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

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

LCLS-II SRF Requirements LCLS-II consists of 35 cryomodules each with 8 cavities – total of 280 cavities The 280 1.3 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

LCLS-II HE SRF Requirements LCLS-II HE adds 20 cryomodules additional cryomodules to the LCLS-II linac The 160 1.3 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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