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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 fs 50 fs 21 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

Injection RF amplitude RF phase jitter noise noise

  • 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