Neil Thompson, David Dunning STFC Daresbury Laboratory

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

• To borrow modelocking concepts from conventional
  cavity lasers and apply them to amplifier FELs to
  generate ultrashort FEL pulses

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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

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Brief summary of conventional cavity mode
locked lasers
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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 ??s2pc/s
• No of modes q bandwidth/mode spacing

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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 ??s
  sidebands overlap neighbouring modes which then
  couple and phase lock.
• The output consists of one dominant repeated
  short pulse.

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Why modelock? gtgt 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,
TiSapphire 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)
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Mode formation locking in a SASE FEL amplifier

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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
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Modal structure of Spontaneous Emission
Starting from 1D wave equation derive spontaneous
emission spectrum for N modules
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Locking the generated modes
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Simulations (3D) in XUV X-Ray
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XUV Parameters
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3D Simulation Results SASE XUV-FEL _at_ 12.4nm
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Mode-Coupled SASE XUV-FEL _at_ 12.4nm
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Mode-Locked SASE XUV-FEL _at_ 12.4nm
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XUV Output Comparison
SASESpike FWHM 10s
Mode-CoupledSpike FWHM 1 fs
Mode-LockedSpike FWHM 400 as
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X-ray Parameters
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Mode-locked X-ray SASE FEL amplifier
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Modelocked Amplifier FEL Animation
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Stability
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Comparison to other attosecond FEL schemes
Table courtesy Riccardo Bartolini
Short pulses generated via optical synthesis
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Application of technique to HHG Amplification in
FELs
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Amplified HHG without modes attosecond
structure washed out
HHG
Proceedings FEL 2006 Also - New Journal of
Physics 9, 82 (2007)
B W J McNeil, J A Clarke, D J Dunning, G J
Hirst, H L Owen, N R Thompson, B Sheehy and P H
Williams,
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Amplified HHG with modes attosecond structure
retained!
Proceedings FEL 2008
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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 (_at_12.4nm) FWHM of each pulse 400 as
• In X-Ray (_at_0.15nm) FWHM of each pulse 23 as
  _at_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.
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Thank You
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1D enhanced frequency range model _at_ 12.4nm
Spike width FWHM 57as !(1.4 optical cycles)
450 as same as Genesis _at_12.4nm
More modes now, therefore shorter spikes