Field Emission Minimization for LCLS-II Andrew Burrill 14-15 September, 2017

Background LCLS-II is installing 37 SRF cryomodules into the existing SLAC tunnel. Each CM contains 8 SRF cavities 35 – 1.3 GHz CMs (16 MV/m operational gradient) Required to be field emission free to 17 MV/m in vertical test 2 – 3.9 GHz CMs (14 MV/m operational gradient) Electron source is a normal conducting RF gun with a Cs2Te photocathode. Gun sits adjacent to first CM with ~ 2m between them. Performance of the first CM is critical to overall emittance of the machine. All components within 20m of the cryomodule must conform to SLAC low particle UHV specifications. Based on response time of Fast valve and mean speed of nitrogen at 20 C. After the SRF section there is a 2.3 km bypass line Burrill - FE Minimization for LCLS-II

Cavity & CM Testing In vertical test 2/3rd of the cavities show no FE up to 24 MV/m Those that do field emit above 17 MV/m, none should exceed 1 R/hr, and most that do are re-rinsed prior to CM assembly In CM testing FE onset degrades CM testing shows an increase in FE from vertical test. Distribution in cavities is random. Some FE starts as low as 8 MV/m Situation is complicated by: Assembly of high power RF input coupler Copper plated bellows between cavities The fact that JLab and FNAL do the assembly in a different manner CM assembly is negatively impacting field emission. Burrill - FE Minimization for LCLS-II

Cavity and CM Assembly Cavities String Assembly Arrive from vendors under vacuum ready for testing. At FNAL tested without opening the valve JLab actively pumps No notable difference in performance with 2 methods Cavities that do show field emission were usually recovered with HPR w/o full disassembly New finding that some cavities FE now gets worse after just a single rinse and require full disassembly?!? String Assembly FNAL assembles high power couplers to cavities, leak check and then string assembly. No HPR before string assembly. String assembly and leak check and then backfilled with N2 until end of CM assembly. JLab HPR of each cavity and then assembly of high power coupler and assembly into full string One leak check on entire string only. Kept under vacuum after this point. Burrill - FE Minimization for LCLS-II

Cleanroom Assembly (opportunity for particle introduction) Burrill - FE Minimization for LCLS-II

Cavity Performance in F1.3-02 *Note: 24 MV/m is the VTS administrative limit **21 MV/m was CMTF administrative limit ***50mR/h wall radiation detector Unit test sum = 133 MV Burrill - FE Minimization for LCLS-II

LCLS-II Accelerator Installation CM35 BC1 L = 62 m R = 10 mm BC2 L = 106 m Injector source Length = 2.1 m Radius = 20 mm LH L = 71 m L0 (1x) L1 (2x) HL L2 (12x) L3 (20x) LTU L3 extension L = 20 m R = 24 mm Differential pumping Diag. Line Helium Distribution line CM01 CM2,3 3.9GHz CM04 CM15 CM16 Particle-Free Zone Burrill - FE Minimization for LCLS-II

LCLS-II Cleanliness Region System Description Definitions of Cleanliness Regions:Particle Free ESD Section 6.2.4 LCLS-II Cleanliness Region Description Surface Contamination Level Blow Off Testing (counts/ft3 of air at a minimum flow of 1 cfm) Reference A Cryo Beamline Vacuum Surfaces (Internal Surfaces) Level 1 0.3 µm bin ≤ 10 counts 0.5 µm bin ≤ 5 counts 1 µm bin 0 counts 3 µm bin 0 counts 5 µm bin 0 counts 10 µm bin 0 counts LCLSII-4.5-ES-0055-R1 B Warm Beamline Vacuum Surfaces (Internal Surfaces) Injector/Gun (Internal Surfaces) Level 10 0.3 µm bin ≤ 15 counts 0.5 µm bin ≤ 10 counts 1 µm bin ≤8 count 3 µm bin ≤5 counts 5 µm bin ≤3counts 10 µm bin ≤1 counts C Cryo Beamline Components, (External Surfaces) Level 25 Max 100 particles/ft3 sum of all bins 0.3 µm and larger   Warm Beamline Assembly parts and tools Max 100 particles/ft3Sum of all bins 0.3 µm and larger Cryo Beamline Assembly Jigs and Fixtures D Warm Beamline Assembly Jigs and Fixtures Level 50 Max 200 particles/ft3 Sum of all bins 0.3 µm and larger E Warm Beamline Components and Assemblies, within clean zones during installation Level 100 Max 1000 particles/ft3 Sum of all bins 0.3 µm and larger Burrill - FE Minimization for LCLS-II

System Description Fundamental Requirements: Particle Free ESD, LCLSII-1.1-ES-0476, Section 6.1 Released written procedures Processes, procedures and facilities validated by testing. Integration, service or maintenance on Cryo beamline vacuum in ISO Class 4 Clean Zones. Final cleaning of “Particle-free” vacuum components in ISO Class 4 (FED- STD Class 10) Cleanrooms Test equipment used to validate “particle-free” components, processes, procedures, and facilities shall have valid calibration at the time of use. Personnel entering the Clean Rooms or Clean Zones while “particle-free” hardware is exposed must be trained cleanroom users. Burrill - FE Minimization for LCLS-II

Technical Status – Design for PF Managing Transitions - Fast Shutter System Fast shutters inserted to protect CMs from events in warm beamline sections Helium Distribution line Differential pumping Diag. Line L0 (1x) L1 (2x) HL L2 (12x) L3 (20x) LTU CM01 CM2,3 3.9GHz CM04 CM15 CM16 CM35 LH L = 32 m R = 10 mm BC1 L = 92 m R = 10 mm BC2 L = 140 m R = 10 mm L3 extension L = 20 m R = 24 mm Injector source Length = 2.1 m Radius = 20 mm Fast signal: commercially available cold-cathodes for 3 to 4 ms from 10-11 to 10-7 Torr pressure rise. Minimum distance between Fast Sensor and Fast Shutter, 20 m, is based on mean speed of nitrogen at 35 C. Detailed implementation criteria are defined in an Engineering Note (LCLSII-1.1-EN-0521) Fast shutter: commercially available, radiation resistant, UHV standard, <10 ms (from signal to low conductance). Burrill - FE Minimization for LCLS-II

Installation – HTR – DIAG0 – COL0 CM01 Installed last Burrill - FE Minimization for LCLS-II

CM Beamline Installation tooling Process Development Scheduled for July 2017 CM Installation begins November 2017 First BLA Install Feb 2018 Burrill - FE Minimization for LCLS-II

LCLS-II has leveraged XFEL experience Summary LCLS-II has leveraged XFEL experience FE degradation in CM testing is not understood Tooling designed to minimize chance for particulate generation New tooling for tunnel installation designed and testing begins soon. Many issues that need to be monitored and improved We have a long way to go. Burrill - FE Minimization for LCLS-II