1. 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

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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.

3. <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
detect timing,
“bin” data by time
probe
pump
jitter
statistics
wasted
shots
valid data range

4. Sources of jitter in a UED system
Assume RF gun-based to achieve <50 fsec bunches for UED
RF Control
Master Clock
Laser control
Laser
HV Modulator
Buncher
Sample
Beam diags
Gun
Dispersive drift
Electron beam:
Gun voltage Amp+phase
Buncher Amp+phase
PC laser arrival time
Timing distribution:
Master clock jitter
Link jitter
Laser:
Oscillator phase noise
Amplifier

5. Jitter from electron bunch compression
d  DE/E z
sdi
szi d z
‘space charge chirp’
V = V0sin(kz)
Dtrf-laser
late
early
Dtrf-laser
Dtrf-laser
Dtrf-laser 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.
Dtsample
Path-Length Energy-
Dependent Beamline

6. RF field stability: low-level RF control
Master Clock
Forward, Reverse and Cavity power probes
HV Modulator
Buncher
Sample
Beam diags
Gun
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.

7. 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.

8. Example: LCLS Linac (F.J. Decker)
0.35 deg to 0.03 deg
Un-SLEDed, HV=340kV ?
Sample images
BC1: E =250 MeV
HV=300kV 8
LCLS Jitter Status in 2012

9. 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

10. How good does the clock have to be?
signal path A
clock
experiment
signal path B
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

11. Optical clocks are good enough
Song, et al, Opt. Expr. 19, (2011)
<0.1fs jitter above 500KHZ
~10-15 freq. stability
Kubina et al, Opt. Expr. 13, 904 (2005)
RF and optical frequencies, at exact integer multiples
Commercially available
2e6, 2e6+1...
reprate
amplitude RF
optical
100MHZ
200THz
frequency
Menlo Systems

12. Pulsed lasers are naturally quiet
Er:fiber laser:
J. A. Cox et al,
Opt. Lett. 35, 3522 (2010)
<1fs above 100kHz
Electro-optic modulators have ~1MHz BW

13. Stabilized optical link timing distribution
VCO or laser
transmitter
receiver
RF phase
detect,
correct
wRF CW
laser AM FS
wRF Rb
ref
optical
delay
sensing
wRF
RF clock controls remote oscillator
~10fs is about the limit
0.01 degree phase error
10fs at 3GHz
Currently used in LCLS and
Out-of-loop resuts:
Controlling VCXO, 200m fiber
delay error, fs
8.4fs, 20 hours to 2kHz (loop BW)
time, hours

14. Synching mode-locked lasers with RF
n*frep
Trep
slave
Master Clock BP
ML Laser
Basic Phase-locked loop Df H
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.)

15. Laser-laser synchronizationDf
ML Laser
Trep BP H
master
slave
n*frep
Detection and bandpass filter
ML Laser
Shelton (14GHz)
Bartels (456THz)
present
work (5THz)
repetition rate
n*frep
carrier/envelope
offset
m*frep+fceo
frequency
Shelton et al, O.L. 27, 312 (2002)
Bartels et al, O.L. 28, 663 (2003)

16. 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:
laser
amplitude
39kHz
resonance
DAC
ADC
step
response
phase

17. RF locking results with tisaf
In-loop measurement compared with difference between two externally referenced measuements
FFT of noise
In-loop:
21fs RMS
1Hz to 170kHz
Jitter spectral density
of laser and reference
Integrated
RMS jitter
Out-
of-
loop:
control
bandwidth
26fs RMS
30Hz to 170kHz

18. Effect of amplifiers on CEP
3mJ
6fs
100kHz
Schultze et al,
Opt. Exp. 18, (2010)
88as
240as
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

19. Out-of-loop lock diagnostics
Compare ML phase with measured buncher phase
RF Control
Master Clock
Laser control
Laser
HV Modulator
Buncher
Beam diags
Gun
Dispersive drift

20. 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.

21. 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”
100Hz
100kHz

22. 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.
Parameter
Value
Energy
750
keV
Charge
1-3x105 fC
laser spot (rms) μm
repetition rate
1-106 Hz
emittance
min. bunch length (rms)
100 fs

23.
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.
Timing
diagnostics
PC laser
Laser heater
RF control
Stabilized link
Seed lasers
Stabilized link
User lasers
Master

24. NGLS Approach: RF and BB Feedback
GUN
0.8 MeV
Heater
100 MeV
BC1
210 MeV
BC2
685 MeV
SPREADER
2.4 GeV L0 L1 Lh L2 L3
CM1
CM2,3
3.9
CM4
CM9
CM10
CM27 ΔE ΔE ΔE
Δστ
Δστ
ΔEτ SP SP SP SP
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)

25. 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.