1. Cryomodule Design for CW Operation 3.9 GHz considerations
E. Harms on behalf of the Fermilab LCLS-II Cryomodule Design Team
TTC meeting at TRIUMF
Working Group 4 Session 4
7 February 2019

2. Introduction
Coupler heating a significant concern in cw regime compared to pulsed operation
3.9 GHz for LCLS-II an example
Nikolay Solyak, et al source of information in succeeding slides
Harms - CW Cryomodule Design| TTC 7 Feb 2019

3. 3.9 GHz at LCLS-II Two 8-cavity cryomodules
16 MV/m nominal gradient, 41 MV/cryomodule
3rd generation design
FLASH (4-cavity built by Fermilab)
XFEL (8-cavity built by LASA/DESY)
Harms - CW Cryomodule Design| TTC 7 Feb 2019

4. 3.9 GHz power coupler for LCLS-II CW operation
Fix coupling; Q=2.7e7 (2.4e7-3.e7)
Cylindrical cold window (as 1.3GHz)
Waveguide warm window
XFEL/FNAL coupler for pulse operation (Pmax = 45kW, DF=1.3%: Pavrg < 0.6 kW).
LCLS-II: Pmax = 1.8 kW cw in quasi – TW regime:
SS Inner Conductor + bellows plating
T max K
Losses 80K
Losses 4K
30 microns plating
~1000
9.2
0.8
100 microns plating
507
9.3
150 microns plating
427
9.4
Tmax ~1000K w/o modification (warm part inner conductor Cu plating 30um)
Harms - CW Cryomodule Design| TTC 7 Feb 2019

5. LCLS-II Main Coupler Design (modification from XFEL design)
1. Cold part (upper picture)
Dimensions of ceramic window
Length of antenna (QL=2.7e7 vs.1.5e7)
Material of antenna - copper (vs. SS)
=> Reduce antenna heating and Qext variation (HTS result)
2. Warm outer part  No changes
3. Inner conductor of warm section
Cu plating increased from 30 to 150 um.
Convolutions in inner conductor bellows reduced from 20 to 15
3 existing warm sections rebuilt to prototype: Tested at RT, power tests, HTS
Cold part
Warm part (outer conductor)
Warm part (inner conductor) and WG box with warm window
Harms - CW Cryomodule Design| TTC 7 Feb 2019

6. HTS 3.9 GHz system Integration test - coupler thermometry
HTTP6
HTTX30
HTTXM4
HTTX29
Hardware:
Cavity - 3HRI03
Coupler #2, antenna trimmed to 43.1 mm
QL (RT) = 1.54e+7
QL(2K /static) = 1.70e+7
QL(2K/dynam)= 1.05e+7 1kW SW
Coupling depends on RF power (due to antenna heating). Copper antenna (vs. SS) will fix that problem
Harms - CW Cryomodule Design| TTC 7 Feb 2019

7. Thermal simulations: Effect of antenna material, Copper vs. SS
Antenna: SS+50 μm Cu covered / Solid copper
Parameters: Pin= 1 kW SW ON-resonance: 10μm outer plating, ε=9.8, tan=3e-4, roughness 10: warm inner: 120 μm Cu coating of SS inner conductor, 15 bellow conv.
TIR ~ 411/430 K
HTS: 150K
(CM: 110 K)
Tmax ~ 469/475 K
320K
10K
short
Port: 1kW
Tant = 481/183 K
Temperature distribution along inner conductor vs. Z
Harms - CW Cryomodule Design| TTC 7 Feb 2019

8. Coupler assembled at Cavity and Tested at HTS
0.8 kW SW, OFF-resonance
0.8 kW, SW, ON-resonance
TCT100~167K
CT flange~160K
PRF=0.8 kW
IR~390K
TIR~385K
ΔTbraid
T80K_shield
Expected maximum Temperatures in CM:
<130K at 50K flange – (from 170K at HT)
<400K at inner conductor bellows  (from <420K at HTS)
Note: HTS uses LN2 line (90K) vs. CM He line (40K), all T related to this line will be by ~40K lower in CM
Harms - CW Cryomodule Design| TTC 7 Feb 2019

9. Thermal simulation summary
Temp.boundaries:10/150/320 K; RF power = 1kW, SW.
Antenna material
T max, K
coupler
Tprobe K
Tmax, K antenna
Pflux, W
@ 4K
@ 80K
ΔL, μm cold-warm
ΔL, μm dyn-static
ΔQ/QL %
Solid Cu
Static
-57
On-resonance
475
430
183
0.87
13.4 22
0.6
Off-resonance
428
411
174
0.75
9.8 15
0.4
SS + 50 m Cu plated
0.57
1.43
-123
ON-resonance
469
481
1.05
15.3
873
25*
447
422
386
0.9
11.2
591 16
w/o thermal radiation
*Confirm at HTS test
Copper antenna reduce temperature at tip and provides smaller Qext deviation in dynamic regime to compare with SS antenna
Harms - CW Cryomodule Design| TTC 7 Feb 2019

10. Temperature and power dissipation at 5K flange
ΔT=HTTX30-HTTX29
Braid temp gradient
5K – end. 
Cryo heat load
Forward power (W)
OFF-resonance
ON-resonance
Pstatic = 0.7W;
Pdyn (on/off) = 1.9/1.6W;
Measured
Power flux
~14W
Temperature, K
~ 12W
Pdyn (on/off) = 14/12 W;
80 K – end. 
392K
> 387K
Note: expected Tmax (inner conductor bellow) ~20K above IR meas. (simul).
Infra-red sensor Temperature (inner conductor, warm part)
Harms - CW Cryomodule Design| TTC 7 Feb 2019

11. Thermal intercept design in 3.9GHz CM
Coupler uses same straps as in 1.3GHz cryomodule
power flux less than in 1.3GHz coupler
same straps are used as for 1.3GHz system (L~150mm, S=120mm2; 0.29 W/K)
Harms - CW Cryomodule Design| TTC 7 Feb 2019

12. Main Coupler Summary
Three coupler prototypes (warm part) were built and tested:
QC inspection and RF measurements
2 couplers were HP processed at warm test stand at 2kW cw.
1 assembled on dressed cavity and tested in HTS at 1kW SW (integrated system test)
Thermal design verified in HTS test and incorporated in CM design.
Coupler procurement in progress (8 couplers are received)
Harms - CW Cryomodule Design| TTC 7 Feb 2019

13. 9/25/2015
HOM coupler design
LCLS-II modified XFEL (INFN/FNAL) design to reduce risks of tuning and heating problems in CW operation.
Reduce beam pipe and bellow diameter from 40 to 38mm to move trapped parasitic mode away to improve the tuning of HOM notch frequency.
Modification of HOM coupler to reduce heating
F-part modification  less penetration of antenna inside HOM to reduce heating
Increase wall thickness of HOM hat to 1.3mm to prevent cracks and vacuum leak
Shorter length of HOM feedthrough antenna (Fermilab vs. XFEL)
In XFEL design lowest mode are closer to operation node (min ~10-20 MHz vs. 100MHz in simul.)
beam pipe aperture 38 mm allows to detune the HOM by ~ 100 MHz up.
Harms - CW Cryomodule Design| TTC 7 Feb 2019

14. HOM F-part modification to reduce antenna heating
9/25/2015
HOM F-part modification to reduce antenna heating
Reduce penetration to beam pipe. Increase length of bump in F-part
XFEL/FLASH
LCLS-II design
G = 3.2e8 
G = 1.74e9 
A. Lunin/khabiboulline
Current design HOM antenna quenches at ~20 MV/m in VTS. Expected that quench limit will even lower in CW regime at HTS and CM.
RF power dissipation on HOM antenna reduced by factor of 5.4 after modification
Harms - CW Cryomodule Design| TTC 7 Feb 2019