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Synchronization

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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Synchronization

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  1. Synchronization • Graeme Hirst • STFC Central Laser Facility • Rutherford Appleton Laboratory

  2. s Synchronization • Graeme Hirst • STFC Central Laser Facility • Rutherford Appleton Laboratory

  3. Requirements • Users need well-determined delays between multipleoutput pulses • Measure-and-bin may bean option if natural jittermatches users’ needs

  4. A synchronisation challenge • Mairesse et al,Attosecond synchronization ofhigh-harmonic soft X-rays,Science 302 1540 (2003)

  5. Requirements • 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

  6. 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

  7. Timing subsystems • PHOTON GENERATION AND TRANSPORT • Includes IDs generating photons from electron bunches and also conventional lasers • MASTER CLOCK • Provides the timing reference across the facility via an associateddistribution system • ELECTRON GENERATION AND TRANSPORT • Includes the electron source, the accelerating cavities and their RF drivers and the beam-transport magnets, their psus and supports • PASSIVE STABILITY • 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)

  8. 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)

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

  10. Photon generation - IDs • Timing follows the electron bunches • SASE FELs • 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

  11. 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)

  12. 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 Mirror Isolated vacuumenvelope Isolating bellows Braces Stable pillarfrom floor

  13. 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:

  14. 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

  15. -2s -1s m 1s 2s Electron generation and transport • Energy (MeV) • sE/E = 10-3, st = 125fs • Time (ps)

  16. Electron bunch arrival time sensitivities • The quadrature sum of phase and amplitudejitter is ~150 fs

  17. 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

  18. 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

  19. Further reading http://repositories.cdlib.org/lbnl/LBNL-61165/ http://www.4gls.ac.uk/documents.htm http://xfel.desy.de/tdr/index_eng.html

  20. Conclusions • 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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