Summary of the maximum SCRF voltage in XFEL Nick Walker (MPY, DESY) TTC Meeting - Vancouver, 5.02.2019

The European XFEL Cold Linac(s) 24 RF stations (A2-A25) 4 cryomodules per RF station (klystron) 32 TESLA 1.3-GHz cavities per RF station (Injector A1 - 1 cryomodule, 8 cavities) Design electron energy 17.5 GeV Pulsed operation: 1.4 ms @ 10 Hz 750 ms fill time, 650 ms beam pulse (‘flat top’) Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

Cryomodule Testing and Installation AMTF All cryomodules were cold-tested in AMTF Operational limits set for individual cavities Quench Field Emission (X-RAY monitor threshold) 31 MV/m –power limit (administrative) Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

Cryomodule Testing and Installation AMTF All cryomodules were cold-tested in AMTF Operational limits set for individual cavities Quench Field Emission (X-RAY monitor threshold) 31 MV/m –power limit (administrative) An average of 27.5 MV/m achieved XFEL spec: 23.6 MV/m Average cryomodule max. voltage: 228 MV Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

Cryomodule Testing and Installation AMTF All cryomodules were cold-tested in AMTF Operational limits set for individual cavities Quench Field Emission (X-RAY monitor threshold) 31 MV/m –power limit (administrative) An average of 27.5 MV/m achieved XFEL spec: 23.6 MV/m Average cryomodule max. voltage: 228 MV Cryomodule Waveguide Distribution System (WDS) tailored to match forward power to each cavity Within practical limits (phase adjustment) 6 cavities (in 5 modules) detuned operationally Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

Cryomodule Testing and Installation AMTF All cryomodules were cold-tested in AMTF Operational limits set for individual cavities Quench Field Emission (X-RAY monitor threshold) 31 MV/m –power limit (administrative) An average of 27.5 MV/m achieved XFEL spec: 23.6 MV/m Average cryomodule max. voltage: 228 MV Cryomodule Waveguide Distribution System (WDS) tailored to match forward power to each cavity Within practical limits (phase adjustment) 6 cavities (in 5 modules) detuned operationally Four modules selected for RF station Module ‘pairs’ balanced (power requirement) Module power balance adjusted to match max. voltage (up to 3dB) courtesy S. Chroba & V. Katalev Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

AMTF estimates of final performance Max. beam energy: 19.3 GeV Assumes exit L2 at 2.4 GeV 17.8 GeV without A24, A25 Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

AMTF estimates of final performance Max. beam energy: 19.3 GeV Assumes exit L2 at 2.4 GeV 17.8 GeV without A24, A25 As of June 23, 2017: 14.7 GeV without A24 and A25 Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

AMTF estimates of final performance Max. beam energy: 19.3 GeV Assumes exit L2 at 2.4 GeV 17.8 GeV without A24, A25 As of June 23, 2017: 14.7 GeV without A24 and A25 Re-evaluate AMTF estimate including errors: Gradient: 2 MV/m WDS: 0.02 dB Klyst arm: 0.04 dB Max beam energy: 15.4 GeV without A24, A25 Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

Achieving 17.5 GeV Maximum Gradient Task Force Systematic evaluation of each RF station in L3 A6-A23 (A24, A25) Determination of limiting cavities (quench) Other limiting factors Radiation in tunnel (dark current) Cryogenic instabilities Waveguide sparking (resolved) Power limitations Commissioning of stations A24 and A25 Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

Single cavity performance (AMTF v operations) Quench limits of 76 cavities were determined operationally Not practical to re-measure all 800 Generally one per module (up to 3) Only 3 cavities identified as having degraded from AMTF test Two were not limiting station performance An additional 12 cavities were detuned operationally See next slide Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

Detuned cavities A: detuned after module tests or early commissioning O: detuned during MGTF studies 12 cavities A6, A7, A8 and A12 operating with only 29 cavities (3 detuned) Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

Achieved voltage and limiting factor MGTF tuning achieved 91% of AMTF projection (Not including A25) Well within error bars Two stations limited by tunnel radiation (A9, A12) dark current Overall voltage reduced Still on a learning curve with radiation measurements A14 limited by cryo load so-called soft quenching Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

Current focus: operationally maintaining max gradients Example A8 Station voltage generally reduced during XFEL operations Normally after experiencing difficulty recovering from a station trip Primary causes Quench detection Power related (klystron, modulator trips) Coupler interlocks (e.g. temperature) Currently evaluating operational margin MGTF set 1 MV/m below observed quench limits Assumed reasons for unstable operation Tune state Qload Control system related (timing etc) MV date Typical reduction 50—100 MV per station Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

Example (simulation) A21 M1 example A21.M1 VSUM = 870 MV Qload = 4.6×106 Includes LFD and static pre-tuning A21.M1 Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

Example (simulation) A21 M1 example Now include VSUM = 870 MV Qload = 4.6×106 Includes LFD and static pre-tuning Now include 50 Hz RMS random detuning 0.1×106 RMS Qload error Easy to ‘use up’ 1 MV/m margin Difficult to reproduce MGTF configuration A21.M1 Also can talk strongly to field emission / dark current Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019

In Summary Initial achieved station voltages were consistent with AMTF projections including errors MGTF carefully studied and tuned each station individually, eventually achieving >90% of AMTF ‘design value’. Currently running with 22 cavities detuned 12 detune as a result of the MGTF studies Tunnel radiation (dark current) continues to be studied. Focus now on maintaining identified max limits operationally Root causes of trips etc. Nick Walker - TTC Meeting - UBC, Vancouver, 5.02.2019