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Attosecond Pulse Trains from FEL Amplifiers. Brian McNeil, Neil Thompson, David Dunning & Brian Sheehy. Workshop on X-Ray FEL R&D LBNL October 23 - 25, 2008. Outline. Brief summary of ‘conventional’ cavity mode locked lasers Mode formation & locking in a SASE FEL amplifier
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Brian McNeil, Neil Thompson, David Dunning & Brian Sheehy
Workshop on X-Ray FEL R&D LBNL October 23 - 25, 2008
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.
*E. Goulielmakis et al., Science 320,1614 (2008)
Nw = 50 q =107
s= δ + Nwλ
n = 2
n = 3
Example: λ=12.4nm, N = 12, δ= 551nm s = 700nm# of modes: q = 2.35Axial Modes from an amplifier FEL
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
The analysis demonstrates that the axial modes generated are formally identical to those of a cavity.
The ‘cavity’, however, is significantly shorter, so that only a few modes may fall under the gain bandwidth.
This now allows coupling via a modulation introduced at a relatively large frequency.
Similarities to DOK: V. N. Litvinenko, Nucl. Instrum. Methods Phys. Res., Sect. A 304, 463 (1991).
Interaction of e-beam and laser in modulator simulated
Modulated e-beam propagated in undulator, including SHOTNOISE.
Energy Modulation converted into BEAMFILE
Chicane dispersion applied with 4-dipole chicane (IBFIELD, IMAGL, IDRIL)
Radiation delayed using OFFSETRADF and ALIGNRADF parameters
Repeat Until Saturation
SASESpike FWHM ~ 10fs
Mode-CoupledSpike FWHM ~ 1 fs
Mode-LockedSpike FWHM ~ 400 as
Mode locking mechanism
Typical FEL amplifier schematic: (4GLS XUV-FEL)
FEL amplifiers are broken into a series of undulator sections. Between these sections it is necessary to accommodate phase-shifters, electron focussing elements and beam positioning monitors. Inclusion of electron bunch delay chicanes should not significantly affect this generic design. Note, typically the electron delay chicane will be independent of energy and of total length ~12-15cm. The chicanes are therefore easy to incorporate into an undulator lattice.
Chicane Magnet stability:
Path length change:
E.g. in XUV case 2nd term is factor 10-5 smaller
*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)
P. M. Paul, et al. Science 292, 1689 (2001)
and his the resonant harmonic of the HHG seed
E.g. for operation at the h=65th harmonic of a Ti:Sapphire drive laser with :
HHG amplification mechanism
HHG amplification mechanism with energy modulated beam at multiple of mode spacing
Current computational codes e.g. GenesisSimulated spectrum
For averaged FEL codes the minimum sample rate is:
Freq. range for non-aliasing:
=> Freq. range that e.g. Genesis can simulate properly without aliasing is:
Pulse lengths able to be modelled are limited!
Spike width FWHM = 57as !(~1.4 optical cycles)
450 as: same as Genesis @12.4nm
More modes now, therefore shorter spikes:
“Potential advantages”: relatively easy to implement – modulator undulator and chicane inserts between undulator modules. Shorter pulses (~23as @ 1.5Å) in a train with variable time structure.
“Practical difficulties”: more difficult to identify at this stage – more modelling required.
Opens up possibility of stroboscopic interrogation of matter using light with the spatiotemporal resolution of the atom.