutron sources,[10] as well as in industrial applications such as wastewater treatment[11] and medical isotope production.[12] In addition, the maximum secondary electron yield (SEY) of Nb$_{3}$Sn is very similar to Nb,[8,13] which means that the multipacting and field emission behaviors in Nb$_{3}$Sn thin film cavities should be comparable to that normally observed in Nb cavities. Thus, the optimized structures of the Nb cavities can be directly applied to the Nb$_{3}$Sn cavities. As a result, Nb$_{3}$Sn is the most promising new alternative material to Nb for SRF application. The RF performance of the SRF cavity at cryogenic temperatures significantly depends on the post-treatment process. Many efforts have been made to establish the post-treatment process of the Nb$_{3}$Sn thin film cavities. The oxypolishing treatment was explored firstly by Hillenbrand and Martens from Siemens AG,[14–16] of which the Nb$_{3}$Sn thin film was anodized in NH$_{3}$ solution and the oxide layer was subsequently dissolved in HF solution. The results show that, when the surface layer of the Nb$_{3}$Sn thin film is contaminated, the oxypolishing treatment can significantly improve the unloaded quality factor $Q_{0}$, otherwise, the oxypolishing treatment has only minor influence on $Q_{0}$. Researchers[17] at University of Wuppertal reported a 50% degradation of the low field $Q_{0}$ and the onset field of $Q$-slope after light oxipolishing. Posen[8] repeated the oxypolishing treatment of the Nb$_{3}$Sn thin film cavity at Cornell University to remove the possible excess Sn on the surface. However, the $Q_{0}$ at low fields was reduced and the $Q_{0}$ degradation with the increasing accelerating field ($Q$-slope) was even stronger at lower fields. The results reported by Siemens AG, University of Wuppertal and Cornell University all show that defects limit the maximum field of the Nb$_{3}$Sn thin film cavities. To remove the performance-limiting defect, Posen carried out a light removal of 30–50 nm for the Nb$_{3}$Sn thin film by both HF rinse and centrifugal barrel polishing (CBP).[8] However, some residues appeared on the surface of Nb$_{3}$Sn grains after the HF rinse, leading to a stronger increase in the $Q$-slope. The performance degradation after CBP is even more serious than that after the HF rinse. So far, the only effective post-treatment method for Nb$_{3}$Sn thin film cavities is high pressure rinsing (HPR) with de-ionized water to prevent field emission. Previous experience from the Nb cavities indicates that a single effective post-treatment method can only improve the performance of the cavity to some extent. A combination of many effective post-treatment methods must be employed to maximize the RF performance of the cavity.[18] The aim of the present study is to explore and expand the effective post-treatment method for Nb$_{3}$Sn thin film cavities. The observation that the low temperature baking treatment after HPR can improve the RF performance of the Nb$_{3}$Sn thin film cavities will be reported in detail. In our work, many 1.3 GHz single cell TESLA shape Nb$_{3}$Sn thin film cavities were coated by the vapor diffusion method. The coating process is described as follows: the Nb substrate cavities are heated up in a deposition system with a tin source, as the saturated tin vapor arrives at the inner surface of the cavity and reacts with Nb, a thin layer of Nb$_{3}$Sn material at the inner surface of the cavity will be formed. Before the coating, the 1.3 GHz single-cell TESLA-shaped Nb substrate cavity labeled as X2X4 underwent standard surface treatment of the Nb cavity, including heavy buffered chemical polishing (BCP) of $\sim $130 µm, degassing at 800 ℃ for 3 hours, light BCP of $\sim $20 µm and HPR. Initially, the X2X4 Nb substrate cavity was coated by a recipe adopted from Siemens AG and Jefferson Lab (JLab)[19] with both the Nb cavity and the tin source kept at 1200 ℃ and labeled as X2X4-1.
cpl-38-9-092901-fig1.png
Fig. 1. (a) Recipe of the first coating with both the Nb cavity thermocouples and the tin source thermocouple at 1200 ℃; (b) recipe of the second coating with the Nb cavity thermocouples at 1100 ℃ and the tin source thermocouple at 1250 ℃.
After the vertical tests before and after low temperature baking treatment, the Nb$_{3}$Sn thin film of X2X4-1 cavity was removed by BCP with about 40 µm material removal. Then the cavity labeled as X2X4-2 was recoated using a recipe based on the temperatures and times specified by University of Wuppertal[20] and Cornell University,[8] the temperature of the Nb substrate cavity and the tin source recorded by thermocouples were 1100 ℃ and 1250 ℃, respectively. The detailed data of temperature and pressure during the two coating processes are shown in Fig. 1. Scanning electron microscopy (SEM) experiments were performed on the test samples of both the first coating and the second coating processes to reveal the surface features of the Nb$_{3}$Sn thin films. As seen in Fig. 2, the grain size of the Nb$_{3}$Sn films grown by the two coatings is 1–2 µm. However, the surface of the Nb$_{3}$Sn film grown during the first coating process has defects such as holes. In contrast, the Nb$_{3}$Sn film grown in the second coating process has a smooth surface and full crystal grains. Furthermore, Table 1 shows the EDS spectrum that the average Sn content of the first coating samples is about 22.11 atomic percent, which is obviously smaller than the 23.90% atomic percent tin of the second coating samples.
cpl-38-9-092901-fig2.png
Fig. 2. (a) The surface morphology of the first coating sample (a) and the second coating sample (b).
cpl-38-9-092901-fig3.png
Fig. 3. Magnetization measurements of the Nb$_{3}$Sn films.
Table 1. Composition of the Nb$_{3}$Sn films coated by the two coating processes.
Coating process First coating Second coating
Position 1 2 3 1 2 3
Sn content (at.%) 22.15 22.03 22.16 23.90 24.02 23.79
Average Sn content 22.11 23.90
The magnetization was measured using a commercial physical property measurement system (Quantum Design PPMS-9) to determine the superconductivity of the coated Nb$_{3}$Sn films, and the results are shown in Fig. 3. For the Nb$_{3}$Sn film from the first coating process, the magnetic moments began to decrease rapidly at 17.35 K ($T_{\rm C,onset}=17.35$ K) and stabilize at 11.59 K ($\Delta T_{\rm C}=5.76$ K). $T_{\rm C,onset}$ of the Nb$_{3}$Sn film from the second coating process is 18.05 K and its $\Delta T_{\rm C}$ is about 0.76 K. This agrees with the previous studies that a higher Sn content will yield a higher transition temperature. The difference between the properties of the Nb$_{3}$Sn films deposited in the first and the second coating processes is attributed to the difference in the coating conditions. The tin vapor pressure controlled by the temperature of the Sn source determines the rate at which tin arrives at the cavity surface. However, the rate of interdiffusion between niobium and tin is controlled by the cavity temperature. Thus, the recipe of the first coating process led to a lower Sn content, a surface of defects and a smaller film thickness. The X2X4-1 Nb$_{3}$Sn cavity was first tested at Peking University (PKU). Prior to the test, the cavity was treated with HPR to clean the surface. No other treatments were performed. During the test, the magnetic field was effectively shielded below 5 mG, a slow cooldown of about $\sim $5.2 min/K was achieved from 25 K to 15 K to minimize the thermal current induced magnetic flux, minimizing the contribution of the trapped magnetic flux to the residual resistance of the cavity.[21] The test conditions of PKU are similar to those of Cornell University, JLab and Fermi National Accelerator Laboratory (FNAL). However, the vertical test results in Fig. 5 show that the $Q_{0}$ is only $\sim $$2.7 \times 10^{9}$ at 4.2 K in the low field region. Moreover, the $Q$-slope is serious at the low field region ($E_{\rm acc} < 4$ MV/m), and the maximum accelerating field $E_{\rm acc,max}$ is limited to 7.3 MV/m.
cpl-38-9-092901-fig4.png
Fig. 4. (a) Cooldown rate of the X2X4-1 Nb$_{3}$Sn cavity at IMP before and after low temperature baking treatment. (b) The earth's magnetic field has not been effectively shielded during the test of the X2X4-1 Nb$_{3}$Sn cavity at IMP.
After HPR and re-assembly, the X2X4-1 cavity was re-tested at 4.2 K and at IMP. The ambient magnetic field at the equator was as high as 22 mG and the ambient magnetic field near the beam tube even reached 50 mG during the initial test. As depicted in Fig. 4, the cooldown rate crossing 18 K before the low temperature baking is $\sim $5.5 min/K, which is similar to the rate obtained at PKU. The $Q_{0}$ is degraded to $\sim $$1.3 \times 10^{9}$ at a low field region due to the much higher ambient magnetic field. However, the $E_{\rm acc,max}$ up to 10.4 MV/m is achieved as a result of the larger capacity of the cryogenic system at IMP. Nevertheless, the low field $Q$-slope is still observed. Based on our previous testing experience, the low field $Q$-slope may be caused by the insufficient degassing of the cavity. Thus, the cavity was warmed up and in situ baked at 100 ℃ for 48 hours at the experimental stand. Then, the cavity was cooled down with a rate of $\sim $0.8 min/K in the temperature range of 25–15 K and tested again under the same magnetic shielding conditions as that before baking. Unexpectedly, although the $E_{\rm acc,max}$ remained unchanged, the $Q_{0}$ at low field region was increased to $\sim $$4.4 \times 10^{9}$ as shown in Fig. 5. Furthermore, the low field $Q$-slope was eliminated and the $Q_{0}$ in the intermediate field region was increased by $\sim $8 times.
cpl-38-9-092901-fig5.png
Fig. 5. Significant improvement of the RF performance of the X2X4-1 Nb$_{3}$Sn cavity after the low temperature baking at 100 ℃ for 48 hours.
cpl-38-9-092901-fig6.png
Fig. 6. Vertical test conditions of the X2X4-2 Nb$_{3}$Sn cavity before (a) and after (b) low temperature baking treatment.
To further verify the low temperature baking effect of the Nb$_{3}$Sn thin film cavity described above, the Nb$_{3}$Sn thin film of the X2X4-1 cavity was removed by BCP, recoated and labeled as X2X4-2. After being treated with HPR to clean the surface, the X2X4-2 Nb$_{3}$Sn cavity was vertically tested at IMP. It should be noted that the shielded ambient magnetic field is less than 5 mG with the magnetic foil installed inside the testing dewar this time. A cooldown rate of $\sim $2.6 min/K with the temperature gradient of $\sim $2.4 K across the cell was achieved as shown in Fig. 6. On the one hand, the Nb$_{3}$Sn film by the second coating has a higher $T_{\rm C,onset}$ and a smaller $\Delta T_{\rm C}$ than that of the first coating process. On the other hand, the vertical test conditions of the X2X4-2 Nb$_{3}$Sn cavity is also better than that of the X2X4-1 Nb$_{3}$Sn cavity. Therefore, the $Q_{0}$ of the X2X4-2 Nb$_{3}$Sn thin film cavity at low field reaches $8.8 \times 10^{9}$, degrading to $3.1 \times 10^{9}$ at the quench field of $E_{\rm acc,max}=16.11$ MV/m. Then the X2X4-2 Nb$_{3}$Sn cavity was warmed up, removed from the experimental stand to the clean room and baked at 100 ℃ for 48 hours. After the baking, the X2X4-2 Nb$_{3}$Sn cavity was vertically tested at 4.2 K again. During this test, the cooldown rate in the temperature range of 25–15 K reached $\sim $11.5 min/K, which is obviously slower than that before baking treatment. However, the X2X4-2 Nb$_{3}$Sn cavity was close to the liquid helium injection port, leading to a slightly larger temperature gradient across the cell of 2.7 K. Under such conditions, the $Q_{0}$ of the X2X4-2 Nb$_{3}$Sn cavity was increased by 11% to $9.8 \times 10^{9}$ at low field, and by about 20% in the field range between 1 MV/m and 8 MV/m. When $E_{\rm acc}$ is greater than 8 MV/m, field emission occurred due to the pollution caused by the exposure of valve sealing interface to the atmosphere during the cavity removal from the experimental stand and its transport to the clean room. Therefore, the X2X4-2 Nb$_{3}$Sn cavity after baking exhibited a strong $Q$-slope above the onset field of $E_{\rm acc}=8$ MV/m and quenched at $E_{\rm acc,max}=15.51$ MV/m. The comparison of the RF performance of the X2X4-2 Nb$_{3}$Sn cavity before and after low temperature baking is shown in Fig. 7.
cpl-38-9-092901-fig7.png
Fig. 7. Vertical test results of the X2X4-2 Nb$_{3}$Sn cavity before and after low temperature baking at IMP.
Low temperature baking has become one of the standard post-treatment processes for the pure Nb cavities, and the mechanism has been studied over the past several years, but still remains unclear. Interestingly, we have, for the first time, observed that low temperature baking can improve the RF performance of the Nb$_{3}$Sn SRF cavity. The present research shows that low temperature baking can significantly improve the RF performance of the Nb$_{3}$Sn thin film cavities with poor film quality under poor vertical test conditions. In the case of Nb$_{3}$Sn thin film cavity with better film quality and under better vertical test conditions, i.e., the magnetic field is shielded below 5 mG, slow cooling can be achieved in the temperature range of 25–15 K, although the performance improvement is small, the performance is still improved after the low-temperature baking. Research on the mechanism of the low temperature baking effect of the Nb$_{3}$Sn thin film cavity is underway. Firstly, the $Q_{0}$ of both the X2X4-1 cavity and the X2X4-2 cavity is improved after the low temperature baking, but the maximum accelerating gradient is still the same as before baking, which may be caused by local defects of extremely thin Nb$_{3}$Sn layer that cannot fully screen RF currents from Nb implied by both the temperature record and the reference. We can conclude that the low temperature baking may work by changing the surface composition or configuration, not by changing the surface morphology. Secondly, low temperature baking can improve the RF performance of the X2X4-1 cavity more significantly, but the improvement of the X2X4-2 cavity is obviously smaller. As the surface morphology of the test samples shown in Fig. 2, the surface of the test samples from the first coating process has defects such as holes. In contrast, the Nb$_{3}$Sn film grown in the second coating process has a smooth surface and full crystal grains. Considering that the inner surface of the Nb$_{3}$Sn cavity was treated with HPR prior to the test, dried and then assembled, it is possible that the low temperature baking process may work by removing the water adsorbed on the inner surface of the cavity. The defects such as holes on the inner surface of the X2X4-1 cavity are more likely to absorb water, and it may be difficult to completely remove them by natural drying. This may explain why the low temperature baking has a more significant effect on the X2X4-1 cavity. Thirdly, the RF field is mainly distributed within three penetration depths on the inner surface of the cavity. Therefore, the low temperature baking of Nb$_{3}$Sn cavity may also work by affecting surface element composition or material parameters, which has the similar mechanism of the pure Nb cavity.[22] In summary, this study observes that the low temperature baking can improve the RF performance of the Nb$_{3}$Sn cavity, which will be an effective supplement to the effective post-treatments for the Nb$_{3}$Sn cavity. The in-depth exploration of the mechanism and comprehensive optimization of the technique of the low temperature baking effect for the Nb$_{3}$Sn cavity is being carried out to provide the optimal and reliable low temperature baking conditions. Meanwhile, the low temperature baking may reduce the strict requirements for the coating and operation of the Nb$_{3}$Sn cavity, which is of great significance to its engineering application. For the detailed coating process and material analysis of the test samples, please see the Supplemental Material. Acknowledgments. The authors are indebted to Jiankui Hao, Lin Lin and Fang Wang at Peking University for their help in the vertical test. References Review of third and next generation synchrotron light sourcesObservation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHCNitrogen and argon doping of niobium for superconducting radio frequency cavities: a pathway to highly efficient accelerating structuresSuccessful Nitrogen Doping of 1.3 GHz Single Cell Superconducting Radio-Frequency CavitiesUnprecedented quality factors at accelerating gradients up to 45 MVm −1 in niobium superconducting resonators via low temperature nitrogen infusionMeasurement of bunch length evolution in electron beam macropulse of S-band linac using coherent edge radiationDesign of 6Mev linear accelerator based pulsed thermal neutron source: FLUKA simulation and experimentSuperconducting Nb3Sn cavities with high microwave qualitiesOn properties of superconducting Nb3Sn used as coatings in RF cavitiesA Superconducting Nb3Sn Coated Multicell Accelerating Cavity
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