Femtosecond Pump / Probe Operation and Plans at the LCLS Josef Frisch for the LCLS Commissioning team

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) 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

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 Genesis Simulation for over compression: 5fs FWHM ΔT=1.1fs ΔT=1.9fs ΔT=4.2fs we typically operate here

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 0-6 mm 0.25 mm pulses not coherent time (fs) Power (GW) 10 5 0-150 fs 2 fs

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

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

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.

Experiment Requirements Pump 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

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

Conventional Timing System Stabilized transmitter Stabilized Receiver Femtosecond Laser Laser Amplifier X-rays Undulator Beam time pickup Experiment E-beam dump ~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.

Timing Jitter in LCLS ~ 1km Master Source Phase Shift Stabilized link ~20fs stability 10ps drift over hours 10fs jitter, 50fs stability Few fs jitter in a few ms High power RF Phase Detect Laser ~50fs RMS jitter shot to shot ~50fs jitter Feedback RF in compressor sets beam time Accelerator Phase cavity FEL Experiment Experiment data corrected offline with phase cavity data report 50-100fs stability

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 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.

EO Beam Time Measurement (Several versions, simplified concept shown) Short pulse laser Free space or fiber-optic Detector Output intensity depends on relative timing of laser plulse and E-beam Bunch fields F. Loehl et al DESY/FLASH Electric field from bunch 6 femtosecond timing noise published (Believe ~3 fs achieved) Electro-optical intensity modulation Allows direct conversion from beam timing to optical signal: significant advantage for some types of timing systems

Long Distance Timing Transmission Adjust Delay Transmitter Use fibers: Low loss as high transmitter frequency Good directional couplers Low cost mirror Timing Signal Feedback 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 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 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

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.

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! (A. Winter et al). DESY optical master oscillator

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 Compressor RF Structure RF Structure Beam time pickup Gun 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.

Other Limits Ground Motion SASE process 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 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

Cross Correlator (very preliminary) (SXR) M. Beye B. Schlotter 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!

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 delayed relative to X-rays. Need to use 2 bunches, one generates THz, second X-rays. THz X-rays FEL For hard X-rays can use crystals to delay to match the THz Timing error limited by mechanical stability THz X-rays FEL 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.