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An X-Ray FEL Oscillator with ERL-Like E-Beams


Laser-driven DC gun being developed at Cornell ... diamond may be used for most cases: ... Gun technologies. SCSS: CeB6, thermionic, pulsed gun. LBL 50 MHz, ... – PowerPoint PPT presentation

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Title: An X-Ray FEL Oscillator with ERL-Like E-Beams

An X-Ray FEL Oscillator with ERL-Like E-Beams
  • Kwang-Je Kim
  • ANL Univ. of Chicago
  • May 23, 2008
  • Seminar at Wilson Lab
  • Cornell University

Next Generation X-Ray Sources
  • High-gain FELs (SASE) will provide an enormous
    jump in peak brightness from the 3rd generation
  • Intense, low emittance bunches Q 1 nC, IP
    several kA, exn 1 mm-mr
  • LCLS, European X-FEL, SCSS, Fermi,..
  • Multi-GeV Energy Recovery Linacs (ERLs) will
    provide high average brightness with low
    intensity, ultra-low emittance bunches at high
    rep rate
  • exn 0.1 mm-mr, Q20 pC, t2ps, frep1.3 GHz, IAV
    up to 100 mA
  • Cornell, MARS, KEK-JAERI, APS,..
  • ERLs have so far been regarded only as a
    spontaneous emission source
  • We show that an X-ray FEL Oscillator (X-FELO) for
    l 1-Å based on high energy ERL beams is feasible
    with peak spectral brightness comparable to and
    average spectral brightness much higher than
    SASEs (to be published in PRL)

ERL Plans Cornell, KEK/JAERI , APS II
Cornell ERL
APS II concept
X-Ray Cvities for Oscillators History
  • X-ray FEL Oscillator (XFEL-O) using Bragg
    reflector was first proposed by R. Colella and A.
    Luccio at a BNL workshop in 1984.
  • This was also the workshop where a high-gain
    FEL(SASE) was proposed by R. Bonifacio, C.
    Pellegrini, and L. M. Narducci
  • X-Ray optical cavities to improve the performance
    of high-gain FELs have been studied recently
  • Electron out-coupling scheme by B. Adams and G.
    Materlik (1996)
  • Regenerative amplifier using LCLS beam ( Z. Huang
    and R. Ruth, 2006)

Current and Future X-Ray Sources
Electron Beam Qualities Enabling X-FELO
  • Laser-driven DC gun being developed at Cornell
    for frep1.3 GHz
  • Thermionic cathod and bunch manipulation for
    frep1-100 MHz
  • sDE1.4 MeV, tel2ps

Principles of an FEL Oscillator
  • Small signal gain G DPintra/Pintra
  • Start-up (1G0) R1 R2 gt1 (R1 R2 mirror
  • Saturation (1Gsat) R1 R2 1
  • Synchronism
  • Spacing between electron bunches2L/n ( L
    length of the cavity)

Bragg Mirrors
  • Requiring total loss per pass to be lt 20, the
    reflectivity of each Bragg mirror should be well
    over 90
  • Possible crystal candidates are
  • Diamond
  • Highest reflectivity hard ( small Debye-Waller
  • Multiple beam diffraction in exact backscattering
    needs to be avoided ( can use as a coupling
  • Sapphire
  • High reflectivity without multiple beam
  • Small thermal expansion coefficient and large
    heat conductivity at T40K

Backscattering Reflectivity's for Sapphire and
Diamond ( Perfect Crystals)
Diamond C(220) ReflectionE04.92 keV
Sapphire Reflectivity _at_ 14.3 keV
Sapphire Crystal Quality
Back-reflection topographs of HEMEX sapphire
wafers cut from different boules show different
dislocation densities (a) 103 cm-2, (b) much
lower dislocation density. Sample area
illuminated by x-rays is 2.1 x 1.7 mm2
Chen, McNally et al., Phys. Stat. Solidi. (a) 186
(2001) 365
X-Ray Focusing
  • Focusing is required to adjust the mode profile
  • Bending the Bragg mirrors for a desired curvature
    (50m) may destroy high-reflectivity
  • Possible options
  • Grazing-incidence, curved-mirrors for non
    backscattering configuration
  • Compound refractive lenses of high transmissivity
    can be constructed ( B.Lengeler, C. Schroer, et.
    Al., JSR 6 (1999) 1153)

Options for XFEL-O Cavities (Y. Shvydko)
Al2O3xAl2O3 _at_14.3 keV RT0.87, Gsat15,
T3 CxCxmirror _at_12.4 keV RT0.91, Gsat10,
T4 Al2O3xAl2O3xSiO2_at_ 14.4125 keV RT0.82,
Gsat22 , T4
Gain Calculation
  • Analytic formula for low signal including
    diffraction and electron beam profile
  • Sufficiently simple for Mathematica evaluation if
    electron beam is not focused, distributions are
    Gaussian, and ZRayleigh b
  • Steady state GENISIS simulation for general
    intra-cavity power to determine saturation power
    (Sven Reiche)

Saturation As circulating power increases, the
gain drops and reaches steady state when gainloss
E7 GeV, ?1Å Q19 pC (Ip3.8A), Nu3000 Mirror
reflectivity90 Saturation power19 MW
E7 GeV, ?1Å Q40 pC (Ip8 A), Nu3000 Mirror
reflectivity80 Saturation power21 MW
Examples XFEL-O
Electrons are not focused but matched to the
optical mode determined by cavity configuration
st2 ps, sDE1.4 MeV, ZRb1012 m
Simulation of Oscillator Start-up
  • Time-dependent oscillator simulation using GENO
    (GENESIS for Oscillator) written by Sven
  • Taking into account FEL interaction (GENESIS),
    optical cavity layout, and mirror bandwidth
  • To reduce CPU
  • Follow a short time-window (25 fs)
  • Track a single frequency component for all
    radiation wavefronts since other components are
    outside the crystal bandpass
  • Even with these simplifications, one pass takes
    about 2 hr

Start-up Simulation (Reiche)
Pessimistic case
Ip4 A, mirror loss10 Effective net gain6
Super-mode Analysis (adapted from G. Dattoli, P.
  • Describes gain and spectrum narrowing in the
    exponential gain regime taking into account the
    profiles of I(z-ct) and Gmono(w)
  • Eigenmode Gauss-Hermite function
  • topt(2teltM)1/2/g1/4 tM1/(2swM) swMmirror
  • Amplitude growth rate of the fundamental mode
  • L00.5(g-a)-(0.5u/tM)2-0.5g1/2(tM/tel) upulse
  • hswM2.8 meV, tel2 ps
  • Bandwidth of the fundamental mode
  • hswopt2.3 meV ?De/e2 10-7!!

  • Reduction in gainlt1
  • Pulse to pulse overlap ult20 fs
  • Cavity detuning (tolerance on cavity length)lt3mm
  • Change in optical axislt0.1mode angle
  • angular tolerance of crystals lt8 nrad
  • LIGO technology?

X-FELO Repetition Rate
  • frep 1.5 MHz when one x-ray pulse stored in 100
    m optical cavity
  • I60 mA (Q40 pC), P0.4 MW ?May not need ERL
  • frep100 MHz with ERL?
  • Thermal loading on crystals is tolerable
  • Electron rms energy spread increases from 0.02
    to 0.05
  • With increased energy spread, the loss in the ERL
    return pass becomes 2 10-5
  • These problems may be solved by increasing the
    minimum recovery energy to 30 MeV (higher than
    usual 10 MeV)

Tunability with two crystals
  • Tuning range is very limited (lt2 10-6) due to the
    need to keep 2f lt 4 mr for high reflectivity of
    grazing incidence miror

Tunable Cavity Scheme (KJK)
  • For tuning increase H and decrease S keeping the
    round trip path length the same
  • L100m, H01m, S00.1m?De/e5 10-4 (Hmax3.3 m)
  • L100m, H01m, S02m ?De/e1 (Hmax14.3 m)
  • With this scheme, diamond may be used for most
  • With 2fmax60 degree (444) for 12ltelt15 keV
  • (220) for 5ltelt6 keV

Gun technologies
SCSS CeB6, thermionic, pulsed gun
Cornell laser-driven 750 kV, DC
LBL 50 MHz, laser driven
Ultra-Low Emittance lt50 MHz Injector(P.
Ostroumov, Ph. Piot, KJK)
  • Use a small diameter thermionic cathode to
    extract low emittance beam
  • Provide 500 kV extracting voltage using low
    frequency 50 MHz room temperature RF cavity
  • Using chicane and slits form a short 1 nsec
  • Remove energy modulation by a 6th harmonic cavity
  • Use a pre-buncher an booster buncher to form low
    longitudinal emittance of the bunched beam
  • Accelerate to 50 MeV using higher harmonic SC
  • Use an RF cosine-wave chopper to form any
    required bunch repetition rate between 1 MHz and
    50 MHz. (ANL Invention Application, IN-06-093)

Performance of X-FELO
  • Spectral range 5 keVlteglt20 keV
  • Full transverse and temporal coherence in 1 ps
  • (Dn/n)FWHM2.5 10-7 hDn2 meV (rms)
  • Tunable
  • 109 photons ( 1 mJ) /pulse
  • Peak spectral brightnessLCLS
  • Rep rate 1-100 MHz ? average spectral brightness
    (1026 -1029) /(mm-mr)2(0.1BW)
  • The average spectral brightness is higher by a
    factor of
  • 105-107 than other future light sources
    considered so far, ERL-based or high-gain
  • Current APS about 100-1000 less than ERL

Science Drivers for XFEL-O
  • Inelastic x-ray scattering (IXS) and nuclear
    resonant scattering (NRS) are flux limited
    experiments! Need more spectral flux in a meV
  • Undulators at storage rings generate radiation
    with 100-200 eV bandwidth. Only 10-5 is used,
    the rest is filtered out by meV monochromators.
  • Presently _at_ APS 5 109 photons/s/meV (14.4
  • XFEL-O is a perfect x-ray source for
  • high-energy-resolution spectroscopy (meV IXS, neV
    NRS, etc.), and
  • imaging requiring large coherent volumes.
  • Expected with XFEL-O 1015 photons/s/meV (14.4
    keV) with 107 Hz repetition rate.

Concluding Remarks
  • A X-FELO around 1-Å is feasible with high-quality
    e-beams contemplated from future ERLs
  • However, the rep rate of an X-FELO can be 100 MHz
    or lower to 1 MHz?Injector may be less
  • An X-FELO is new type of future light sources (
    in addition to high-gain FELs and ERLs)