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Synchronization. Graeme Hirst STFC Central Laser Facility Rutherford Appleton Laboratory. s. Synchronization. Graeme Hirst STFC Central Laser Facility Rutherford Appleton Laboratory. Requirements. Users need well-determined delays between multiple output pulses

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  • Graeme Hirst

  • STFC Central Laser Facility

  • Rutherford Appleton Laboratory




  • Graeme Hirst

  • STFC Central Laser Facility

  • Rutherford Appleton Laboratory



  • Users need well-determined delays between multipleoutput pulses

  • Measure-and-bin may bean option if natural jittermatches users’ needs

A synchronisation challenge

A synchronisation challenge

  • Mairesse et al,Attosecond synchronization ofhigh-harmonic soft X-rays,Science 302 1540 (2003)



  • Photoinjector lasers toaccelerator RF

  • Machine diagnosticse.g. EO sensors

  • Users need well-determined delays between multipleoutput pulses

  • Measure-and-bin may bean option if natural jittermatches users’ needs

  • XUV-FEL electronsto seed laser

  • VUV-FEL electronsto intracavity photons

Erl specifics

  • Dtrms = (∫ d)½

  • £

  • System to be synchronised

  • 2p0

  • Actuator

  • Phase noise (dB/Hz)

  • Mixer

  • PLLElectronics

  • £

  • Noise frequency  (Hz)

  • Output

  • RF Ref

ERL specifics

  • ERLs can deliver both ultrashort electron bunches and high bunch rates

  • The availability of ultrashort bunches drives the requirement for femtosecond synchronisation

  • Operation at high bunch rate enables low-jitter synchronisation

  • The PLL roll-off frequency must be set below the Nyquist limit of the spectrum from the mixer

Timing subsystems

Timing subsystems


  • Includes IDs generating photons from electron bunches and also conventional lasers


  • Provides the timing reference across the facility via an associateddistribution system


  • Includes the electron source, the accelerating cavities and their RF drivers and the beam-transport magnets, their psus and supports


  • The effect of PLL control tends to be to suppress phase noise by a factor

  • The system optimisation process should therefore begin by minimising the free-running jitter and drift using passive techniques

  • In practice the use of passive techniques is likely to becost-limited(100fs corresponds to 30mm)

Clock and distribution

  • It is widely accepted that the lowest noise pulsed clocks are based on modelocked fibre lasers phase-locked to microwave synthesisers

  • Dtrms ~10fs (1kHz – Nyquist)

  • Er-doped fibre laser

  • Microwave synthesiser

  • Laser locked to synthesiser

  • Winter et al,High precision laser master oscillators for optical timing distribution systemsin future light sources,FEL06 paper TUPCH029 (2006)

Clock and distribution

  • In principle most applications do not require an absolutely stable clock

  • But clock timing must be stable over the response times of system elements:

  • 100s of nanoseconds (difference in clock distribution path lengths)

  • microseconds (photon lifetime inside cavity FELs)

  • milliseconds (RF field lifetime in high Q SCRF cavities)

Clock and distribution1

  • Using RF-modulated cw laser beams interferometric techniques have stabilised distribution path lengthsto a few fs over many hours.



  • A scheme based on a pulsed laser has delivered ~10fs jitter performance in a working accelerator environment. Electrical RF recovery has been demonstrated at a similarlevel.

Clock and distribution

  • Advantages of optical timing signals include the option of direct laser seeding and also the possibility of very high resolution optical timing measurement

  • Distribution systems based on length-stabilised telecoms fibre products are being developed for several facilities*

  • *See e.g. Wilcox et al,Fermi timing and synchronization system,LBNL report LBNL-61165

Photon generation ids

Photon generation - IDs

  • Timing follows the electron bunches


  • As above, but the temporal profile varies with noise and also with the peak electron current

  • Bending magnets and undulators

  • Cavity FELs

  • Again these follow the electron bunches except that HF noise is filtered out by the optical cavity lifetime

  • Seeded FELs

  • Timing controlled by the seed sourceso good synchronisation required

Photon generation lasers

  • Commercial fibre lasers offer ~70fs synchronisation and the performance of ultra-stable home-built ones reaches down to 10fs

Photon generation - lasers

170 fs cross-correlation  <75 fs RMS jitter over 2 minute averaging time

  • Commercial free-space lasers are now capable of sub-100fs synchronisation* and custom systems can reach 20fs†

*Unpublished data for Micra/Synchro-Lock AP courtesy of B Wheelock, Coherent Inc†D J Jones et al, Rev Sci Instr 73 (8) 2843 (2002)

Photon transport

Photon transport

  • Propagation is generally in vacuum, so timing depends on positional stability of the mirrors

  • Floor-mounted components can, with no special stabilisation, be used for visible interferometry over >10m, provided simple passive steps are taken to avoid vibration. This corresponds to <300nm movement over 10m.

  • Issues include floor stability (must either be sufficiently thick or well-bonded to bedrock) and control of vibration transmitted through vacuum envelope

  • Scaling suggests3mm over 100m should be practical

  • Interferometrycan be used tomonitor slowmovements

  • Dynamic heatloading mayneed to becompensated


Isolated vacuumenvelope

Isolating bellows


Stable pillarfrom floor

Electron generation and transport

fRF = 1.3 GHzC = 100Sin = 100 fsR56 = 0.15 msA/A = 10-4sf = 0.01°

1 fs50 fs21 fs

  • To check this model’s validity it was extended to several elements, representing an early 4GLS XUV-FEL design

  • The equations were solved without normalisation to mean time and energy, revealing the full effects of parameter changes on the bunch timing

  • The remaining limitations are still numerous and serious:

St2 =54 fs

  • The electron distributions in energy and time at the gun are unrealistic

  • Beam-disruptive effects (wakefield, CSR etc) are not included

  • Elements’ positional instabilities and magnet psu noise are not included

  • The use of mean bunch times is an oversimplification,given the dependence of FEL gainon peak current

Electron generation and transport

St2 = (1/C)2.Sin2 + (R56.sA/c.A)2 + (1-1/C)2.(sf/wRF)2

InjectionRF amplitudeRF phasejitternoisenoise

  • The simplest model of electron bunch acceleration and compression reveals the difficulty of delivering electrons with well-controlled arrival times:

Electron generation and transport1

Electron generation and transport

  • RF Gun: 4MeV output, uncorrelated Gaussians in E (0.3% s) and t (5ps s)

  • Injector linac: 185MeV, -7deg at 1.3GHz

  • 3w cavity: 23MeV, 177deg

  • Merge: R56 = 0.25m, T566/R56 = -1.5

  • Main linac: 595MeV, 5deg

  • Spreader and arc: R56s equal and opposite (no net effect)

  • Final linac: 50MeV, -90deg

  • Final compressor: R56 = 0.31m, T566/R56 = -1.5

Electron generation and transport2






Electron generation and transport

  • Energy (MeV)

  • sE/E = 10-3, st = 125fs

  • Time (ps)

Electron bunch arrival time sensitivities

Electron bunch arrival time sensitivities

  • The quadrature sum of phase and amplitudejitter is ~150 fs

Electron generation and transport3

Electron generation and transport

  • Reoptimisation of the machine’s design and/or working point

  • Arrival time sensitivities might be traded off against final energy spread and peak gradient, via reduced R56s and accelerating further from peak

  • With an electron timing jitter of ~150 fs fewer than one in four of the 4GLS XUV-FEL shots would be properly seeded

  • Possible solutions might include:

  • Using well-synchronised conventional lasers to adjust the electron bunch timing (cf bunch slicing in storage rings)

  • Improving the control of the cavity RF amplitudes and phases

  • High-resolution timing sensors could be used as inputs to the RF feedback control systems

Electron generation and transport4

Electron generation and transport

  • Hacker et al,Large horizontal aperture BPMfor use in dispersive sectionsof magnetic chicanes,EPAC 2006, paper TUPCH022 (2006)

  • Electron bunches are sensed using stripline field monitors

  • High bandwidth (12GHz) fibre-coupled electro-optic modulators convert the electrical signals to timing signals using thedistributed optical clock

  • 30fs timing resolution has been demonstratedand 10fs may be practical

Further reading

Further reading



  • Exploiting the short electron bunches available from ERLs will require synchronisation systems working at the fs level

  • Clocks and distribution systems, laser synchronisation, optical-to-RF recovery andfree-space photon transport have all been demonstrated with ~10fs performance

  • Electron arrival time stability of ~10fs has yet to be demonstrated but high-resolution sensors coupled to PLL controllers appear very promising

  • Tues May 15thTi:S laserlocked to SRS

  • Thurs May 17thHome-built 1550nm fibre laser lockedto RF clock

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