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

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

3.9 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

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

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

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

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

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

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

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

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

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

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

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