Vertical Test Results of 9-Cell Cavities for LCLS-II Dan Gonnella For the LCLS-II Project

Outline Introduction to LCLS-II Challenges in Production Cavity Vertical Test Results Summary & Outlook for ILC

Outline Introduction to LCLS-II Challenges in Production Cavity Vertical Test Results Summary & Outlook for ILC

Introduction to LCLS-II The SRF linac is closely based on the XFEL/ILC/TESLA Design It 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 2.5 x 1010 at 16 MV/m and 2 K 2.7 x 1010 at 16 MV/m and 2 K Q specification lowered in VT due to addition of stainless steel blank on short side of cavity – adds ~0.8 nΩ of Rres

Cavity Preparation In order to achieve the ambitious Q specification, all 9-cell cavities for LCLS-II are prepared with nitrogen-doping Original recipe: 140 μm bulk EP 800oC degas for 3 hours in vacuum 2 minutes at 800oC in 20-30 mTorr of N2 6 minutes at 800oC in vacuum 5-7 μm light EP

Cavity Preparation In order to achieve the ambitious Q specification, all 9-cell cavities for LCLS-II are prepared with nitrogen-doping Original recipe: 140 μm bulk EP 800oC degas for 3 hours in vacuum 2 minutes at 800oC in 20-30 mTorr of N2 6 minutes at 800oC in vacuum 5-7 μm light EP

Cavity Preparation In order to achieve the ambitious Q specification, all 9-cell cavities for LCLS-II are prepared with nitrogen-doping Original recipe: 140 μm bulk EP 800oC degas for 3 hours in vacuum 2 minutes at 800oC in 20-30 mTorr of N2 6 minutes at 800oC in vacuum 5-7 μm light EP Cavities are being produced by Research Instruments GmbH (RI) and Ettore Zanon S.p.a (EZ) Niobium was procured from Tokyo Denkai and OTIC Ningxia

Outline Introduction to LCLS-II Challenges in Production Cavity Vertical Test Results Summary & Outlook for ILC

Challenges in Production Early LCLS-II cavities were plagued by two main issues that limited performance below the required cavity spec: Poor flux expulsion in the raw niobium sheet material Improper fabrication techniques at one of the cavity vendors

Q vs E for First Article Cavities Some Great Cavities Some Good Cavities All these cavities tested in very low (0-1 mG) magnetic fields

Residual Resistance in First Articles Rres higher than in the prototyping phase TD Expected 2-3 nΩ, found 5-9 nΩ Spread is a result of differences in ambient magnetic fields

Issues with Original Recipe Material from both vendors showed worse flux expulsion when treated at 800oC in single-cell cavity tests than material used during the prototyping and R&D stage All material meets specifications This means that magnetic field specifications would need to be tighter in order to minimize the need for efficient flux expulsion Cryomodule results thus far have shown ambient magnetic fields less than the LCLS-II CM Spec of 5 mG – further improvement on this would be difficult 140 μm bulk EP was insufficient to remove the damage layer, adding additional Rres Worse flux expulsion is an independent phenomena from N-Doping – It impacts all cavity preparation methods

Cavity Preparation In order to achieve the ambitious Q specification, all 9-cell cavities for LCLS-II are prepared with nitrogen-doping Updated recipe: 200 μm bulk EP 900oC degas for 3 hours in vacuum for TD cavities 950/975oC degas for 3 hours in vacuum for NX cavities 2 minutes at 800oC in 20-30 mTorr of N2 6 minutes at 800oC in vacuum 5-7 μm light EP

Residual Resistance After Recipe Change Spread lower due to near 100% FE TD After Increasing Bulk EP and Degas Temperature, Rres is consistently ~2 nΩ

Q vs E for First Article Cavities

Q vs E for First Article Cavities Highest Q0 Ever Obtained in Production SRF Cavities Cavities now consistently meet Q0 specification

Improper Cavity Fabrication Q Slope Onset 12-14 MV/m Cavities produced at Vendor A showed significantly lower quench fields than those produced at Vendor B Additionally a strong Q slope was present in many of the cavities, suggesting normal conducting inclusions were present LCLS-II project staff were placed on site to evaluate and fix the underlying problems Subset of cavities, behavior observed in most

Vendor A – Surface Roughness This Issue was Fixed Data from Y. Trenikhina EP at Vendor A was producing a very rough surface Roughness was as high as 15 μm It was found that EP temperature was leading to the EP no longer being in the “polishing regime” but in the “etching regime”

Vendor A – Grinding Technique Visual inspection of the interior of Vendor A cavities after fabrication showed the presence of many defects Some pits had visible normal conducting media embedded Theorized that aggressive grinding on the dumbbell surface led to the “burying” of media This normal conducting media would then be revealed after bulk EP All procedures were reworked with LCLS-II staff to improve the fabrication procedures Additionally, it was found that a dirty weld stackup procedure was leading to weld defects which further limited cavity gradient

Changes to Vendor A Procedures After evaluation and rewriting of Vendor A cavity fabrication procedures, performance for Vendor A cavities significantly improved Performance is now in line with cavities produced at Vendor B and what we would expect Cavities that were fabricated with improper procedures will be reworked

Outline Introduction to LCLS-II Challenges in Production Cavity Vertical Test Results Summary & Outlook for ILC

Quench Field Results - Vendor A Maximum Eacc Category <Emax> [MV/m] Original Grinding 17.6 Restart New Weld-Stackup Rework

Quench Field Results - Vendor A Maximum Eacc Category <Emax> [MV/m] Original Grinding 17.6 Restart 22.4 New Weld-Stackup Rework

Quench Field Results - Vendor A Maximum Eacc Category <Emax> [MV/m] Original Grinding 17.6 Restart 22.4 New Weld-Stackup 24.0 Rework

Quench Field Results - Vendor A Maximum Eacc Lesson Learned: Great care and vendor oversight is necessary to ensure good cavity fabrication Category <Emax> [MV/m] Original Grinding 17.6 Restart 22.4 New Weld-Stackup 24.0 Rework 20.0

Quench Field Results - Vendor B Maximum Eacc Vendor B cavities show an average quench field of 22.8 MV/m

Quench Field Results By Material Maximum Eacc Vendor B cavities show an average quench field of 22.8 MV/m No significant difference in quench fields for cavities made from different materials

Q0 Results - Vendor A Q0 at 16 MV/m Cavities made by Vendor A with the original grinding procedures showed low Q0 primarily due to strong Q slope Improved procedures has resulted in an average Q0 of 3.2x1010

Q0 Results - Vendor B Q0 at 16 MV/m Q0 performance strongly depends on material and heat treatment due to differences in flux expulsion

Q0 Results - Vendor B Q0 at 16 MV/m Q0 performance strongly depends on material and heat treatment due to differences in flux expulsion Material/Heat Treatment <Q0> TD/800 2.5x1010 TD/900 NX/900 NX/950/975

Q0 Results - Vendor B Q0 at 16 MV/m Q0 performance strongly depends on material and heat treatment due to differences in flux expulsion Material/Heat Treatment <Q0> TD/800 2.5x1010 TD/900 3.3x1010 NX/900 NX/950/975

Q0 Results - Vendor B Q0 at 16 MV/m Q0 performance strongly depends on material and heat treatment due to differences in flux expulsion Material/Heat Treatment <Q0> TD/800 2.5x1010 TD/900 3.3x1010 NX/900 2.6x1010 NX/950/975

Q0 Results - Vendor B Q0 at 16 MV/m Q0 performance strongly depends on material and heat treatment due to differences in flux expulsion Lesson Learned: Flux expulsion must be verified on a material batch level to ensure Q0 meets specification Material/Heat Treatment <Q0> TD/800 2.5x1010 TD/900 3.3x1010 NX/900 2.6x1010 NX/950/975 3.5x1010

Outline Introduction to LCLS-II Challenges in Production Cavity Vertical Test Results Summary & Outlook for ILC

Summary & Outlook for ILC Early LCLS-II cavity production faced two main challenges to meeting the required specifications In order to meet gradient spec, strict vendor oversight and reworking of procedures was necessary This will become even more important for ILC which requires higher gradients than LCLS-II In order to meet the Q0 spec, a recipe change was required twice mid-production All future niobium should first be checked for flux expulsion on single-cells More long term: identify a material spec that governs flux expulsion

Summary & Outlook for ILC Both cavity vendors are now consistently producing cavities that exceed the very ambitious LCLS-II specifications Average Q0 for the project has been demonstrated to be well above 3x1010 Cavities show an average quench field of ~23 MV/m, consistent with expectations on nitrogen-doped cavities produced with the LCLS-II recipe