1. LCLS-II Cavity Production and Vertical Testing
Fermilab and JLab: 3 cavities/cool-down (IB-1 VT2 and VT3, Test-lab D5) Compare with DESY/XFEL AMTF 4 cavities/cool-down
LCLS-II/Cryo systems Weekly meeting

2. Cryogenic Systems Scope:
Component
Count
Parameters
Linac Cryomodules
1.3GHz - 35 ea
3.9 GHz - 2 ea
Linac – 4 cold sections
8 cavities per cryomodule (1.3 & 3.9 GHz)
1.3 GHz Cryomodule (CM)
8 cavities/CM
13 m long. Cavities + SC Magnet package
+ BPM
3.9 GHz Cryomodule
6.2 m long. Cavities + BPM
Additional Cryomodules
1.3 GHz: 4 production + 1 spare
3.9 GHz: 1 spare
1.3 GHz 9-cell cavity
320 each
16 MV/m; Q0 ~ 2.7e10 (avg); 2.0 K;
gradient reach to 19 MV/m (No Q-spec);
bulk niobium sheet - metal
3.9 GHz 9-cell cavity
24 each
13.4 MV/m; Q0~2.0e9
Cryoplant (CP1/CP2)
2 each
4.5 K / 2.0 K cold boxes; K; K; 3.7 kW nom. tot. load
Spare compressors
2 Warm He Comp. 1 spare Cold Comp.
Cryogenic Distribution System (CDS)
210 m vacuum-jacketed line, 2 each distribution boxes, 6 each feedcap / 2 each endcap
No use of “spares”
LCLS-II DOE Status Review, Jun 13-15, 2017

3. Background - SRF Cavities
LCLS-II Specification: Q0≥ Eacc = 16 MV/m in 5 mG remnant field
Additionally the cavities must reach 19 MV/m in VT – accounts for errors in gradient measurement
Specification designed to reduce 2K cryogenic load, and thus operating cost of machine.
Made possible by Nitrogen doping of SRF cavities.
Comes with 2 trade-offs.
Losses from trapped flux in doped cavities can be up to 3.6 times higher than un-doped cavities
Reduction in maximum achievable gradient of cavity – not an issue for LCLS-II
Remedied by:
Improved magnetic hygiene and shielding
Optimized design and cooldown procedures
Gonnella – March 17, LCLS-II Collaboration Meeting

4. Performance Summary for the first 11 cryomodules
Data Source
Average Q
Avg Gradient [MV/m]
F1.3-1 (pCM)
CM Test
3.0x1010
18.2
J1.3-1 (pCM)
3.3x1010 *
132.8
F1.3-2
2.1x1010
20.0
J1.3-2
Vertical Test
2.1x1010 **
21.5
F1.3-3
3.31x1010
23.2
J1.3-3
3.61x1010
22.2
F1.3-4
3.33x1010
23.0
J1.3-4
3.62x1010
23.9
F1.3-5
3.01x1010
21.6
J1.3-5
2.69x1010
22.7
F1.3-6
2.82x1010
Totals
2.86x1010
21.9
pCMs
Retesting after shipping test now in-progress
800 C bake
900 C bake
* Q measured on 2 of 8 cavities (JLab pCM Jan 2017)
** VT Q scaled based upon measured field in the first 2 CMs.
Q0 Spec = 2.7 x 1010 (ensemble average)
LCLS-II DOE Status Review, Jun 13-15, 2017

5. Yield of Production Cavities
Received
129
42% of total 304
Tested
107
Total qualified 81
76% yield
Qualified as received 46
Qualified after re-HPR 35
43% (2 re-HPR 2x)
LCLS-II DOE Status Review, June 2017

6. Field Emission
Uncontrolled processes at both vendors lead to high rates of field emission
JLab, SLAC, and FNAL personnel worked with personnel at both vendors to develop and implement clean manufacturing procedures.
Both vendors were required to revise procedures and submit to JLab for approval
Improvements were made to procedures at cavities 17, 53, and 73.
Cavities 1-17: 75% FE
Cavities 17-53: 47% FE
Cavities 53-73: 23% FE
Cavities 73+: 9% FE (below 19 MV/m)
Rerinsing is effective for cavities with FE. Out of over 30 cavities with FE, only two had to be rerinsed twice.

7. Significant capabilities beyond our initial consideration.
Cavity Q0 Performance in VT
Cavity Gradient Reach in VT
19 MV/m
LCLS-II Spec
16 MV/m
800/140 in Low Bamb
900/200 in 5-10 mG
Sheet type
Sheet type

8. Q = G/ Rs where RS = RBCS + R0 + RTF
Background - Obtaining and preserving Q ≥ 2.7e10 at 16 MV/m, 2K
Q = G/ Rs where RS = RBCS + R0 + RTF
RTF= s * η* Bamb
Trapped magnetic flux
residual
Doping
Intrinsic
residual
Sensitivity to mag field
Flux trapping percentage
Total Rs budget for Q~ 2.7e10 = 10 nanoOhms
pCM material:
RBCS ~ 4.5 nΩ +
R0 ~ 1-2 nΩ +
RTF ~ 1.4*(<0.2)*B =
if B ~5 mG = 7.5 nΩ => Q ~3.5e10
if B ~1 mG = 6.5 nΩ => Q ~4e10
Gonnella – March 17, LCLS-II Collaboration Meeting

9. Q = G/ Rs where RS = RBCS + R0 + RTF
How to obtain and preserve Q > 2.7e10 at 16 MV/m, 2K?
Q = G/ Rs where RS = RBCS + R0 + RTF
Trapped magnetic flux
residual
Doping
Intrinsic residual
Total Rs budget for Q~ 2.7e10 = 10 nanoOhms
pCM material:
RBCS ~ 4.5 nΩ +
R0 ~ 1-2 nΩ +
RTF ~ 1.4*(<0.2)*B =
production material:
RBCS ~ 4.5 nΩ +
R0 ~ 4-5 nΩ +
RTF ~ 1.4*(>0.7)*B =
if B ~5 mG = 7.5 nΩ => Q ~3.5e10
if B ~1 mG = 6.5 nΩ => Q ~4e10
if B ~5 mG = 14 nΩ => Q ~ 1.9e10
if B ~1 mG = 10 nΩ => Q ~ 2.7e10
Two issues to be fixed: 1) higher intrinsic R0; 2) Flux expulsion
Grassellino | Director's review

10. 2014, Q degradation risk: discovery of slow cooldown trapping all flux, fast helping expulsion
Slow and homogeneous cooldown through Tc does not provide enough force to expel magnetic flux from the material
Fast cooldown with larger thermogradients helps expelling magnetic flux
Dressed N doped nine cell cavity vertical test at T=2K
A. Romanenko, A. Grassellino, O. Melnychuk, D. A. Sergatskov, J. Appl. Phys. 115, (2014)
A. Romanenko, A. Grassellino, A.Crawford, D. A. Sergatskov, Appl. Phys. Lett. 105, (2014)
D. Gonnella et al, J. Appl. Phys. 117, (2015)
M. Martinello, M. Checchin, A. Grassellino, A. Romanenko, A. Crawford, D. A. Sergatskov, O. Melnychuk, J. Appl. Phys. 118, (2015)
Grassellino | Director's review

11. 2014: Q degradation risk – Flux expulsion via fast cooling needed to maintain low residual resistance
Full expulsion
% flux trapping
Full trapping
Bottom-top cell
Low thermogradient detrapping threshold seen in early cavities (Wah Chang)
A. Romanenko, A. Grassellino, O. Melnychuk, D. A. Sergatskov, J. Appl. Phys. 115, (2014)
A. Romanenko, A. Grassellino, A.Crawford, D. A. Sergatskov, Appl. Phys. Lett. 105, (2014)
D. Gonnella et al, J. Appl. Phys. 117, (2015)
M. Martinello, M. Checchin, A. Grassellino, A. Romanenko, A. Crawford, D. A. Sergatskov, O. Melnychuk, J. Appl. Phys. 118, (2015)
Grassellino | Director's review

12. 2015: Later Studies: Worrisome Expulsion Trend Found With Batches of AES Cavities
Findings triggered production material assessment
S.Posen et al, J. Appl. Phys. 119, (2016)
Wah Chang
Tokyo Denkai
7:30
AES Single Cells Batch 1
AES Single Cells Batch 2
10 K thermogradient: 30% vs 100% flux expulsion
Grassellino | Director's review

13. 900 C (versus 800C) improves expulsion
2015: Bulk heat treatment has effect: can convert from poor to strong expulsion via 900C treatment
Complete Expulsion
S.Posen et al, J. Appl. Phys. 119, (2016)
900 C (versus 800C) improves expulsion
Based on available cooldown flow rates > 30 g/sec per CM [1] , DT achievable in tunnel > 5K, from horizontal test studies [2, 3]
Complete Trapping
[1] T.Peterson et al, LCLSII-4.5-EN-0479-R0
[2] D. Gonnella et al , J. Appl. Phys. 117, (2015)
[3] A. Grassellino et al, Proceedings of SRF15, MOPB028
Grassellino | Director's review

14. Flux expulsion statistics.

15. pCM Measurement of Temperature and Magnetic Field
45-deg tilted fluxgate sensor
Transverse fluxgate sensor measuring transverse field
Cernox sensor
Helium Inlet
Helium Return
Four Cavities Fully Instrumented
G. Wu et al. | 2017 TESLA Technology Collaboration at MSU

16. Q0 under Different Cooldown Mass Flow
LCLS-II/Cryo systems Weekly meeting

17. Q0 under Different Cool Down Mass Flow
Temperature difference from top of the cavities and bottom of cavities
 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 
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
G. Wu et al. | 2017 TESLA Technology Collaboration at MSU

18. Cryomodule cooldown (1)

19. Cryomodule cooldown (2)