Main Coupler R&D at DESY TESLA Technology Collaboration (TTC) Meeting, Milano, 6-9 February 2018. Main Coupler R&D at DESY Denis Kostin, DESY 8.02.2018

Outline E-XFEL Main Coupler R&D Main Coupler for CW Operation

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 TiN coated ceramic windows. Motorized antenna tuning (10mm) allows for Qext adjustment (106..107).

E-XFEL Main Coupler Design Geometry Copper Plating part inner [μm] inner RRR outer [μm] outer RRR cold solid copper 10±20% bellow: ±30% 30..80 warm 30±20% RF Power peak power rep.rate equivalent CW power 150 kW 10 Hz 1.9 kW

E-XFEL Main Coupler Push-Rod Bellow Problem [1] All capacitors are replaced by coax gaskets: no push-rod leaks (RF is ON) since then problem solved FPC push-rod leak: leak spot discovered E-XFEL FPC FPC coax-gasket: capacitors are replaced FPC simulation: leak spot may have a contact-discharge CST MWS simulation

E-XFEL Main Coupler Push-Rod Bellow Problem [2] possible solution with capacitor (to use HV bias): push-rod isolation inside the bellow to prevent the electrical contact.

E-XFEL Main Coupler Actuator Optimization possible actuator optimization: increase the moving range over 10 mm; improve the motor axle connection; shift the peek axle connection to the push-rod outwards for easy access and removal; in case of not implementing the HV bias consider the design without the peek axle.

E-XFEL Main Coupler Parts Problems [1] FPC test/operation problems examples # Problem Number Repair Action 1 T70K 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 T70K 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 4 T70K overheating in the test warm parts disassembled and checked: no problem found, re-assembled 5 warm part damaged by the discharge with not tightened bias capacitor screws 1 warm part replaced, 1 warm part cleaned 6 waveguide part damaged by a warm part RF discharge during the test (see p.3) warm part replaced 7 actuator axle broken 8 axles replaced push-rod leak 26 push-rods replaced 9 push-rod cold end not connected push-rods re-assembled

E-XFEL Main Coupler Parts Problems [2] (XTL) position module coupler part status comments 1 A4.M4.C4 XM72 CPI-CP-024 shorted T70K → 200K, high e-1 signal (discharge), cold part problem. AMTF: push-rod replaced, no other problems. 2 A12.M4.C1 XM53 THRI-WP-706 T70K → 150K, discharge, warm part problem. No problem / observations in AMTF. 3 A13.M2.C8 XM51 THRI-WP-683 OK high e-2 signal – conditioned. 4 A16.M1.C1 XM69 THRI-CP-880 high e-1 signal – conditioned. 5 A16.M2.C1 XM60 CPI-CP-028 T70K → 150K high e-1/2 signals, cold part problem. Problem seen in AMTF, T70K → 115K (some overheating). 6 A16.M2.C2 CPI-WP-030 high e-2/3 signals – conditioned. 7 A16.M4.C7 XM66 THRI-WP-323 8 A19.M3.C4 XM-2 AC3H26 testing T70K overheating – no discharge. Observed in AMTF as well. 9 A19.M3.C6 AC3H38 conditioned (5hr) up to 150kW FT-pulse (25MV/m) 10 A20.M4.C1 XM65 THRI-WP-470 high T70K, cpl.vac. increased, high e-2 signal bursts, warm part problem. 11 A21.M2.C5 XM92 THRI-WP-155 T70K overheating – no discharge. WP was already replaced in AMTF – same problem. 5 FPCs are not conditionable and showing T70K overheating – shorted / disconnected from RF.

Main Coupler CW operation [1] Qload increase is needed N cavity Qload.max 1 AC114 2.80×107 2 AC156 1.93×107 3 AC146 2.15×107 4 AC154 2.02×107 5 AC157 2.00×107 6 AC158 1.85×107 7 AC151 1.78×107 8 AC152 2.03×107 XM-3 @ CMTB CW run example Pfor = (Eacc×L)2/(4×Rsh/Q×Qload) limited by a motor/mechanics Pfor ~ 1/Qload test with 20 MV/m simulated data 5 mm T70K increases

Main Coupler CW operation [2] 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 achieved Qload.max warm part / 70K window shifted by 10 mm using special iso-vac flange

Main Coupler CW operation [3] – Copper Plating with the CW RF power in a range of 10..15 kW 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. inner conductor outer conductor

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); 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.