World Wide Fundamental Power Coupler meeting 4 CERN

World Wide Fundamental Power Coupler meeting #4, CERN, June 5 -6, 2018. Fundamental Power Coupler at DESY: E-XFEL Update and R&D Denis Kostin, DESY 5. 06. 2018

Fundamental Power Coupler at DESY: E-XFEL Update and R&D Outline E-XFEL Main Coupler R&D Main Coupler for CW Operation Main Coupler for SRF Injector cavity Denis Kostin, DESY. WWFPC #4, CERN, June 5 -6, 2018 2

Fundamental Power Coupler at DESY: E-XFEL Update and R&D Denis Kostin, DESY. WWFPC #4, CERN, June 5 -6, 2018 3 E-XFEL Main Coupler E-XFEL Input RF power coupler consists of warm, cold and waveguide main parts. Coaxial coupler is made of copper and copper plated (10/30µm) stainless steel with alumina Ti. N coated ceramic windows. Motorized antenna tuning ( 10 mm) allows for Qext adjustment (106. . 107).

Fundamental Power Coupler at DESY: E-XFEL Update and R&D Denis Kostin, DESY. WWFPC #4, CERN, June 5 -6, 2018 E-XFEL Main Coupler Parts Problems: Summary FPC test/operation problems examples # Problem 1 T 70 K over temperature caused by a loose contact between warm and cold parts inner conductors (inner screw problem) 18 13 warm parts replaced, 5 warm parts cleaned, cold parts cleaned 2 T 70 K over temperature caused by warm part problem, RF discharge in some cases 10 warm parts replaced 3 not conditionable RF discharge in a warm part 11 warm parts replaced 4 T 70 K overheating in the test 4 5 6 warm part damaged by the discharge with not tightened bias capacitor screws waveguide part damaged by a warm part RF discharge during the test (see p. 3) Number 2 Repair Action warm parts disassembled and checked: no problem found, re-assembled 1 warm part replaced, 1 warm part cleaned 1 warm part replaced 7 actuator axle broken 8 axles replaced 8 push-rod leak (caused by RF power coupling in PR-bellow) 26 push-rods replaced 9 push-rod cold end not connected 11 push-rods re-assembled No push-rod bellow leaks since all capacitors replaced by the copper gaskets, since over a year operation. 102 E-XFEL modules (816 couplers) were tested in AMTF. 4

Fundamental Power Coupler at DESY: E-XFEL Update and R&D Denis Kostin, DESY. WWFPC #4, CERN, June 5 -6, 2018 E-XFEL Main Coupler Parts Problems: Operation N position module coupler part status comments 1 A 4. M 4. C 4 XM 72 CPI-CP-024 shorted T 70 K → 200 K, high e-1 signal (discharge), cold part problem. AMTF: push-rod replaced, no other problems. 2 A 12. M 4. C 1 XM 53 THRI-WP-706 shorted T 70 K → 150 K, discharge, warm part problem. No problem / observations in AMTF. 3 A 13. M 2. C 8 XM 51 THRI-WP-683 OK high e-2 signal – conditioned. 4 A 16. M 1. C 1 XM 69 THRI-CP-880 OK high e-1 signal – conditioned. 5 A 16. M 2. C 1 XM 60 CPI-CP-028 shorted 6 A 16. M 2. C 2 XM 60 CPI-WP-030 OK high e-2/3 signals – conditioned. 7 A 16. M 4. C 7 XM 66 THRI-WP-323 OK high e-2/3 signals – conditioned. 8 A 19. M 3. C 4 XM-2 AC 3 H 26 testing 9 A 19. M 3. C 6 XM-2 AC 3 H 38 OK 10 A 20. M 4. C 1 XM 65 THRI-WP-470 shorted high T 70 K, cpl. vac. increased, high e-2 signal bursts, warm part problem. 11 A 21. M 2. C 5 XM 92 THRI-WP-155 shorted T 70 K overheating – no discharge. WP was already replaced in AMTF – same problem. T 70 K → 150 K high e-1/2 signals, cold part problem. Problem seen in AMTF, T 70 K → 115 K (some overheating). T 70 K overheating – no discharge. Observed in AMTF as well. conditioned (5 hr) up to 150 k. W FT-pulse (25 MV/m) 5 FPCs are not conditionable and showing T 70 K overheating – shorted / disconnected from RF. Couplers operation stable since over a year of E-XFEL operation – no other shorted (not used) couplers. 97 modules (776 couplers) are currently installed in E-XFEL. 5

Fundamental Power Coupler at DESY: E-XFEL Update and R&D Denis Kostin, DESY. WWFPC #4, CERN, June 5 -6, 2018 6 E-XFEL Degraded Cavities: Coupler Interlock Effect C 7/C 8. M 4. A 5 source cavity gradient A 5: cavity dark current generation A 5: multiple coupler spark IL event: Dark current, sourced by degraded cavity C 8/C 7. M 4. A 5 generates a strong gamma radiation pulse (bremsstrahlung) which affects semiconductor light detectors of the couplers WG-spark interlock. Coupler technical IL event switches off the RF power – but this is not the coupler problem: coupler IL event was triggered simultaneously for many couplers in A 5, mostly M 3. A 5 and M 2. A 5. A possible degradation of the spark IL sensor (A 5. M 2. C 6) was observed – probably caused by the gamma radiation.

Fundamental Power Coupler at DESY: E-XFEL Update and R&D Denis Kostin, DESY. WWFPC #4, CERN, June 5 -6, 2018 E-XFEL High Gradient: Coupler 70 K Temperatures RF station A 11 operated with 900 MV – average cavity accelerating gradient over 28 MV/m, maximum setting. Coupler 70 K cold window temperatures reached at max. near 150 K – can be tolerated in the operation. 7

Fundamental Power Coupler at DESY: E-XFEL Update and R&D Denis Kostin, DESY. WWFPC #4, CERN, June 5 -6, 2018 E-XFEL Coupler Conditioning: Last RF Stations Cold coupler conditioning took 2 days + additional week for decreasing the vacuum pressure. Conditioning was needed to ensure the modules operation – modules were installed, but not actively used for the EXFEL operation for over a year time. cavity/coupler vacuum A 24 cavity/coupler vacuum A 25 8

Fundamental Power Coupler at DESY: E-XFEL Update and R&D Denis Kostin, DESY. WWFPC #4, CERN, June 5 -6, 2018 Main Coupler CW operation: Loaded Q limited by a motor/mechanics N cavity Qload. max 1 AC 114 8. 4× 107 2 AC 156 6. 8× 107 3 AC 146 1. 1× 107 4 AC 154 9. 1× 107 5 AC 157 7. 5× 107 6 AC 158 6. 6× 107 7 AC 151 2. 6× 107 8 AC 152 2. 6× 107 9 Qload increase was needed XM-3 @ CMTB CW run example Pfor = (Eacc×L)2/(4×Rsh/Q×Qload) Pfor ~ 1/Qload test with 20 MV/m simulated data 5 mm T 70 K increases

Fundamental Power Coupler at DESY: E-XFEL Update and R&D Denis Kostin, DESY. WWFPC #4, CERN, June 5 -6, 2018 10 Main Coupler CW operation: Shifted Coupler Warm Part shift to achieve higher Qload shift antenna by +5 mm outwards: Qload. max 2× 107 → 5 × 107 1 8. 4× 107 2 6. 8× 107 3 1. 1× 107 4 9. 1× 107 5 7. 5× 107 6 6. 6× 107 7 2. 6× 107 8 2. 6× 107 achieved Qload. max warm part / 70 K window shifted by 10 mm using special iso-vac flange we plan to do it on another 2 E-XFEL modules XM 46, XM 50

Fundamental Power Coupler at DESY: E-XFEL Update and R&D Denis Kostin, DESY. WWFPC #4, CERN, June 5 -6, 2018 outer conductor inner conductor Main Coupler CW operation: New Copper Plating 150µm with the CW RF power in a range of 10. . 15 k. W inner conductor overheating will be an issue, with a maximum temperature on the warm inner conductor bellow; to decrease the inner conductor temperature it must be gas cooled from inside or copper-plated with increased layer thickness ( 150µm) and RRR (50); coupler tests with warm parts having 150µm copper re-plated inner conductor are in preparation. Parts are ordered now (RI). 11

Fundamental Power Coupler at DESY: E-XFEL Update and R&D Denis Kostin, DESY. WWFPC #4, CERN, June 5 -6, 2018 SRF Injector Cavity Coupler: Development started at DESY Gaussian pulse current excitation EM field decay Qload calculation 12

Fundamental Power Coupler at DESY: E-XFEL Update and R&D Denis Kostin, DESY. WWFPC #4, CERN, June 5 -6, 2018 Conclusions Main coupler design for the European-XFEL linac is well established and proven. Possible R&D could be push-rod and actuator design improvements. Additional study should look for the warm parts overheating issue (E-XFEL: 35 WPs replaced); All 25 E-XFEL RF stations 776 couplers are conditioned, 5 couplers are shorted (not in use). Last 2 installed RF stations couplers A 24 and A 25 were conditioned recently, conditioning was apparently needed and took 2 days – the stations were not used in linac operation before; Dark current, sourced by cavities C 8/C 7. M 4. A 5 generates a strong gamma radiation pulse (bremsstrahlung) which affects semiconductor light detectors of the couplers WG-spark interlock. Coupler technical IL event switches off the RF power – but this is not the coupler problem: coupler interlock event was triggered simultaneously for many couplers in 4 modules of station A 5, mostly modules #3 and #4. A possible coupler interlock R&D here; Station A 11 was run at max. SP = 900 MV – reached cold window temperature of 150 K, still not an issue, but shows, that proper coupler cooling could be rather critical; Main coupler CW application is another R&D direction – pushing to higher Qload and achieving better thermal properties with increased copper plating thickness are the possible applications. SRF Injector cavity coupler development was started at DESY recently; 13