1. First operation of the XFEL linac with the 2K cryogenic system
T.Paetzold2, B. Petersen1, T. Schnautz1, C. Ueresin3 & J.Zajac3
1Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany
2Linde Kryotechnik/Engie, Hamburg, Germany
3Linde Kryotechnik, Pfungen, Switzerland
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2. Cold compressors Overview
(General) challenges
Requirements on XFEL cold compressors
Cold compressor bypass operation
Mass flow compensation
Temperature adjustment
Stand alone operation
Pressure Stability
Current Problems
Summary
Outlook
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3. Cold compressors Operational challenges
Cold compressors are turbomachines and hence very sensitive towards massflow-, temperature- and pressure- changes in the 2K-return flow (but the massflow-stability is most important)
Surge line
Choke operation: Should be avoided as cryo capacity decreases
Surge operation: Not possible - breakdown of operation

4. Cold compressors Requirements
Requirements on XFEL 2K cold compressor system:
2K Pressure stability: < 1%
High availability
Exchange of cold compressor motors for maintenance after hrs.
Suction pressure: 24mbar mbar
Set-pressure shall be regulated automatically
30% turn down capacity
Discharge pressure: 700mbar mbar…
… by reducing the suction pressure of warm screw compressors
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5. Cold compressors Requirements
Required abilities to optimize operational issues
Cold compressor system shall allow:
Easy adaption on different operation modes
Extension of the specific operation field
Stand alone operation
Re-connection of XFEL Linac/injector at sub atmospheric pressure
Reduction of recovery time
Cold compressor bypass system can fulfill these requirements
Cold compressors
CB44
CC-bypass
XFEL
Linac & Injector
Refrigerating plant
Before you start editing the slides of your talk change to the Master Slide view:
Menu button “View” > Slide Master:
Edit the following items:
1. On the Title slide (second master slide)
a) Click to add title of your talk
b) Click to add subtitle (conference, location, name of the speaker, date)
2. On the Slide Master (first master slide)
a) The 1st row in the violet header: Delete the existent text and write the title of your talk into this text field b) The 2 rows in the footer area: Delete the text and write the information regarding your talk (same as on the Title Slide) into this text field.
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6. Cold compressors Bypass operation
Bypass operation: Massflow compensation and temperature adjustment
2K supply
(~4.5K)
2K supply (linac)
Cold compressors (require ~106g/s)
Cold compressor bypass
Mass flow compensation
Temperature adjustment
Stand alone operation
Cold compressor bypass functions:

7. Cold compressors Operational challenges
Bypass operation: Massflow compensation and temperature adjustment
2K return flow: 70g/s
2K supply
(~4.5K)
2K supply (linac)
2K return (linac)
Cold compressors (require ~106g/s)

8. Cold compressors Operational challenges
Bypass operation: Massflow compensation and temperature adjustment
2K return flow: 70g/s
2K supply
(~4.5K)
2K supply (linac)
2K return (linac)
Cold compressors (require ~106g/s)
20K return
(coldbox)

9. Cold compressors Operational challenges
Bypass operation: Massflow compensation and temperature adjustment
2K return flow: 70g/s
2K supply
(~4.5K)
2K supply (linac)
2K return (linac)
Cold compressors (require ~106g/s)
20K return
(coldbox)
CC-bypass has to produce: 30g/s

10. Cold compressors Operational challenges
Bypass operation: Massflow compensation and temperature adjustment
Real CC mass flow: 2K return flow + CC-bypass flow
2K return flow: 70g/s
2K supply
(~4.5K)
2K supply (linac)
2K return (linac)
Cold compressors (require ~106g/s)
20K return
(coldbox)
CC inlet temperature is adjusted by use of two heat exchangers in CC-bypass
CC-bypass has to produce: 30g/s

11. Cold compressors Mass flow compensation
Bypass operation: Massflow compensation
(12:00)
(10:00)
110 g/s
50 g/s
70 g/s
(06:00)
(12:00)
110 g/s
50 g/s
70 g/s
Required massflow
Real massflow
Bypass flow
2K return flow
Surge shutdown
Required massflow
Real massflow = (2K return flow + bypass flow)
Bypass flow
Massflow bypass
2K return flow (linac & injector)
Conclusion:
CC-bypass operation delivers reasonable reactions on changes in 2K return flow
Extent of changes in 2K return determines limits of massflow compensation

12. Cold compressors Recovery after shutdown / stand alone (bypass) operation
Bypass operation: Recovery after shutdown
Normal operation configuration
Warm He pumps
AMTF
XFEL Coldbox
Distribution box
XLVB
XFEL
Linac & Injector
Cold compressors
CB44
CC-bypass
2K return flow
1000mbar
29mbar

13. Cold compressors Recovery after shutdown / stand alone (bypass) operation
Bypass operation: Recovery after shutdown
Operation configuration after surge – beam operation not possible, but linac can be kept cold at 30mbar
1000mbar
29mbar
Warm He pumps
AMTF
Cold compressors
CB44
CC-bypass
XFEL Coldbox
Distribution box
XLVB
XFEL
Linac & Injector
2K return flow
1000mbar
1000mbar
Sudden shutdown!

14. Cold compressors Recovery after shutdown / stand alone (bypass) operation
Bypass operation: Recovery after shutdown
Operation configuration after surge – beam operation not possible, but linac can be kept cold at 30mbar
1000mbar
29mbar
Warm He pumps
AMTF
Cold compressors
CB44
CC-bypass
XFEL Coldbox
Distribution box
XLVB
XFEL
Linac & Injector
2K return flow
1000mbar
1000mbar → 29mbar
Stand alone bypass operation - Internal pump down

15. Cold compressors Recovery after shutdown / stand alone (bypass) operation
Bypass operation: Recovery after shutdown
Normal operation configuration after internal pumpdown and re-connection of XFEL linac/injector
1000mbar
29mbar
Warm He pumps
AMTF
XFEL Coldbox
Distribution box
XLVB
XFEL
Linac & Injector
Cold compressors
CB44
CC-bypass
2K return flow
1000mbar
29mbar
Re-connection of linac at sub atmospheric pressure
Conclusion:
The recovery time after a cold compressor shutdown is minimized by
the possibility of the stand alone CC-bypass operation and
the re-connection of the XFEL linac/injector at sub atmospheric pressure
(5 hours, 2-3 well trained cryo operators).

16. Cold compressors Pressure stability
Specified pressure stability: 1% (30.6 mbar +/- 0.3 mbar)
Cascaded regulation for pressure adjustment in 2K circuit is used
The pressure controller of the linac determines the set point of another pressure controller, which regulates the suction pressure of the cold compressors
Automatic heat load compensation (AHLC) is used: Changes in the 2K return flow – caused by dynamic RF operation - are determined automatically and compensation takes place in the linac by automatic heating in the 2K liquid helium.
Sudden RF-shutdown
(10 GeV)
Required massflow for CCs
30.9 mbar
Real massflow
(2K return flow + bypass massflow)
2K pressure
30.6 mbar
Massflow bypass
1% - pressure stability
30.3 mbar
2K return flow from linac
(09:00)
(16:00)
(09:00)
(16:00)
Conclusion:
Cold compressors deliver a pressure stability better than 0.3%, which is much better than internally specified.
Heat load changes –caused by dyn. RF operation- can be compensated perfectly well by AHLC

17. Cold compressors Motor failures
Current problems/challenges with cold compressors: CC-motor failures
CC-motors are equipped with ceramic ball bearings
Three of four motors show bearing problems
Specification: lifetime of cold compressor motors of at least hrs.
Observed average lifetime of the (three) motors is far below specification
Due to the bearing failures, the specified lifetime could not yet be achieved
New motor design has been tested successfully at manufacturer and is currently in operation at DESY for > 2200 hrs. without failures
Root cause analysis by Linde Kryotechnik
Conclusion:
Downtime, caused by cold compressor issues, has to be reduced significantly
Duration of interruption in case of motor failure: 2.5 days
New CC-motor design is very promising, impact on lifetime has to be verified

18. Cold compressors Summary
XFEL cryogenic system: Summary
Most promi
Cold compressors deliver very high 2K pressure stability (0.3%)
Dynamic procedures in the XFEL linac can be compensated quite well by CC-bypass operation
Most promi
Sensibility of cold compressors could be eased by
Bypass operation in combination with
automatic heat load compensation
Even a sudden RF-shutdown from 10 GeV does not affect the stability of the cold compressor operation
Most promi
Recovery time after shutdown is optimized by the possibility …
… to use of warm helium pumps to keep the linac at 30mbar (static)
… of stand alone CC-bypass operation (internal pumpdown)
… to re-connect XFEL linac/injector to the cold compressors at sub atmospheric pressure
Problem solving related to lifetime of CC-motors is ongoing
New motor design seems to increase lifetime of CC-motors → more experience must be gained

19. Cold compressors Outlook
XFEL 2K cold compressors: Outlook
Most promi
Dynamic procedures have to be (partly) automated
Dynamic procedures at XFEL cryo are still very complicated, time consuming and personnel-intensive
Pumpdown
Restart of cold compressors
Cryo operators have to be trained intensively
Operators not only need to know how to operate the plant, they also have to understand why they have to do it in a certain way
Operating instructions have to be optimized
Different compressor-wheels will be defined to match the real 2K return flow
Required massflow for cold compressors will be reduced
Operational costs will be reduced

20. Cold compressors Outlook
XFEL 2K cold compressors: Outlook
Most promi
Dynamic procedures have to be (partly) automated
Dynamic procedures at XFEL cryo plant (e.g. restart of cold compressors) are still very complicated, time consuming and personnel-intensive
Cryo operators have to be trained intensively
Operators not only need to know how to operate the plant, they also have to understand why they have to do it in a certain way
Operating instructions have to be optimized
Different compressor-wheels will be defined to match the real 2K return flow
Required massflow for cold compressors will be reduced
Operational costs will be reduced

21. Thank you for your attention
Acknowledgements
In particular, we thank:
Linde Kryotechnik (C. Ueresin, L. Decker, K. Rautert, …)
Linde/Engie cryo operating team (J. Zajac, T. Paetzold, …)
Hecht cryo & gas experts (H. Hecht, …)
Many more…
Thank you for your attention