european xfel and coherence properties of the radiation from x-ray free electron lasers

European XFEL and coherence properties of the radiation from x-ray free electron lasers

• 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.

SLAC Accelerator Seminar, May 20, 2010

E.A. Schneidmiller, M.V. Yurkov

DESY, Hamburg

European x-ray free electron laser

Part 1:

An overview of current status of the European XFEL.


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.


• 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.


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


• XFEL work packages


• 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.


• String and module assembly at Saclay


• Accelerator module test facility in Hamburg.


WEB cameras allow to follow construction progress on-line:

http://www.xfel.eu/project/webcams/


• 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.


• 2014: First electron beam.

• 2015: Start user operation.

• Start-up version:


Scientific instruments

SASE 1 tunable, planar

0.1 nm


0.1 – 0.4 nm

U 2

U 1

e-

electrons 17.5 GeV

Exp

erim

ents


0.4 – 1.6 nm

e-


• 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

DKS, Nucl. nstrum. and Methods 193(1982)415

• 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).


• 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.


• 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.

spectrum

power

Strict definitions of statistical characteristics

Statistics and probability distributions

z = 0.1 zsat

• Transverse (bottom) and longitudinal (top) distributions of the radiation intensity exhibit rather chaotic behaviour.

z = 0.5 zsatz = zsat


SaturationLinear regime 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.


SSY, The Physics of Free Electron Lasers

SASE FEL, linear regime: general features

Linear regime Linear regime

Saturation Saturation

Probability distribution of the energy in the radiation pulse

Probability distribution of the energy after narrow band monochromator

Statistics and probability distributions:Experimental results from TTF FEL/FLASH

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

1.4 mm-mrad

SASE1 @ 0.1 nm

SSY, Opt. Commun. 281(2008)1179; 281(2008)4727 ; New J. Phys. 12(2010)035010 .

0.6 mm-mrad

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.


• 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.

z-s intensity distribution

SSY, Opt. Commun. 186(2000)185

Degree of transverse coherence

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.

Degree of transverse coherence z-s intensity distribution

2/ = 0.5 … 4

XFEL with planar undulator: odd harmonics

SSY, Phys. Rev. ST Accel. Beams 9(2006)030702


Evolution of probability distributions for the 1st and the 3rd harmonics

Linear

Saturation

Coherence time: 1st, 3rd, 5th

Average spectra: 1st, 3rd, 5th


Summary of XFEL coherence

• 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

European XFELLCLS

Thank you for your attention!


Part 3:

Perspective extensions of the European XFEL.

EXFEL: Undulator concept

SSY, TESLA FEL 2004-02SSY, TESLA FEL 2004-02

• 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.

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 2004-02SSY, 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 2004-02SSY, 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)050702SSY, 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)357E.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 2004-02SSY, TESLA FEL 2004-02

Future extension: control of SASE process by SASE switches

Magnetic switch

RF switch

SSY, TESLA FEL 2004-02SSY, TESLA FEL 2004-02

EXFEL: circular polarization at full power and with high degree of coherence at SASE1/2/3

Planar undulator Helical undulator

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.

planar helical

EXFEL: SASE4 - dedicated beamline for operation in the “water window” and VUV wavelength range (1.6 - 6.4 nm )


0.1 nm


0.1 – 0.4 nm

U 2

U 1

e-

electrons 17.5 GeV

Exp

erim

ents


0.4 – 1.6 nm

e-

SASE4: WW/VUV

•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 2004-05SSY, TESLA FEL 2004-05


Prototypes of undulators

1D- Handbook: main properties of SASE FEL

european xfel and coherence properties of the radiation from x-ray free electron lasers

Documents

xray free electron lasers

new xray laser facility

european xfel gmbh

laser heater system

cold vacuum

cold magnets

amtf operation

cryomodules assembly