Introduction to Free Electron Lasers and Fourth-Generation Light Sources. 黄志戎 (Zhirong Huang, SLAC)

Transcription

1 Introduction to Free Electron Lasers and Fourth-Generation Light Sources 黄志戎 (Zhirong Huang, SLAC)

2 FEL References K.-J. Kim and Z. Huang, FEL lecture note, available electronically upon request Charles Brau, Free Electron Lasers (Academic Press, 1990), slightly outdated but good basics Saldin, Schneidmiller, Yurkov, The Physics of Free Electron Lasers (Springer, 1999), more SASE but much more technical Web Resources LCLS CDR, LCLS science, European XFEL TDR, Spring-8 Compact SASE Source CDR,

3 Lecture Outline Introduction FEL mechanism SASE principle Temporal and transverse characteristics SASE experiments and projects Seeding options ultra-short pulses (if time available)

4 Free Electron Lasers Produced by the resonant interaction of a relativistic electron beam with a photon beam in an undulator Tunable, Powerful, Coherent radiation sources First operation of a free-electron laser at Stanford University Today 22 free-electron lasers operating worldwide 19 FELs proposed or in construction More info at

5 FEL oscillators Single pass FELs (SASE or seeded)

6 Vision of Science SLAC Report 611 Atomic, molecular and optical science Aluminum plasma classical plasma G=1 G =10 dense plasma G =100 High energy density science high density matter Density (g/cm -3 ) t=τ Coherent-scattering studies of nanoscale fluctuations t=0 Program developed by international team of scientists working with accelerator and laser physics communities Nano-particle and single molecule (non-periodic) imaging the beginning... not the end Absorption Resonance Raman t0 t 1 t2 t3 t 4 t5 Diffraction studies of stimulated dynamics (pump-probe)

7 Structural Studies on Single Particles and Biomolecules Requirements: High peak brightness High photon density 230 fs or shorter pulses Fast array detectors Single Molecule: >10 12 photons Measurement of static properties Is the goal- understanding dynamics is prerequisite Clusters of ~100 molecules with known orientation can be attempted first

8 Undulator Radiation λ 1 λ u forward direction radiation (and harmonics) undulator parameter K = 0.94 B[Tesla] λ u [cm] LCLS undulator K = 3.5, λ u = 3 cm, e-beam energy from 4.3 GeV to 14 GeV to cover λ 1 = 1.5 nm to 1.5 Å Can energy be exchanged between electrons and copropagating radiation pulse?

9 Resonant condition UVSOR FEL, Okazaki, Japan

10 Resonant interaction x K0 /γ 0 λu FEL interaction λ1 = λu 1 + 2γ K0 z Use variables energy η=( =(γ-γ 0 )/γ 0 phase θ=( =(k r +k u )z-ω r t radiation wavenumber undulator wavenumber arrival time at undulator distance

11 Pendulum equations Longitudinal electron motion in combined undulator and radiation fields described by pendulum equations =1 for helical undulator for planar undulator η θ Π 0 Π 2 Π 3 Π λ r θ

12 Three FEL modes

13 Self-amplified spontaneous emission (SASE)

14 X-ray FEL requires extremely bright beams Power grows exponentially with undulator distance z. For a 1-D, mono-energetic beam FEL Pierce parameter ρ peak current radiation power beam power 17 ka emittance beta function SASE power reaches saturation at ~ 20 L G FEL performance depends exponentially on e-beam qualities

15 Beam focusing Focusing of electron beam in the undulator π π

16 x K0 /γ 0 Resonant condition Emittance effect λu ψ λ1 = λu 1 + 2γ K0 electron with an angle ψ z Require average change in λ 1 over gain length << λ 1 Emittance requirement Smaller β x increase beam density, ideally

17 Slippage and FEL slices Due to resonant condition, light overtakes e-beam by one radiation wavelength λ 1 per undulator period Interaction length = undulator length optical pulse electron bunch optical pulse electron bunch z Slippage length = λ 1 undulator period (100 m LCLS undulator has slippage length 1.5 fs, much less than 200-fs e-bunch length) Each part of optical pulse is amplified by those electrons within a slippage length (an FEL slice) Only slices with good beam qualities (emittance, current, energy spread) can lase

18 SASE temporal spikes Due to noisy start-up, SASE has many intensity spikes LCLS spike ~ 1000 λ 1 ~ 0.15 μm ~ 0.5 fs! From one spike to another, no phase correlation Each spike lases indepedently, depends only on the local (slice) beam parameters LCLS pulse length ~ 200 fs with ~ 400 SASE spikes ~ x-ray energy fluctuates 5% 1 % of of X-Ray X-Ray Pulse Pulse Length Length

19 Transverse coherence Spontaneous undulator radiation phase space is the incoherent sum of the electron phase space, consists of many spatial modes X 2πε x x SASE: higher-order modes have stronger diffraction + FEL gain is localized within the electrons selection of the fundamental mode (gain guiding) fully transversely coherent even ε x > λ 1 /4 π λ 1 /2 (diffraction limit)

20 LCLS transverse mode simulation from S. Reiche Z=25 m Z=37.5 m Z=50 m Z=62.5 m Z=75 m Z=87.5 m m ε n = 1.2 μm, γ=28000, λ 1 =1.5 Ǻ, ε n /γ = ε x,y = 3.6 λ 1 /(4π)

21 # of Peak Brightness Enhancement From Storage Ring Light Sources To SASE B = #of photons Ω x Ω y Ω z Undulator in SR (Ω i - phase space area) SASE Enhancement Factor αν photons N lc ~ 10 6 e αν e N lc Ω x Ω y (2πε x ) (2πε y ) ( λ 2) Ω Z Δω ω σ Z = ps c Δω ω σ Z = fs c compressed 2 10 B N lc : number of electrons within a coherence length l c

22 Nonlinear harmonic generation FEL instability creates energy and density modulation at λ, Near saturation, strong bunching at fundamental λ produces rich harmonic components E small signal, linear regime λ near saturation, nonlinear regime λ t Coherent harmonics drive by fundamental λ (E n E 1n ) gain length = L G /n (n is harmonic order) similar transverse coherence spikier temporal structure Theory and simulations predicts third harmonic reaches up to 1% of fundamental at saturation

23 SASE Demonstration Experiments at Longer Wavelengths IR wavelengths: UCLA/LANL (λ = 12μ, G = 10 5 ) LANL (λ = 16μ, G = 10 3 ) BNL ATF/APS (λ = 5.3μ, G = 10, HGHG = 10 7 times S.E.) Visible and UV: LEUTL (APS): E e 400 MeV, L u = 25 m, 120 nm λ 530nm VISA (ATF): E e = 70 MeV, L u = 4m, λ = 800 nm TTF (DESY): E e < 300 MeV, L u = 15 m, λ = nm SDL (NSLS): E e < 200 MeV, L u = 10 m, λ = nm TTF2 (DESY): E e ~ 450 MeV, L u = 27 m, λ = 30 nm All Successful, TTF2 (FLASH) is in user operation mode

24 LEUTL FEL A B C σ t (ps) I (A) ε n (μm) σ δ (%) λ (nm) Observations agree with theory/ computer models (S. Milton et al., Science, 2001) Optical energy [a.u.] A B C Distance [m]

25 Energy (nj) April 20, Nonlinear Harmonic Radiation at VISA* Fundamental Nonlinear Harmonic Energy vs. Distance z(m) Associated gain lengths L f = 19cm L2 = 9.8cm L 3 = 6.0cm 2nd harmonic L n = L g / n * A. Tremaine et al., PRL (2002) 3rd harmonic Mode (n) Energy Comparison Wavelength (nm) Energy (μj) % of E Using the relation of 2nd and 3rd harmonic energies as given by Z. Huang and K.J.Kim K E 2 = γk u σ x 2 K2 K 3 2 b2 b 3 b -bunching parameters K n -Coupling coefficients 2 E 3

26 Observations at TTF FEL* Statistical fluctuation Transverse coherence after double slit after cross Y [mm] Intensity [arb.units] X [mm] X [mm] * V. Ayvazyan et al., PRL (2002); Eur. Phys. J. D (2002)

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30 LCLS must extend FEL wavelength by another two orders of magnitude from 13 nm 1 nm 1 Å 6 MeV 135 MeV 250 MeV σ z 0.83 mm σ z 0.83 mm σ z 0.19 mm σ δ 0.05 % σ δ 0.10 % σ δ 1.6 % Linac-0 L =6 m rf gun L0-a,b Linac-1 L 9 9 m rf 25 ϕ rf Linac-X L =0.6 m rf = 160 ϕ rf Linac-2 L 330 m rf 41 ϕ rf 4.30 GeV σ z mm σ δ 0.71 % Linac-3 L 550 m rf 0 ϕ rf 13.6 GeV σ z mm σ δ 0.01 %...existing linac DL1 L 12 m R b,c,d X BC1 L 6 6 m mm R b 24-6d SLAC linac tunnel BC2 L 22 m mm R a 30-8c Commission in Jan Commission in Jan DL2 L =275 m 56 0 R 56 undulator L =130 m research yard

31 Accelerator issues RF photocathode gun 1 μm normalized emittance, reasonable peak current Emittance preservation in linacs (SLC experiences) Bunch compression coherent synchrotron radiation microbunching instability (mitigated by a laser heater) Machine stability energy jitter (wavelength jitter) bunch length and charge jitters (FEL power jitter) transverse jitters (power and pointing jitters) Undulator straight trajectory to μm level (beam-based alignment) undulator parameter tolerance (e.g., ΔK/K ~ 10-4 )

32 Beyond SASE SASE x-ray FELs such as the LCLS will lay the foundation for next-generation x-ray facilities Due to its noisy startup, SASE is transversely coherent but temporally chaotic (LCLS 1.5 Å simulation by S. Reiche) temporal spectral Monochromator can be used to select a single mode, but flux is reduced (by ~600) and intensity fluctuates 100% Various schemes to improve temporal coherence proposed

33 Methods to improve temporal coherence High-gain harmonic generation (HGHG), starting from a seed laser at longer wavelengths ( nm) Two-stage self seeding: derive the x-ray seed from monochromized SASE for the next-stage amplification Regenerative amplifier FEL: feedback monochromized SASE for regenerative amplification

34 L.-H. Yu, PRA44, 5178 (1991) seed laser electrons... HGHG Principle Modulator D Radiator λ 1 λ h =λ 1 /h to next stage

35 L. H. Yu et al., PRL91, (2003) Advantages of HGHG Narrow bandwidth Intensity, a.u. HGHG SASE 10 4 Stable central wavelength Fourier transform limited Wavelength, nm BNL SDL FEL results Larger ratio of output/spontaneous radiation Short pulse (20fs) Stable Intensity from shot to shot Can be cascaded to short wavelength laser E-beam

36 Cascading to shorter wavelengths Whole bunch harmonic cascade: each stage energy modulation must be smaller than the next stage FEL parameter ρ Fresh bunch technique: Shift laser pulse from one part of an electron bunch (used part, with large energy spread) to a fresh part of the electron bunch Electron bunch Laser pulse Before Shifter After Shifter This makes it possible to use large energy modulation: Bunching parameter ~ order of 1 Time jitter between laser and e-beam must be less than 100 fs

37 Fermi FEL at Sincrotrone Trieste (Italy) Linearizer X-band cavity Beam switchyard Bunch compressor Bunch compressor FEL-1 FEL-2 Injector Linear accelerator Vertical transport line FEL Spectral range covered by two undulator lines FEL 1: 100 ~40 nm (12 30eV) single stage FEL 2: ~40 10 nm (30 124eV) two stages Also BESSY HGHG FEL with wavelength range 40 nm - 1 n (

38 V. Miltchev (DESY) Two-stage self-seeding option* Schematic view of the seeding option for FLASH Basic requirements: 1) The 1 st section operates in linear high-gain regime, <P SASE >~10MW 2) The micro bunching is smeared out after the magnetic chicane 3) The monochromator resolution Δω/ω ) The seeding power P SEED ~10kW >> shot noise power P SHOT ~10W 5) The seed pulse is amplified to saturation in the 2 nd undulator section * J. Feldhaus et al. / Optics Communications 140(1997)

39 V. Miltchev (DESY) Electron beam optics 1, 2) 1 st undulator section bypass 22 m 2 nd undulator section 14.5 m 30 m 3 tuning bypass undulator bypass dipole vertical focusing quadrupole vertical defocusing quadrupole sextupole tuning bypass dipole for each radiation wavelength λ R tune the quad strength to achieve linear regime in the 1 st section use the bypass magnets to match to the optics in the 2 nd section Minimizing CSR and optics effects 1) B. Faatz et. al., NIM A475, 603 (2001) 2) R. Treusch et. al. "The Seeding Project for the FEL in TTF Phase II", HASYLAB annual report 2001

40 Regenerative Amplifier FEL (RAFEL) RAFEL: high-gain, small feedback, multi-bunch scheme Demonstrated in IR (~16 μm, LANL): NIMA429, 125 (1999) Proposed for VUV FELs DESY: B. Faatz et al., NIMA 429, 424 (1999) Daresbury 4GLS: N. Thompson et al., FEL2005

41 X-ray RAFEL We propose and analyze an x-ray RAFEL using narrowbandwidth Bragg crystals* Bragg mirror x-ray Bragg mirror e-beam chicane undulator e-beam Bragg mirror x-ray Alternative backscattering geometry may also be used * Z. Huang & R. Ruth, PRL96, (2006)

42 Bragg s law Diamond crystals as Bragg Mirrors Diamond (high heat load, low absorption) at 60 degree 1 C (400), 1.55 Ǻ π-polarized C (511), 1.2 Ǻ σ-polarized 1 reflectivity XOP simulations reflectivity Δω/ω r x x 10-6 Δω/ω r

43 10 1 A possible RAFEL configuration for LCLS 1 radiation energy at undulator end (mj) relative rms energy fluctuation number of x-ray passes output power power (GW) time (fs)

44 Methods to generate ultra-short x-ray pulses X-ray manipulation of a frequency-chirped SASE E-beam manipulation: selective emittance spoiling

45 X-ray Pulse Slicing Instead of compression, use a monochromator to select a slice of the chirped SASE ω monochromator short x-ray slice t compression Single-stage approach SASE FEL Monochromator

46 Two-stage Pulse Slicing Slicing before saturation reduces power load on monochromator Second stage seeded with sliced pulse (microbunching removed by bypass) Allows small bandwidth for unchirped bunches Chicane SASE FEL Monochromator FEL Amplifier C. Schroeder et al., NIMA483, 89 (2002) Larger FEL bandwidth than at saturation when slicing, potentially longer x-ray pulse length than 1-stage Synchronization between sliced pulse and the part of electrons having the right energy

47 Minimum Pulse Duration The rms pulse duration σ t after the monochromator ω / u σ ω σ ω u t S. Krinsky & Z. Huang, PRST-AB6, , (2003) Minimum pulse duration is limited to for either compression or slicing / u σ ω SASE bandwidth reaches minimum (~ρ= ) at saturation, minimum rms pulse duration = 6 fs for 1% E-chirp

48 E-beam manipulation for fs and as x-rays Large x-z correlation inside a bunch compressor chicane LCLS BC2 2.6 mm rms Easy access to time coordinate along bunch 0.1 mm rms

49 P. Emma et al. PRL92, (2004) Slotted-spoiler Scheme 1 μm emittance 15-μm m Be Be foil 6 μm emittance 1 μm emittance

50 fs fsand as x-ray pulses A full slit of 250 μm unspoiled electrons of 8 fs (fwhm) 2~3 fs x-rays at saturation (gain narrowing of a Gaussian electron pulse) 2 fsec fwhm stronger compression + narrower slit (50 μm) 1 fs e as x-rays (close to a single coherence spike!)

51 the beginning... not the end