1. Photo injector dark current at FLASH and efforts for reduction at FLASH and XFEL Svem Lederer Unwanted Beam Workshop 2012 Berlin, Dec 18, 2012

2. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 2 Outline > FLASH > XFEL > Standard actions to minimize Field Emission (FE) > Attempts to reduce dark current transport > CO 2 (dry ice) cleaning > Cathode handling

3. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 3 FLASH > Normal conducting 1.3 GHz RF gun > Ce 2 Te cathode > Nd:YLF based ps photocathode laser > sFLASH undulators (variable gap) > THz Undulator > FEL Experimental Hall > 3 rd harmonic module 3.9 GHz > Diagnostics and matching > Deflecting cavity LOLA 315 m Bunch Compressors Soft X-ray Undulators sFLASH 5 MeV150 MeV450 MeV1250 MeV Accelerating Structures Diagnostics FEL Experiments RF stations LOLA RF Gun Laser Photon Diagnostics Beam Dump 3 rd harmonics

4. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 4 FLASH > FLASH uses TESLA technology > 7 accelerating modules: each with 8 superconducting 9-cell cavities, 1.3 GHz > Pure Nb, operated at 2 K > Energy gain (nominal) 200 MeV per module > Burst mode: 800 μs at 10 Hz > Efficient acceleration: high Q ~ 10 10 (loaded Q = 2 x 10 6 ) > Electron beam energy ~375 MeV - 1.25 GeV Energy gain ~25 MV/m 1 m

5. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 5 FLASH Gun body with water cooling Solenoid Bz_peak ~ 0.17 – 0.2 T Bucking coil Photo cathode (Cs 2 Te, QE 1-15%) Coaxial RF coupler Laser 262 nm, length 6.5 ps rms Electron bunch 0 – 3 nC, 5 MeV Mirror in vacuum > 1.3 GHz copper cavity, 1 ½ cell  RF power 4 MW, RF pulse length 850 µs, 10 Hz (34 kW)  Driven with laser on Cs 2 Te photocathode > Electron beam parameters:  Charge 0…3 nC, bunch length ~2 mm rms  Emittance <1.5 mm mrad @ 1 nC  Trains of thousands of bunches/sec

6. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 6 QuantityValue electron energy10.5/14/17.5 GeV macro pulse repetition rate10 Hz RF pulse length (flat top) 600  s bunch repetition frequency within pulse4.5 MHz bunch charge0.02 – 1 nC electron bunch length after compression (FWHM)2 – 180 fs Slice emittance0.4 - 1.0 mm mrad beam power500 kW # of modules (containing eight 9-cell superconducting 1.3 GHz cavities) 101 accelerating gradient for 17.5 GeV23.6 MV/m # of 10 MW multi-beam klystrons27 average klystron power (for 0.03 mA beam current at 17.5 GeV) 5.2 MW photon wavelength0.05 – 4 nm XFEL

7. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 7 XFEL E 4 = 17.5 GeV BC0 BC1 BC2

8. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 8 Dark current issues > Dark current produced from the RF-gun or cathode  Losses before first module yield x-ray radiation causing damages on electronic devices  If accelerated by first 1.3 GHz module, the following 3.9 GHz module can “act as collimator” Severe increase of cry-load which can even prevent from operation  If further transported to bunch compressors High activation of beam line components Damages on electronic devices  If further transported High activation of collimators Losses in undulator section can harm permanent magnetic field quality Activated components are a personal safety issue!

9. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 9 Standard actions to minimize FE > All vacuum components hydrocarbon free > 100 % check by means of RGA

10. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 10 Standard actions to minimize FE > Preparation in clean room class 100 > Machine installations in local clean rooms

11. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 11 Standard actions to minimize FE > Clean room assembly places

12. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 12 Standard actions to minimize FE > Even if all parts are prepared and mounted particle free there will always be particles inside the cold and warm vacuum! > For abs. pressure p > 1 mbar and differential pressure Δp > 1 mbar (e.g. opening of valves, start pumping) => movement of particles observed > For abs. pressure p no movement of particles observed > Automatic pumping / venting unit  Constant flow of 3 l/s of nitrogen or argon, by means of mass flow controllers.  Venting to 1100 mbar, to ensure always to have a small over pressure (known conditions).

13. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 13 Standard actions to minimize FE > Slow venting and pumping recipient Pump cart

14. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 14 Attempts to reduce dark current transport > FLASH RF-gun section RF-gun dc-kicker dc-collimator

15. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 15 Attempts to reduce dark current transport – dc collimator > “simple” pin-hole (8 mm diameter) right in front of first superconducting module FLASH Possible upgrade for XFEL and FLASH Currently no operation w/o usage of collimator possible!

16. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 16 Attempts to reduce dark current transport – dc kicker > Dark current kicker

17. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 17 > Dark current images on 3GUN screen (Ce:YAG screen with FC) > Dc kicker smears the dark current distribution out > Kicked dark current is buried in the dc collimator Attempts to reduce dark current transport – dc kicker Kicker ON Kicker OFF collimator

18. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 18 Attempts to reduce dark current transport – dc kicker > … but it can also make things worse in terms of steering dark current trough the dc collimator: dc kicker ON dc kicker OFF TD spectrum of dark current measured after 2 nd bunch compressor

19. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 19 Attempts to reduce dark current transport for XFEL > Increased dc kicker length from 200 mm to 300 mm > Increased distance between kicker and collimator increases efficiency of the dc kicker > Probably different dc collimator than FLASH > Continuous monitoring of dark current from RF-gun by means of dark current monitor right in front the first accelerating module > Cold collimator between first 1.3 GHz accelerating module and 3.9 GHz module

20. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 20 > Additional final cleaning technique for particles + film contaminations > Mechanical, thermal + chemical cleaning forces > Local, dry, without residues > Successfully applied to 1-3-cell Nb cavities, but transfer to 9-cell cavities still missing > Routinely applied to copper gun cavities at 1.3 GHz + 3.9 GHz (“REGAE”) and their power couplers CO 2 (dry ice) cleaning

21. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 21 CO 2 (dry ice) cleaning

22. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 22 Cathode handling > Mechanical polishing to mirror finish followed by cleaning in ultrasonic bath > Achieved surface roughness in the order of 20 – 30 nm Ra > Quality checks  Optical inspection by eye – incredibly good for scratches  Reflectivity measurements  Non-destructive roughness measurements by 3d laser scanning microscope

23. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 23 Cathode handling > All cathode handling in clean room class 100 > All parts (except cathodes) to be installed in the system are cleaned in the clean room for machine vacuum system > Check by particle counter and blowing by dry ionized air

24. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 24 Cathode handling > Result of wrong handling (missed blowing by nitrogen of #97, #71 and #54)  Unusable cathodes

25. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 25 Cathode handling > Load-lock-system is a major source of particles  Cathodes are moved through rail system  Cathodes are moved through opened valves  Cathodes are inserted into and removed from carrier > Insertion into and removal from RF-gun is a particle source too

26. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 26 Cathode handling > Continuous development of cathode carries > Old design:  CuBe2 “sliding” blocks have drawback, that each time they get in contact with rail system, one has friction of clean planar surfaces

27. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 27 Cathode handling > Newest carrier design  Aim to have only ball bearings in contact with rail system

28. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 28 Cathode handling SAFEM (Scanning Anode Field Emission Microscope) > 3d anode movement > Field up to 200 MV/m > Mechanical conditions for cathodes as in RF-gun > Should serve as QA for cathodes in future.

29. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 29 Conclusions > Field emission of RF-gun and cathodes is a severe issue may stop operation of FLASH and XFEL > Attempts to reduce dark current transport  Dc collimator  Dc kicker  CO 2 (dry ice) cleaning > Cathode handling is always an issue. Currently we are far away from being particle free.

30. Sven Lederer| Photo injector dark current at FLASH and efforts for mitigation of dark current at FLASH and XFEL| Dec 18th, 2012 | Page 30 Thanks to all colleagues for their support and transparencies! THE END!