S. Spampinati, J.Wu, T.RaubenhaimerS. Spampinati, J.Wu, T.Raubenhaimer

Future light source Future light source

MarchMarch, 2012, 2012

Simulations for the HXRSS experiment Simulations for the HXRSS experiment with the 40 pC beamwith the 40 pC beam

Focus on

Pulse characteristic in the SASE undulatorPulse characteristic in the seeded undulatorSeed used power

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Presentation aimsComments on simulations vs experimentsDerive model of pulse evolution in SASE and seeded undulatorfrom experimental observationBenchmark with start to end simulationsTry ideal simulations model to match with experimentTry to understand seeding efficiency and match with simulations

Start to end simulation are a guidanceSome beam parameter (current beam profile) can be measured, even if indirectly, to confirm simulationsOnly few accelerator configurations and beams can be simulated completely Accelerator and beam change from shift to shift and from shot to shotIdeal models can be used to catch physics

Comments on simulations vs experiments

Measurements of current of fs beam profile beam energy spectrometer (Z.Huang, K.Bane, Y.Ding, and P Emma, Phys. Rev. ST Accel. Beams 13, 092801 (2010) )The beam measured is not exactly the beam in the undulator but 1-1 correspondence existsThe beam profile change from shot to shot

R. Iverson, H. Loos, Z. Huang, H.-D. Nuhn, Y. Ding, J. Wu, S. Spampinati T.O. Raubenheimer,

24/1/2012

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Pulse in the SASE undulator Measured quantity: 2.5m gain length and ≈20 µJ at crystal Short length with low energy: short pulse length Back extrapolation of measured power: Considering a shot noise power of some KW the pulse length should be shorter then fsStart to end simulation confirm very short pulse formation 3.7 m gain length, energy ≈5µJ. Than we try simulations with a beam more bright

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SASE undulator (U 3-15) with a more brighter beam

Beam parameter: energy spread 4 MeV, emittance 0.40.Energy at crystal ~20µJ in very narrow pulses, gain length 3.1 m

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Seed energy

Letargy

Evolution of the pulse in the seeded undulator

Energy along seeded undulator (active length on x)Fitting the data with exponential curvegain length can be short like 3.5 m3.5 lethargy lengthAmplified energy is around 1 nJ considering interaction from the startIf the seed peak power is of the order of MW the FEL pulse lengthis <= 1fs (is gain length measurement in the seeded part correct?)

experimental spectral relative bandwidth FWHM 8-5*10^-5 (FWHM PULSE DURATION)* (FWHM PULSE DURATION)=0.442.8-4*fs FWHM pulse duration for a Gaussian Fourier transform pulseLasing from a small part of the beam or chirp on the FEL pulse. FEL pulse longer then 2.8 fs Then considering a gain length of 3.5 m the seed power is more like 0.2-0.3 MW. This level of power prevent saturation even for such short gain length. FEL pulse in the second undulator is longer than the SASE in the firstundulator

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Evolution of the pulse in the seeded undulator (continue)

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Seeded undulator start to end simulation

For the optimum detuning of the second undulator the core starts to contribute to the pulse energy and this produce the shorter gain length Gain length 5 m Starting from 2MW seed power production of ≈250µJ

Different colors for different tapering

Wakes in the undulator no chirp Gain length 5 mLasing on all the beam Same detuning of the second undulator for narrow spectrum maximum energySeems very difficult to have gain length shorter than 5m

Simulation with high current beam

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Lasing from horns in the SASE undulators. Then the core starts to lase even if the horns still dominate

High current in the horn reduces gain length in the SASE undulator. It seems, from the experiments, that the Horns are very bright

Shorter Gain length in the experiment shorter than the simulated one

The shorter seed gain length observed in the experiments (<5m) requires a seed power below 0.2 MW

Gain length ≈ 5m is more compatible with MW level seed power

END