Transcript European XFEL and coherence properties of the radiation from x

SLAC Accelerator Seminar, May 20, 2010
European XFEL and coherence properties of the
radiation from x-ray free electron lasers
E.A. Schneidmiller, M.V. Yurkov
DESY, Hamburg
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Part 1: An overview of current status of the European XFEL.
Part 2: Properties of the radiation from x-ray free electron lasers:
Statistical properties.
Longitudinal and transverse coherence.
Higher harmonics.
European x-ray free electron laser
Part 1:
An overview of current status of the European XFEL.
European x-ray free electron laser
Technical challenges of the European XFEL:
• Superconducting accelerator with high average power of about 600 kW  potential
for high average power/brillance of the radiation.
• Multiple undulator beamlines and independent operation for several user
stations/instruments  need in variable-gap undulators.
• … and many more have been already described in previous talks by our colleagues.
Here we briefly highlight recent progress of the project in general.
European x-ray free electron laser
• The European X-Ray Free-Electron Laser Facility
GmbH, as the company was officially named upon
its registration into the Commercial Register of the
Hamburg District Court on 08 October 2009. The
new company with limited liability under German
law is listed as number HRB 111165. Company has
been created by DESY as unique shareholder.
• On 30 November 2009, representatives from
Denmark, Germany, Greece, Hungary, Italy,
Poland, Russia, the Slovak Republic, Sweden and
Switzerland signed the “Convention concerning the
Construction and Operation of a European X-ray
Free-Electron Laser Facility”. France joined
convention in February, 2010. Spain and China are
considering the document.
• The costs for the construction and commissioning
of the new X-ray laser facility amount to 1082
million Euro (price levels of 2005). As the host
country, Germany (the federal government,
Hamburg and Schleswig-Holstein) covers 54
percent of these costs. Russia bears 23 percent
and the other international partners between 1 and
3.5 percent each.
European x-ray free electron laser
Robert Feidenhans'l, Professor at the University of
Copenhagen is the Chairman of the Council of the
European XFEL GmbH starting February 23rd, 2010.
Council (Shareholders)
Chair: R. Feidenhans'l
European x-ray free electron laser
• XFEL work packages
European x-ray free electron laser
• In-kind contributions (not complete – just to get an impression):
• France: power couplers, cavity strings assembly, cryomodules assembly,
BPMs system.
• Italy: Nb cavities, Cold masses for cryomodules, 3.9 GHz accelerator
cryomodule.
• Poland: HOM couplers, cold vacuum, warm vacuum, cryogenics for AMTF,
RF for AMTF, AMTF operation, cold magnets.
• Russia: cryogenics, beam dump, beam diagnostics, magnetic elements
(dipoles quadrupoles, etc), connector cables for pulse transformers, cold
and warm vacuum, test benches for cryomodules at AMTF, transverse
deflecting structure.
• Spain: undulators for SASE3, cold magnets, power supplies.
• Sweden: heat load investigations on diffractive optics, laser heater system
for injector, fiducial marking of undulator quadrupoles.
• Switzerland: BPMs, intra-bunchtrain feedback system.
European x-ray free electron laser
• String and module assembly at Saclay
European x-ray free electron laser
• Accelerator module test facility in Hamburg.
European x-ray free electron laser
WEB cameras allow to follow construction progress on-line:
http://www.xfel.eu/project/webcams/
European x-ray free electron laser
• Two tunnel boring machines will be required to
construct the 5777 metres of tunnel for the European
XFEL. The largest of the two machines, which has an
external diameter of 6.17 metres, passed the factory
acceptance test in the first week of February.
• April 29, 2010: The five big parts of the tunnel boring
machine arrived to Hamburg port. The crane lifts up
the 51-tonne cutting wheel that will later excavate the
soil.
• June 30, 2010: First tunnel and borer christening
ceremony.
European x-ray free electron laser
• 2014: First electron beam.
• 2015: Start user operation.
• Start-up version:
Scientific instruments
SASE 2
U2
tunable, planar
0.1 – 0.4 nm
electrons
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e
U1
17.5 GeV
SASE 1
tunable, planar
0.1 nm
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SASE 3
tunable, planar
0.4 – 1.6 nm
e
Experiments
European x-ray free electron laser
European x-ray free electron laser
• Experience of LCLS and latest PITZ results are analyzed at DESY/XFEL.
• XFEL user community is forming. Series of instrumentation workshop
formulated extended user requests (extended wavelength range,
polarisation issues, detectors, etc).
• Possible change of the parameters space of the European XFEL is under
discussion. The process is in progress, and it is too early to announce final
result.
• We can only say that these changes will not be revolutionary: most probably
energy will be reduced, and undulator parameters will be re-optimized
correspondingly.
• Here we invite you to talk about optimization and properties of the radiation
from optimized x-ray free electron lasers.
Properties of the radiation from XFELs
Part 2:
Properties of the radiation from x-ray free electron lasers: from
qualitative physical picture to engineering design.
Incoherent radiation versus coherent radiation of
modulated electron beam
Qualitative look at the radiation properties
TTF FEL, 2001
• Self Amplified Spontaneous Emission (SASE) FEL is an attractively simple device:
it is just a system consisting of a relativistic electron beam and an undulator only.
• SASE FEL is capable to produce high power and high quality radiation (in terms of
coherence properties).
DKS, Nucl. nstrum. and Methods 193(1982)415
Qualitative look at the radiation properties
• Longitudinal coherence is formed due to slippage effects (electromagnetic
wave advances electron beam by one wavelength while electron beam
passes one undulator period). Thus, typical figure of merit is relative
slippage of the radiation with respect to the electron beam on a scale of
field gain length  coherence time.
• Transverse coherence is formed due to diffraction effects. Typical figure of
merit is ratio of the diffraction expansion of the radiation on a scale of field
gain length to the transverse size of the electron beam.
Qualitative look at the radiation properties
spectrum
power
• Radiation generated by SASE FEL consists of wavepackets (spikes). Typical
duration of the spike is about coherence time c.
• Spectrum also exhibits spiky structure. Spectrum width is inversely proportional to
the coherence time,  » 1/c, and typical width of a spike in a spectrum is inversely
proportional to the pulse duration T.
• Amplification process selects narrow band of the radiation, coherence time is
increased, and spectrum is shrinked. Transverse coherence is improved as well due
to the mode selection process.
Strict definitions of statistical characteristics
Statistics and probability distributions
z = 0.1 zsat
z = 0.5 zsat
z = zsat
• Transverse (bottom) and longitudinal (top) distributions of the radiation
intensity exhibit rather chaotic behaviour.
Statistics and probability distributions
Linear regime
Saturation
Deep nonlinear regime
• Probability distributions of the instantaneous power density (top) and of the
instantaneous radiation power (bottom) look more elegant and seem to be
described by simple functions.
Statistics and probability distributions
SSY, The Physics of Free Electron Lasers
SASE FEL, linear regime: general features
Statistics and probability distributions:
Experimental results from TTF FEL/FLASH
Probability distribution
of the energy in the
radiation pulse
Linear regime
Saturation
Probability distribution of the
energy after narrow band
monochromator
Linear regime
Saturation
V. Ayvazyan et al., Nucl. Instrum. and Methods A 507 (2003)368
Qualitative look at the evolution of the radiation
properties in XFEL
Radiation power
Brilliance
Degree of transverse coherence
Coherence time
• Radiation power continues to grow along the undulator length.
• Brilliance reaches maximum value at the saturation point.
• Degree of transverse coherence and coherence time reach their
maximum values in the end of exponential regime.
Optimized XFEL
SSY, Opt. Commun. 235(2004)415, 281(2008)1179; 281(2008)4727.
Optimized XFEL at saturation
0.6 mm-mrad
SASE1 @ 0.1 nm
1.4 mm-mrad
SSY, Opt. Commun. 281(2008)1179; 281(2008)4727 ; New J. Phys. 12(2010)035010 .
Qualitative look at the transverse coherence
2/ = 2.5,  = 0.65
t = 2 fs, 2.7 fs, 3 fs, 4.1 fs
2/ = 4.5,  = 0.4
t = 1.7 fs, 2.4 fs, 3.1 fs, 4.2 fs
Transverse coherence
Contribution to the total saturation power of the radiation modes with
higher azimuthal indexes 1, 2, 3, 4… grows with the emittance.
Transverse coherence
Degree of transverse coherence
z-s intensity distribution
2/ = 0.5 … 4
• In the case of large emittance the degree of transverse coherence degrades due to poor mode
selection.
• For small emittances the degree of transverse coherence visibly differs from unity. This
happens due to poor longitudinal coherence: radiation spikes move forward along the electron
beam, and interact with those parts of the beam which have different amplitude/phase.
• Longitudinal coherence develops slowly with the undulator length thus preventing full
transverse coherence.
SSY, Opt. Commun. 186(2000)185
Transverse coherence
Degree of transverse coherence
z-s intensity distribution
2/ = 0.5 … 4
• Poor longitudinal coherence is also responsible for the fast degradation of the transverse
coherence in the nonlinear regime.
• In the linear exponential regime group velocity of spikes (/ ds/dz) is visibly less than the
velocity of light due to strong interaction with the electron beam. In the nonlinear regime group
velocity of spikes approaches velocity of light due to weak interaction with the electron beam.
• Radiation spikes move forward faster along the electron beam and start to interact with those
parts of the beam which were formed due to interaction with different wavepackets.
• This process develops on the scale of the field gain length.
XFEL with planar undulator: odd harmonics
SSY, Phys. Rev. ST Accel. Beams 9(2006)030702
XFEL with planar undulator: odd harmonics
Evolution of probability distributions
for the 1st and the 3rd harmonics
Linear
Saturation
XFEL with planar undulator: odd harmonics
Coherence time: 1st, 3rd, 5th
Average spectra: 1st, 3rd, 5th
Summary of XFEL coherence
European XFEL
LCLS
• Parameters of an optimized SASE FEL in the saturation are universal functions of the
only parameter, 2/ .
• The best transverse coherence properties are achieved for 2/ ~ 1.
• At smaller values of the emittance the degree of transverse coherence is reduced
due to strong influence of poor longitudinal coherence on a transverse one. At large
values of the emittance the degree of transverse coherence degrades due to poor
mode selection.
• XFEL driven by low energy (or, bad emittance) electron beam suffers from bad
transverse coherence. Asymptotically degree of transverse coherence scales as
Thank you for your attention!
European x-ray free electron laser
Part 3:
Perspective extensions of the European XFEL.
EXFEL: Undulator concept
• Three undulators cover continuously wavelength range 0.1-1.6 nm at fixed electron
energy.
• Each undulator provides three different modes of operation using the same
undulator structure (conventional SASE, high-power, frequency doubling).
• Use of dispersion sections to control amplification process.
• All undulators are planar, variable-gap devices with an identical mechanical design.
SSY, TESLA FEL 2004-02
EXFEL: High power (sub-TW) mode of operation
• Use of a dispersion section for effective beam bunching;
• Application of undulator tapering for effective increase of radiation power in the
nonlinear regime.
SSY, TESLA FEL 2004-02
EXFEL: Two-color mode of operation
• Use of a dispersion section for effective beam bunching at the 2nd harmonic;
• 2nd part of the undulator is tuned to the second harmonic;
• Application of undulator tapering for effective increase of radiation power in the
nonlinear regime.
SSY, TESLA FEL 2004-02
Generation of attosecond pulses: 100 GW option
Slicing of electron bunch with fs-laser
SASE process
SSY, Phys. Rev. ST AB 9(2006)050702
Self-seeding option
• Self-seeding scheme is still planned for installation at FLASH
E.L. Saldin et al. NIMA 475(2001)357
Future extension: generic XFEL beamline
• Operation at a fixed energy in one electron beamline
• Control of SASE process in undulators is performed by SASE switches
• Three SASE undulators in a row cover wavelength range 0.1-1.6 nm
• Extended possibilities for generation of high power (sub-TW level) radiation
• Implementation of attosecond mode of operation (in “parasitic mode”)
• Extended possibilities for harmonic generation (use of frequency doubler)
SSY, TESLA FEL 2004-02
Future extension: control of SASE process by
SASE switches
RF switch
Magnetic
switch
SSY, TESLA FEL 2004-02
EXFEL: circular polarization at full power and with
high degree of coherence at SASE1/2/3
Planar undulator
Helical undulator
planar
helical
 Helical afterburner. Electron beam gains density modulation in the planar undulator. This density
modulation (scalar quantity) serves as a seed for FEL process in the helical undulator producing
radiation with helical polarization.
 At the moment this option is discussed for SASE3. In the case of smaller emittance we can
discuss replacement in the future of the last modules of SASE1/SASE2 with helical modules.
SASE4:
WW/VUV
SASE 2
U2
tunable, planar
0.1 – 0.4 nm
electrons
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e
U1
17.5 GeV
SASE 1
tunable, planar
0.1 nm
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SASE 3
e
Experiments
EXFEL: SASE4 - dedicated beamline for operation in the
“water window” and VUV wavelength range (1.6 - 6.4 nm )
tunable, planar
0.4 – 1.6 nm
• Can be placed in one of the tunnels for spontaneous undulators U1 or U2.
• Uses spent beam after SASE2.
• Extremely high energy in the radiation pulse, about two orders of magnitude above project
value of FLASH (500 uJ).
SSY, TESLA FEL 2004-05
European x-ray free electron laser
Prototypes of undulators
1D- Handbook: main properties of SASE FEL