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Neil Thompson, David Dunning STFC Daresbury Laboratory Brian M c Neil University of Strathclyde

Attosecond Pulse Trains from FEL Amplifiers. Neil Thompson, David Dunning STFC Daresbury Laboratory Brian M c Neil University of Strathclyde Brian Sheehy Sheehy Scientific Consulting. The Basic Idea.

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Neil Thompson, David Dunning STFC Daresbury Laboratory Brian M c Neil University of Strathclyde

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  1. Attosecond Pulse Trains from FEL Amplifiers Neil Thompson, David DunningSTFC Daresbury Laboratory Brian McNeilUniversity of Strathclyde Brian SheehySheehy Scientific Consulting

  2. The Basic Idea..... To borrow modelocking concepts from conventional cavity lasers and apply them to amplifier FELs to generate ultrashort FEL pulses

  3. Outline • Brief summary of ‘conventional’ cavity mode locked lasers • generation and locking of modes • why modelocking is important • Mode generation & locking in a SASE FEL amplifier • 3D & 1D simulations in XUV & X-Ray • Comparison with other attosecond FEL schemes • Application of technique to HHG Amplification in FELs • Summary

  4. Brief summary of ‘conventional’ cavity mode locked lasers

  5. n = 1 ω n=1 n = 2 s s n >> 2 * Where cavity modes come from* perimeter = s s “It is the fixed time delay or time shift between successive round trips that gives the axial mode character to a laser output signal” - Siegman • Envelope is atomic linewidth: gain bandwidth of medium • Mode spacing ∆ωs=2πc/s • No of modes q = bandwidth/mode spacing

  6. Sidebands How cavity modes are locked • The modes are locked by establishing a fixed phase relationship between the axial modes. • Application of modulation (e.g. cavity length modulation, gain modulation, frequency modulation) causes axial modes to develop sidebands. • If modulation frequency is at mode spacing Δωs sidebands overlap neighbouring modes which then couple and phase lock. • The output consists of one dominant repeated short pulse.

  7. Why modelock? >> Ultrashort pulse generation! 1963: mode-locking discovered This history of short pulse generation in ‘conventional’ lasers has developed from the first mode-locked lasers, through dye-lasers, Ti:Sapphire and now to High Harmonic Generation in gas jets. Since 1964, pulse durations have been reduced by ~ 5 orders of magnitude to ~130 as and very recently* to ~80 as. 2000: new technology: HHG 1986: 6 fs plateau *E. Goulielmakis et al., Science 320,1614 (2008)

  8. Mode formation & locking in a SASE FEL amplifier* *

  9. Electron delayδ s= δ + Nwλ Nw period undulator n=1 n = 2 n =1 n = 3 s s ω Generating modes in an amplifier FEL • Axial modes are synthesised by repeatedly delaying electron bunch in magnetic chicanes between undulator modules • Produces a sequence of time-shifted copies of radiation from one module, and hence axial modes • Modes locked by modulating the input electron beam energy at the mode spacing The spectrum is the same as a ring cavity of length s. Have synthesized a ring cavity of length equal to the total slippage between modules

  10. simulated total delay delay in chicane slippage in one undulator module number of modules Example: mode spacing Modal structure of Spontaneous Emission Starting from 1D wave equation derive spontaneous emission spectrum for N modules:

  11. Sidebands Locking the generated modes

  12. Simulations (3D) in XUV & X-Ray

  13. XUV Parameters

  14. Spike FWHM ~ 10fs 3D Simulation Results: SASE XUV-FEL @ 12.4nm

  15. Spike FWHM ~ 1 fs Spike FWHM ~ 10fs Mode-Coupled SASE XUV-FEL @ 12.4nm

  16. Spike FWHM ~ 1 fs Spike FWHM ~ 400 as From conventional cavity analysis: Mode-Locked SASE XUV-FEL @ 12.4nm

  17. XUV Output Comparison SASESpike FWHM ~ 10s Mode-CoupledSpike FWHM ~ 1 fs Mode-LockedSpike FWHM ~ 400 as

  18. X-ray Parameters

  19. Spike FWHM ~ 23 as Mode-locked X-ray SASE FEL amplifier

  20. Modelocked Amplifier FEL: Animation

  21. Stability

  22. All these schemes select a small slice of beam and then arrange for only this slice to lase. FEL pulse length is then dependent on slippage, and typically giving ~ 250 as for LCLS parameters Comparison to other attosecond FEL schemes Table courtesy Riccardo Bartolini Short pulses generated via optical synthesis

  23. Application of technique to HHG Amplification in FELs

  24. Amplified HHG without modes*: attosecond structure washed out HHG *B W J McNeil, J A Clarke, D J Dunning, G J Hirst, H L Owen, N R Thompson, B Sheehyand P H Williams, Proceedings FEL 2006 Also - New Journal of Physics 9, 82 (2007)

  25. Input HHG Seed Amplified HHG Seed Amplified HHG with modes: attosecond structure retained! Proceedings FEL 2008

  26. Conclusions • Application of mode-locking techniques, stolen from ‘conventional’ cavity lasers, indicate possibility of generating attosecond pulse trains from FEL amplifiers • Method tested using full 3D simulation code used in design of e.g. XFEL and LCLS: • In XUV (@12.4nm) FWHM of each pulse ~ 400 as • In X-Ray (@0.15nm) FWHM of each pulse ~ 23 as @12.4nm • In comparison with other attosecond FEL proposals pulse widths about an order of magnitude shorter - BUT IN TRAIN • Spacing within train easily adjustable by varying field strength in chicanes • Method (without energy modulation) can also be employed to directly amplify HHG pulses while retaining their attosecond structure Opens up possibility of stroboscopic interrogation of matter using light with the spatiotemporal resolution of the atom.

  27. Thank You

  28. 1D enhanced frequency range model @ 12.4nm Spike width FWHM = 57as !(~1.4 optical cycles) 450 as: same as Genesis @12.4nm More modes now, therefore shorter spikes:

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