Simulations for the HXRSS experiment
with the 40 pC beam
S. Spampinati, J.Wu, T.Raubenhaimer
Future light source
March, 2012
Presentation aims
Comments on simulations vs experiments
Derive model of pulse evolution in SASE and seeded undulator
from experimental observation
Benchmark with start to end simulations
Try ideal simulations model to match with experiment
Try to understand seeding efficiency and match with simulations
Focus on
Pulse characteristic in the SASE undulator
Pulse characteristic in the seeded undulator
Seed used power
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Comments on simulations vs experiments
Start to end simulation are a guidance
Some beam parameter (current beam profile) can be measured, even
if indirectly, to confirm simulations
Only few accelerator configurations and beams can be simulated
completely
Accelerator and beam change from shift to shift and from shot to shot
Ideal models can be used to catch physics
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 exists
The beam profile change from shot to shot
24/1/2012
R. Iverson, H. Loos, Z. Huang, H.-D. Nuhn, Y. Ding, J. Wu, S. Spampinati
T.O. Raubenheimer,
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 fs
Start 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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Evolution of the pulse in the seeded undulator
Seed energy
Letargy
Energy along seeded undulator (active length on x)
Fitting the data with exponential curve
gain length can be short like 3.5 m
3.5 lethargy length
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Amplified energy is around 1 nJ considering interaction from the start
If the seed peak power is of the order of MW the FEL pulse length
is <= 1fs (is gain length measurement in the seeded part correct?)
Evolution of the pulse in the seeded undulator (continue)
experimental spectral relative bandwidth FWHM 8-5*10^-5
 (FWHM PULSE DURATION)* (FWHM PULSE DURATION)=0.44
2.8-4*fs FWHM pulse duration for a Gaussian Fourier transform pulse
Lasing 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 first
undulator
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Seeded undulator start to end simulation
Different colors for
different tapering
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
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Simulation with high current beam
Wakes in the undulator no chirp
 Gain length 5 m
Lasing on all the beam
Same detuning of the second undulator for narrow spectrum
maximum energy
Seems very difficult to have gain length shorter than 5m
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
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END