1. CW Operation of XFEL Modules
Outline
Limitations of the XFEL cryo module for high duty cycle and CW
Module tests
Dynamic heat load for the cw operation
Dynamic heat load measurements vs. duty factor
Maximum gradient test for the lp operation
Vector sum stabilization with RF-feedback
Wolf-Dietrich Möller, slides are from Jacek Sekutowicz
TTC topical meeting on CW SRF
12th – 14th June 2013, Cornell University

2. Limitations for the XFEL Module and Linac 1
The XFEL main linac consists out of 7 cryo strings,
every string includes 12 cryo modules,
every cryo module contains 8 cavities.
2.2 K forward
5 K forward
40 K forward
2-phase tube:
240 W (20W/cryomodule) is the estimated limit for one cryo string.
2 K, Gas Return Tube
80 K, Return
8 K, Return
1st limit: Heat load at 2K for each cryomodule must not exceed 20 W.

3. Limitations for the XFEL Module and Linac 2
Each cryomodule houses 8 cavities (in total there will be 808 cavities in the linac)
Cavity was designed for ca. 1% DF.
End groups (FPC and HOM couplers) are cooled by means of heat conduction.
HOM coupler
FPC
Dissipation on the HOM coupler antennas is the main sources of the heat for the end groups.

4. Limitations for the XFEL Module and Linac 3
Experience with vertical tests (cavities immersed in the superfluid bath):
Without HOM antennae:
Eacc up to 45 MV/m in the cw mode
With HOM antennae:
Eacc up to 40 MV/m in the lp mode
DF = 10 %
2nd limit: Heating of the HOM couplers must not lead to quenching of the end-cells.

5. Limitations for the XFEL Module and Linac 4
Other, “practical” limits:
3rd limit: The main XFEL linac will not be rebuilt for the new operation modes
Minor modifications in the cryomodule (cavities) have to be done now, prior to the series production.
Linac can be extended with 12 cryomodules from the injector section.
4th limit: An upgrade of the cryogenic plant should be “doable”
5th limit: New RF-sources will be added to the klystrons we plan to use for the nominal operation.
The new sources should fit in the tunnel. We plan to have a single RF-source/CM.

6. CW and Long Pulse Operation Modes
Proposed new operation modes for the XFEL:
Continuous wave operation for Eacc ≤ 7 MV/m
Long pulse operation at Eacc > 7 MV/m with DF ~ (7/Eacc)2
Example of long pulse operation at Eacc = 14 MV/m
750 ms
frep = 1Hz
~100 fs 1s
250 ms
4 µs for 1 nc
Bunch quality as high as for the nominal short pulse operation.

7. Dynamic heat load for the cw operation, 1
For experiments we equipped CryoModule Test Bed with cw operating RF-source (IOT).
Example #1 of the cw run
Goal: Measurement of dynamic 2K heat load for the cw operation.
Conditions:
7 cavities in operation (C#7 had detuned HOM coupler. It was operable <4MV/m).
End-groups were thermally connected to the 2K tube.
< Qload> = 1.5E7, 3 dB resonance width 87 Hz.
Eacc= 5.6 MV/m.
Pout IOT ~ 4.4 kW, RF-power/coupler ~ 550 W.
Feedbacks: RF off and piezo on.
Estimated dynamic heat load and measured static load:
For assumed Qo = 1.7E10 at this gradient dynamic heat load should be 13.7 W.
Cryomodule static loss 4.5 W.

8. Dynamic heat load for the cw operation, 2
2K dynamic loss at Eacc= 5.6 MV/m
Dynamic HL = 16 W
< Eacc > = 5.6 MV/m
Reference 5 W
Eacc =0 MV/m
Measured dynamic heat load for 7 cavities was 16 W.

9. Dynamic heat load for the cw operation, 3
Conclusion:
The difference of 2.3 W between measured and estimated HL can be attributed to warming of 14 end-groups, which is W/(end-group) .
Projecting this result on 8 cavities operating at 1.8 K, one might expect cw operation at Eacc close to 7 MV/m if Qo at that gradient is 3.4 E10.

10. Dynamic heat load measurements vs. duty factor, 1
Example #2 of the long pulse run
Goal: Measurement of the dynamic 2K heat load vs. DF for lp operation.
Conditions:
<Eacc> = 8.1 MV/m, Rep. rate 0.7 Hz.
6 cavities in operation, (C#7 detuned HOM coup., C#8 quenched at 9.3 MV/m at larger DF).
Feedbacks: RF off, piezo feedback and bias on.
Test Result: for the DF range from 15% to 47%.
DF =47%
DF =31%
DF =25%
DF =21%
DF =39%
DF =15%
Reference 1W; 0 MV/m

11. Dynamic heat load measurements vs. duty factor, 2
Test Result, cont.: Diagram showing measured and estimated Heat Load (HL) at 2K.

12. Dynamic heat load measurements vs. duty factor, 3
Conclusion:
The cryomodule (6 cavities) could be operated very stable at 8.1 MV/m at up to 650 ms long flat top + 33 ms filling. No anomalous behavior was observed up to this flat top and 47% DF.
The difference between measured and estimated heat load for long pulses can be attributed to higher operating temperature of 12 end-groups. These additional heat is ~280 mW/end-group at 650 ms long flat top.
Projecting this result for 8 cavities operating at 1.8 K, DF = 14%, one might expect to reach 20 MV/m for the 20 W/cryomodule with repetition rate of 1 Hz.

13. Maximum gradient test for the lp operation, 1
Example #3 of the lp run
Goal: Measurement of the dynamic 2K heat load at higher gradients.
Conditions:
Maximum IOT Pout was ca. 18 kW, due to strong reflection in the input circuit of IOT and reflection in the RF-power distribution system.
Eacc= 10.5 MV/m (6 good cavities) + C# 8 was operated at 8 MV/m.
DF= 17 %, repetition rate 0.7 Hz and flat top ca ms.
Feedbacks: RF off, piezo on.
Estimated dynamic heat load and measured static load:
For assumed Qo = 1.7E10 at this gradient, the dynamic heat load is 5.3 W.
Static heat load of the tested cryomodule was 4.5 W.

14. Maximum gradient test for the lp operation, 2
Test Result: System was operating stable for ca. 2h, no anomalous behavior was observed.
Reference 1 W; 0 MV/m
Dynamic HL = 6.3 W
< Eacc > = 10.5 MV/m
Conclusion:
As before, 1 W difference between the dynamic measured and estimated heat load is due to losses in the end-groups, ca W/(end-group).
Again, projecting this result for 8 cavities operating at 1.8 K, one should be able to reach 21.5 MV/m at DF=17%, flat-top ~140 ms for the budget of 20 W/CM.

15. Vector sum stabilization with RF-feedback
Goal: Stability of the vector sum for the cw operation with the new µTCA LLRF.
Conditions: Eacc = 3.5 MV/m, mode cw, piezo feedback off, bias on.
Test Result: 1s 1s
RF-feedback off
Standard deviations for:
Amplitude = 1.5E-3
Phase = °
phase
Vector sum
RF-feedback on
Standard deviations for:
Amplitude = 6.2 E-5
Phase = °
Vector sum
phase
Conclusion:
New µTCA RF-feedback improves amplitude and phase stability by factor of 24 and 51 respectively, and fulfills spec for the XFEL linac.

16. conclusion
The experiments showed feasibility of more flexible operation for the XFEL linac.
Following must be demonstrated in near future with coming XFEL-like cryomodules:
Benefit to operate at 1.8 K (can we get to higher gradients for 20W/cryomodule ?).
Higher gradients than 10.5 MV/m in lp operation (maximum Eacc vs. DF).
Superconducting RF gun needs more R&D
Test stand for SRF guns should be ready in 2014.
Other superconductors or photo-emitters should be studied.
R&D programs for 1.3 GHz and 3.9 GHz cavities should give an answer by the end of 2014 how high in gradient we can go for the cw mode.

17. Acknowledgements
Thank you Jacek, that you allowed me to present your experiment.
Jacek:
I want to express my gratitude to all Colleagues contributing to the experiments:
DESY: V. Ayvazyan, J. Branlard, W. Cichalewski, M. Ebert, N. Engling, J. Eschke, T. Feldmann, A. Goessel, W. Jalmuzna, D. Kostin, M. Kudla, D. Makowski,
F. Mittag, W. Merz, C. Müller, R. Onken, A. Piotrowski, K. Przygoda, I. Sandvoss, A. Sulimov.