Ernest Orlando Lawrence Berkeley National Laboratory

1
Overview of laser, timing, and synchronization
issues John Corlett, Larry Doolittle, Bill
Fawley, Steven Lidia, Bob Schoenlein, John
Staples, Russell Wilcox, Sasha Zholents LBNL
2
Scientific goal - application of ultrafast x-ray
sources to study dynamics with high-resolution

• Diffraction and spectroscopy
• Nuclear positions and electronic, chemical or
  structural probes

Time-resolved x-ray diffraction
Time-resolved EXAFS NEXAFS
Plus photoelectron spectroscopy, photoemission
microscopy, etc

• Access new science in the time-domain x-ray regime

3
Pump-probe experiment concept

• Ultrafast laser pulse pumps a process in the
  sample
• Ultrafast x-ray pulse probes the sample after
  time ?t
• Ultrafast lasers an integral part of the process
• X-rays produced by radiation in an electron
  accelerator

4
Pump-probe experiment concept

• Both laser and x-ray pulses should be stable in
  temporal and spatial distributions
• Parameters and quality of x-ray pulse determined
  by the electron beam
• Accelerator parameters
• Synchronization between laser and x-ray pulses,
  ?t, should ideally be known and controllable - to
  the level of the pulse duration itself 10 fs

5
Many projects around the world are addressing the
need for ultrafast x-rays, in different ways

• LCLS linac SASE (construction)
• BNL DUV FEL linac HGHG (operational)
• DESY TTF-II linac SASE (construction)
• SPPS linac spontaneous emission from short
  bunches (operational)
• ALFF linac SASE
• BESSY FEL linac HGHG
• European X-ray FEL linac SASE
• Daresbury 4GLS ERL HGHG spontaneous
• LUX recirculating linac HGHG spontaneous
• Cornell ERL ERL spontaneous
• MIT-Bates X-ray FEL linac HGHG SASE
• Arc-en-Ciel recirculating linac / ERL HGHG
  SASE
• FERMI_at_Elettra linac HGHG
• BNL PERL ERL spontaneous

6
What are the difficulties in achieving x-ray beam
quality?

• X-rays are produced by electrons emitting
  synchrotron radiation in an accelerator
• The electron beams are manipulated by rf and
  magnetic systems
• The x-ray beam quality is limited by the electron
  beam quality in many ways
• Electron bunch charge, energy, emittance, energy
  spread, bunch length, position,
• At the radiator!
• Production of high-brightness bunches is tough
  enough
• Emission process, space charge, rf focusing, .
• Then we must accelerate and otherwise manipulate
  the bunches before they reach the radiating
  insertion device
• Many opportunities to degrade the electron bunch
• Space charge, rf focussing, emittance
  compensation, CSR, geometric wakefields, rf field
  curvature, resistive wall wakefields, optics
  aberrations, optics errors, alignment, rf phase
  errors, rf amplitude errors,

7
Synchronization

• In addition to the electron bunch properties, the
  need for synchronization of the x-ray pulse to a
  reference signal - the pump - is required for
  many experiments
• Time between pump signal and probe x-ray pulse
• Predictable or measurable
• Stable to pump probe pulse durations
• This presents additional demands on the
  accelerator, instrumentation, and diagnostics
  systems
• Various techniques may be employed to enhance
  synchronization
• Slit spoiler for SASE
• Seeding
• HGHG
• ESASE (Enhanced SASE)
• e- bunch manipulation x-ray compression
• Measurement of relative x-ray - pump laser timing
• Electro-optic sampling of electron bunch fields
• Time-resolved detection of x-ray and laser pulses
  at the sample

8
The roles of lasers, timing,and synchronization
in an ultrafast x-ray facility

• Laser systems
• Generate the high-brightness electron beam in an
  rf photocathode gun
• Produce the pump signals at the beamline
  endstations
• Timing system
• Provides reference signals to trigger (pulsed)
  accelerator systems
• Provides reference waveforms to synchronize rf
  systems
• Provides reference waveforms to synchronize
  endstation lasers
• Synchronization
• To control and determine the timing of the x-ray
  pulse with respect to a pump pulse
• Requires stable systems in the x-ray facility,
  connected by a stable timing system including
  stable timing distribution systems
• The timing system only has to be stable enough
  for all of the components connected to it to
  follow its timing jitter (to the required level)
• Phase noise ? timing jitter
• The majority of the timing jitter must be within
  the bandwidth of the accelerator laser systems
  such that they can follow
• Local feedback around rf laser systems
• Lock to timing system master oscillator

9
Phase noise and timing jitter
10
Some space and time parameters for a conceptual
ultrafast x-ray facility
Length scale 100s m Time scale µs Equivalent
bandwidth 100s kHz

• 10 fs 3 µm at c
• Thermal expansion for ?T 0.1C in Cu over 100 m
• 170 µm or 570 fs
• Similar magnitude effect from refractive index
  change in optical fiber

11
Some rf systems parameters for a conceptual
ultrafast x-ray facility

• 10 fs 5x10-3 rf phase L-band
• Cavity filling time (Q104) 2 µs
• Bandwidth 100 kHz
• Cavity filling time (Q107) 2 ms
• Bandwidth 100 Hz
• 10 fs 1x10-2 rf phase S-band
• Cavity filling time (Q104) 1 µs
• Bandwidth 300 kHz

12
Synchronize rf systems to a master oscillator

• Noise sources
• Microphonics
• Thermal drift
• Electronic noise
• Digital word length

• Control phase and amplitude of the rf fields
  experienced by the electron beam
• The master oscillator must have a phase noise
  spectrum such that the majority of the timing
  jitter is accumulated within the bandwidth of the
  rf systems
• Local feedback ensures that the rf systems follow
  jitter in the master oscillator

13
Choice of master oscillator

• rf crystal oscillator has low noise close to
  carrier
• Laser has low noise above 1 kHz
• Mode locked laser locked to good crystal
  oscillator provides a suitable master oscillator
• Active mode-lock cannot respond rapidly to
  perturbations

14
Although there are very good low-noise sapphire
loaded cavity oscillators
http//www.psi.com.au/pdfs/PSI_SLCO.pdf
15
Phase noise spectrum requirement

• Master oscillator phase noise within bandwidth of
  feedback systems can be corrected
• Residual uncontrolled phase noise plus noise
  outside feedback systems bandwidth results in
  timing jitter and synchronization limit

16
Laser synchronization
D.J. Jones et al., Rev. Sci. Instruments, 73,
2843 (2002).
time (sec)

• two independent psec Mira 900-P (Coherent)
  lasers
• PLLs at 80 MHz (n1) and 14 GHz (n175)

• Sub-femtosecond timing jitter has been
  demonstrated between two mode-locked Tisapphire
  lasers
• Limit is electronic noise (under favorable
  conditions)

17
Sophisticated laser systems are an integral
component of an FEL facility
FEL seed lasers
Multiple beamline endstation lasers
Photocathode laser
18
Lasers may be synchronized to a common master
oscillator
FEL seed lasers
Laser master oscillator
Multiple beamline endstation lasers
Photocathode laser
19
rf systems need to be synchronized to a common
master oscillator
FEL seed lasers
Laser master oscillator
Multiple beamline endstation lasers
Photocathode laser
20
rf signals for the accelerator may also be
derived from the laser master oscillator
FEL seed lasers
Laser master oscillator
Multiple beamline endstation lasers
Photocathode laser
Accelerator RF signals
21
rf photocathode gun
22
rf photocathode laser
UV pulse time profile

• UV pulse on cathode

• W. S. Graves, MIT-Bates (DUV FEL, Brookhaven)

23
rf photocathode laser
Bunch production, acceleration, and compression

• UV pulse on cathode
• Non-uniformity exacerbates space-charge effects
• Temporal non-uniformity induces micro-bunching

• W. S. Graves, MIT-Bates (DUV FEL, Brookhaven)

24
Laser pulse shaping influences the emitted
electron bunch
photo- switch
Deformable mirror
polarizer
Pockels Cell
Pulse Amplitude Stabilizer Patent LLNL (R.
Wilcox)
25
Laser-driven photocathode - one of the many laser
systems
H. Tomizawa, JASRI
R. Cross, J. Crane, LLNL

• Need high reliability
• Integrated systems
• hot spare system attractive
• Develop techniques for pulse shaping

26
rf gun phase and amplitude

• Cathode
• Cs2Te

• Laser
• 1 µJ, 35 ps, 10 kHz, 266 nm
• Spatial and temporal control to provide
  low-emittance electron bunches

• RF field
• 64 MVm-1 at cathode

• LUX rf gun concept as an example
• Assume 5 of bunch length (1 psec) jitter
• Primary drivers are launch phase, cell 1 gradient
  and bunch charge (laser intensity)
• Assumed uncorrelated disturbances three most
  significant parameter tolerances are (rms
  values)
• Launch phase 0.43 degree
• Cell 1 gradient 1.4 variation
• Bunch charge 36 variation

27
Nominal LCLS Linac Parameters for 1.5-Å FEL
Single bunch, 1-nC charge, 1.2-mm slice
emittance, 120-Hz repetition rate
6 MeV ?z ? 0.83 mm ?? ? 0.05
250 MeV ?z ? 0.19 mm ?? ? 1.6
4.54 GeV ?z ? 0.022 mm ?? ? 0.71
14.1 GeV ?z ? 0.022 mm ?? ? 0.01
135 MeV ?z ? 0.83 mm ?? ? 0.10
Linac-X L 0.6 m ?rf -160?
rf gun
Linac-1 L ?9 m ?rf ? -25
Linac-2 L ?330 m ?rf ? -41
Linac-3 L ?550 m ?rf ? -10
new
Linac-0 L 6 m
undulator L 130 m
21-1b 21-1d
X
21-3b 24-6d
25-1a 30-8c
...existing linac
BC-1 L ?6 m R56? -39 mm
BC-2 L ?22 m R56? -25 mm
DL-1 L ?12 m R56 ?0
LTU L 275 m R56 ? 0
SLAC linac tunnel
research yard
(RF phase frf 0 is at accelerating crest)
P. Emma, SLAC
28
Jitter Tolerance Levels in the LCLS
X-
X-band
and test the budget with jitter simulations
rms Dt-jitter 109 fs
sz jitter 14 rms

• P. Emma, SLAC

29
SASE FEL output

• The SASE FEL process arises from noise

Saturation
Half way along undulator
Radiation intensity build-up along undulator
http//www.roma1.infn.it/exp/xfel/SaseXfelPrincipl
es/Sasexfelprinciples.pdf
30
Slit spoiler defines radiating region of bunch
0.1 mm (300 fs) rms
Easy access to time coordinate along bunch
50 mm
x, horizontal pos. (mm)
2.6 mm rms
z, longitudinal position (mm)
LCLS BC2 bunch compressor chicane (similar in
other machines)
Paul Emma, SLAC
31
Add thin slotted foil in center of chicane
BEFORE FOIL
1-mm emittance
AFTER FOIL
5-mm emittance
1-mm emittance
Paul Emma, SLAC
32
Timing determination from Electro Optic sampling
-developing techniques at the SPPS
A. Cavalieri
Principle of temporal-spatial correlation
single pulse
Line image camera
EO xtal
analyzer
polarizer
Er
width
centroid
30 seconds, 300 pulses sz 530 fs 56 fs
rms Dt 300 fs rms
33
ESASE - Enhanced Self-Amplified Spontaneous
Emission
SASE
Bunching
Modulation
Acceleration
A. Zholents - Wednesday
34
Enhanced Self-Amplified Spontaneous Emission
P0 235 GW With a duty factor 40, Paverage
6 GW
x-ray macropulse

• Each micro-pulse is temporally coherent and
  Fourier transform limited
• Carrier phase is random from micro-pulse to
  micro-pulse
• Pulse train is synchronized to the modulating
  laser

35
Harmonic generation scheme -coherent source of
soft x-rays
Developed and demonstrated by L.-H. Yu et al, BNL
e- bunch
modulator
radiator
laser pulse
bunching chicane
e-beam phase space
Input
Output
np
-np
In a downstream undulator resonant at l0/n,
bunched beam strongly radiates at harmonic via
coherent spontaneous emission
Energy-modulate e-beam in undulator via FEL
resonance with coherent input radiation
Dispersive section strongly increases bunching at
fundamental wavelength and at higher harmonics
L.-H. Yu et al, High-Gain Harmonic-Generation
Free-Electron Laser, Science 289 932-934
(2000) L.H. Yu et al., "First Ultraviolet High
Gain Harmonic-Generation Free Electron Laser",
Phys. Rev. Let. Vol 91, No. 7, (2003)
36
Cascaded harmonic generation scheme
seed laser pulse
disrupted region
radiator
radiator
modulator
modulator
head
tail
Low e electron pulse
Unperturbed electrons
Delay bunch in micro-orbit-bump (50 mm)
seed laser pulse
3rd - 5th harmonic radiator
3rd - 5th harmonic radiator
modulator
modulator
37
User has control of the FEL x-ray output
properties through the seed laser

• OPA provides controlled optical seed for the free
  electron laser
• Wavelength tunable
• 190-250 nm
• Pulse duration variable
• 10-200 fs
• Pulse energy
• 10-25 µJ
• Pulse repetition rate
• 10 kHz
• Endstation lasers seeded by or synchronized to
  Tisapphire oscillator

Q-switched NdYAG (2w)
Tisapphire Oscillator lt100 fs, 2 nJ lt50 fs jitter
Tisapphire Regenerative Amplifier
Optical Parametric Amplifier
grating compressor
grating stretcher
gt10 conv. efficiency
1 mJ, 800 nm, 10 kHz
RF derived from optical from master oscillator
Endstation synch.
38
Seeding with XUV from high harmonics in a gas jet
(HHG)

• Coherent EUV generated up to 550 eV
• R. Bartels et al, Science 297, 376 (2002), Nature
  406, 164 (2000)

Gas jet
Harmonic emission
E field
30
25
20
15
10
5
0
Time(fs)
I. Christov et al, PRL 78, 1251, (1997)
J. Zhou et al, PRL 76(5), 752-755 (1996)
H. Kapteyn, JILA/Uni. Colorado/NIST
39
Seeding multiple cascades from a single electron
bunch allows 10 kHz operation in LUX concept
FEL optical pulses
e-beam

• Optical pulses overlap different part of bunch
  for each beamline
• Timing jitter influences number of cascades that
  can be served by a single bunch
• CSR effects in the arcs introduce few fs jitter
  for few charge variation

40
Laser-manipulation produces attosecond x-ray
pulses in harmonic cascade FEL
spectral broadening and pulse compression
800 nm
e-beam
e-beam
2 nm light from FEL
two period wiggler tuned for FEL interaction at
800 nm
time delay chicane
harmonic-cascade FEL
1 nm coherent radiation
dump
e-beam
chicane-buncher
2 nm modulator
end station
1 nm radiator
end station
A. Zholents, W. Fawley, Proposal for Intense
Attosecond Radiation from an X-Ray Free-Electron
Laser, Phys. Rev. Lett. 92, 224801 (2004)
41
Ultrafast x-ray pulses by electron bunch
manipulation and x-ray compression
50 fs
2 ps
42
Synchronize deflecting cavities and pump laser
for hard x-ray production
crab cavity 3.9 GHz
Master Oscillator Laser
laser pulse
Dy
x-rays
electron bunch
Dt
Dt

• Synchronization dependent on phase of deflecting
  cavity
• Phase lock to master oscillator
• Fast feedback systems around scrf
• Extend frequency response of the system

43
Typical end station concept
Precisely timed laser and linac x-ray pulses
10 m

• Active laser synchronization
• Independent oscillators at each endstation
• Complete independence of endstation lasers
• Wavelength, pulse duration, timing, repetition
  rate etc.

44
Beamline endstation lasers
chirped-pulse amplification
Q-switched NdYAG (2w)
Tisapphire Oscillator lt100 fs, 2 nJ lt50 fs jitter
Tisapphire Regenerative Amplifier
Optical Parametric Amplifier
grating compressor
grating stretcher
gt1 mJ, 800 nm, 10 kHz
RF derived from optical master oscillator
typical regenerative amplifier
20 passes ? DL1 µm (Dt66 fs)

• interferometric stabilization
• cross-correlate with oscillator (compress first)
• temperature stabilize (Zerodur or super-invar)

45
All-optical timing system to achieve
synchronization between laser pump and x-ray probe

• Laser-based timing system
• Stabilized fiber distribution system
• Interconnected laser systems
• Active synchronization
• Passive seeding
• rf signal generation
• 2050 fs synchronization

46
Timing distribution
47
Timing distribution - fiber systems developed fro
distribution of frequency standards
D. Jones, UCB/JILA
Mixer/amplifier noise floor
48
Modelocked fiber laser oscillatorrf stabilized
Modelocked Fiber Laser Oscillator RF Stabilized

• Phase-lock all lasers to master oscillator
• Derive rf signals from laser oscillator
• Fast feedback to provide local control of
  accelerator rf systems
• Synchronization 10s fs

49
SummaryLasers, timing,and synchronization

• Laser systems under development at many
  institutions
• Applications for improved light-source operations
• Photocathode laser, timing system master
  oscillator, FEL seed laser, endstation pump laser
• Manipulation of e- beam by laser has great
  potential
• HHG power increasing, wavelength decreasing
• Ultra-stable timing systems with optical fiber
  distribution systems under development
• Application of techniques to accelerator
  environments and requirements is to be
  demonstrated
• 10s fs synchronization seems achievable