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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

Outline and Acknowledgment

- MaRIE Signature Science Facility
- 50-keV X-ray FEL
- Issues with Energy Diffusion
- Pre-bunched X-ray FEL
- Electrons Channeling in Si Crystals
- MEDUSA Simulation Results
We thank Henry Freund (SAIC) for the use of MEDUSA FEL simulation code, and Paul Channell (LANL) for helpful discussion.

MaRIE(Matter-RadiationInteractionsinExtremes)

- The Multi-probe Diagnostic Hall will provide unprecedented probes of matter.
- X-ray scattering capability at high energy and high repetition frequency with simultaneous proton imaging.

- The Fission and Fusion Materials Facility will create extreme radiation fluxes.
- Unique in-situ diagnostics and irradiation environments comparable to best planned facilities.

- The M4 Facility dedicated to making, measuring, and modeling materials will translate discovery to solution.
- Comprehensive, integrated resource for controlling matter, with national security science infrastructure.

LANSCE Accelerator

Why 50 keV XFEL?

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.

MaRIE XFEL Temporal Pulse Format

Two RF pulses separated by an adjustable delay

~1 ms

1 - 100 ms

Each macropulse consists of 10-100 micropulses

30 ns

Each micropulse has ~1011 x-ray photons

<1 ps

0.3 ns

Linear Accelerator

Beam Conditioning

Undulator

X-rays Beam

Electron Beam Dump

MaRIE 50-keV XFEL Baseline Concept2

(1.264 x10cm)

Energy diffusion limits the maximum beam energy and undulator lengthEnergy diffusion due to quantum fluctuations*

2

gK0N

3

2

dg

u

g

2

2

l (1 + 1.33K0 + 0.4K0 )

u

* 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

- Start up with coherent bunched beam radiation.
P(z) = Pc e

- With higher start-up radiation intensity and optical guiding, FEL will saturate in shorter undulator length, thus reducing energy diffusion.
Energy diffusion ~ Lu

z/LG

Pre-bunched beam FEL minimizes energy diffusion by reducing undulator lengthStart-up in SASE and Pre-bunched FEL on the order of a radiation wavelength

- SASE starts up with spontaneous radiation from randomly distributed N electrons in a bunch
Radiation intensity ~ N

- Pre-bunched beam FEL starts up with coherent radiation from periodically bunched beam with N electrons
Radiation intensity ~ N2

Coherent start-up with pre-bunched beams

- shortens the saturation length
- reduces energy spread due to quantum fluctuations
- extends XFEL to higher photon energy (shorter l)

g on the order of a radiation wavelength

g

g

f

f

f

How 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.

Axial Channeling of Relativistic Electrons in Silicon Crystals

Energy loss rate vs. electron incident angle

Interplanar spacing

1.36 Å

1.92 Å

2.35 Å

Lindhard angle

30 mrad

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.

Experimental Results of GeV Electron Channeling in Thin Silicon Crystals

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

MaRIE XFEL Baseline Parameters Silicon Crystals

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

Time-independent Simulations for Silicon Crystals50-keV SASE FEL with MEDUSA

FEL peak power (blue) shows saturation at z > 115 m

FEL mode radius (red) shows no optical guiding at z < 70 m

Time-independent Simulations for Silicon CrystalsPre-bunched Beam FEL with MEDUSA

FEL peak power (blue) shows saturation at z = 20 m

FEL mode radius (red) shows optical guiding at z < 2 m

Undulator Length

Power (W) Silicon Crystals

Undulator length (m)

Pre-bunched beam start-up power is 300,000 times SASE start-up noisePre-bunched beam XFEL saturates while SASE is still in lethargy region.

Time-dependent MEDUSA Calculations for Pre-bunched Beam XFEL at 50 keV

Power vs z for slice # 146

Peak power vs slice number

Power (GW)

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.

Effect of Normalized rms Emittance on SASE and Pre-bunched XFEL

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.

Conclusions XFEL

- Los Alamos is studying the feasibility of a 50-keV X-ray FEL for materials science research.
- We propose to pre-bunch the electron beam periodically at the radiation wavelength before injecting the electron beam into the undulator.
- The periodically bunched beam 50-keV XFEL saturates in 20 meters of undulator length, while SASE takes 115 meters to saturate.
- Unlike SASE, the periodically bunched beam FEL does not require exceptionally small rms emittance.

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