A 50-keV X-ray Free-Electron Laser with Periodically Bunched Electron Beam Dinh Nguyen, Steve Russell, Bruce Carlsten and Cris Barnes Los Alamos National Laboratory XFEL Workshop Lawrence Berkeley Laboratory October 23-25, 2008. Outline and Acknowledgment. MaRIE Signature Science Facility
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A 50-keV X-ray Free-Electron Laser withPeriodically Bunched Electron BeamDinh Nguyen, Steve Russell, Bruce Carlsten and Cris BarnesLos Alamos National LaboratoryXFEL WorkshopLawrence Berkeley LaboratoryOctober 23-25, 2008
We thank Henry Freund (SAIC) for the use of MEDUSA FEL simulation code, and Paul Channell (LANL) for helpful discussion.
MaRIE seeks to probe insidemultigranular samples of condensed matter that represent bulk performance properties with sub-granular resolution. With grain sizes of tens of microns, "multigranular" means 10 or more grains, and hence samples of few hundred microns to a millimeter in thickness. For medium-Z elements, this requires photon energy of 50 keV or above.
This high energy also serves to reduce the absorbed energy per atom per photon in the probing, and allows multiple measurements on the same sample. Interest in studying transient phenomena implies very bright sources, such as an XFEL.
Two RF pulses separated by an adjustable delay
1 - 100 ms
Each macropulse consists of 10-100 micropulses
Each micropulse has ~1011 x-ray photons
Electron Beam DumpMaRIE 50-keV XFEL Baseline Concept
(1.264 x10cm)Energy diffusion limits the maximum beam energy and undulator length
Energy diffusion due to quantum fluctuations*
l (1 + 1.33K0 + 0.4K0 )
* Z. Huang and K.J. Kim, “Review of x-ray free-electron laser theory”
Phys. Rev. Special Topics – Accelerators and Beams, 10, 034801 (2007)
Periodically bunch electron beams with density modulations on the order of a radiation wavelength
P(z) = Pc e
Energy diffusion ~ Lu
z/LGPre-bunched beam FEL minimizes energy diffusion by reducing undulator length
Radiation intensity ~ N
Radiation intensity ~ N2
Coherent start-up with pre-bunched beams
g on the order of a radiation wavelength
fHow do we pre-bunch electron beam with a period of 0.25 Angstrom?
Si crystal as a spatial mask
with 1-2 Å spacing
Chicane transforms energy modulations in x into density modulations in z with a compression factor
D.C. Nguyen and B.E. Carlsten, “Amplified coherent emission with electron beams prebunched in a masked chicane” Nucl. Instr. Meth. Phys. Res. A 375 (1996) 597-601.
Energy loss rate vs. electron incident angle
V.M. Biryukov, Y.A. Shesnokov and V.I. Kotov, “Crystal channeling and its applications at high energy accelerators” Springer Press, Berlin 1997.
J.F. Bak et al., “Channeling radiation from 2 to 20 GeV/c electrons and positrons. Pt. 2. Axial case” Nucl. Phys. B (1988-06-13) Vol.302, iss.4, p.525-558.
S.P. Fomin et al., “Features of Angular Distributions of 1 GeV Electrons Scattered by Thin Silicon Monocrystals” Problems of Atomic Science and Technology. 2001, pp. 138-143
Beam energy 35.32 GeV
Bunch charge 0.25 nCBunch length (FWHM) 74 fs (at undulator)
Peak current 3.4 kA (at undulator)
Transverse emittance 0.2-0.6 mm (at undulator)
rms Energy spread 0.01%
Undulator period (lu) 3 cm
Undulator parameter (Krms) 2.624
Undulator total length 20 m
Wavelength 0.0248 nm
Pierce parameter 1.77 x 10-3
1D Gain length 77.7 cm
Saturation power (prebunched) 1.7 x 1010 watts
Micropulse energy 1.2 mJ
# photons per micropulse 1.5 x 1011
FEL peak power (blue) shows saturation at z > 115 m
FEL mode radius (red) shows no optical guiding at z < 70 m
FEL peak power (blue) shows saturation at z = 20 m
FEL mode radius (red) shows optical guiding at z < 2 m
Power (W) Silicon Crystals
Undulator length (m)Pre-bunched beam start-up power is 300,000 times SASE start-up noise
Pre-bunched beam XFEL saturates while SASE is still in lethargy region.
Power vs z for slice # 146
Peak power vs slice number
Time-dependent calculations show pre-bunched beam FEL to have smooth temporal profile (spectral profile is also expected to be smooth) instead of the usual SASE spiky profile.
Very low emittance is not necessary for pre-bunched XFEL as it is for SASE. Pre-bunched FEL can accept emittance up to 1 mm-mrad.