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 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).

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

yy

0 180

LB=1.80 mB=1.60 T

Chicane BPM for energy measurement

s

LT=14.3 m

9 GeV9 GeV

BPMProf. Monitor

Max dispersion 45 cm

Max dispersion 45 cm

SPPS

SPPS chicane energy jitter

Incoming orbit jitter in the chicane25 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 rms0.06%

• linac linac phasephase 0.1 deg-S rms 0.1 deg-S rms• linac linac voltagevoltage 0.1% rms 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 linac phasephase 0.1 deg-S rms 0.1 deg-S rms• linac linac voltagevoltage 0.1% rms 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 and bunch arrival time variations…time variations…

……and bunch arrival and bunch arrival time variations…time variations…

0 0 0.26 psec rms 0.26 psec rms0 0 0.26 psec rms 0.26 psec rms

Simulate bunch length Simulate bunch length variations…variations…

Simulate bunch length Simulate bunch length variations…variations…

82 82 20 fsec rms 20 fsec rms82 82 20 fsec rms 20 fsec rms

Pulse-to-pulsePulse-to-pulse jitter estimates based on machine stabilityjitter estimates based on machine stabilityP. Emma

Far-Infrared Detection of Wakefields from Ultra-Short Bunches

Wakefield diffraction radiation wavelength comparable

to bunch length

pyrometer

foilLINAC

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 LayoutOTR 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

0res CT T T

BW limited pulse Short chirp

Long chirp

Temporal profile

Spectral profiles

Timing jitter moves centroid of spectrumTiming 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