1. The LCLS-II HE High Q0 and Gradient R&D Program
Dan Gonnella, for the LCLS-II HE R&D Collaboration
2019 TTC Workshop
February 6, 2019

2. Outline
LCLS-II HE SRF Requirements
Niobium Material Experience
Nitrogen-Doping and LCLS-II
Pushing the Limits of High Q and High Gradient SRF
R&D Plan
R&D Results
Summary

3. LCLS-II SRF Requirements
LCLS-II consists of 35 cryomodules each with 8 cavities – total of 280 cavities
The GHz 9-cell cavities have a very ambitious Q specification:
Additionally, cavities must reach 19 MV/m
2.7 x 1010 at 16 MV/m and 2 K

4. LCLS-II HE SRF Requirements
LCLS-II HE adds 20 cryomodules additional cryomodules to the LCLS-II linac
The GHz 9-cell cavities have a very ambitious Q specification:
Additionally, cavities must reach 24.5 MV/m
2.7 x 1010 at 20.8 MV/m and 2 K

5. LCLS-II cavity production is nearly complete (>90%)
LCLS-II Status
LCLS-II cavity production is nearly complete (>90%)
Nitrogen-doping has been a tremendous success in regards to reaching high Q0 both in vertical test and in the cryomodule
LCLS-II cavities were treated with the 2/6 nitrogen doping recipe:
3 hours at 900oC in vacuum (or 950o, or 975o)
2 minutes at 800oC in 25 mTorr of N2
6 minutes at 800oC in vacuum
5-7 μm light EP

6. LCLS-II Results: Q0 Q0 at 16 MV/m Vendor RI EZ* <Q0> at 16 MV/m
3.4x1010
3.3x1010
Nearly all cavities made with “good” material have Q’s above the LCLS-II spec at 16 MV/m
TD/900, NX/950/975
VT results, 3-5 mG
134 Cavities Shown
*EZ data shown after procedural modifications to improve cavity fabrication
Note that the spec has been adjusted to 2.5x1010 to account for flange losses present in VT

7. LCLS-II Results: Q0 at HE Gradient
Q0 at 21 MV/m
Vendor RI
EZ*
<Q0> at 16 MV/m
3.4x1010
3.2x1010
<Q0> at 21 MV/m
3.3x1010
Nearly all cavities made with “good” material have Q’s above the LCLS-II spec at 16 MV/m
Of the cavities that make it to 21 MV/m, all except for 1 have a Q0 higher than 2.5x1010 at 21 MV/m
Q0 in LCLS-II cavities is more than sufficient for HE
TD/900, NX/950/975
VT results, 3-5 mG
110 Cavities Shown
*EZ data shown after procedural modifications to improve cavity fabrication
Note that the spec has been adjusted to 2.5x1010 to account for flange losses present in VT

8. LCLS-II Results: Q0 at HE Gradient
Q0 at 21 MV/m
Vendor RI
EZ*
<Q0> at 16 MV/m
3.4x1010
3.2x1010
<Q0> at 21 MV/m
3.3x1010
Nearly all cavities made with “good” material have Q’s above the LCLS-II spec at 16 MV/m
Of the cavities that make it to 21 MV/m, all except for 1 have a Q0 higher than 2.5x1010 at 21 MV/m
Q0 in LCLS-II cavities is more than sufficient for HE
Q0 with 2/6 is More Than Adequate to Meet LCLS-II HE Specifications
TD/900, NX/950/975
VT results, 3-5 mG
110 Cavities Shown
*EZ data shown after procedural modifications to improve cavity fabrication
Note that the spec has been adjusted to 2.5x1010 to account for flange losses present in VT

9. LCLS-II Results: Gradient Reach
Quench Field
Vendor
<Equench> [MV/m] RI
22.9
EZ*
23.3
Note that ~50% of the cavities shown were admin limited at 24 MV/m. Of those not admin limited, 32% exceeded the HE VT Spec of 24.5 MV/m
LCLS-II cavities must reach >19 MV/m in VT
Both LCLS-II cavity vendors produce cavities with average quench fields of ~23 MV/m in VT
However, of the 216 cavities shown, 52 (24%) do not reach >21 MV/m in VT (HE operating gradient)
Gradient reach in LCLS-II cavities is not sufficient for HE
TD/900, NX/950/975
VT results, 3-5 mG
216 Cavities Shown
*EZ data shown after procedural modifications to improve cavity fabrication
Only ~32% of cavities exceed the LCLS-II HE VT Spec of 24.5 MV/m

10. LCLS-II Results: Gradient Reach
Quench Field
Vendor
<Equench> [MV/m] RI
22.9
EZ*
23.3
Note that ~50% of the cavities shown were admin limited at 24 MV/m. Of those not admin limited, 32% exceeded the HE VT Spec of 24.5 MV/m
LCLS-II cavities must reach >19 MV/m in VT
Both LCLS-II cavity vendors produce cavities with average quench fields of ~23 MV/m in VT
However, of the 216 cavities shown, 52 (24%) do not reach >21 MV/m in VT (HE operating gradient)
Gradient reach in LCLS-II cavities is not sufficient for HE
Gradient Reach with 2/6 Recipe is NOT SUFFICIENT to Reach LCLS-II HE Specification
TD/900, NX/950/975
VT results, 3-5 mG
216 Cavities Shown
*EZ data shown after procedural modifications to improve cavity fabrication
Only ~32% of cavities exceed the LCLS-II HE VT Spec of 24.5 MV/m

11. LCLS-II Results: Gradient Reach
Quench Field
Vendor
<Equench> [MV/m] RI
22.9
EZ*
23.3
Note that ~50% of the cavities shown were admin limited at 24 MV/m. Of those not admin limited, 32% exceeded the HE VT Spec of 24.5 MV/m
LCLS-II cavities must reach >19 MV/m in VT
Both LCLS-II cavity vendors produce cavities with average quench fields of ~23 MV/m in VT
However, of the 216 cavities shown, 52 (24%) do not reach >21 MV/m in VT (HE operating gradient)
Gradient reach in LCLS-II cavities is not sufficient for HE
Gradient Reach with 2/6 Recipe is NOT SUFFICIENT to Reach LCLS-II HE Specification
A RECIPE CHANGE IS NEEDED
TD/900, NX/950/975
VT results, 3-5 mG
216 Cavities Shown
*EZ data shown after procedural modifications to improve cavity fabrication
Only ~32% of cavities exceed the LCLS-II HE VT Spec of 24.5 MV/m

12. LCLS-II HE R&D Plan
In order to meet the requirements of LCLS-II HE, an R&D effort has been developed
We will develop a cavity processing method to consistently produce cavities that reach 24.5 MV/m in VT with a Q0 of 2.7x1010 at 21 MV/m
This effort is being carried out by the three labs that participated in the original LCLS-II R&D: FNAL, JLab, and Cornell University

13. Nitrogen-Doping Improvement
We have pursued two (mostly) parallel paths:
Focusing on the light doping regime which should theoretically lead to:
Higher quench fields
Higher RBCS (lower Q0)
Lower sensitivity of residual resistance to trapped flux (less dependence on flux expulsion to maintain high Q0)
Focusing on longer anneal times
Early results at the end of the LCLS-II High Q0 R&D at JLab demonstrated high quench fields
Though to produce a more uniform doped layer

14. JLab R&D Plan and Progress
JLab prepared 3 single-cells with a new doping demonstrating longer anneal times:
At 800oC: 3 minutes with nitrogen, 60 minutes anneal time
Each cavity received 5 μm EP and was tested
Following the cavity test, each cavity received an additional 5 μm EP

15. JLab Single-Cell Performance
3/60 results are very promising
Of the first 3 (5 μm EP), 1 cavity did not meet HE spec, 1 was FE limited but met gradient spec, and 1 far exceeded spec
After an additional 5 μm EP, 2 of the 3 cavities met the HE spec
Suspected that the third cavity is defect limited – will not be used in future studies
FE Limited

16. JLab Single-Cell Performance
3/60 results are very promising
Of the first 3 (5 μm EP), 1 cavity did not meet HE spec, 1 was FE limited but met gradient spec, and 1 far exceeded spec
After an additional 5 μm EP, 2 of the 3 cavities met the HE spec
Suspected that the third cavity is defect limited – will not be used in future studies
See talk by Ari Palczewski later for more details
FE Limited

17. FNAL R&D Plan and Progress
FNAL prepared 3 single-cells with a lighter doping:
At 800oC: 2 minutes with nitrogen, no anneal
Each cavity received 5 μm EP and was tested
Success of the JLab 3/60 recipe refocused the FNAL R&D to reproduce results
3 cavities were reset and given the 3/60 recipe + 5 μm (1 cavity received 10 μm) EP
An additional single-cell cavity was given the 3/60 recipe and then an EP study was completed:
Successive tests after 5, 10, and 15 μm total EP

18. FNAL Single-Cell Performance – 2/0
All 3 cavities treated with the 2/0 recipe meet LCLS-II HE specification
1 of the 3 is borderline (also poor flux expelling material)
Trapped flux sensitivity was found to be about the same as for the standard LCLS-II 2/6 recipe

19. FNAL Single-Cell Performance – 3/60
Multiple Cavities with 3/60
EP Study on 3/60
3/60 cavity performance confirmed at FNAL – successful demonstration on single-cells at two independent facilities
Of the cavities given 5 or 10 μm EP, 100% pass the HE spec
15 μm EP resulted in good quench field, but lower Q0 than the HE spec
Q0’s of more than 5x1010 at 21 MV/m and 2.0 K were demonstrated – some of, if not the highest Q’s ever recorded at 2 K
Spread in trapped flux sensitivities between the 4 cavities: all lower or about the same as LCLS-II 2/6 recipe

20. 9-Cell Studies at FNAL
Due to the promising results on single-cell cavities, FNAL supplied 3 9-cell cavities to be used for earlier-than-expected 9- cell demonstration
These 3 cavities all received the 2/0 doping and 7 μm EP
Unfortunately, performance was less than stellar, with two of the cavities quenching earlier
One cavity met the gradient spec, but had low Q0 due to poor flux expelling material
9-Cells with 2/0

21. CAV0018 Quench Field (MV/m) CAV0017 Quench Field (MV/m)
9-Cell Studies at FNAL
Cell #
CAV0018 Quench Field (MV/m)
CAV0017 Quench Field (MV/m) 1
19.2 20 2
Either (=22-23) or (=34)
26.4 3
>25
>30 4
Either (=24.88) or (>28.46)
>27 5
>34.2
32.8 6
Either (>24.88) or (>28.46) 7 8
>26.4 9
>19.2
>20
Both 9-cell cavities which showed low quench fields, were dominated by low quench on one side of the cavity – the side closest to the nitrogen inlet in the furnace

22. See talk by Daniel Bafia next for more details
9-Cell Studies at FNAL
Cell #
CAV0018 Quench Field (MV/m)
CAV0017 Quench Field (MV/m) 1
19.2 20 2
Either (=22-23) or (=34)
26.4 3
>25
>30 4
Either (=24.88) or (>28.46)
>27 5
>34.2
32.8 6
Either (>24.88) or (>28.46) 7 8
>26.4 9
>19.2
>20
See talk by Daniel Bafia next for more details
Both 9-cell cavities which showed low quench fields, were dominated by low quench on one side of the cavity – the side closest to the nitrogen inlet in the furnace

23. Path Forward and 6 Month Look Ahead
12 9-cell cavities are being treated with the two candidate recipes:
8 will be given 3/60
4 given 2/0
Remaining 4 will be reset and retreated at a later time
These results, along with prototype 9-cells built from scratch will demonstrate readiness for LCLS-II HE production
We expect the first production-level 9-cell results in 2-3 months
A prototype cryomodule will begin construction at the end of the year

24. Summary
LCLS-II has demonstrated very high Q0 and good quench fields in the SRF cavities however, the gradient reach is not sufficient for LCLS-II HE
An R&D effort is underway to improve on the gradient performance of the LCLS-II cavities while maintaining high Q0
Results on single-cell have demonstrated two candidate recipes that consistently meet LCLS-II HE specs
9-cell results at FNAL have been less stellar, however there are some confounding issues that must be sorted out
At the conclusion of the R&D effort, we will define a new cavity processing method to be used in LCLS-II HE