Opportunities and Challenges for X -ray Free Electron Lasers for X-ray Ultrafast Science J. Hastings Stanford Linear Accelerator Center June 22, 2004 European XFEL Laboratory
How Short is short? defined by New York Traffic Commissioner T.T. Wiley in 1950 as: the time between the light turning green and the guy behind you honking. -W. Safire, New York Times,, March 7, 2004
Ultrafast Sources and Science: Synchrotrons X-ray sources: Current lasers: Laser plasmas XFEL s Ultrafast lasers Acoustic phonons Science: Vibrations (Optical phonons) Strings, Particle Chemistry and Biochem Cosmology Collisions Electron dynamics harpo yacto zepto atto femto pico nano micro milli 10-27 10-24 10-21 10-18 10-15 10-13 10-9 10-6 10-3 Pulse duration (seconds)
1878: E. Muybridge at Stanford Tracing motion of animals by spark photography E. Muybridge L. Stanford Muybridge and Stanford disagree whether all feet leave the ground d at one time during the gallop E. Muybridge, Animals in Motion,, ed. by L. S. Brown (Dover Pub. Co., New York 1957).
Scattering experiments The challenge: to know pulse widths and delay
From Storage Rings to Linacs HEP SR Pulse Length Storage rings 100-30 ps Single pass linear colliders Single pass linacs Free electron lasers (FELs) Energy recovery linacs (ERLs) 100-1 fs 300 as??
Peak Brilliance of FEL s 1 Å photons per phasespace volume per bandwidth X-Ray ~10 9 10 3 by e quality, ~10 9 10 6 by FEL gain long undulators courtesy T. Shintake
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
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)
Observations at TTF FEL* Statistical fluctuation Transverse coherence after double slit after cross * V. Ayvazyan et al., PRL (2001); Eur. Phys. J. D (2002)
Temporal Characteristics E(t)= j E 0 (t-t j ), t j is the random arrival time of j th e - N u l E 0 : wave packet of a single e - l c ~ 100-1000 l < bunch length l c c 2 σ ω λ π N ρ u Sum of all packets E(t) bunch length
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 I 1 = I M L M L is NOT a constant, decreases due to increasing coherence in the exponential growth, increases due to decreasing coherence after saturation)
Saturation Saturation Exponential Gain Regime Exponential Gain Regime Undulator Regime Undulator Regime THE DREAM Select a single spike 1 % of X-Ray Pulse 1 % of X-Ray Pulse Electron Bunch Electron Bunch Micro-Bunching Micro-Bunching 0.2 fs 0.9 fs ~ 300 attoseconds 2.5 fs
LCLS Linac Parameters for 1.5-Å FEL 7 MeV σ z 0.83 mm σ δ 0.2 % rf gun Linac-0 L =6 m new new...existing linac 150 MeV 250 MeV σ z 0.83 mm σ z 0.19 mm σ δ 0.10 % σ δ 1.8 % Linac-X L =0.6 m ϕ rf =180 Linac-1 Linac-2 L =9 m L =330 m ϕ rf = 38 ϕ rf = 43 DL-1 L =12 m R 56 0 21-1b 1b 21-1d 1d single bunch, 1-nC, 120-Hz X BC-1 L =6 m 56 = 36 mm R 56 21-3b 24-6d SLAC linac tunnel Linac-3 L =550 m ϕ rf = 10 25-1a undulator 30-8c L =120 BC-2 m L =22 m 56 = 22 mm DL-2 L =66 m R 56 = 0 R 56 4.54 GeV σ z 0.022 mm σ δ 0.76 % 14.35 GeV σ z 0.022 mm σ δ 0.02 % FFTB hall (RF phase: φ rf rf = 0 at accelerating crest) Two stages of bunch compression
Magnetic Bunch Compression Ε/Ε σ z0 chirp Ε/Ε or overcompression Ε/Ε undercompression z z z σ E /E σ z V = V 0 sin(ωτ) z = R 56 Ε/Ε RF RF Accelerating Voltage Path Path Length-Energy Dependent Beamline
Add thin slotted foil in center of chicane y coulomb scattered e e unspoiled e coulomb scattered e 2 x x E/E t PRL 92,, 074801 (2004). 15-µm m thick Be foil P. Emma, M. Cornacchia, K. Bane, Z. Huang, H. Schlarb, G. Stupakov, D. Walz (SLAC)
Genesis 1.3 FEL code x-ray Power z 60 m ~10 10 photons 2 fs fwhm Power (GW) (<1 fs possible)
Pulse Length Control With Stepping Motor Tapered slot width allows pulse length control with simple stepping motor Double slit might be used to generate two precisely timed pulses
Chirped Photon Beams Ε/Ε h/2 z photons ω electrons σ ω /h σ ω h t
SASE Pulse Compression σ ω e - energy e SASE frequency h time time B. Sheehy In addition, an optical compressor used for X-rays X (~1 Å) ) may require long (>100 m) paths. frequency σ t time
Monochromator Pulse Slicing ω σ ω h σ ω /h σ m t monochromator bandwidth Monochromator bandwidth, σ m,, limits pulse, as as does does intrinsic photon bandwidth, σ ω S. Krinsky, Z. Huang, PR ST AB, 6,, 050702 (2003).
Seed Self-seeding option at the TESLA VUV FEL No Seed
Short pulses: Chirped pulse slicing chirp Ε/Ε z SASE FEL Electron bypass Monochromator FEL Amplifier Output radiation Energychirped electron beam 1 st Undulator 2 nd Undulator Frequencychirped radiation Input radiation Ε/Ε 10 3 Pulse Slicing
The Sub-Picosecond Pulsed Source (SPPS) Short Bunch Generation in the SLAC Linac Damping Ring (γε 30 µm) 50 ps RTL SLAC Linac 1 GeV 9 ps 0.4 ps 20-50 GeV Add Add 12-meter chicane compressor in in linac linac at at 1/3-point (9 (9 GeV) FFTB <100 fs Existing bends compress to <100 fsec 1.5% ~1 Å 30 ka 28 GeV 80 fsec FWHM
R&D at SPPS Towards X-Ray X FELs Wakefields of micro-bunch Develop bunch length diagnostics RF phase and voltage stability of linac Emittance growth in compressor chicane (CSR) X-ray optics and transport
P. Muggli, M. Hogan
1.6 Autocorrelation minimum bunch length (with ϕ-jitter) CTR 60 50 Power Spectrum (a.u.) 10 0.1 0.001 10-5 10-7 10-9 10-11 10-13 10-15 10-17 w Filtering Mylar resonances 10 0 CTRFSpecSigmaz20Mylar12.5_3 10 100 1000 Wavelength (µm) 10-1 10-2 10-3 10-4 SigmazMylar12.7_3WandBS Filter Amplitue (a.u.) 1.2 0.8 Bunch σ z (µm) 40 30 20 w/o Filtering 0.4 10 s z» 9 mm CombinedCTRInterferogramsSm 0-100 -50 0 50 100 z (µm) 0 0 5 10 15 20 25 30 35 40 Autocorrelation σ z (µm) P. P. Muggli et et al. al. Gaussian bunch: s z fi 18 mm
Ti:s Ti:s laser Electro-Optical Sampling 200 µm m ZnTe crystal e Single-Shot Shot <300 fs Timing Jitter (20 Shots) e temporal information is encoded on transverse profile of laser beam 170 fs rms Adrian Cavalieri et al., U. Mich.
Adrian Cavalieri et al., U. Mich.
X-ray induced electron emission spectra Synchronization reaction cell I laser excited E s nominal Position Sensitive Detector Pump Laser (3 mm diameter) Tuned to signature e - energy position yields relative arrival time many psec range with fsec resolution Measurement of of X-ray X-ray Timing Timing Using Using Spatial Coincidence With With Probe Probe Laser Laser timing jitter is is not not a limitation 80 fs X-ray pulse John Arthur
Chirped Laser pulse assisted Laser Auger assisted Auger decay decay Auger decay direct measure of x-ray pulse duration dn dw W kin Broadening of sidebands sidebands at?w = hν laser Auger electron bunch ~ t x-rays (convoluted with Auger decay time) W 1 W 2 0 Laser pulse W h J. M. Schins et al., Phys. Rev. Lett. 72, 2180 (1994) T. E. Glover et al., Phys. Rev. Lett. 74, 2468 (1996) W bind R. Kienberger, SLAC
In Summary XFELs will provide peak brilliance at 1 Å? Full transverse coherence TTF FEL? Multi-GW peak power ~10 12 photons per pulse LEUTL? Pulse duration ~ 100 fs? Pulse repetition rates ~ 10 100 hz
Final Comment For LCLS, slice emittance >1.8 µm will not saturate ε = 1.2 N µm ε = 2.0 N 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