Genfa Wu, presented by S. Posen TTC 2019, TESLA Technology Collaboration 5 February 2019

1. Genfa Wu, presented by S. Posen TTC 2019, TESLA Technology Collaboration 5 February 2019 Magnetic Field Sensors and Measurements in Cryomodules

2. Outline • Introduction • Magnetometers and Thermometers • Strategy of Instrumentation • Magnetic Fields in Cryomodules • Summary S. Posen | TTC at Vancouver 2019

3. Outline • Introduction • Magnetometers and Thermometers • Strategy of Instrumentation • Magnetic Fields in Cryomodules • Summary S. Posen | TTC at Vancouver 2019

4. Introduction • High Q Requirement in SRF applications • Reduce cryoplant size • Reduce operational cost • Reduce cryomodule cost of large helium circuit • Reduce the risk of high mass flow induced vibrations • Magnetic Field Trapping in Niobium Cavities • Increases Rs, increases heatload • Magnetic Field Management is Important for Cryomodules • Magnetic hygiene to reduce component residual magnetic field • Magnetic shielding to attenuate earth magnetic field • Demagnetization of cryomodule to reduce magnetization during cryomodule fabrication (Welding, magnetic tools, installation, etc.) • Thermal design to reduce thermo-electric current • Fast cool down to expel remaining true residual magnetic field Magnetometer is important instrumentation S. Posen | TTC at Vancouver 2019

5. Outline • Introduction • Magnetometers and Thermometers • Strategy of Instrumentation • Magnetic Fields in Cryomodules • Summary S. Posen | TTC at Vancouver 2019

6. Magnetometers and Thermometers • Requirement of Magnetometers • Direct measurement of magnetic field • High sensitivity (nT) • Sufficient range • Cryogenic temperature compatible • Small size • Vector measurement, preferred • Thermometers to help understand thermal behaviors • Cernox thermometers Bartington Fluxgate meets the spec, except its single-axis and relatively large size S. Posen | TTC at Vancouver 2019

7. Outline • Introduction • Magnetometers and Thermometers • Strategy of Instrumentation • Magnetic Fields in Cryomodules • Summary S. Posen | TTC at Vancouver 2019

8. Strategy of Instrumentation • Measure the Residual Magnetic Field at Room Temperature • Measure the Residual Magnetic Field at Cryogenic Temperature • Measure the Dynamic Magnetic Field during Cool Down • Measure the Dynamic Magnetic Field during Cavity Superconducting Transition and during Quench • Strategic Locations Based on Cryomodule Design S. Posen | TTC at Vancouver 2019

9. Strategy of Instrumentation – LCLS-II pCM Cell #1 Thermometer Cell #9 Thermometer 45-deg tilted fluxgate sensor Helium Return Beam Axis Helium Inlets Cell #1 Thermometer Transverse fluxgate sensor measuring transverse field Cell #9 Thermometer Four Cavities Fully Instrumented Five Fluxgate Sensors between the Magnetic Shields S. Posen | TTC at Vancouver 2019

10. Strategy of Instrumentation – PIP-II 650 MHz pCM M. Martinello, S. Posen, S. Chandrasekaran Helium Return DRAFT Vertical and transverse fluxgate sensors Axial fluxgate sensor Cell #1 Thermometer Cell #5 Thermometer Beam Axis Cell #5 Thermometer Behind equator Cell #1 Thermometer Cell #5 Thermometer Helium Inlet Four Cavities Fully Instrumented Helium Inlet Four Fluxgate Sensors on couplers S. Posen | TTC at Vancouver 2019

11. Strategy of Instrumentation – PIP-II 650 MHz pCM • In order to understand flux expulsion efficiency during cooldowns, simulations suggested that the flux-gates need to be placed as follow: • Longitudinally between irises (Bsc / Bnc = 0.18) • Vertically between irises (Bsc / Bnc = 0.85) • In this way the variation of the field (Bsc/Bnc) during the SC transition, after complete Meissner effect, is maximized and the fraction of field trapped/expelled can be estimated. • The simulations, courtesy of Iouri Terechkine, take into account REAL magnetic field environment in cryomodule and integrate the results within the active length of the fluxgate Martina Martinello

12. Strategy of Instrumentation – PIP-II 650 MHz pCM • In order to understand flux expulsion efficiency during cooldowns, simulations suggested that the flux-gates need to be placed as follow: • Longitudinally between irises (Bsc / Bnc = 0.18) • Vertically between irises (Bsc / Bnc = 0.85) • In this way the variation of the field (Bsc/Bnc) during the SC transition, after complete Meissner effect, is maximized and the fraction of field trapped/expelled can be estimated. • On the other hand, placing the fluxgate at +45o gives Bsc/Bnc=0.93 • And, similarly, placing the fluxgate at -45o gives Bsc/Bnc=1.1 Martina Martinello

13. Strategy of Instrumentation – PIP-II 650 MHz pCM • In order to detect B generated by thermo-currents a transverse fluxgate is needed Example of magnetic field generated by thermo-current during a fast cooldown of an LCLS-II cryomodule Martina Martinello

14. Outline • Introduction • Magnetometers and Thermometers • Strategy of Instrumentation • Magnetic Fields in Cryomodules • Summary S. Posen | TTC at Vancouver 2019

15. Residual Magnetic Field at Room Temperature Double layer magnetic shield and vigorous magnetic hygiene reduced residual magnetic field to ~1 mG. Fields up to 46 mG discovered after major assembly & installation work • Cryo-piping welds, warm coupler install, etc. • Most likely due to re-magnetization of the vacuum vessel & magnetic shields Cryomodule successfully demagnetized using in-place coils S. Chandrasekaran, TTC 2017 S. Posen | TTC at Vancouver 2019

16. Residual Magnetic Field at Room Temperature • Final ambient magnetic fields just before cooldown S. Chandrasekaran, TTC 2017 S. Posen | TTC at Vancouver 2019

17. Static Magnetic Field at Cryogenic Temperature CAV1 Temperature CAV5 transverse B 45-deg B CAV1 transverse B CAV8 transverse B CAV4 transverse B Static thermal magnetic field is trapped during superconducting transition (Slow cool down) S. Posen | TTC at Vancouver 2019

18. Static Magnetic Field Resets at Room Temperature Static magnetic fields were restored to near zero as cryomodule warmed up S. Posen | TTC at Vancouver 2019

19. Temperatures and Magnetic Fields During Fast Cool Down CAVITY #4 Top Cernox Bottom Cernox 45-deg B 9.2 K During a fast cool down under 80 g/s mass flow: Bottom of cavity sees faster drop of the temperature. A temperature gradient persists during transition. Large temperature difference induces strong transverse magnetic field in addition to existing fields also caused by cryomodule thermal currents. Strong transverse field reduced to “normal” amplitude during SC transitions. 45-degree sensor sees relatively small to negligible field. Cool down lasts about three minutes. S. Posen | TTC at Vancouver 2019

20. Cavity Q0 Comparison of LCLS-II F1.3-01 (Fermilab pCM) * TB9AES028 was at 14 MV/m Slow cool down results the trapping of static thermal magnetic field Fast cool down reduces flux trapping of remaining thermoelectric current induced field S. Posen | TTC at Vancouver 2019

21. Summary • Magnetic Field Measurement in LCLS-II pCM was very successful. • We now understands better how magnetic field evolves in a cryomodule, which helped LCLS-II cryomodules reach high Q routinely in production. • Details of thermo-electric current induced field is still unknown. S. Posen | TTC at Vancouver 2019