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

2. Outline Introduction Magnetometers and Thermometers
Strategy of Instrumentation
Magnetic Fields in Cryomodules
Summary
8/3/2019
S. Posen | TTC at Vancouver 2019

3. Outline Introduction Magnetometers and Thermometers
Strategy of Instrumentation
Magnetic Fields in Cryomodules
Summary
8/3/2019
S. Posen | TTC at Vancouver 2019

4. Introduction Magnetometer is important instrumentation
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
8/3/2019
S. Posen | TTC at Vancouver 2019

5. Outline Introduction Magnetometers and Thermometers
Strategy of Instrumentation
Magnetic Fields in Cryomodules
Summary
8/3/2019
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
8/3/2019
S. Posen | TTC at Vancouver 2019

7. Outline Introduction Magnetometers and Thermometers
Strategy of Instrumentation
Magnetic Fields in Cryomodules
Summary
8/3/2019
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
8/3/2019
S. Posen | TTC at Vancouver 2019

9. Strategy of Instrumentation – LCLS-II pCM
Cell #1
Thermometer
45-deg tilted fluxgate sensor
Transverse fluxgate sensor measuring transverse field
Helium Inlets
Helium Return
Cell #9
Cell #1 Thermometer
Beam Axis
Four Cavities Fully Instrumented
Five Fluxgate Sensors between the Magnetic Shields
8/3/2019
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
Cell #5 Thermometer
Behind equator
Beam Axis
Cell #5 Thermometer
Cell #1 Thermometer
Helium Inlet
Four Cavities Fully Instrumented
Helium Inlet
Four Fluxgate Sensors on couplers
8/3/2019
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
2/1/2019

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
2/1/2019

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
2/1/2019

14. Outline Introduction Magnetometers and Thermometers
Strategy of Instrumentation
Magnetic Fields in Cryomodules
Summary
8/3/2019
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
8/3/2019
S. Posen | TTC at Vancouver 2019

16. Residual Magnetic Field at Room Temperature
Final ambient magnetic fields just before cooldown
Fluxgate orientation
Cavity #
Magnetic field component (mG)
Inside cavity helium vessel
45° 1
1.25 4
0.66 5
0.05 8
0.23
Transverse
1.43
0.29
0.14
0.15
Outside cavity helium vessel
Axial
1.22 2
0.67
0.96 7
0.60
0.24
S. Chandrasekaran, TTC 2017
8/3/2019
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)
8/3/2019
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
8/3/2019
S. Posen | TTC at Vancouver 2019

19. Temperatures and Magnetic Fields During Fast Cool Down
Top Cernox
Bottom Cernox
45-deg B
9.2 K
CAVITY #4
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.
8/3/2019
S. Posen | TTC at Vancouver 2019

20. Cavity Q0 Comparison of LCLS-II F1.3-01 (Fermilab pCM)
Usable Gradient [MV/m]
Fast Cool Down (80 g/s)
Slow Cool Down (<8 g/s)
TB9AES021
18.2
2.6E+10
1.8E+10
TB9AES019
18.8
3.1E+10
1.5E+10
TB9AES026
19.8
3.6E+10
3.3E+10
TB9AES024
20.5
2.1E+10
TB9AES028
14.2
1.9E+10
TB9AES016
16.9
2.0E+10
TB9AES022
19.4
TB9AES027
17.5
2.3E+10
Average
3.0E+10
Total Voltage
148.1 MV
* 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
8/3/2019
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.
8/3/2019
S. Posen | TTC at Vancouver 2019