Exceptional Events During the Operation of the European XFEL Mathieu Omet Vancouver, 5 February 2019

Contents Introduction Sudden field emission Effects of phase modulation during filling Actual quench limit determination Stable quench condition High cryo loads Summary

The European X-ray Free Electron Laser (XFEL) Soft and hard X-ray light experiments ~800 TESLA-type cavities Resonance frequency 1.3 GHz 32 cavities per XTL RF station Design energy 17.5 GeV Pulsed operation 10 Hz

Overview of L2 Cold quadrupole magnet 3 x 12 x

Investigation of A5.L2 What happened Fast ramp up of L2 to higher than previous SP most likely caused dark current event SC quads were off, allowing dark current transport along L2 Resulting situation A5.M4.C8: Energy stored in cavity is dissipated in dark current. Other cavities show beam loading while dark current transport. A5.M4.C7: Showed same behavior when M4.C8 was detuned. ΔE ≈ 2 MV/m ΔE ≈ 2 MV/m ΔE ≈ 26 MV/m

Investigation of A5.L2 Dark current hitting the wall up stream created gamma radiation, which was detected by coupler waveguide spark detectors of multiple modules triggering the technical interlock (TIL)

Investigation of A5.L2 Results Conditioning of cavities was done separately by disabling the TIL and a slow gradient increase Conditioning successful New vector sum voltage limit: 830 MV Limited by quench of M4.C2

Feed Forward Phase Modulation during Filling Goal: reduction of required power Method: Phase modulation of feed forward table during filling I Q I Q 

Feed Forward Phase Modulation during Filling Example: A24 at 870 MV Power reduction due to FF phase modulation: 1.2 MW (~19%) Reflections drastically reduced (A20: traces with, persistence without FF phase mod.)

Further Impact of Feed Forward Phase Modulation during Filling (A8) Change of gradient distribution High gradients are increased, low gradients are decreased Has to be quantified for every RF station limiting cavity

A20: Development of a Quench over three Pulses Pulse 1: Last nominal: Emax ≈ 19.9 MV/m  Actual quench limit Pulse 2: QL changes slightly (see decay), Emax higher than before Pulse 3: Due to lower QL the filling yields higher gradient and quench is clearly visible: Emax ≈ 20.8 MV/m Pulse 4: Quench Pulse by pulse evaluation necessary! Sample and hold of Emax missleading

Soft quenching: Example of A14 20.09.17 07.03.18 Consistent results Quench behavior reproducible Higher cryo load can be tolerated to a certain level We are operating this station without M3.C5 in the soft quench regime M3.C5 creates too much cryo load VS limit at 680 MV Corresponding cavity gradients marked in the plots by dashed lines

Soft quenching: Example of not acceptable example from the cryo perspective (A25.M3.C4) Cavity gradient A25.M3.C4 A25.M3.C4 Cavity temperatures ~63% Helium levels at A25 ~11.5 g/s ~59.8% Helium flow rate at A25 ~9.2 g/s >20 W additional load (A25.M3.C4) >10 W additional load (hautfarbener plot)

Differences in cavities (A23.M4.C6) Gradient of A23.M4.C6 Differences in cavities (A23.M4.C6) 1.9954 K Strong temperature dependence on gradient of A23.M4.C6 Different thermal connection Larger change in Q0 due to differences from cavity to cavity Larger change in QL due to differences from coupler to coupler 1.9940 K Cavity temperatures A23.M4.C6 A23.M4.C3 A23.M4.C2 A23.M4.C4 A23.M4.C8 A23.M4.C7 A23.M4.C1 1.9930 K ΔT ≈ 36 mK 1.9920 K 1.9910 K Δt ≈ 3 m 1.9900 K Cavity wird in einem Bereich erhöhter Kryolast betrieben. Daher starke Änderungen auch bei kleinen HF-Lastwechseln. (Softquenchregime?)

RF Operation interrupted due to cryo turbine failure (A14.M1.C8) Cryo OK gone due to cryo turbine failure RF interruption of about 45 minutes High cryo load of A14.M1.C8 VS voltage was lowered from 550 MV to 500 MV in order to establish stable cryo operation more quickly ΔT ≈ 17 mK Δt ≈ 45 m Hautfarbene Kavität macht auch erhöhte Kryolast

Summary After start-up the last two cavities of L2 showed sudden field emission Both cavities could be conditioned and the original limits were restored Set point phase modulation shows great potential of saving power Changes cavity gradient distribution Setup has to be done carefully not to reduce maximal possible beam energy For actual quench limit determination evaluation of consecutive pulse shapes necessary Long time operation in stable quench condition possible Cryo plant can handle additional cryo load to a certain level Cryo loads differ from cavity to cavity Large and fast fluctuation can be challenging for cryo plant operation

Thank you very much for your attention! Questions?