Tuning and characterization of short electron
and FEL radiation pulses at FLASH
during shifts 19(a)-21(m).01.2011
E. Schneidmiller and M. Yurkov (SASE & MCP)
C. Behrens, W. Decking, H. Delsim , T. Limberg, R. Kammering (rf & LOLA)
N. Guerassimova and R. Treusch (PGM & GMD)
Characterization techniques
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• Electron pulse: LOLA (pulse shape), toroids (charge), pyro
detectors (signal related to bunch shape and charge).
• Photon pulse: Pulse energy (GMD and MCP), measurements
of statistical fluctuations (MCP), spectral measurements
(PGM).
• A lot of data has been recorded. Here we present only brief online analysis and preliminary conclusions.
DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov
Statistical fluctuation method
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Ackermann et al.,
NaturePhoton.1(2007)336
DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov
Statistical fluctuation method
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• Fluctuations / instability of the machine / electron beam parameters also
contribute to the fluctuations of the radiation pulse energy.
• Then one needs a lot of statistics to eliminate those contributions (gating);
measurement may take from few minutes to one hour; data analysis takes a
while and needs experts
• At saturation the pulse is usually longer than in exponential gain regime; so,
the method gives a lower estimate for pulse duration at saturation
• Very useful in combination with other methods (single-shot spectra etc.) and
start-to-end-simulations
• Method was used at TTF1 and FLASH, recently also at LCLS
DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov
Single-short spectra
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• In an ideal case (monochromatic electron beam) and linear mode of SASE
operation average number of spikes in spectrum is nearly the same as the
number of spikes (wavepackets) in the time domain.
• Width of the spike in the spectrum is inversely proportional to the pulse
length.
• Thus, qualitative analysis of spectra measured in the linear regime may give
a hint for qualitative estimation of pulse length and number of modes in the
radiation pulse.
DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov
Results: pulse energy and number of modes
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• We tuned SASE @ 14 nm to maximum radiation energy with different charges 150
pC, 250 pC, and 500 pC.
• Pulse energy level at the full undulator length was:
•
150 pC
25-35 uJ
250 pC
35 uJ
500 pC
>200 uJ
• Then SASE process has been suppressed in the undulator modules 5 and 6 (by
means of orbit kick) in order to deal with linear regime of SASE FEL operation.
• Characterizations for the number of modes has been performed with statistical
measurements using MCP07 detector. Measured number of modes in the linear
regime:
DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov
Results: number of modes and pulse duration
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• Measurements of fluctuations in the end of the linear regime gives us an estimate for
the number of the radiation modes (spikes) M in the pulse.
• This allows to estimate radiation pulse length as T_rad ~ M x coherence time. We
estimate coherence time as 5 fs (similar to our estimate in 2007 Nature Photonics
paper).
• Pulse lengthening in the nonlinear regime is defined by two effects. The first one is
slippage estimated to be about 10 fs when saturation occurs in the middle of the 5th
undulator module. The second effect relates to the lasing to saturation of the lowcurrent tails of the bunch. Pulse lengthening in this case depends on the shape of the
tails. For gaussian-like bunch shape we estimate pulse lengthening by 30% to 50%.
Thus, we have the following results:
150 pC
250 pC
500 pC
sigma = 60%
sigma = 59%
sigma = 29%
M = 2.8
M=3
M = 12
T_rad ~ 15 fs (+10 fs + 7 fs)
T_rad ~ 15 fs (+10 fs + 7 fs)
T_rad ~ 60 fs (+10 fs + 30 fs)
Here numbers in brackets refer to the pulse lengthening in the nonlinear regime.
Our experience tells us that practical accuracy of this estimate is about factor of 2.
DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov
Spectra @ 150 pC, linear regime
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• Statistical measurements agree qualitatively with spectral measurements .
• 150 pC
sigma = 60%
DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov
M = 2.8
T_rad ~ 15 fs (+10 fs + 7 fs)
LOLA images @ 150 pC
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DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov
Spectra @ 250 pC, linear regime
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• 250 pC
sigma = 59 %
M=3
T_rad ~ 15 fs (+10 fs + 7 fs)
• Not a good agreement. Drift of the machine parameters? – There was a big time
interval between spectral and statistical measurements.
DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov
LOLA images @ 250 pC
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DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov
Spectra @500 pC, linear regime
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• 500 pC
sigma = 29%
DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov
M = 12
T_rad ~ 60 fs (+10 fs + 30 fs)
LOLA images @ 500 pC
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DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov
How to tune SASE with short pulse
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I. Simplified procedure (1 shift):
a) Choose small charge (between 100 and 200 pC).
b) Tune gun+laser (incl. iris), rf parameters, optics and orbit for maximum pulse energy.
c) With high probability the pulses are short. If users complain, take single-shot spectra.
If estimated pulse length is too big, go to
II. More thorough procedure (2 shifts):
a) Choose small charge (between 100 and 200 pC).
b) Set up compression (if possible with the help of experts), check phase space with LOLA.
Important indication is energy chirp due to space charge (opposite to that from RF).
c) Tune SASE as in I.b).
d) Do LOLA measurement.
e) Take single-shot spectra (optionally also statistics in linear regime).
f) Iterate if necessary.
At least, our trial attempts resulted in short pulse length. Characterization of electron pulse shape
with LOLA resulted in a confined and nearly symmetric shape. Final result proving ultra-short
pulse duration is spectral measurement showing small number of spikes.
DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov
Summary
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• Tuning of short pulses of about coherence length is possible with linearized
compression scheme.
• For radiation wavelength of 14 nm short pulse mode of operation is achieved in the
range of bunch charges 150 pC – 250 pC. Even without thorough optimization
saturation pulse energy is 25 – 35 uJ – similar to that reachable with the roll-off
compression scheme during past years.
• Stability of the SASE level in the saturation was nearly the same as during typical
user runs with the roll-off compression scheme (about 35% at 150 pC). It can be
improved significantly (down to 25%) by stabilization of the bunch charge – it
fluctuated a lot during our measurements, about 7 % rms at 150 pC.
• Readings of pyro detectors were too noisy at small charge. Dedicated tuning of pyro
detectors for small charges will help a lot.
• We should find more dedicated time to pass procedure for the short-pulse-SASE
tuning and characterization jointly with accelerator and photon experts for different
wavelengths (especially long). Goal: tuning dedicated regimes for users
requesting short pulses at specific wavelengths.
DESY, Beam Dynamics Meeting, January 24, 2011
E.A. Schneidmiller, M.V. Yurkov