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

2. 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

3. 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

4. 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

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

6. 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

7. 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

8. 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

9. 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

10. 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 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

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

12. 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

13. 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