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Femtosecond Pump / Probe Operation and Plans at the LCLS

Josef Frisch for the LCLS Commissioning team

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Ultra-Fast ScienceSome 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)

Measurement resolution is set by the length of the pump / probe pulses AND the accuracy of the time delay between the pump and probe.

H. Edgerton

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Short Bunch Operation at LCLSPhase = +1 deg

Phase =+0.5 deg

Phase = 0 deg

Phase = -0.5 deg

Phase = -1 deg

ΔT=5.0fs

ΔT=2.3fs

ΔT=1.1fs

ΔT=1.9fs

ΔT=4.2fs we typically operate here

Genesis Simulation for over compression: 5fs FWHM

Low charge (20pC) operating mode for very short pulses

No direct pulse length measurement available, but believed to be < 5fs FWHM

P. Emma, M. Cornacchia, K. Bane, Z. Huang, H. Schlarb (DESY), G. Stupakov, D. Walz

Narrow or Double X-Ray Pulses from a Slotted Foil

PRL 92, 074801 (2004).

time (fs)

Pow

er (G

W)

0

10

5

0-150 fs

2 fs

0-6 mm

0.25 mm

pulses not coherent

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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 Quantemoptik / ATLAS

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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 RequirementsPump Laser

X-ray beam

X-rays to detector

System evolves from pump to probe time

Ideally would scan time difference

Generally OK to let jitter vary the timing and measure shot-to-shot

Timing jitter relative to an external clock isn’t important

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Sources of Timing Jitter

Gun RF off crest

Laser

RF RF

Bunch Compressor

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

Undulator Beam time pickup E-beam

dump

Femtosecond Laser

Laser Amplifier

ExperimentX-rays

Stabilized transmitter

Stabilized Receiver

~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

Master Source

High power

RF

RF in compressor sets beam time

~ 1km

Few fs jitter in a few ms

~50fs RMS jitter shot to shot

10ps drift over hours

Phase Detect

Phase cavity

Accelerator FEL

Phase Shift

FeedbackLaser

Experiment

Experiment data corrected offline with phase cavity data report 50-100fs stability

10fs jitter, 50fs stability

~50fs jitter

Stabilized link ~20fs stability

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Beam arrival time cavity (LCLS)Similar to a cavity BPM but use the monopole modePhase 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

RMS difference between cavities ~12 femtoseconds RMS at 250pC, 25 femtoseconds at 20pC. Drift is ~100 femtoseconds p-p over 1 day.

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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

Electric field from bunch

Electro-optical intensity modulation

Bunch fields

Output intensity depends on relative timing of laser plulse and E-beam

(Several versions, simplified concept shown)

6 femtosecond timing noise published

(Believe ~3 fs achieved)

F. Loehl et alDESY/FLASH

Short pulse laser

Detector

Free space or fiber-optic

Allows direct conversion from beam timing to optical signal: significant advantage for some types of timing systems

Long Distance Timing Transmission

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mirror

Transmitter

Timing Signal

Compare forward and reflected signals

Adjust Delay

Feedback

Use fibers:Low loss as high transmitter frequencyGood directional couplersLow cost

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 feedbackPulses detected at the receiver end are used for timing

Carrier scheme (LBNL, used at LCLS)Frequency stabilized laser used in an interferometerInterferometer determines fiber lengthControl fiber length with feedback (feed forward in this case).

Excellent resolution – based on optical wavelength

Difference between phase and group velocity is important an must be compensated

Pulses allow direct locking to experiment laser

Both systems work at <20fs over 100M fibers

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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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Laser Stabilization• Conventional Ti:Sapphire laser oscillators can

be locked to ~50fs to a RF reference.– Several limitations:

• 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.• The pulsed DESY / FLASH system allows

direct optical cross correlation between the experiment laser and the timing system!

DESY optical master oscillator

(A. Winter et al).

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.

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Feedback

Gun RF Structure

RF Structure

CompressorBeam 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)

Optical / X-ray Cross Correlator

GaAs or similar

Laser

X-rays Reflected optical beam measured on array sensor

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 20

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Cross Correlator (very preliminary)

W. Schlotter et al. (LCLS)

(SXR)M. BeyeB. Schlotter

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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!

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

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FEL X-rays

THz THz delayed relative to X-rays. Need to use 2 bunches, one generates THz, second X-rays.

FEL X-rays

THzFor hard X-rays can use crystals to delay to match the THz

Timing error limited by mechanical stability

Plans to test both schemes at SLAC / LCLS.

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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.