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

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

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

LCLS-II Q0 Specification Cryomodule Q0 ≥ 2.7e10 @ 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

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

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

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

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

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

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

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

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

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

Q0 under Different Cooldown Mass Flow CM03 TD 900C F1.3-01 Wah-Chang 800C CM04 TD 900C CM05 TD 900C J1.3-01 Wah-Chang 800C Specification CM07 NX 900C CM02 TD 800C TTC 2018 Milano, February 6, 2018

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

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

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

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

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

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

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

Coupler Temperatures Slow Cool Down Q0 ~ 2.9e10 Fast Cool Down Q0 ~ 3.2e10 CM04 1.49 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

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

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

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

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

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

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

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

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

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

LCLS-II Magnetic Shield Design Magnetic Shielding Material is CryoPerm 10 The baseline permeability is >12,000@45K Actual permeability measured around 45,000@RT 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

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: 464233 - REV 1 - Specification and Measurement Procedures of Magnetic Properties of Parts for LCLS II Cryomodule Assembly LCLS-II FAC Review, Sept 26-28, 2017

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

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

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

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

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

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

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

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

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