A. Zholents, PQE, January, 2006 X-ray free electron lasers Alexander Zholents LBNL An introduction to the afternoon session John Arthur, SLAC Applications of the intense coherent x-ray pulses from LCLS John Corlett, LBNL Proposals and concepts for future FELs Andrew Sessler, LBNL Transverse-longitudinal correlations: FEL performance and emittance exchange

A. Zholents, PQE, January, 2006 X-ray FEL essentials Layout of XFEL at DESY (Germany) Electron beam production system: Q=1nC,  n =1 mm-mrad Electron beam delivery system: E=20 GeV, I peak =5kA Electron beam utilization for emission of x-rays: l=1Å,  =100fs,  E=1mJ More info about future x-ray FEL facilities in the talk of John Corlett and use of these facilities in the talk of John Arthur in the following session m courtesy T. Limberg

A. Zholents, PQE, January, 2006 u SNSNSNSNSNSNSNSN NSNSNSNSNSNSNSNS 1. Undulator B y – peak magnetic field Emission of x-rays 2. Electron beam 3. Laser (not always needed) undulator parameter vzvz

A. Zholents, PQE, January, 2006 NSNSNSNSNSNSNSNS SNSNSNSNSNSNSNSN FELs “power” is in bunching * Radiation power Electrons should stay bunched within *) Motz 1953; Phillips 1960, Madey 1971

A. Zholents, PQE, January, 2006  E/E z z V = V 0 sin(  t ) RF Accelerating Voltage Voltage  z = R 56  E/E Path Length-Energy Dependent Beamline Path Length-Energy Dependent Beamline  E/E z z z z ‘chirp’ RF “buncher” courtesy P.Emma

A. Zholents, PQE, January, 2006 Energy modulation of electrons in the undulator by the laser light Electron trajectory through undulator Undulator period N S S N S S N N e - l u Light Magnetic field in the undulator BB 0 sin(k u z) E k B E k B V V Laser wavelength Laser “buncher” FEL resonance condition While propagating one undulator period, the electron is delayed with respect to the light on one optical wavelength =

A. Zholents, PQE, January, 2006 Laser pulse energy Spontaneous emission energy Field energy in the far field region of undulator radiation in the presence of the laser field: Laser field Electron spontaneous emission Energy modulation  E EE ()() Number of undulator periods Number of optical cycles  is the electron phase relative to the laser wave at the undulator entrance Laser “buncher” (2) Laser pulse width  z

A. Zholents, PQE, January, 2006 Laser “buncher” (3)  E 2 a =1/137 photon energy (in 50 fs FWHM pulse)  Numerical example: h L  L  R A L 5 nJ  E ≈ 150 keV to be compared with uncorrelated electron beam energy spread of ~ 100 keV K = 3 ( for K >> 1 ) Spontaneous emission energy (30-th harmonic of Ti:Sapphire laser produced using High-order Harmonic Generation) 45eV Laser pulse energy = = =

A. Zholents, PQE, January, 2006 modulator bunching chicane XUV light N u =100 radiator N u = nm, 5 nJ, 50 fs = 100 kW 30 nm, 30 MW Laser “buncher” (4) Optical klystron* *) Skrinsky, Vinokurov,1977 e-e- e-e- XUV light fragment of e-beam: modulation fragment of e-beam: bunching

A. Zholents, PQE, January, 2006A. Zholents, San-Diego, April, 2004 bunching chicane Laser light time delay chicane 240 nm 48 nm e-e- modulator radiator e-e- light modulator radiator 48 nm e-e- light 12 nm Harmonic cascade FEL * Position of FEL pulse in full electron beam pulse Unperturbed electrons ~100-fs seed laser pulse tail head radiator modulator Fresh bunch technique   phase energy  evolution of e-beam phase space New undulator resonant at L / n, and bunched beam radiates at n-th harmonic bunching chicane *) Csonka 1980; Kincaid 1980; Bonifacio 1990; L.-H. Yu 1990  z

A. Zholents, PQE, January, 2006 Power vs. z and  -  scatterplots At each modulator, radiation interacts with “fresh” e- At each harmonic upshift (modulator to radiator), macro- particle phase multiplied by n Bunching effects of dispersive section visible in change from Z=6 m in 48- nm modulator to Z=0.4 m scatterplot in 12-nm radiator Z=0 m Z=1.8 m Z=3.6 m Z=0 m Z=0.4 m Z=3 mZ=6 m 240-nm modulator 1.0 GW 4.0 GW 48-nm radiato r 48-nm modulato r GW 12-nm radiato r GW 48-nm modulator Energy (MeV) q (radians) -p-p +p+p -5 p +5 p -p-p +p+p q (radians) +4 p -4 p q (radians) Z=5.4 m Z=3.4 m Z=4.4 m Z=2.4 m GINGER simulations W. Fawley, LBNL

A. Zholents, PQE, January, 2006 GINGER simulation of 4- stage cascade configuration (240 nm  1 nm); W. Fawley Input laser seed initialized with broadband (a) phase noise (b) amplitude noise Fields resolved in simulation on 240 nm/c temporal resolution or better Results: Noise reaches minimum at 48-nm stage (slippage aveg.) In later stages noise increases due to harmonic multiplication of low frequency components RMS phase noise d  (t)/dt after removal of average component d  (t)/dt ( A.U.) (a) (b) 12 nm4 nm1 nm 48 nm 240 nm EXIT Noise evolution from imperfect seed * *) Saldin et al., 2002 Noise can be a problem at 1 Å

A. Zholents, PQE, January, 2006 Self-Amplified Spontaneous Emission FEL * *) Kondratenko, Saldin 1980; Bonifacio, Pellegrini, Narducci 1984 Similar to optical lasers, SASE x-ray FEL starts from spontaneous emission but  avoids use of mirrors courtesy S. Reiche courtesy Z. Huang Density modulation (shot noise at start or microbunching latter) drives energy modulation and vice-versa Instability reaches saturation after all electrons are microbunched (or rate of de-bunching equals rate of bunching) gain length P rad ≈rP beam

A. Zholents, PQE, January, 2006 The FEL parameter r Small diffraction, radiation field interacts locally with the electron beam, i.e. optical guiding* (some similarity with fiber optics) No guiding, strong diffraction I A = 17 kA e-beam emittance beta-function for K>>1 peak current light emittance; x – x-ray wavelength Key parameters: beam energy spread causes de-bunching *) Moore 1984; Scharlemann, Sessler, Wurtele 1985

A. Zholents, PQE, January, 2006 Transverse coherence When spontaneous undulator radiation consists of many spatial modes, i.e. incoherent sum of individual electron emissions But FEL gain is localized within the electrons and higher-order modes have stronger diffraction : gain guided selection of fundamental mode results in fully transverse coherence even at x X’ e-beam light beam (diffraction limited) courtesy S. Reiche Radiation field at different locations along the undulator

A. Zholents, PQE, January, 2006 Temporal coherence Bunch length Cooperation length (slice): SASE output exhibits “chaotic light” properties Number of longitudinal modes: M ≈ (bunch length)/slice Fluctuation in the x-ray pulse energy ~ 1/√M Slice properties, i.e. slice peak current, emittance and energy spread define performance

A. Zholents, PQE, January, 2006 Temporal coherence (2) M decreases as coherence builds up during the exponential gain reaching minimum at saturation (~200 at LCLS) courtesy W. Fawley

A. Zholents, PQE, January, 2006 Production of bright electron beams: generation e-beam peak brightness unites in a single expression key parameters for x-ray FELs Peak brightness of different photocathode e-guns (2002) courtesy P. Piot ~100 A,  n =1 mm-mrad rapid acceleration near to the cathode to avoid space charge dilution DESY-Zeuthen new generation of e-guns

A. Zholents, PQE, January, 2006 Production of bright electron beams: preservation q Non-linear effects in bunch compression: rf waveform, T 566 q Longitudinal and transverse wakefields in accelerator q Space charge effects (mainly longitudinal) q Coherent synchrotron radiation (CSR) and emittance excitation q Resistive wall wakefields in undulators Physics phenomena affecting the e-beam while acceleration and compression q Jitter in the rf phase and amplitude in accelerating structures q Intensity and timing jitters in photocathode gun laser q Misalignment of rf structures and magnetic elements q Power supply ripples Technical issues

A. Zholents, PQE, January, 2006  E / E = 0  x = R 16 (s)  E/E bend-plane emittance growth e–e–e–e– R zzzz coherent radiation for  z overtaking length: L 0 ~(24  z R 2 ) 1/3  E / E < 0 s s xx xx  Powerful radiation generates energy spread in bends  Causes bend-plane emittance growth (short bunch worse)  Energy spread breaks achromatic system  L0L0L0L0 l ~ CSR wake is strong at very small scales (~1  m) Coherent Synchrotron Radiation (CSR) courtesy P. Emma

A. Zholents, PQE, January, 2006 Initial density modulation induces energy modulation through longitudinal space charge forces, converted to more density modulation by a compressor l t Current 1% 10%  growth of slice energy spread (and emittance) l t Energy Space charge Gain=10 compression Longitudinal space charge, CSR and microbunching instability c ourtesy Z. Huang saturation due to overmodulation stops the growth

A. Zholents, PQE, January, 2006 Spectral dependence of the gain of the microbunching instability Microbunching instability (2) Entire machine with its accelerating sections, drifts and chicanes acts as an amplifier for initial density perturbation and can be characterized by a spectral gain function (in an analogy to the FELs) * *) Z. Huang et. al, Phys. Rev. ST –Acc. and Beams, v.5, (2002) 1400 l (mm) Instability increases rms energy spread by a factor of 5-10 FERMI FEL project

A. Zholents, PQE, January, 2006 Laser heater as an instrument for a suppression of microbunching instability 1,2 1) E.L. Saldin, E.A. Schneidmiller, and M.V. Yurkov, DESY Report No. TESLA-FEL , ) Z. Huang, et. al, Phys. Rev. ST –Acc. and Beams, V.7, , (2004). 800 Laser heater 2.5 keV Laser heater 5 keV Laser heater 7.5 keVLaser heater 10 keV Laser “heater” (laser-e-beam interaction induce energy spread) provides “Landau damping” effect through controlled increase of the energy spread at the beginning of acceleration

A. Zholents, PQE, January, 2006 Alignment errors and orbit distortions are responsible for transverse wakefields produced by e-beam, and transverse wakefields twist e-beam into a banana shape courtesy P. Emma  s z   Slice emittance is not affected Centroid shift and  variation can be important Wakefields Other wakefields: Longitudinal wakes, Resistive wall wakes, Surface roughness wakes also do not affect slices and produce similar global variations that nevertheless can be dangerous for FEL performance

A. Zholents, PQE, January, 2006 Pushing over the limits … further improvements can be obtained by using: 1) electron beam conditioning* 2) enhanced SASE * Talk of J. Wurtele, Wednesday evening

A. Zholents, PQE, January, 2006 *) Sessler, Whittum, Yu in 1992 allows relaxed emittance requirement in FEL z y undulator  E/E y without conditioning z y  E/E y with conditioning z y Electron beam conditioning* provides correlation of electron transverse amplitudes with electron energies to prevent de-bunching of electrons (more in Sessler’s talk in the following session)

A. Zholents, PQE, January, 2006 e-beam Caution: approximately one half of electrons have wrong sign of correlation !!! Proposed scheme gains factor of 10 5 in efficiency by utilizing laser and wiggler for electron energy modulation instead of RF cavities: Laser-assisted electron beam conditioning * *) Zholents 2005

A. Zholents, PQE, January, 2006 Example: LCLS-like FEL with 2 times of LCLS emittance 1- no conditioning, 2 - ideal conditioning (all electrons), 3 - proposed conditioner. GENESIS simulations Beam parameters: energy = 14 GeV peak current = 3.4 kA, energy spread = 1.2 MeV, emittance = 2.4 mm-mrad, beta-function = 20 m. Laser-assisted electron beam conditioning (2)

A. Zholents, PQE, January, 2006 E ~ 4.5 GeV Laser peak power ~ 10 GW (“easy”) Short wiggler, ~ 10 periods BunchingAccelerationModulation fs pulse L ~0.8 to 2.2  m Electron beam after bunching Peak current, I/I 0 z / L kA E ~ 14 GeV One optical cycle ESASE-Enhanced Self-Amplified Spontaneous Emission* *) Zholents 2004 Laser pulse width

A. Zholents, PQE, January, 2006 “Start-to-End” simulations 3-m FODO lattice period –drifts+quads occupy ~ 0.5m –not compatible with current LCLS lattice design ESASE (2) Example for LCLS with  =12 m ESASE cases saturate by 50 m with times power contrast over unmodulated part of the electron bunch - the opportunity for an absolute synchronization of a probe x-ray pulse to a pump laser pulse courtesy W. Fawley “standard” LCLS ESASE

A. Zholents, PQE, January, 2006 Combined field of two lasers Energy modulation of electrons produced in interaction with two lasers Attosecond x-rays using ESASE* *) Zholents, Penn 2005

A. Zholents, PQE, January, 2006 Peak current after chicane one laser two lasers Attosecond x-rays using ESASE (2)

A. Zholents, PQE, January, 2006 background from 100 fs e-bunch side peaks main peak attosecond pulse x-ray pulse energy growth over the length of the undulator 35 GW Attosecond x-rays using ESASE (3) main peak side peaks entire bunch ~350as

A. Zholents, PQE, January, 2006 Summary  X-ray FELs are as good as the electron beam is, i.e.: peak current slice emittance slice energy spread  Production of a high-brightness electron beams and preservation of the electron beam quality is affected by: space charge coherent synchrotron radiation microbunching instability various wake fields  Laser-assisted manipulation of electrons in the phase space is a promising concept for future FELs : electron beam conditioning enhanced self-amplified spontaneous emission

A. Zholents, PQE, January, 2006 Outlook: future FEL-based multi-user x-ray facility Future facility will be as much laser beam based as electron beam based and will have a multi -FEL x-ray production “farm”. This “farm” will be fed by a high- repetition rate linac (up to MHz) equipped with a high brightness source of electrons. Optical lasers will be used for a production and shaping the electron bunches and for seeding the x-ray radiation. An advent of high-average power lasers will boost high- repetition rate FELs. High repetition rate linac FEL “farm” Laser(s)

A. Zholents, PQE, January, 2006 Use of FELs will expand beyond FELs based on the SASE method ( in a construction phase at present ) towards FELs producing laser-like nearly Fourier transform limited x-ray beams at various wavelengths with controlled pulse duration, bandwidth, and polarization. Outlook: future FEL-based multi-user x-ray facility

A. Zholents, PQE, January, 2006 Gratefully acknowledged: K. Bane, J. Corlett, M. Cornaccia, P. Craevich, S. DiMitri, D. Dowel, P. Emma, W. Fawley, W. Graves, J. Hasting, Z. Huang, K.- J. Kim, S. Leone, S. Lidia, G. Penn, R. Schoenlien, A. Sessler, J. Staples, G. Stupakov, J. Wu, J. Wurtele, M. Zolotorev Thank you !

A. Zholents, PQE, January, 2006 courtesy B. Faatz

A. Zholents, PQE, January, 2006