1. Modified TTF3 Coupler Design and Related R&D for LCLS-II
Chris Adolphsen
Presented by Marc Ross 12/2/14 at the KEK TTC Meeting
for the Coupler Team: Jeff Tice, Karen Fant, Dale Miller, Faya Wang, Chris Nantista, Liling Xiao and Zenghai Li (SLAC) and Andy Hocker, Nikolay Solyak, Ken Premo, Tug Arkan, Andrei Lunin, Ivan Gonin and Timer Khabiboulline (FNAL)

2. Outline Overview and Specs Shorter Antenna Thicker Plating
Waveguide Assembly
Coupler Results from a FNAL HTS Cavity Measurement
Processing and Instrumentation
Summary of Coupler Work at SLAC
Summary

3. Cavity Power Coupler Use basic DESY 2006 TTF3 design, but
Shift Qext range higher
Improve cooling of warm section so can run at 7 kW with full reflection
Modify waveguide assembly (use flex rings, perhaps an aluminum WG box and push-pull antenna positioner with coarse/fine manual control)

4. LCLS-II Coupler Technical Specs
Item
Spec
Comment
Design
DESY TTF3
Defined by SLAC drawings
Max Input Power
7 kW CW
Max Reflected Power from Cavity
Assume would be able to run with full reflection
Minimum Qext Foreseen
1e7
Allows 16 MV/m with no beam and 6.6 kW input
Maximum Qext Foreseen
5e7
Match for 0.3 mA beams at 16 MV/m, 26 Hz BW
Reduction in Antenna Length
8.5 mm
Maintain 3 mm rounding
Range of Antenna Travel
+/- 7.5 mm
Nominal defined by bellow
Predicted Qext Min Range
3.6e6 – 4.7e6 – 7.5e6
Includes +/- 5 mm transverse offsets
Predicted Qext Max Range
1.0e8 – 1.1e8 – 1.5e8
Warm Section Outer Cond Plating
10 um +/- 5 um, RRR =
Nominal EuXFEL
Warm Section Inner Cond Plating
150 um +/- 10 um, RRR =
Increase to limit temp rise < 150 degC for 7 kW, full ref.
Cold Section Outer Cond Plating
10 um +/- 5 um, RRR = 30-80
Center Conductor HV Bias
Optional
Use flex copper rings that can be replaced with existing capacitor rings if HV bias needed
Warm and Cold e-Probe Ports
Yes
Will not instrument – do not expect MP
Warm Light Port
Will not instrument – do not expect arcs
Motorized Antenna No
Adjust manually
Cold Test and RF Processing
With 7 kW input, low fields and no multipacting bands – will instead process in-situ

5. Shorter Coupler Antenna
Qext ~ 4e7
Shortened Antenna
Qmin
Qmid
Qmax
Original coupler*
1E6
4.0E6
2.0E7
Tip cut by 10 mm
8E6
4.0E7
2.0E8
Tip cut by 8.5 mm
6E6
2.5E7
1.4E8

6. Coupler Heating
Inner conductor temperature for 15 kW TW operation for various thicknesses of the warm section inner conductor copper plating
Plating Thickness
Limit to 450 K (bake temp)

7. Effective location of the short (mm)
7 kW Full Reflection Simulations
Simulations assume 100 um inner conductor plating and no resistivity increase with plating roughness
3D case includes heating in the warm window
Location = 33 mm corresponds to on-resonance operation (no beam)
Effective location of the short (mm) 2D 3D

8. Effect of Copper Roughness on Resistivity –Empirical formula used by circuit board designers
Rs = Rs_ideal (T,RRR) * Ksr (roughness, skin depth(Rs_ideal(T,RRR)))
Wrong
Empirical, Max = 2
1.3 GHz Skin Depth (um)
RRR = 10
RRR = 100
T = 300 K
1.8
T = 70 K
0.8
0.6
T < 20 K
0.2

9. Theoretical Approach at SLAC
Developing better theory that depends on feature height to separation ratio – expect this ratio to be << 1, which plot below shows will have a minor effect on heating
G. Stupakov

10. Thicker Copper Plating Qualification
Increase copper plating thickness on warm section inner conductor from 30 um to 150 um Recently had 3 ILC sections modified in this way – two will be used in HTS tests at FNAL
Cross section of inner conductor bellow in a test section: measure um copper thickness variation

11. FNAL HTS Test Schedule

12. Calibration of IR sensor on Warm Section at FNAL
T T2
thermocouplers
Nitrogen

13. Calibration of IR Sensor on Modified Warm Section
Emissivity of Cu plated SS tube heated at 900 C is ± 0.03.
A 10% emissivity error produces a systematic error of 2.10 C.

14. Arcing at Waveguide Contact
When processing ILC couplers, discovered that the waveguide ‘capacitor’ mating surface had arced in some of the warm sections

15. Copper Flex Rings
Made plug-compatible copper flex rings to replace non-flexible HV capacitance rings to get better WG-to-Window contact
Adopted by EuXFEL for CPI couplers

16. Aluminum Waveguide Box (Not in Baseline)
Serge Prat, TTC08

17. Setting Qext Will use manual knob, not motor to move antenna
Need to set antenna to 0.5 mm accuracy to get Qext in the e7 range
Probably send people into SLAC tunnel to iterate on Qext during commissioning (i.e. can’t rely on mechanical tolerances)

18. First 6 kW CW Operation at FNAL HTS
Used shorter antenna and warm section with 150 um plating
Found coupler temp higher than expected due to poor thermal tie-down
Will add a SS split-ring washer (a la XFEL) to make the thermal contract between the copper plate and 70 flange better - also increase the number of braids from 2 to 4
Andy Hocker, FNAL

19. Expected Inner Conductor Temps with Ideal and Measured Boundary Temps
2D Simulations (blue and green lines) and IR measurement (red dot) with 3D Prediction (purple star)

20. Measured Qext Range
Found Qext range shifted lower as if antenna location ~ 3 mm closer to beam line – perhaps 300 K to 2K flange length in the HTS shorter than that in a CM
Nominal
= either 1 mm or 1.25 mm per turn
Andy Hocker, FNAL

21. Production Coupler RF Processing and Instrumentation
Will not low-power test couplers
Will not pulse power process the couplers
Will not CW process the couplers
Will not instrument e-probe ports
Will not instrument light port
Will monitor coupler vacuum manifold pressure
Time (hr)
Power (MW)
Only high power pulsed rf sources currently available at SLAC. For the couplers needed for the two LCLS-II prototype CMs, do not process as above, but run only at 40 kW (below the lowest multipacting band at 50 kW) with 1.1 ms pulses at 1% duty – do not see change in vacuum pressure with rf on or off.

22. Summary of 2014 Coupler Work at SLAC Thus Far
Increased inner conductor plating thickness in three ILC warm sections – CPI is modifying two more
Five pairs of ILC cold sections have had antenna shortened, and have been baked and rf processed.
Have contract with CPI to clean, assembly, bake and rf process 8 couplers for EuXFEL
Four coupler pairs received at SLAC
All pairs cleaned and assembled – CPI visited to learn techniques
Two pairs baked + RGA scanned
One pair fully processed
RFQ for new couplers out soon – already posted on US Federal Business Opportunities web page.

23. Summary
Modified TTF3 coupler design for CW operation appears to work, but thermal anchoring is an issue in the HTS setup.
Still considering whether to cold and/or hot test production couplers for LCLS-II