Frequency and Field Resolved TEM Mode Cavity Performance Characterizations

This study focuses on the performance characterizations of TEM mode cavities with different treatments, including mid-T bakes and other processes. Investigations include the impact on Q0 and Eacc, sensitivities to magnetic fields, and strategies for improving medium field Q slope. The research aims to explore enhancements in low-frequency cavities operating at 4.2K, providing insights into different cooldown schemes and mitigating flux.

• TEM mode cavities
• Frequency dependence
• Field sensitivity
• Low-frequency operation
• Magnetic field

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1. Mid-T Bakes on Coaxial Multi-mode Cavities Philipp Kolb, RuthAnn Gregory, Daniel Hedji, Mattias McMullin, Zhongyuan Yao, Arthur Blackburn, Tobias Junginger, Robert Laxdal Frequency and field resolved TEM mode cavity performance characterizations with different treatments Based on 10.3389/femat.2023.1244126 Philipp Kolb

2. Outline Motivation Cavities Mid-T bakes vs 120C/48h on BCP ed cavities Field dependence Frequency dependence Sample studies External field sensitivity Summary & Outlook 12/9/2024 Philipp Kolb TTC 2023 2

3. Motivation Great improvements on Q0 and Eacc in recent years N2 doping & infusion, two-step baking, mid-T bakes, Nb3Sn But research is focused on 1.3GHz, elliptical cavities for big projects like LCLS-II, SHINE, ILC operating around 2.0K. Question: Can we increase ?0 in low frequency, TEM mode cavities like QWR & HWR, typically operating at low frequency, 4.2K, and are BCPed, not EPed? Question: Do different geometry leads to different sensitivities to magnetic fields? Does a different cooldown scheme help expel magnetic flux? Question: Do we find the origin and mitigation strategies of medium field Q slope in TEM cavities? 12/9/2024 Philipp Kolb TTC 2023 3

4. TEM Cavity equivalent to single cell elliptical cavities Single cell 1.3GHz elliptical 1.3GHz nine-cell elliptical cavity cavity ? ISAC-II QWR FRIB HWR 12/9/2024 Philipp Kolb TTC 2023 4

5. Coax Test Cavities 40cm Designed and build two coax cavities, QWR and HWR, with a similar purpose & size as a 1.3GHz single cell cavity. No beam ports on cavity, simplifying geometry. RRR grade Nb for cavity walls, reactor grade for ports/bottom plate (QWR), No NbTi -> less sources of contamination. More details in: Phys. Rev. Accel. Beams 23, 122001 20cm Phys. Rev. Accel. Beams 23, 122001 Some notes: ?? (= ????(?) + ????) is field distribution corrected (see Delayen et al.), better comparable to ??= ?/?0 from elliptical cavities. All data shown is with external magnetic field compensated to ???? < 0.5??, unless otherwise stated. Delayen et al 217MHz 648MHz 389MHz 778MHz 1166MHz 12/9/2024 Philipp Kolb TTC 2023

6. Mid T bake - 250 to 400C Mid T bakes between 250 and 400 C on EP ed elliptical 1.3GHz cavities have been shown to increase ?0 with rising RF field. Rise in ?0 is attributed to a decrease in ???? with low ????. Very promising heat treatment for heavy ion accelerators like the ISAC-II LINAC at TRIUMF, operating at 4K where ???? is typically dominant. Q: does this translate to BCP ed cavities with low frequencies in a different geometry? H. Ito et al, arXiv:2101.11892v1 F. He et al, arXiv:2012.04817 12/9/2024 Philipp Kolb TTC 2023 6

7. Cavity Treatments Discussed 120C/48h QWR Baseline: 800C for 5-6h for hydrogen degassing 15?m flash BCP HPR and clean assembly 120C/48h: Baseline + In-situ bake for 48h inside cryostat no air exposure before measurement Mid-T bakes - 400(300)C/3h Surface reset with 15?? BCP UHV furnace heat treatment Air exposure before HPR and clean assembly No chemistry after heat treatment 400C/3h HWR 12/9/2024 Philipp Kolb TTC 2023 7

8. All fixed T cavity measurements 217MHz 389MHz 648MHz 778MHz 1166MHz Top 4K Bottom 2K All with external mag field < 0.1?T during cooldown. 12/9/2024 Philipp Kolb TTC 2023 8

9. Fitting to cooldown data Collecting data down to ~1.5K ?/?0 converted in ?? taking into account non-uniform field distribution. Noticeable bump at 2.17K for higher RF field measurements superfluid transition not considered here. Fit parameters give ???? directly and are used to calculate ????. 12/9/2024 Philipp Kolb TTC 2023 9

10. Field Dependence Modeling 2K Rres 4K Shown example from 220MHz: 2 ?? ?0 ???? is modeled as ?????? = ?0 1 + ? with ?0as a normalization constant (100mT arbitrarily chosen). Used to quantify slope not due to any specific physics model. No specific model for ?????? 12/9/2024 Philipp Kolb TTC 2023 10

11. 2 ?? ?0 ????= ?0 1 + ? Field Dependence ? At 4K: ? typically decreases with higher frequency. At 2K: no clear trend emerges. ? at 4K and 2K are of similar order of magnitude. Changes in perceived Q slopes are caused by reduced ?0. To get low Q slopes, high frequency seems to be a necessary ingredient. 12/9/2024 Philipp Kolb TTC 2023 11

12. Evaluating Frequency Dependence Example here from ???? at 4K for the 120C bake. ? = ? ?? Theoretical predictions by SRIMP (? 1.75 1.85) depend on material parameters ????,?, , temperature and is individually evaluated for the treatments. 12/9/2024 Philipp Kolb TTC 2023 12

13. Frequency Dependence ? = ? ?? ???? follows pretty much anomalous NC losses with ? ~ 0.66. At low field generally matches to prediction by SRIMP (? 1.75 1.85) but high field much different especially for the mid-T bakes. MB resistance is a low field theory not that surprising that some differences at higher fields show up. 12/9/2024 Philipp Kolb TTC 2023 13

14. Sample Measurements ~25mm Samples were produced to witness cavity treatments as close as reasonable possible. EDX and SEM were used as surface probes SEM found some carbon-based contaminants on the surface of mid-T bakes samples, but not 120C or baseline. The mid-T bakes increased the carbon counts and decreased oxygen. Also used in ???? measurements at TRIUMF E. Thoeng, TUIXA04, SRF 2023 to probe magnetic screening profiles (E. Thoeng, TUIXA04, SRF 2023) 12/9/2024 Philipp Kolb TTC 2023 14

15. Magnetic Field Sensitivity HWR results at 389MHz Cooldown dynamics critically important and interplay with geometry important. Fast cooldown can help with expelling flux. Other ways to manipulate cooldown to be explored Orientation of field relative to cavity can make a difference 120C bake shows higher sensitivity to horizontal than vertical fields. Mid-T bakes seem more sensitive than the low-T bake similar to N doping. Geometry of coaxial cavities leads to non-uniform trapping. MSc thesis of Ruth Gregory graduating later this year. 12/9/2024 Philipp Kolb TTC 2023 15

16. Summary and outlook Coaxial multi-mode cavities are a useful tool for studying SRF cavity performance Unique insights into frequency dependence Data at low rf fields does follow usual BCS theory but unusual behaviour at higher fields - also temperature dependent Mid-T bakes on BCPed surfaces do show decreased Q slope at high frequencies but overall, the usual 120C low temperature bake produces better performance Influence of surface roughness BCP vs EP needs to be investigated on low frequency cavities. Sensitivity to magnetic field is a complex topic due to the interplay between geometry and cooldown. Different cooldowns to be investigated. 12/9/2024 Philipp Kolb TTC 2023 16

17. Thanks! Thanks go to the people who make things happen in the background: Bhalwinder Waraich, Ruminder, Sekhon, James Keir, Devon Lang, David Kishi, Johnson Cheung, Ben Matheson And you for your attention! 12/9/2024 Philipp Kolb - TTC 2022 17

18. Some references Summary paper on cavities and methodology: P. Kolb et al: Phys. Rev. Accel. Beams 23, 122001 Mid-T bake results: P. Kolb et al: Front. Electron. Mater Sec. Superconducting Materials Cavity design: Z. Yao et al SRF2017, LINAC2018 First QWR cold test results: P. Kolb, TTC 2019 Vancouver Further QWR test results: P. Kolb et al, SRF2019 Baseline results for QWR & HWR: P. Kolb TTC@Cern 2020 Results from 120C/48h low temperature bake: P. Kolb TTC 2021 12/9/2024 Philipp Kolb - TTC 2022 18

19. Frequency dependence theoretical prediction To determine theoretical predictions Per treatment 1. Fit lowest field ??? data from one mode using SRIMP to full BCS model extract 0,?,???? 2. Use BCS parameters to calculate ???? at different frequencies and T 3. Extract frequency dependence parameter x: ????= ? ?? T [K] Baseline 120C 400C 300C 4.2 1.80 1.82 1.77 1.82 2 1.78 1.79 1.74 1.78 Theoretical predictions for frequency dependence parameter x 12/9/2024 Philipp Kolb TTC 2023 19