1. Cool Down Studies of LCLS-II Cryomodules
Genfa Wu
February 6, 2018
TTC 2018 Milano

2. Thermal Management and Challenges
Outline
Introduction
Thermal Management and Challenges
Reducing Field Trapping during Superconducting Transitions.
Fast Cool Down to Expel any Remaining Magnetic Field
Thermal Currents
Cool Down Rate
Reduce thermoelectric currents
TTC 2018 Milano, February 6, 2018

3. Thermal Management and Challenges
Outline
Introduction
Thermal Management and Challenges
Reducing Field Trapping during Superconducting Transitions.
Fast Cool Down to Expel any Remaining Magnetic Field
Thermal Currents
Cool Down Rate
Reduce thermoelectric currents
TTC 2018 Milano, February 6, 2018

4. LCLS-II Q0 Specification
Cryomodule Q0 ≥ 16 MV/m Average Magnetic Field < 5mG (Amplitude)
Magnetic Field Sources:
Ambient field
Thermoelectric current
Dynamic thermoelectric current during fast cool down
Static thermoelectric current regardless of cool down rate
TTC 2018 Milano, February 6, 2018

5. Q0 Measurement of LCLS-II Cryomodules at FNAL
Q0 with Slow Cool Down (<5g/s))
Q0 with Fast Cool Down (30-80g/s)
Material
F1.3-01
2.1E+10
3.0E+10
WC 800C
F1.3-02
TD 800C
F1.3-03
2.6E+10
3.5E+10
TD 900C
F1.3-04
2.9E+10
3.2E+10
F1.3-05
2.5E+10
3.1E+10
F1.3-06
2.3E+10
Mostly NX 900C
F1.3-07
2.4E+10
2.7E+10
J1.3-01
Average
2.8E+10
New Record Q0 of 1.3GHz CW Cryomodule is now 3.5e10 (±11%)
TTC 2018 Milano, February 6, 2018

6. Thermal Management and Challenges
Outline
Introduction
Thermal Management and Challenges
Reducing Field Trapping during Superconducting Transitions.
Fast Cool Down to Expel any Remaining Magnetic Field
Thermal Currents
Cool Down Rate
Reduce thermoelectric currents
TTC 2018 Milano, February 6, 2018

7. Magnetic Field Management
Good Flux Expellable Material
Ambient Field
Magnetic Shield Design
Magnetic Hygiene
Active Coil Field Cancellation
Thermoelectric Currents
Improve thermal profiles
Flux Expulsion by Fast Cool Down
Ambient Field
Magnetic Shield Design
Magnetic Hygiene
Active Coil Field Cancellation
Thermoelectric Currents
Must improve thermal profiles
Flux Expulsion by Fast Cool Down
Poor Flux Expellable Material
TTC 2018 Milano, February 6, 2018

8. Poor flux expulsion material in some cryomodules
Challenges
Poor flux expulsion material in some cryomodules
Poor thermal profile induced ambient thermoelectric current (static thermoelectric current)
No instrumentation to troubleshoot issues
Q0 measurement to calculate trapped field
TTC 2018 Milano, February 6, 2018

9. Thermal Management and Challenges
Outline
Introduction
Thermal Management and Challenges
Reducing Field Trapping during Superconducting Transitions.
Fast Cool Down to Expel any Remaining Magnetic Field
Thermal Currents
Cool Down Rate
Reduce thermoelectric currents
TTC 2018 Milano, February 6, 2018

10. Cryomodule Cool Down – Magnetic Field
Fast Cool Down from 45K
CAV4 Longitudinal B
CAV4 Transverse B
+5 mG
-5 mG
Maximum +386, -210 mG
Static thermoelectric magnetic field is primarily caused by cryomodule’s intrinsic thermal gradients.
TTC 2018 Milano, February 6, 2018

11. Static Magnetic Field Resets at Room Temperature
Temperatures
Fluxgate Readings at 300 K
Fluxgate Readings at 2 K
Static thermoelectric magnetic fields were restored to near zero as cryomodule warmed up
TTC 2018 Milano, February 6, 2018

12. Dynamic Thermoelectric Current vs. Static Thermoelectric Current
Meissner Transition
Slow Cool Down
Fast Cool Down
CAV5 transverse B
45-deg B
CAV1 transverse B
45-deg B
CAV8 transverse B
Transverse B
Static B
CAV4 transverse B
CAV1 Temperature
Dynamic B
Dynamic thermoelectric magnetic fields disappear before superconducting transition
US-Japan ILC cost reduction workshop, 12/6/2017

13. Thermal Management and Challenges
Outline
Introduction
Thermal Management and Challenges
Reducing Field Trapping during Superconducting Transitions.
Fast Cool Down to Expel any Remaining Magnetic Field
Thermal Currents
Cool Down Rate
Reduce thermoelectric currents
TTC 2018 Milano, February 6, 2018

14. Q0 under Different Cooldown Mass Flow
CM03 TD 900C
F Wah-Chang 800C
CM04 TD 900C
CM05 TD 900C
J Wah-Chang 800C
Specification
CM07 NX 900C
CM02 TD 800C
TTC 2018 Milano, February 6, 2018

15. Q0 under Different Cool Down Mass Flow
Temperature difference from top of the cavities and bottom of cavities in pCM
 Mass Flow
80 g/s
47 g/s
25 g/s
Slow Cool Down
Cavity
 T(K)
CAV1
4.1
3.3
2.6
≤0.08
CAV4
4.9
4.7
4.4
CAV5
7.0
7.1 7
CAV8
5.6
2.89
Average Q0
 2.99e10
 ~2.46e10
 ~2.26e10
2.06e10 
45-deg tilted fluxgate sensor
Transverse fluxgate sensor measuring transverse field
Cernox sensor
Helium Inlet
Helium Return
Flow injected at each cavity bottom, return to GHRP between CAV4 and CAV5.
Linac cryogenic capacity 120 g/s
Sensors are at limited locations.
Fast cool down with sufficient mass flow can achieve effective flux expulsion
TTC 2018 Milano, February 6, 2018

16. Thermal Management and Challenges
Outline
Introduction
Thermal Management and Challenges
Reducing Field Trapping during Superconducting Transitions.
Fast Cool Down to Expel any Remaining Magnetic Field
Thermal Currents
Cool Down Rate
Reduce thermoelectric currents
TTC 2018 Milano, February 6, 2018

17. Thermoelectric Currents
Thomas Seebeck
1821
Seebeck coefficient varies from metal to metal
Thermocouple Temperature Measurement
TTC 2018 Milano, February 6, 2018

18. Potential Sources of Thermal Electric Current
Magnet
Bi-metal transitions
Other circuits such as tuner motor thermal strap, HOM thermal straps, 2-phase pipe chimney, magnet thermal straps, etc.
Coupler
and HOM
Tuner motor
TTC 2018 Milano, February 6, 2018

19. Coupler Maybe the Primary Source
55 K
5 K
2 K
293 K
Courtesy of R. Fischer, J. Kaluzny, T. Peterson.
Very speculative
Other circuits such as tuner motor thermal strap, HOM thermal straps, 2-phase pipe chimney, magnet thermal straps, etc.
Thermal Shield
Coupler
2-phase Chimney
Coupler
2-phase Chimney
Coupler
2-phase Chimney
Coupler
2-phase Chimney
Coupler
2-phase Chimney
Coupler
2-phase Chimney
Coupler
2-phase Chimney
Coupler
2-phase Chimney
Cavity
Cavity
Cavity
Cavity
Cavity
Cavity
Cavity
Cavity
TTC 2018 Milano, February 6, 2018

20. Trapped Field in LCLS-II Cryomodules
Trapped Field Obtained during Slow Cool Down
Calculated using Q0 and assumed 1.4 n/mG sensitivity
TTC 2018 Milano, February 6, 2018

21. Coupler Temperatures CM01 3.21 mG Coupler T (K) Coupler T (K)
Trapped Field (mG) 95 7
3.2 81 8
3.4 68 3
1.5
TTC 2018 Milano, February 6, 2018

22. Coupler Temperatures Slow Cool Down Q0 ~ 2.9e10
Fast Cool Down Q0 ~ 3.2e10
CM mG
Coupler T (K)
Coupler T (K)
Trapped Field (mG) 95 7
3.2 81 8
3.4 68 3
1.5
TTC 2018 Milano, February 6, 2018

23. Coupler Temperatures CM07 1gps 3.4 mG Coupler T (K) Coupler T (K)
Trapped Field (mG) 95 7
3.2 81 8
3.4 68 3
1.5
TTC 2018 Milano, February 6, 2018

24. Coupler Flange Temperature Affected by 45K Circuit
45K Circuit may have changed Seebeck current
1 gps
2.4e10
75 gps
2.5e10
83 gps
2.6e10
83 gps
2.7e10
TTC 2018 Milano, February 6, 2018

25. Summary
Thermoelectric currents are better characterized, but not well controlled.
Dynamic thermal currents during fast cool down were not harmful.
Static thermal currents are not understood yet.
Higher cool down rate improves the field expulsion
If material does not expel flux well
Fast cool down becomes less effective, but still beneficial if material can expel some flux.
Thermal profile must be improved (test is in progress)
Improve coupler thermal strapping
Lower coupler thermal intercept temperature
Use coupler power to equalize the coupler temperature
TTC 2018 Milano, February 6, 2018

26. Extra slides
LCLS-II Meeting on CM/SRF Topics, 11/17/2017

27. Coupler Heating is Negligible to Q0 Performance Change
N. Solyak and G. Wu
Mass flow remained constant despite coupler heats up to 140K at its 50K flange
TTC 2018 Milano, February 6, 2018

28. Temperature Profile of F1.3-07
Gate valve
Magnetic shield
70 g/s
Helium vessel
1 g/s
75 g/s
Soaking may not bring benefit after 11-days
TTC 2018 Milano, February 6, 2018

29. RI: Q0 Results
RI has completed fabrication of original order of 133 cavities
121 cavities have been tested so far at JLab and Fermilab
TD Cavities 900/200 preparation consistently exceed LCLS-II spec
Slide by Dan Gonnella of SLAC
TTC 2018 Milano, February 6, 2018

30. RI: Q0 Results
RI has completed fabrication of original order of 133 cavities
121 cavities have been tested so far at JLab and Fermilab
TD Cavities 900/200 preparation consistently exceed LCLS-II spec
NX Cavities at 900oC have middling results
Slide by Dan Gonnella of SLAC
TTC 2018 Milano, February 6, 2018

31. RI: Q0 Results
RI has completed fabrication of original order of 133 cavities
121 cavities have been tested so far at JLab and Fermilab
TD Cavities 900/200 preparation consistently exceed LCLS-II spec
NX Cavities at 900oC have middling results
950 and 975oC have further improved upon the NX cavities’ performance
Slide by Dan Gonnella of SLAC
TTC 2018 Milano, February 6, 2018

32. LCLS-II Magnetic Shield Design
Magnetic Shielding Material is CryoPerm 10
The baseline permeability is
Actual permeability measured around
Design allows the access to tuner motor and piezos.
Double layers in the middle
One layer End Caps
LCLS-II FAC Review, Sept 26-28, 2017

33. Magnetic Hygiene - Components
Permeability is not enforced
Any tool that measures permeability will magnetize the components inadvertently. De-magnetization may not be cost effective for many parts.
Permeability measurement is intrinsically inaccurate for non flat and irregular surfaces.
Remnant Field
Measurement is done for all components that is within 3-in distance from cavity.
Measurement is done at contact and 1-in distance.
Working specification is provided for parts screening
Demagnetization
Parts will be demagnetized or rejected based on the value and demagnetization labor cost. ( Be careful with the tools )
Demagnetization of vacuum vessels
Demagnetization of assembled cryomodules
Technical Division Controlled Document:
REV 1 - Specification and Measurement Procedures of Magnetic Properties of Parts for LCLS II Cryomodule Assembly
LCLS-II FAC Review, Sept 26-28, 2017

34. Magnetic Hygiene - In-situ demagnetization
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. Chandrasekeran, TTC 2017
LCLS-II FAC Review, Sept 26-28, 2017

35. Magnetic fields obtained in prototype CM at Fermilab CMTF
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. Chandrasekeran, TTC 2017
LCLS-II Meeting on CM/SRF Topics, 11/17/2017

36. Slow Cool Down Cool Down Rate ~3 K/hour During transition,
T on cells ≤0.08K,
T on end groups ≤0.18K
Transition
Slow cool down created a uniform temperature on cavity
LCLS-II FAC Review, Sept 26-28, 2017

37. Static Thermal B Field Meissner Transition
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)
LCLS-II FAC Review, Sept 26-28, 2017

38. Temperatures and Magnetic Fields During Fast Cool Down
Top Cernox
Bottom Cernox
45-deg B
9.2 K
CAVITY #4
Transverse
Transverse B
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.
45-degree sensor sees relatively small to negligible field.
Cool down lasts about three minutes
LCLS-II FAC Review, Sept 26-28, 2017

39. Temperature Gradient and B field at Cavities
TABLE I. Temperature difference and B field for four cavities when cavity bottom passed Tc during fast cool down under 80 g/s mass flow
Cavity number
Top Bottom Temperature Difference [K]
Longitudinal B field [mG]
Transverse B field [mG] 1
4.1 5 4
4.3 7
6.8 2 8
5.1
Remnant field from other components cannot be measured other than at the sensor locations.
LCLS-II FAC Review, Sept 26-28, 2017

40. Cryomodule Cool Down – Magnetic Field
Fast Cool Down from 45K
CAV4 Longitudinal B
CAV4 Transverse B
Maximum +386, -210 mG
Static thermal magnetic field is primarily caused by cryomodule’s intrinsic thermal gradients.
LCLS-II FAC Review, Sept 26-28, 2017

41. Temperatures and Magnetic Fields During Fast Cool Down
Top Cernox
Top Cernox
Bottom Cernox
Bottom Cernox
CAVITY #4
45-deg B
45-deg B
CAVITY #1
Transverse B
Transverse B
9.2 K
9.2 K
Top Cernox
Bottom Cernox
Transverse B
45-deg B
45-deg B
CAVITY #8
Transverse B
Top Cernox
CAVITY #5
Bottom Cernox
9.2 K
9.2 K
LCLS-II FAC Review, Sept 26-28, 2017

42. Dynamic Thermal Current vs. Static Thermal Current
Dynamic thermal magnetic field not harmful
Induced from cavity and helium vessel thermal gradient during fast cool down
Dissipates quickly as the cavity reaches equilibrium before superconducting transition and can be expelled during fast cool down
Does not appear during slow cool down
Static thermal magnetic field can be trapped if there is no flux expulsion
Induced from various current loops in a cryomodule
Requires fast cool down to achieve flux expulsion
Proper heat treatment of niobium cavity material is a necessity.
LCLS-II FAC Review, Sept 26-28, 2017