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SPPS, Beam stability and pulse-to-pulse jitter

Zeuthen Workshop on Start-to-End Simulations of X-ray FEL’s. SPPS, Beam stability and pulse-to-pulse jitter. Patrick Krejcik For the SPPS collaboration. August 18-22, 2003. Long term stability dominated by RF phase drifts.

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SPPS, Beam stability and pulse-to-pulse jitter

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  1. Zeuthen Workshop on Start-to-End Simulations of X-ray FEL’s SPPS, Beam stability and pulse-to-pulse jitter Patrick Krejcik For the SPPS collaboration August 18-22, 2003

  2. Long term stability dominated by RF phase drifts Measurement of the phase variations between two adjacent linac sectors over a period of several days Measurement of phase variations seen along the linac main drive line over a period of several days.

  3. 0.5 deg. S-band klystron phase variation over several minutes Phase variations measured at the PAD of a single klystron over a period of minutes. Each point is an average over 32 beam pulses.

  4. Machine Feedback Systems • Low level RF compensation of drifts • Only as good as phase reference system • Low noise master oscillator • Reference phase distribution system must also be free of drifts. • Interferometric stabilization of a long phase reference line against low frequency drifts introduces noise at higher frequencies

  5. Pulse-to-pulse jitter • Cannot be corrected by feedback • Machine needs to meet XFEL stability requirements for long enough to allow beam tuning and feedbacks to work

  6. Klystron phase stable to <0.1 deg. S-band over ~10 sec. Pulse-to-pulse phase variations, and histogram, measured at PAD of a single klystron shows 0.07-degree S-band rms variation over 17 seconds. Pulse-to-pulse relative amplitude variations measured at the PAD of a single klystron shows 0.06% rms variation over 2 sec (horizontal axis is in 1/30-sec ticks).

  7. Beam based jitter measurements

  8. Linac orbit jitter dependance on BNS phase 0 deg. (on crest) -10 deg. (opposite phase to optimum BNS damping) SPPS 3 nC charge per bunch

  9. SPPS Charge jitter 0.023% rms

  10. s s y y Beam Based Measurement of Relative Phase Jitter Between Bunch and the Transverse Deflecting Cavity Phase deviations calculated from transverse kick measured by fitting BPM orbit downstream of cavity

  11. Chicane BPM for energy measurement 9 GeV Max dispersion 45 cm LB=1.80 m B=1.60 T BPM Prof. Monitor s SPPS LT=14.3 m

  12. SPPS chicane energy jitter

  13. Incoming orbit jitter in the chicane25 microns rms

  14. Beam-Based Feedback Systems • Orbit steering in linac, undulator launch etc • Respond with fast steering correctors • Beam energy measured at BPM in high dispersion region in chicanes, undulator dog leg. • Correct with two klystrons with opposing phases so there is no net phase change -f f

  15. Energy feedback at chicane responding to a step energy change Klystron on Klystron off Energy measured at a dispersive BPM, Actuator is a pair of klystron phase shifters

  16. Energy jitter from chicane feedback system 5.6 MeV rms 0.06%

  17. Pulse-to-pulsejitter estimates based on machine stability Simulate bunch length variations… …and bunch arrival time variations… 0  0.26 psec rms 82  20 fsec rms P. Emma • linac phase 0.1 deg-S rms • linac voltage 0.1% rms • DR phase 0.5 deg-S rms • Charge jitter of 2% rms

  18. Far-Infrared Detection of Wakefields from Ultra-Short Bunches Shortest bunch in FFTB with slight over-compression in linac foil LINAC Wakefield diffraction radiation wavelength comparable to bunch length FFTB pyrometer GADC

  19. Jitter in bunch length signal over 10 seconds ~10% rms

  20. Bunch Length Feedback Systems • Needs fast, pulse-by-pulse relative bunch length measurement • THz radiation from bunch wakefields detected as diffraction radiation, transition radiation • THz radiation from CSR in BC and DL bends • Signal is monotonically increasing with decreasing bunch length • BL feedback responds by changing RF phase upstream of BC • Requires that energy is independently being held constant by orbit-based feedback

  21. Bunch Length Feedback Systems • SPPS has demonstrated bunch length optimization with feedback • At 10 Hz response time ~1 min. • Present system uses dither control • More sophisticated system would use THz detectors with different BW’s to normalise signal without dithering • Multiple bunch compressors require independent monitoring and control

  22. Dither feedback control of bunch length minimization – L. Hendrickson Bunch length monitor response Feedback correction signal “ping” optimum Linac phase Dither time steps of 10 seconds

  23. Bunch arrival timing jitter • Synchronisation of electron bunch (linac RF) with laser for user experiments • Coarse timing wrt RF bucket • Sub picosecond (femtosecond!?) synchronisation • Time-stamping each bunch

  24. OTR Layout OTR Screen mirror OTR light also provides timing signal for RF synchronisation with experimental laser Photodiode Pyrometer

  25. Laser timing compared to OTR

  26. Electro optic sampling with chirped laser pulse BW limited pulse Short chirp Long chirp Spectral profiles Temporal profile Timing jitter moves centroid of spectrum

  27. Stability of the x-ray beam

  28. Undulator launch feedback rms angle jitter 5 microradians

  29. SPPS X-ray jitter, seen at the end of the monochromator

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