SPPS, Beam stability and pulse-to-pulse jitter

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SPPS, Beam stability and pulse-to-pulse jitter
Zeuthen Workshop on Start-to-End Simulations of
X-ray FELs

• Patrick Krejcik
• For the SPPS collaboration

August 18-22, 2003
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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.
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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.
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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

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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

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Klystron phase stable to lt0.1 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).
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Beam based jitter measurements
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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
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SPPS Charge jitter 0.023 rms
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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
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Chicane BPM for energy measurement
9 GeV
Max dispersion 45 cm
LB1.80 m B1.60 T
BPM Prof. Monitor
s
SPPS
LT14.3 m
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SPPS chicane energy jitter
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Incoming orbit jitter in the chicane25 microns
rms
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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
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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
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Energy jitter from chicane feedback system
5.6 MeV rms 0.06
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Pulse-to-pulse jitter estimates based on machine
stability
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

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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
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Jitter in bunch length signal over 10 seconds
10 rms
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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

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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 BWs to normalise signal without
  dithering
• Multiple bunch compressors require independent
  monitoring and control

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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
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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

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OTR Layout
OTR Screen
mirror
OTR light also provides timing signal for RF
synchronisation with experimental laser
Photodiode
Pyrometer
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Laser timing compared to OTR
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Electro optic sampling with chirped laser pulse
BW limited pulse
Short chirp
Long chirp
Spectral profiles
Temporal profile
Timing jitter moves centroid of spectrum
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Stability of the x-ray beam
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Undulator launch feedback rms angle jitter 5
microradians
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SPPS X-ray jitter, seen at the end of the
monochromator