1. J. Rudolph, Helmholtz-Zentrum Berlin EuCARD SRF Annual Review 2010 Bunch Length Measurements and Status of the Slice Diagnostics System for ELBE / SCRF Gun

2. 2 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun  Overview: ELBE and SRF injector  Bunch length measurements: basics, setup, results  Slice diagnostic system: basics of zero-phasing, setup, simulations and simulation results  Present status

3. 3 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Overview: ELBE and SRF Injector Why SRF-Gun?

4. 4 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun SRF Gun operation parameter parameterpresent cavitynew “high gradient cavity” measured ´08ELBEhigh chargeELBEhigh charge final electron energy2.1 MeV3 MeV≤ 9.5 MeV peak field13.5 MV/m18 MV/m50 MV/m laser rep. rate1 – 125 kHz13 MHz2 – 250 kHz13 MHz≤ 500 kHz laser pulse length (FWHM) 15 ps4 ps15 ps4 ps15 ps laser spot size2.7 mm5.2 mm 2 mm5 mm bunch charge≤ 200 pC77 pC400 pC77 pC1 nC max. aver. current1 µA1 mA100 µA1 mA0.5 mA peak current13 A20 A26 A20 A67 A transverse norm. emittance (rms) 3±1 mm mrad @ 80 pC 2 mm mrad7.5 mm mrad1 mm mrad2.5 mm mrad  Design for the SRF-Gun @ ELBE  CW + high rep rate - 500 kHz, 13 MHz  Medium average current 1 – 2 mA (< 10 mA)  High energy (stand alone) - 3½ cells  Low - high bunch charge 80 pC – 1 nC  Low transverse emittance 1 – 3 mm mrad Energy? Bunch length? Emittance?

5. 5 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Bunch length measurements for SRF Gun Analysis  Cherenkov-effect used to convert longitudinal electron distribution to light distribution of identical temporal distribution  Light pulse is transported to streak-camera by a low- dispersive fully reflective beamline  Longitudinal distribution of light pulse is measured Light transport to streak camera?

6. 6 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Streak camera OAP 2500 OAP 175 Board in accelerator tunnel Board in streak camera hutch Source 1 m 2 m 3 m 5 m Radiation protection wall Setup of reflective Cherenkov-beamline Phase range for bunch length measurement?

7. 7 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Faraday-Cup phase scan for bunch length estimation  Measured pulse length of photocathode laser: 6.6 +/- 0.5 ps rms  F-cup phase scan data:  Extraction bunch length: 7.3 +/- 1.1 ps rms  Zero crossing at 33 degL  Bunch length measurements for phase range 30 to 75 degL Machine settings for measurement?

8. 8 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun  Bunch length as function of injection phase  Measurements 30 to 75 degL (zero crossing at 33degL)  Laser pulse length: 6.6 ps rms ( 15.5 ps FWHM)  Laser rep rate: 50 kHz  Bunch charge: ~ 8 pC  Beam current: 0.45 µA (dark current 0.05 µA )  Average field on axis: 7.2 MV/m  Energy of electrons: 3 MeV  4 measurement series, equal conditions:  3 x lens to focus light on streak camera (phase scan 1 to 3)  1 x OAP to focus light on streak camera (phase scan 4) Measurement conditions and machine settings Measurement results?

9. 9 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Measured bunch length as a function of injection phase and ASTRA simulation  Same dependency of phase for all phase scans  Bunch length is between 12 and 32 ps rms  ASTRA Simulation shows much shorter pulses than measurements  factor 5  Energy spread measurement indicates longer bunches: ~ 30 ps rms (details by J. Teichert) Fcup current Cherenkov ASTRA Conclusion?

10. 10 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Summary of bunch length measurements  Bunch length difference of factor 5 between measurement and ASTRA  Energy spread measurements indicate longer bunches  Possible measurement error sources:  Streak-camera adjustment  Misalignment of Cherenkov beamline  Dispersion in Cherenkov beamline (max. 3 ps)  Possible sources for bunch lengthening:  Machine setup  Photocathode laser  pulse length for high power operation  To Do:  Check laser pulse length  Check Cherenkov beamline with photocathode laser  Check machine setup and further simulations Slice emittance measurement?

11. 11 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun The basic principle of zero-phasing for slice diagnostics  Bunch passes cavity at zero-crossing of RF phase  correlation between energy and longitudinal bunch position  Chirped beam send through spectrometer  longitudinal distribution transferred to transverse distribution, position in x on screen corresponds to position in t in longitudinal direction  longitudinal slices accessible  Beam size in y direction for each longitudinal slice measured  Combination with quadrupole scan technique allows to reconstruct the vertical emittance for different slices Spectrometer?

12. 12 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Properties and principle of a Browne-Buechner spectrometer F2 R M0 Q F0 M1 M2 F1 r2 r1 R R R x y green: low energy blue: medium energy red: high energy Spectrometer position?  Uniform field magnet with circular boundary of radius R and deflection angle of 90°  Divergent source is focused onto a hyperbolic surface when placed a distance R from the boundary  Reference energy particles are focused with image length equal to R  Imaging properties hold true for broad range of energy  Barber´s rule defines focus positions  Ideal installing position: non-dispersive beamline section

13. 13 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Possible alternatives for spectrometer installation positions …  “Alternative 2”: spectrometer in dispersive chicane section  “Alternative 1”: spectrometer in straight beamline section spectrometer

14. 14 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Possible alternatives for spectrometer installation positions... continued  “Alternative 3”: spectrometer in extended chicane  “Alternative 4”: spectrometer after added dogleg spectrometer

15. 15 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Possible alternatives for spectrometer installation positions... continued spectrometer  “Alternative 4b”: spectrometer after double bend achromat  “Alternatives 1”, “2”, “4b”: simulation results shown on following slides  “Alternative 3” + “4”: (no detailed simulations)  Trouble controlling dispersion and beta-functions Simulation of measurement?

16. 16 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Elegant simulations  Realistic bunch properties and emittance (1 mm mrad normalized)  Quadrupole scan with 10 quad settings  Beam optics and phase space plots show simulation results for representative quadrupole settings:  “scan 1” (quad start value, black)  “scan 5” (red) and  “scan 10” (quad end value, blue)  Results of different scans somewhere in these limits  Remark: focus plane of spectrometer is plane perpendicular to reference orbit in distance R of magnetic field boundary  not tilted like actual focus plane Beam optics? M. Borland. elegant: A Flexible SDDS – Compliant Code for Accelerator Simulation. Advanced Photon Source, 2000. LS-287.

17. 17 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Comparison of alternative setups: dispersion and beta functions... spectrometer scanning quad spectrometer scanning quad spectrometer scanning quad spectrometer  “Alternative 1”: dispersion rises at spectrometer and reaches 0.4 m @ spectrometer screen  “Alternative 2”: dispersion rises at chicane dipole and is lowered by spectrometer (depends on slope of dispersion)  dispersion non-zero @ spectrometer slit “Alternative 1” “Alternative 2”

18. 18 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Comparison of alternative setups: dispersion and beta functions... continued scanning quad  “Alternative 4b”: dispersion closed after arc for zerophasing, rising again at spectrometer to value of 0.4 m @ spectrometer screen “Alternative 4b” spectrometer scanning quad spectrometer Phase space?

19. 19 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun p-x correlation in spectrometer slit plane...  “Alternative 1”: no p-x correlation, all energies equally cut by spectrometer slit  80 % of particles left  “Alternative 2”: linear p-x correlation caused by dispersion of chicane dipole, correlation cut by spectrometer slit, energy chirp gets lost  4 % of particles left Spec slit open Spec slit closed “Alternative 1” “Alternative 2”

20. 20 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun p-x correlation in spectrometer slit plane... continued  “Alternative 4b”: nonlinearities in p-x correlation caused by 2nd order dispersion, correlation cut by spectrometer slit  head and tail of bunch get lost  50 % of particles left “Alternative 4b” Spec slit openSpec slit closed

21. 21 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun p-x correlation in spectrometer focus plane... Spec slit openSpec slit closed  “Alternative 1”: linear p-x correlation, spectrometer slit sharpens correlation  “Alternative 2”: nonlinear p-x correlation, and loss of energy chirp by slit t-x correlation similar to presented p-x correlation! Energy chirp needs to be adjusted to size of spectrometer screen in focal plane! “Alternative 1” “Alternative 2”

22. 22 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun p-x correlation in spectrometer focus plane... continued “Alternative 4b” Spec slit openSpec slit closed  “Alternative 4b”: nonlinear p-x correlation, correlation cut by slit  linear again but head and tail of bunch missing Energy chirp needs to be adjusted to size of spectrometer screen in focal plane! t-x correlation similar to presented p-x correlation!

23. 23 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Vertical phase space in focus plane...  Focus scanned through spectrometer focus plane by quad scan  Alternative 2: large edge focusing effects because of large beam size in x at spectrometer “Alternative 1” “Alternative 2” Spec slit openSpec slit closed

24. 24 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Vertical phase space in focus plane... continued  Alternative 4b: ? “Alternative 4b” Spec slit openSpec slit closed Conclusions of simulations?

25. 25 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Conclusion of simulations  Simulation proved: method working!  “Alternative 1”:  good solution for slice diagnostics  Advantages of spectrometer can be fully utilised  “Alternative 2”:  Main problem: spectrometer in dispersive section, energy-time correlation lost  Advantages of spectrometer cannot be used  “Alternative 4b”:  2nd order dispersion effects  nonlinear p-x correlation  No slice diagnostics of bunch head and tail Present status?

26. 26 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Present status March 2010 …  User demands for immediate realisation of energy and energy spread measurements of high accuracy  “Alternative 2” is the only possibility that could be realised immediately  Spectrometer installed as “alternative 2” for energy and energy spread measurements, though not appropriate for slice diagnostics  “Alternative 1”: ideal solution, but: beam pipe in spectrometer of small diameter (8 mm)  “Alternative 4b”: nonlinearities, no ideal solution

27. 27 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Present status … continued Summary?

28. 28 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Summary and outlook  Bunch length measurements:  Streak camera measurements: 12 ps... 32 ps rms (depending on phase)  ASTRA simulations: 2 ps … 12 ps rms (depending on phase)  Energy spread measurements indicate: ~ 30 ps rms  Need further considerations  Slice-diagnostics system:  Different setups for spectrometer installation positions simulated  Simulations proved: method appropriate  Spectrometer installed at “alternative 2”  Need spectrometer position in straight beamline  Plan to reinstall at more appropriate “alternative ?”  Outlook:  First slice measurements foreseen for April / May with “alternative 2”  Summer / autumn 2010 installation at “alternative ?”  Slice measurement using all spectrometer imaging properties

29. 29 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun  Thanks to the ELBE crew, the Accelerator Physics group at HZB and especially to A. Arnold, M. Abo-Bakr, T. Kamps, J. Teichert and R. Xiang. Thank you for your attention!

30. 30 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun BACK-UP

31. 31 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun

32. 32 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun t-x correlation in focus plane … Alternative 1 Alternative 2 Spec slit openSpec slit closed

33. 33 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun t-x correlation in focus plane … continued Alternative 4b Spec slit openSpec slit closed

34. 34 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun t-x correlation in focus plane … Alternative 1 Alternative 2 Spec slit openSpec slit closed

35. 35 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Elegant Simulations –alternative 1 – quadrupole scan  left slit open / right closed slit

36. 36 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Elegant Simulations –alternative 2 – quadrupole scan  left slit open / right closed slit

37. 37 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Streak image data analysis for minimizing errors  consideration of single slices of image  Gaussian fit over x-axis  10 slices within rms width  Analysis of single slices  Gaussian fit for intensity profile of each slice  Weighting of rms values with contained intensity  gives bunch length  Unweighted standard deviation and mean value of errors determined by fitting procedure as overall errors time x rms time x

38. 38 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Bunch length and ASTRA (normalized)  Measured values normalized to measured value at 0 degL (16 ps)  ASTRA simulated values normalized to simulated value for 0 degL (2.2 ps)

39. 39 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun ASTRA Simulation der Strahldynamik Bunchladung q = 8 pC für 0.4 μA, E max = 16.5 MV/m für E = 3 MeV. Strahlparameter ohne Raumladung am Faraday Cup (bei ca. 1.2 m)  Extraktion von Ladung zwischen -5 und 90 degL relativ zu zero crossing der HF  Transversale und longitudinale Emittanz, Spotsize, Divergenz steigen ab 60 degL steil an  Bunchlänge raus/rein = 1 bei ca. 40 degL  darunter < 1 (velocity bunching)  darüber > 1 (de-bunching)

40. 40 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun ASTRA Simulation der Strahldynamik Bunchladung q = 8 pC für 0.4 μA, E max = 16.5 MV/m Strahlparameter mit Raumladung am Cherenkov- Monitor (bei ca. 4.4 m)  Simulationsergebnis zum Vergleich mit Messung

41. 41 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun Diskussion möglicher Fehlerquellen  Dispersion in Elektronenstrahlbeamline  Ausrichtung der Streak-Kamera  nicht optimiert  Genauigkeit der für Simulationen angenommenen Maschinenparameter  Dispersion in optischer Beamline  Reflektive Optik, aber Elemente, die Dispersion haben:  Aerogel, Luft, Viewports in Cherenkov-Station (Fused Silica), Viewport zwischen Tunnel und Streak-Raum (BK7)  BK7 Viewport begrenzt Spektrum zu kurzen Wellenlängen (300 nm)  Pulsverlängerung (300 – 750) nm maximal: 2.5 ps, mit Linse etwa 3 ps