ucla x-ray free-electron lasers - unina.itultrafast09.fisica.unina.it/contributi/pellegrini.pdfucla...

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Claudio Pellegrini UCLA

Department of Physics and Astronomy

X-ray Free-electron Lasers

Ultra-fast Dynamic Imaging of Matter II Ischia, Italy, 4/30-5/3/ 2009

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Ischia 5/1/2009 C. Pellegrini 2

Outline

1. Present status of X-ray free-electron lasers (FELs). 2.  Near future developments, using existing accelerator technology,

electron/radiation pulses manipulation, HHG seeding, to produce: shorter pulses; improved longitudinal coherence; higher average brightness; wavelengths<0.1 nm; more compact, less costly FELs.

3  Further future developments, will use new electron sources, advanced laser-plasma accelerator, novel short period undulators for very compact, table-top FELs.

4 Conclusions.

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Peak brightness of existing X-ray FELs, Flash and LCLS

An old plot with superimposed FLASH and the very new LCLS data. Good predictions. Peak power in the 1-10 GW range.

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LCLS first lasing at SLAC, April 2009, at 0.15 nm

Courtesy LCLS group, SLAC

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LCLS: FEL parameters

Wavelength 15 1.5 Å FEL parameter 8.5 4.2 10-4 Cooperation length 282 57 nm Peak saturation power 4 8 GW Average saturation power 0.23 0.23 W Coherent photons/pulse 10.6 1.1 1012 Peak photon flux 31 5.8 1024 Ph/s Peak brightness* 0.28 15 1032 Average brightness* 0.16 4.5 1022 Instantaneous photon ΔE/E 0.07 0.03 % Beam radius, rms. 49 36 µm Beam divergence, rms. 2.4 0.33 µrad Pulse duration, rms, 70 70 fs

*Ph./s/mm2/mrad2/.1%bw

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LCLS: Other parameters

Pulse repetition rate 120 Hz Single spike duration (0.15 nm) 1 fs Number of spikes 200 Spike line width 3x10-4

… and, mostly important, more than109 photons per coherent volume (compare with less than 1 for spontaneous synchrotron radiation sources). This, and the FEL attribute of simultaneous short wavelength and short pulse length, nanometer to sub-nanometer, sub-picosecond to femtosecond, will lead us to explore a new range of phenomena.

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•  Pulse trains of up to 800 µs duration •  Up to 10 Hz repetition rate (currently 5 Hz) •  Fixed gap undulators (Tune with electron beam energy) •  Wavelength range (fundamental): 6-47 nm •  Pulse energy average: 100 µJ •  Peak power: ~ 5 GW •  Pulse duration (FWHM): 10-50 fs •  Spectral width (FWHM): 0.5-1 %

Courtesy Siegfried Schreiber, DESY

Electron beam pulse train (30 bunches, 1 µs spacing, 5 Hz rep rate)

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Transverse coherence at FLASH

Double-slit diffraction pattern at 25.5 nm indicates good level of transverse coherence.

(from XFEL TDR)

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UV to X-ray FEL next developments

Photon energy, keV 0.010-100 Pulse repetition rate, Hz 100-105

Pulse duration, fs <1-1000 Coherence, transverse Very good Coherence length LBunch to LCooperation (300-0.1µm) Peak Brightness 1030 -1034 ph/mm2mrad2s 0.1%bw Average brightness 1018 -1027 ph/mm2mrad2s 0.1%bw Polarization Variable, linear to circular

Flash, LCLS, and the next FELs, like XFEL, SCSS, Fermi, are only the beginning of the road. Present and next generation FEL capabilities can be extended to these parameters:

UCLA Two examples

As an example of FELs development I will discuss two examples:

•  Very short radiation pulses at about 1 and 0.15 nm •  High energy photon FELs, E>10keV


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Example of developments: from 100fs to ultra-short X-ray pulses

Many methods have been proposed to reduce the pulse length to the fs range: slotted spoiler; ESASE; two stage undulator with energy chirped pulse. All these methods select and use part of the electron bunch to lase. Pulse length can be as short as 1fs or less. The number of photons in the pulse is reduce by the number of electron lasing to the total number of electrons. There is a spontaneous radiation pedestal. Another possibility studied recently is to operate the FEL at low charge[1,2]. I use this case to illustrate what can be achieved.

[1] Rosenzweig et al. ”, Nuclear Instruments and Methods, A 593, 39-44 (2008). [2] Reiche, Rosenzweig, Musumeci, Pellegrini, Nucl. Instr. And Methods, A 593, 45-48 (2008).

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Recent studies show that a smaller emittance (x 0.1), larger electron beam brightness (x 10-100) and very short, ~ 1fs or less, electron bunches can be produced by reducing the bunch charge from about 1nC to few, 1 to 10 pC, and using velocity and magnetic bunching .

Low charge electron bunches for ultra-short X-ray pulses

SPARX: E=2 GeV, λ=3 nm, Single spike σB=0.48 µm (1.6 fsec), 2x1010 photons in the pulse.

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LCLS 1pC example: attosecond pulses.

Beam current profile

Single spike at saturation, with 1010 photons.

Beam brightness~ 4x1017 A/m2 rad2 compared to 6x1015 A/m2 rad2

for the 1 nC design case.

Peak power vs. z

λ = 0.15nm,σ E = 10−4 ,σ L = 160nm(530as).

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LCLS at low charge, short pulses


20 pC λ = 0.15nm 20 pC λ = 1.5nm

Y. Ding and the LCLS group, SLAC, and C. Pellegrini, UCLA, 2009 Part. Acc. Conf.

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LCLS at low charge, short pulses

Y. Ding and the LCLS group, SLAC, and C. Pellegrini, UCLA, 2009 Part. Acc. Conf.

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60 KeV, short pulse FEL, using low emittance electron bunches

FEL Parameters

Wavelength 0.02 nm FEL parameter 0.0004 Gain Length 2.7 m Saturation power 1 GW Coherent Photons 108

Pulse length 0.5 fs

The small emittance at 1pC can be used to obtain high energy photons at low beam energy with a short period undulator.

Beam Parameters Energy =11.5 GeV IP =800 A εN=6.x10-8 m σE= 10-4

Undulator λU=0.015 m K=1

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0.02nm, short pulse FEL

Pulse duration~0.5 fs

Power vs undulator length

Spectrum

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….. or a Compact soft X-ray FEL

•  Beam energy 1.4 GeV •  S-band injector+X or C-band linac, length <35 m •  1.5 cm period, K=1, undulator, length 15 m •  Bunch charge, 1-20 pC •  Pulse length, 1 to few fs •  Number of coherent photons/pulse, 1010-1012

•  Linac repetition rate, 120 Hz •  X-ray Pulses in one linac pulse, 1 to 100 •  Synchronization to external laser using the signals from the

photoinjector laser and the coherent radiation from the electron bunch after compression.

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Optical manipulations techniques (1)

Bunching Acceleration SASE Modulation

A. Zholents, Phys. Rev. ST Accel. Beams 8, 040701 (2005)

•  Precise synchronization of x-ray output with the modulating laser •  Variable output pulse train duration •  Increased peak current and shorter x-ray undulator •  Solitary ~100-attosecond duration x-ray pulse

Peak

cur

rent

, I/I 0

z /lL

20-25 kA

< fs section

ESASE

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Example with seed at 30 nm, radiating in the water window First stage amplifies low-power seed with “optical klystron”

More initial bunching than could be practically achieved with a single modulator Output at 3.8 nm (8th harmonic)

300 MW output at 3.8 nm (8th harmonic) from a

25 fs FWHM seed

1 GeV beam 500 A

1.2 micron emittance 75 keV energy spread

Modulator l=30 nm, L=1.8 m

Modulator l=30 nm, L=1.8 m

Radiator l=3.8 nm, L=12 m

100 kW l=30 nm

Lambert et al., FEL04

Optical manipulations techniques

HHG LASER SEED

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Future developments II

•  Plasma laser accelerators, 1 GeV/m or more, would greatly decrease the linac length.

•  Novel electron sources , using plasmas or ultra-cold gases or .?., reducing the emittance below 0.1 mm mrad, and increasing the beam 6-D phase space density would give: –  Reduced beam energy for same wavelength –  Larger FEL parameter -> larger efficiency, photon

number/electron, shorter pulse duration

•  Undulators with small period, and large gap/period ratio, like microwave undulator or other new ideas, would also reduce the beam energy for the same wavelength.

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Conclusions

 FLASH and LCLS are only the beginning of a new class of photon sources which will:

–  Allow the exploration and manipulation of matter at the Angstrom-femtosecond level and the study of nonlinear phenomena

–  Give fully longitudinal and transverse coherence –  Provide order of magnitudes larger peak and average

brightness –  Reduce the cost and size of the sources while extending

their performance

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