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Strategies for achieving femtosecond synchronization in Ultrafast Electron Diffraction. John Byrd R. B. Wilcox, G. Huang, L. R. Doolittle Lawrence Berkeley National Laboratory Workshop On Ultrafast Electron Sources For Diffraction And Microscopy Applications UCLA, December 14-16 2012. - PowerPoint PPT Presentation
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Strategies for achieving femtosecond synchronization in Ultrafast Electron
DiffractionJohn Byrd
R. B. Wilcox, G. Huang, L. R. DoolittleLawrence Berkeley National Laboratory
Workshop On Ultrafast Electron Sources For Diffraction And
Microscopy ApplicationsUCLA, December 14-16 2012
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• We have been focused on synchronization issues at FELs where one of the main issues is stable timing distribution and synchronization of remote lasers.
• I’ll try to concentrate on issues relevant to lab-scale experiments for UED.
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<10fs pump/probe experiments drive timing system design• ≤10fs X-ray pulses already on LCLS, FLASH• Want timing uncertainty ≤ pulse width
– Otherwise pulse is statistically widened– Or, timing range is statistically sampled (then “binned” if measured)– And/or shots are wasted, reducing effective reprate
3valid data range
pumpprobe
detect timing, “bin” data by time
wastedshots
jitterstatistics
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Electron beam:Gun voltage Amp+phaseBuncher Amp+phasePC laser arrival time
Sources of jitter in a UED system• Assume RF gun-based to achieve <50 fsec bunches for UED
Laser
Sample Beam diags
RF Control
HV Modulator
Gun
Buncher
Dispersive drift
Master Clock
Laser control
Timing distribution:Master clock jitterLink jitter
Laser:Oscillator phase noiseAmplifier
Jitter from electron bunch compression
Path-Length Energy-Dependent Beamline
d DE/E
z
sdi
szi
V = V0sin(kz)
d
z
‘space charge chirp’
earlylate
d
z• Relative phase jitter of the electron bunch and RF
is converted to energy jitter.• The time jitter is compressed by the compression
factor• Early and late bunches have different
compression• Overfocused beams begin to increase time jitter.
Dtrf-laser Dtrf-laser
Dtrf-laser
Dtrf-laser
Dtsample
RF field stability: low-level RF control
• Use modern digital RF controller to measure and stabilize the cavity field.– Feedback within RF pulse can only occur for long RF pulses >20 microseconds– Feedback cannot control shot-to-shot variable noise from the RF source
• Modern RF controllers can achieve <10-4 amplitude and 0.01 deg phase stability.
Sample Beam diags
RF Control
HV Modulator
Gun
Buncher
Forward, Reverse and Cavity power probes
Master Clock
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RF source stability• For pulsed RF sources:
– Variable charging of the PFN delivers variation of the high voltage to the klystron
– Variable firing of the thyratron switch– Klystron is often run near saturation so HV variation
usually results in a phase shift. – “Breakdown” in any part of the RF path (klystron, SLED,
waveguide, cavity, load) can cause plasma induced reflections, phase shifts. These “breakdowns” can be well below the limit for an RF trip and may be already a part of “normal” operations.
Example: LCLS Linac (F.J. Decker)
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8
– 0.35 deg to 0.03 deg
LCLS Jitter Status in 2012
Sample images
HV=300kVBC1: E =250 MeV
Un-SLEDed, HV=340kV
?
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RF source stability• For CW or quasi-CW RF sources:
– Klystron must be operated with some overhead to provide feedback control
– AM/PM conversion from variable cavity tuning– HV PS harmonics– RF clock phase noise
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How good does the clock have to be?
• Determined by delay difference tD = tA – tB • High frequency: differential noise, frequency >1/(2tD) • Low frequency: phase delay change Dt = tD x (Df/f)• Example: 200m fiber
– tD is 1mS– High frequency noise above 500kHz < 1fs– Long term frequency drift < 10-9
clock experiment
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Optical clocks are good enough
• RF and optical frequencies, at exact integer multiples
• Commercially available Menlo Systems
Kubina et al, Opt. Expr. 13, 904 (2005)
~10-15 freq. stability
100MHZ 200THz
opticalRF
frequency
ampl
itude
reprate
Song, et al, Opt. Expr. 19, 14518 (2011)
<0.1fs jitter above 500KHZ
2 3 4 5... 2e6, 2e6+1...
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Pulsed lasers are naturally quiet
• <1fs above 100kHz– Electro-optic modulators have ~1MHz BW
J. A. Cox et al, Opt. Lett. 35, 3522 (2010)
Er:fiber laser:
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Stabilized optical link timing distribution
• RF clock controls remote oscillator • ~10fs is about the limit
– 0.01 degree phase error– 10fs at 3GHz
• Currently used in LCLS and Fermi@Elettra
time, hours
dela
y er
ror,
fs
8.4fs, 20 hours to 2kHz (loop BW)
Out-of-loop resuts:
Rbref
AMCWlaser
FS
RF phasedetect,correct
opticaldelay
sensing
wRF
transmitter receiver wRF
Controlling VCXO, 200m fiber
VCO or laser
wRF
Synching mode-locked laserswith RF
ML Laser
Df
Trep
BP
H
slaven*frep
Master Clock
Basic Phase-locked loop
• ML Oscillator is a sub-harmonic of the clock frequency. • Best performance if the photo-detected harmonic of oscillator
frequency is the clock frequency. Otherwise, additional frequency multiplication is needed, reducing resolution.
• Possible AM/PM conversion at the PD• ML oscillator is a dynamic device. Feedback response H should be
designed to dynamic response of oscillator (piezo, piezo driver, etc.)
Laser-laser synchronization
Shelton (14GHz)
Bartels (456THz) presentwork (5THz)
repetition rate n*frep
carrier/envelopeoffset
m*frep+fceo
frequency0
Shelton et al, O.L. 27, 312 (2002)Bartels et al, O.L. 28, 663 (2003)
ML Laser
Df
ML Laser
Trep
BP
Trep
BP
H
master slaven*frep n*frep
Detection and bandpass filter
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Optimizing RF lock for ti:sapphire laser• Use modern control techniques
– Determine open loop transfer function– Add filter to prevent oscillation with high gain (30kHz LPF)
Transfer function:
amplitude
phase
39kHzresonance
laser
DAC
stepresponse
ADC
RF locking results with tisaf• In-loop measurement compared with difference between two
externally referenced measuements
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21fs RMS1Hz to 170kHz
FFT of noise
Jitter spectral densityof laser and reference
control bandwidth26fs RMS
30Hz to 170kHz
Integrated RMS jitter
In-loop:
Out-of-loop:
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Effect of amplifiers on CEP
• CEP thru example optical parametric amp, 240as long term • Dispersion changes CEP
– Carrier and envelope velocity are different– Dispersion controlled to minimize pulse width, thus
stable
Schultze et al,Opt. Exp. 18, 27291 (2010)
3mJ6fs100kHz
88as240as
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Out-of-loop lock diagnostics• Compare ML phase with measured buncher
phase
Laser
Beam diags
RF Control
HV Modulator
Gun
Buncher
Dispersive drift
Master Clock
Laser control
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Post-sample diagnostics• Measure electron charge, position and angle following sample• Use deflecting cavity to measure beam-RF jitter. • Use magnetic spectrometer to measure energy jitter. Should be
correlated to energy jitter induced by timing jitter at buncher.
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Noise measurement and control depends on repetition (sample) rate• High reprate enables high bandwidth feedback
– Control BW ≈ sample rate/10• Integrated jitter above sample rate is “shot to shot”
100kHz
100Hz
A high rep-rate RF gun for UED(Daniele Filippetto)• APEX Phase I RF gun has been built as R&D for a
high rep-rate FEL– CW 187 MHz gun, 750 keV, 1 MHz laser rep-rate
(could be higher), low emittance– Because of low frequency RF gun, beam dynamics
quasi-DC. 1.3 GHz buncher. – Expected RF stability DV/V~10-4 and Df~0.01 deg– Deflecting cavity and spectrometer diagnostics. – High rep-rate allows for broadband RF and beam-
based feedback.– If laser pump/electron probe jitter can be reduced to
<10 fsec, diffraction images can be integrated.– Expected operation in 2013.
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Parameter Value
Energy 750 keV
Charge 1-3x105 fC
laser spot (rms)
50-1000 μm
repetition rate
1-106 Hz
emittance 0.03-0.6 μm
min. bunch length (rms)
100 fs
NGLS@Berkeley• The eventual goal is to provide remote synchronization between all
FEL driver systems: x-rays, lasers, and RF accelerators. Our current focus is to synch user laser systems with timing diagnostics.
Master
PC laser RF controlTiming diagnostics Seed lasers
User lasers
Laser heater
Stabil
ized l
ink
Stabilized link
Stabilized link
Stabilized linkStabilize
d link
Stabilized link
NGLS Approach: RF and BB Feedback
CW SCRF provides potential for highly stable beams…
Measure e- energy (4 locations), bunch length (2 locations), arrival time (end of machine)Feedback to RF phase & amplitude, external lasers
Stabilize beam energy (~10-5 ?), peak current (few %?), arrival time (<20 fs)
3.9CM1 CM2,3 CM4 CM9 CM10 CM27
BC1210 MeV
BC2685 MeV
GUN0.8 MeV
Heater100 MeV
L0 L1 Lh L2 L3
SPREADER2.4 GeV
ΔE Δστ
SP
ΔE Δστ
SP
ΔEτ
SPSP
ΔE
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Conclusions• UED is the ideal setup for pump-probe
– Pump and probe generated by same laser• Laser-RF stability requires careful control of RF and
laser with out-of-loop comparisons. – Greatest potential for improvement. – CW RF can be stabilized to DV/V~10-4 and Df~0.01 deg– Potential for significant improvement in laser lock
• Further improvement using beam-based feedback to stabilize source. – High rep-rate will help.