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

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

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.

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

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

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 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).

Beam based jitter measurements

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

SPPS Charge jitter 0.023% rms

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   y y 

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

SPPS chicane energy jitter

Incoming orbit jitter in the chicane 25 microns rms

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 

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

Energy jitter from chicane feedback system 5.6 MeV rms 0.06%

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

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

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

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

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

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

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

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

Laser timing compared to OTR

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

Stability of the x-ray beam

Undulator launch feedback rms angle jitter 5 microradians

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