Linac Based Photon Sources: XFELS. Coherence Properties. J. B. Hastings. Stanford Linear Accelerator Center

Transcription

1 Linac Based Photon Sources: XFELS Coherence Properties J. B. Hastings Stanford Linear Accelerator Center

2 Coherent Synchrotron Radiation Coherent Synchrotron Radiation coherent power N incoherent power λ 1/3 Wavelength σ z vacuum chamber cutoff Power

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4 Microbunching through SASE Process At entrance to the undulator Exponential gain regime Saturation(maximum bunching) Excerpted from the TESLA Technical Design Report, released March 2001

5 LCLS at SLAC Å LCLS 2 compressors one undulator X-FEL based on last 1-km of existing SLAC linac X-FEL based on last 1-km of existing SLAC linac

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

7 Peak Brilliance of FEL s Peak Brilliance of FEL s photons per phasespace volume per bandwidth X-Ray 1 Å 1 Å 10 6 by FEL gain ~ by e quality, 10 3 by e quality, long undulators courtesy T. Shintake

8 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 λ/2 (diffraction limit) 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/4 π

9 Gain Guiding (LCLS) Z=25 m Z=37.5 m Z=50 m Z=62.5 m Z=75 m Z=87.5 m m Courtesy of S. Reiche (UCLA)

10 Observations at TTF FEL* Statistical fluctuation Transverse coherence after double slit after cross 1 0 Intensity [arb.units] Y [mm] X [mm] X [mm] * V. Ayvazyan et al., PRL (2001); Eur. Phys. J. D (2002) 2 0 2

11 Temporal Characteristics E(t)= j E 0 (t-t j ), t j is the random arrival time of j th e - N u λ E 0 : wave packet of a single e - l c ~ λ < bunch length c l 2 c π λ N σ ω u ρ Sum of all packets E(t) bunch length

12 Longitudinal Modes Due to noise start-up, SASE is a chaotic light temporally with M L coherent modes (M L spikes in intensity profile) M L bunch length coherence length Its longitudinal phase space is M L larger than FT limit (room for improvement) Integrated intensity fluctuation M L M L is NOT a constant, decreases due to increasing coherence in the exponential growth, increases due to decreasing coherence after saturation) I I 1 =

13 Manipulating the Longitudinal Phase Space Produce shorter pulses Fix the central wavelength Alter the longitudinal coherence

14 Manipulating the Longitudinal Phase Space Cut the electron beam: shorter bunches Self-Seed Self seed with chirped electron beam Produce a chirped photon beam: use a monochromator to slice a small time portion of the the beam in energy => a short pulse in time Seed with an external source

15 σ E /E Magnetic Bunch Compression Magnetic Bunch Compression Ε/Ε Ε/Ε Ε/Ε or over- σ compression z0 σ z0 z chirp z V = V 0 sin(ωτ) z = R 56 Ε/Ε RF RF Accelerating Voltage Path Path Length-Energy Dependent Beamline undercompression σ z z

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17 0.55 fs

18 Self Seeding, N = 1

19 Self-seeding option at the TESLA VUV FEL No Seed Seed

20 Short pulses: Chirped pulse slicing chirp Ε/Ε z SASE FEL Electron bypass Monochromator FEL Amplifier Energychirped electron beam 1 st Undulator 2 nd Undulator Frequency -chirped Input radiation radiation Ε/Ε 10 3 Pulse Slicing Output radiation

21 Monochromator: short pulse limit Monochromator with smaller bandwidth slices out shorter pulse δt out = δt in x δω mono /δω chirp But uncertainty principle gives a limit δω mono x δt out 1/2 Output pulse fwhm (fs) 0 Effect of uncertainty principle (E=8 kev) Effect of slicing (1% chirp on 120fs rms pulse) Monochromator resolution δω/ω (rmsx10-5 ) Note that if the uncertainty principle dominates, then the output pulse has complete longitudinal coherence For LCLS at 8 kev with 1% chirp, the minimum pulse length is about 3.5 fs fwhm, using a monochromator resolution of 3.3x10-5 rms. Some practical monochromator options: Crystal reflection rms resolution Output pulse fwhm Ge(111) 14x fs Si(111) 5.7x fs Si(220) 2.5x fs

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25 But, the FEL is a challenge: For LCLS, slice emittance >1.8 µm will not saturate For LCLS, slice emittance >1.8 µm will not saturate ε N = 1.2 µm ε N = 2.0 µm P = P 0 P = P 0 /100 courtesy S. Reiche SASE FEL is not forgiving instead of mild brightness loss, power nearly switches OFF electron beam must meet brightness requirements

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27 SASE FEL Electron Beam Requirements SASE FEL Electron Beam Requirements radiation wavelength transverse emittance: ε Ν < 1 µm at 1 A, 15 GeV peak current undulator period (~1.5 µm realistic goal) energy spread: <0.08% at I pk = 4 ka, K 4, λ u 3 cm, beta function undulator field FEL gain length: 20L g > 100 m for ε Ν 1.5 µm Need to increase peak current, preserve emittance, and maintain small energy spread, all simultaneously AND provide stable operation

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