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Strategies for achieving femtosecond synchronization in Ultrafast Electron

DiffractionJohn 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

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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

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valid 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 DD E/ 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 raten*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 bandwidth

26fs RMS30Hz 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

Stab

ilized

link

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.