1. Summary of the maximum SCRF voltage in XFEL
Nick Walker (MPY, DESY)
TTC Meeting - Vancouver,

2. The European XFEL Cold Linac(s)
24 RF stations (A2-A25)
4 cryomodules per RF station (klystron)
32 TESLA 1.3-GHz cavities per RF station
(Injector A1 - 1 cryomodule, 8 cavities)
Design electron energy 17.5 GeV
Pulsed operation: Hz
750 ms fill time, 650 ms beam pulse (‘flat top’)
Nick Walker - TTC Meeting - UBC, Vancouver,

3. Cryomodule Testing and Installation
AMTF
All cryomodules were cold-tested in AMTF
Operational limits set for individual cavities
Quench
Field Emission (X-RAY monitor threshold)
31 MV/m –power limit (administrative)
Nick Walker - TTC Meeting - UBC, Vancouver,

4. Cryomodule Testing and Installation
AMTF
All cryomodules were cold-tested in AMTF
Operational limits set for individual cavities
Quench
Field Emission (X-RAY monitor threshold)
31 MV/m –power limit (administrative)
An average of 27.5 MV/m achieved
XFEL spec: 23.6 MV/m
Average cryomodule max. voltage: 228 MV
Nick Walker - TTC Meeting - UBC, Vancouver,

5. Cryomodule Testing and Installation
AMTF
All cryomodules were cold-tested in AMTF
Operational limits set for individual cavities
Quench
Field Emission (X-RAY monitor threshold)
31 MV/m –power limit (administrative)
An average of 27.5 MV/m achieved
XFEL spec: 23.6 MV/m
Average cryomodule max. voltage: 228 MV
Cryomodule Waveguide Distribution System (WDS) tailored to match forward power to each cavity
Within practical limits (phase adjustment)
6 cavities (in 5 modules) detuned operationally
Nick Walker - TTC Meeting - UBC, Vancouver,

6. Cryomodule Testing and Installation
AMTF
All cryomodules were cold-tested in AMTF
Operational limits set for individual cavities
Quench
Field Emission (X-RAY monitor threshold)
31 MV/m –power limit (administrative)
An average of 27.5 MV/m achieved
XFEL spec: 23.6 MV/m
Average cryomodule max. voltage: 228 MV
Cryomodule Waveguide Distribution System (WDS) tailored to match forward power to each cavity
Within practical limits (phase adjustment)
6 cavities (in 5 modules) detuned operationally
Four modules selected for RF station
Module ‘pairs’ balanced (power requirement)
Module power balance adjusted to match max. voltage (up to 3dB)
courtesy S. Chroba & V. Katalev
Nick Walker - TTC Meeting - UBC, Vancouver,

7. AMTF estimates of final performance
Max. beam energy: 19.3 GeV
Assumes exit L2 at 2.4 GeV
17.8 GeV without A24, A25
Nick Walker - TTC Meeting - UBC, Vancouver,

8. AMTF estimates of final performance
Max. beam energy: 19.3 GeV
Assumes exit L2 at 2.4 GeV
17.8 GeV without A24, A25
As of June 23, 2017: 14.7 GeV
without A24 and A25
Nick Walker - TTC Meeting - UBC, Vancouver,

9. AMTF estimates of final performance
Max. beam energy: 19.3 GeV
Assumes exit L2 at 2.4 GeV
17.8 GeV without A24, A25
As of June 23, 2017: 14.7 GeV
without A24 and A25
Re-evaluate AMTF estimate including errors:
Gradient: 2 MV/m
WDS: 0.02 dB
Klyst arm: 0.04 dB
Max beam energy: 15.4 GeV
without A24, A25
Nick Walker - TTC Meeting - UBC, Vancouver,

10. Achieving 17.5 GeV Maximum Gradient Task Force
Systematic evaluation of each RF station in L3
A6-A23 (A24, A25)
Determination of limiting cavities (quench)
Other limiting factors
Radiation in tunnel (dark current)
Cryogenic instabilities
Waveguide sparking (resolved)
Power limitations
Commissioning of stations A24 and A25
Nick Walker - TTC Meeting - UBC, Vancouver,

11. Single cavity performance (AMTF v operations)
Quench limits of 76 cavities were determined operationally
Not practical to re-measure all 800
Generally one per module (up to 3)
Only 3 cavities identified as having degraded from AMTF test
Two were not limiting station performance
An additional 12 cavities were detuned operationally
See next slide
Nick Walker - TTC Meeting - UBC, Vancouver,

12. Detuned cavities A: detuned after module tests or early commissioning
O: detuned during MGTF studies
12 cavities
A6, A7, A8 and A12 operating with only 29 cavities (3 detuned)
Nick Walker - TTC Meeting - UBC, Vancouver,

13. Achieved voltage and limiting factor
MGTF tuning achieved 91% of AMTF projection
(Not including A25)
Well within error bars
Two stations limited by tunnel radiation (A9, A12)
dark current
Overall voltage reduced
Still on a learning curve with radiation measurements
A14 limited by cryo load
so-called soft quenching
Nick Walker - TTC Meeting - UBC, Vancouver,

14. Current focus: operationally maintaining max gradients
Example A8
Station voltage generally reduced during XFEL operations
Normally after experiencing difficulty recovering from a station trip
Primary causes
Quench detection
Power related (klystron, modulator trips)
Coupler interlocks (e.g. temperature)
Currently evaluating operational margin
MGTF set 1 MV/m below observed quench limits
Assumed reasons for unstable operation
Tune state
Qload
Control system related (timing etc) MV
date
Typical reduction 50—100 MV per station
Nick Walker - TTC Meeting - UBC, Vancouver,

15. Example (simulation) A21 M1 example A21.M1 VSUM = 870 MV
Qload = 4.6×106
Includes LFD and static pre-tuning
A21.M1
Nick Walker - TTC Meeting - UBC, Vancouver,

16. Example (simulation) A21 M1 example Now include
VSUM = 870 MV
Qload = 4.6×106
Includes LFD and static pre-tuning
Now include
50 Hz RMS random detuning
0.1×106 RMS Qload error
Easy to ‘use up’ 1 MV/m margin
Difficult to reproduce MGTF configuration
A21.M1
Also can talk strongly to field emission / dark current
Nick Walker - TTC Meeting - UBC, Vancouver,

17. In Summary
Initial achieved station voltages were consistent with AMTF projections including errors
MGTF carefully studied and tuned each station individually, eventually achieving >90% of AMTF ‘design value’.
Currently running with 22 cavities detuned
12 detune as a result of the MGTF studies
Tunnel radiation (dark current) continues to be studied.
Focus now on maintaining identified max limits operationally
Root causes of trips etc.
Nick Walker - TTC Meeting - UBC, Vancouver,