FemtosecondPump_Probe

Report
Femtosecond Pump / Probe
Operation and Plans at the LCLS
Josef Frisch for the LCLS
Commissioning team
1
Ultra-Fast Science
Some experiments use multiple images on an
already evolving system
All feet off the ground
Most experiments are pump probe:
Stimulate the system (fire bullet)
Wait
Measure with a probe pulse (flash bulb)
H. Edgerton
Measurement resolution is set by the length
of the pump / probe pulses AND the accuracy
of the time delay between the pump and
probe.
2
Short Bunch Operation at LCLS
Low charge (20pC) operating
mode for very short pulses
No direct pulse length
measurement available, but
believed to be < 5fs FWHM
Phase = +1 deg
Phase =+0.5 deg
Phase = 0 deg
ΔT=5.0fs
Phase = -0.5 deg
ΔT=2.3fs
Phase = -1 deg
ΔT=1.1fs
Genesis Simulation for over
compression: 5fs FWHM
ΔT=1.9fs
ΔT=4.2fs we typically operate here
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Narrow or Double X-Ray Pulses from a Slotted Foil
PRL 92, 074801 (2004).
P. Emma, M. Cornacchia, K.
Bane, Z. Huang, H. Schlarb
(DESY), G. Stupakov, D. Walz
0.25 mm
0-6 mm
pulses not
coherent
Power (GW)
10
5
0
0-150 fs
2 fs
time (fs)
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Low Charge AND Slotted Foil
X-ray spectrum with 20pc operation
– few spikes suggest ~5 fs pulses
With 20pc and slotted foil see
single spike spectrum suggests
very short pulses
No direct measurement but LCLS
may be producing ~1fs X-ray pulses
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Short Pulse Lasers
• Commercial Ti:Sapphire
lasers can produce
pulses as short as 15fs.
(25-50fs more typical).
• High harmonic
generation can produce
~100aS, pulses in the
XUV ~100eV.
• Assume lasers will
produce shorter pulses
in the future
Attosecond XUV generation, Max-Plank_institut fur
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Quantemoptik / ATLAS
Experiment Requirements
• This talk will concentrate on laser pump / X-ray probe
experiments
– Most common experiment at LCLS
• Right now operating with ~10-100 fs X-ray pulses and ~50fs
laser pulse
• In the future we expect few-fs X-rays and few-fs laser
pulses
• Timing control at the few fs level will be required.
– Typical temperature coeficient for either coaxial cables or fiber
optics is 2x10-5/C° ->1 meter is 60 femtoseconds / C°
– Thickness of a sheet of paper = 100fs
• When describing timing drift or jitter, need to be careful to
clarify what reference is used for comparison.
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Experiment Requirements
Pump Laser
X-ray
beam
X-rays to
detector
System evolves from pump to
probe time
Timing jitter relative to an
external clock isn’t
important
Ideally would scan time
difference
Generally OK to let jitter vary the
timing and measure shot-to-shot
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Sources of Timing Jitter
Laser
Gun
RF off crest
Bunch Compressor
RF
RF
Laser pulse is compressed typically 2X in gun, then an additional factor of 100 in the
bunch compressors
Changes in laser time are compressed, so gun laser jitter is not very important. Beam
time is mostly set by the RF in the compression system. (both amplitude and phase
contribute)
Synchronizing the gun laser to the experiment laser doesn’t fix
the jitter
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Conventional Timing System
Stabilized
transmitter
Stabilized
Receiver
Femtosecond
Laser
Laser
Amplifier
Undulator
Beam time
pickup
X-rays
E-beam
dump
Experiment
~100M
Beam pickup typically responds to electric field of bunch: either RF cavities
or electro-optical pickups are used
Stabilization system typically feeds back on the length of the cable / fiber.
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Timing Jitter in LCLS
~ 1km
Master
Source
10ps drift
over hours
Few fs jitter
in a few ms
High
power
RF
Phase
Shift
~50fs RMS jitter
shot to shot
Feedback
Stabilized link ~20fs stability
10fs jitter,
50fs stability
Phase
Detect
~50fs
jitter
Laser
RF in compressor
sets beam time
Accelerator
Phase
cavity
FEL
Experiment data corrected offline with phase cavity data report
50-100fs stability
Experiment
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Beam arrival time cavity (LCLS)
Similar to a cavity BPM but use the
monopole mode
Phase drift from cavity temperature is the
most significant problem
1us time constant, 10-5 /C° temperature
coefficient -> 10ps/C° (!)
Raw Signal
Phase slope gives
cavity temperature
12
RMS difference between cavities ~12 femtoseconds RMS at 250pC, 25
femtoseconds at 20pC. Drift is ~100 femtoseconds p-p over 1 day.
RF Phase Detection Limits
• Oscillators: unlocked timing noise relative to an “ideal”
clock increases with time
– Conventional oscillators: 1fs RMS above 1 KHz
– Sapphire oscillators 1fs RMS above 10Hz
• RF phase measurement (2X thermal noise)
– 1GHz, 1ms, 1mW power -> 20aS (theoretical)
– SLAC summer students actually measured a noise level
corresponding to 30aS in a 1KHz bandwidth
– In a 1MHz bandwidth, still expect 1 fs.
– Phase cavity system noise is about 7fs RMS. (best conditions)
• Electronics noise is not a stringent limit!
• Drift: few fs / °C for mixers.
• Drift: ~30fs / °C for 1 M cable.
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EO Beam Time Measurement
(Several versions, simplified concept shown)
Short pulse laser
Free space or fiber-optic
Detector
Bunch fields
Output intensity depends on
relative timing of laser plulse and Ebeam
Electric field
from bunch
Electro-optical
intensity
modulation
F. Loehl et al
DESY/FLASH
6 femtosecond timing noise
published
(Believe ~3 fs achieved)
Allows direct conversion from beam timing to optical signal: significant advantage
for some types of timing systems
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Long Distance Timing Transmission
Transmitter
Adjust Delay
mirror
Feedback
Use fibers:
Low loss as high transmitter frequency
Good directional couplers
Low cost
Timing Signal
Compare forward
and reflected signals
Envelope scheme (DESY, MIT Bates):
Transmit short (ps) pulses at ~100MHz rate.
Timing of the reflected pulses is used to measure the fiber length.
Control fiber length with feedback
Pulses detected at the receiver end are used for timing
Carrier scheme (LBNL, used at LCLS)
Frequency stabilized laser used in an interferometer
Interferometer determines fiber length
Control fiber length with feedback (feed forward in this case).
Both systems work at <20fs over 100M fibers
Pulses allow direct locking to
experiment laser
Excellent resolution – based on optical
wavelength
Difference between phase and group
velocity is important an must be
compensated
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Optical to Electronic Conversion
• Even with perfect fiber stabilization systems, this can be the
performance limit.
• Photo-diodes: Tradeoff between noise and linearity
– Nonlinearity: Charge extraction -> changes bias voltage -> changes
capacitance -> changes phase delay
– High frequency diodes have small area, low capacitance.
– For S-band (3GHz) diode -> 150fs single shot resolution
– For X-band (12 GHz) diode -> 60fs single shot
• For high repetition rate systems (oscillators) this isn’t too bad:
68MHz, 100us TC -> 1fs (ideal)
• For amplifiers, this is a large problem – single shot measurements
are very difficult.
• Can in principal use an optical resonant cavity (etalon) to average
signals. For Q = 100 -> ~10fs
• Other techniques have been developed for fiber based systems:
Rely on electro-optical mixing between laser and RF signal.
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• Conventional Ti:Sapphire laser oscillators
can be locked to ~50fs to a RF reference.
– Several limitations:
Laser
Stabilization
• Phase detection from photodiodes
• Acoustic noise changing the cavity length
• Pump laser fluctuations change the effective
cavity length through nonlinearities
– Laser chirp pulse amplifier system can add
jitter
• Wavelength changes can change the delay
through the compressors (if the wavelength
response of the amplifier isn’t flat)
• Pulse shape changes with laser power from
changes in amplifier saturation
– Very active area of research both at labs
and in industry.
– At least at LCLS this is the limit to stability.
(A. Winter et al).
DESY optical master oscillator
• The pulsed DESY / FLASH system allows
direct optical cross correlation between
the experiment laser and the timing
system!
17
Superconducting vs RT Accelerators
• The beam timing jitter relative to the accelerator
timing reference system is similar for room
temperature and superconducting accelerators:
30-50fs RMS.
Feedback
Gun
RF
Structure
Compressor
RF
Structure
Beam time
pickup
In an superconducing accelerator the beam timing can be measured for each
pulse at the ~MHz beam rate, much faster than the typical 100us energy
storage time in the accelerator cavities
This allows the use of a fast timing feedback to reduce the timing jitter.
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Other Limits
• Ground Motion
– Tidal stretching is 30um / kilometer.
(100fs/km)
– In principal predictable, but in practice
tricky
– Fast ground motion varies with location.
• Measured at SLAC as 10s of nanometers over
14 M separation.
• Needs more study
• SASE process
– Statistical fluctuations give a minimum
timing jitter of [(1/12)rL]1/2 with r the
slippage distance and L the bunch length.
– If only part of the bunch lasers, X-ray time
will not match electron beam time.
• Location of experimental IP (1 um -> 3fs)
• Looks difficult to reach 1fs even if the
individual technical system problems can
be resolved.
Tides observed in LEP frequency
corresponding to ~2x10-8
(L. Araudon et al, CERN SL/94-07)
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Optical / X-ray Cross Correlator
X-rays
Reflected optical beam
measured on array sensor
Laser
GaAs or similar
Tests at SXR (W. Schlotter et al) have demonstrated <60fs RMS (consistent with 0)
single shot X-ray to laser optical timing measurement.
Note that electronic timing will still be needed for “crude” 100fs timing
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Cross Correlator (very preliminary)
(SXR)
M. Beye
B. Schlotter
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W. Schlotter et al. (LCLS)
Cross Correlation
• Various physics is available, but need to find a
way to operate over the full wavelength range
and with femtosecond resolution
– 250-12 KeV
– Operate at few uJ pulse energies (1fs operation)
• Final version should do cross correlation in the
experimental chamber
– 1fs is 300nm, very difficult to control long lengths at
this level.
• Need to find appropriate physics to use for this
– May need an XFEL to study this physics!
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THz Timing Experiments
• THz pump / X-ray probe
– The high peak current beams used for XFELs can also serve as sources of very
intense THz radiation
– This radiation is precisely timed to the electron beam.
– Unfortunately since the beams are ultra-relativistic the THz can never “catch”
the X-rays
THz
FEL
X-rays
THz
FEL
X-rays
THz delayed relative to X-rays. Need to
use 2 bunches, one generates THz,
second X-rays.
For hard X-rays can use crystals
to delay to match the THz
Timing error limited by
mechanical stability
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Plans to test both schemes at SLAC / LCLS.
Future Timing Systems
• “Conventional” systems presently have 50-100fs
rms timing resolution
• Can probably extend to ~10-30fs RMS
• Conventional lasers now produce <25fs pulses,
with ~100as available from XUV lasers.
• XFELS at <10fs, with <1fs likely in the near future.
• For single femtosecond timing will need new
approaches like direct X-ray / optical cross
correlation.
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