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The X-ray Free-electron Laser: Exploring Matter at the

angstrom-femtosecond Space and Time Scales

C. PellegriniUCLA/SLAC

C. Pellegrini, August 7, 2014

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Because they have the unique capability of generating high intensity, coherent X-ray pulses at angstroms wavelength and femtoseconds pulse duration, the characteristics time and space scale for atomic and molecular phenomena.

C. Pellegrini, August 7, 2014

Why x-ray free electron lasers?

X-ray FELs give us a new window on atomic and molecular phenomena of interest to biology, chemistry and physics, the phenomena of the world around us and of ourselves.

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Development of X-Ray lasers has been a major direction in laser physics almost from the time the first laser was developed in 1960. In the conventional atom-based laser approach this task is extremely difficult, because of the very short lifetime of excited atom-core quantum energy levels. Together with the large energy needed to excite inner atomic levels, 1 to 10 KeV compared to about 1 eV for visible lasers, this leads to a requirement for very intense pumping levels to attain population inversion.

Early work on X-ray lasers

C. Pellegrini, August 7, 2014

Scientists at LLNL used a nuclear weapon to drive an X-ray laser in the Dauphin experiment, apparently with success, in 1980.

Ted Maiman (25 years after first Ruby laser)

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The X-ray free-electron laser (X-ray FEL)a user facility

C. Pellegrini, August 7, 2014

They are the only instruments allowing us to explore matter at the length and time scale typical of atomic and molecular phenomena: Bohr atomic radius, about 1 Å, Bohr period of a valence electron, about 1 fs.

X-ray FELs properties: Tunability, 20-0.1nm Full transverse coherence Longitudinal coherence, near transform limited Pulse duration, few to 100fs Peak Power, 20-100 GW Expandable to TW in the future 1010ph/fs, more at TW level

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Plot from J. Ullrich, A. Rudenko, R. Moshammer, Ann. Rev. Phys. Chem. 63, 635 (2012)

X-ray FELs and other light sources

The jump by 9 orders of magnitude obtained at LCLS in 2009 is a remarkable event.

C. Pellegrini, August 7, 2014

Brilliance, also called brightness, is a measure of the coherence of the photon beam. Improved longitudinal coherence will further increase the brilliance.

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Linac Coherent Light Source at SLAC

Injector

Linac (1 km)

Near Experiment Hall

Far ExperimentHall

Undulator (130 m)

A new era in x-ray sources and science

1.5-15 Å(14-4.3 GeV)

X-ray Transport (200 m)

LLNL

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C. Pellegrini, August 7, 2014

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X-ray FEL physics: One electron of energy E =mc2 γ

Undulator with NU periods and magnetic field on axis BU.The electron has a sinusoidal trajectory around the axis.

Each electron emits a wave train with NU periods

For a case like that of LCLS:

C. Pellegrini, August 7, 2014

Line width

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Superposition of wave trains emitted bymany, Ne, electrons

Synchrotron radiation sourcesDisordered state, single electron wave trains superimpose with random phases. Intensity ~ Ne

X-ray FELOrdered state, all wave trains are in phase. Intensity ~ Ne2

Ne is about109- 1010 . Large possible gain. At 1Å we have about 103 -104 electrons per wavelength. How do we squeeze them in one tenth of the wavelength and have these micro-bunching separated exactly by λ? How do we go from disorder to order? Answer: FEL

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RandomWell bunched

SASE: a beam self-organization effect.

λ

Evolution of power and longitudinal beam density along the undulator from spontaneous radiation to FEL amplified radiation.

The self organization effect can start from the initial noise at the undulator radiation frequency in the electron beam longitudinal distribution, the same that gives the spontaneous radiation. This is a SASE FEL. The instability produces an ordered distribution in the beam, similar to a 1-d relativistic crystal.

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2009: LCLS works!

•The LCLS X-ray pulse duration and intensity can be changed from 100 to a few femtosecond and 1013 to 1011

photons/pulse, over the wavelength range of 4 to 0.12 nm. Emma P. et al. ScienceThe X-ray pulse wavelength, intensity and duration can be optimized for each experiment, something not possible in other X-ray sources.

Transverse coherence: good!Vartanyants et el. Phys. Rev. Lett. 107, 144801, 2011

Longitudinal coherence: good!J. Amann, et al. Nature Photonics, 2012.180

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Hard X-rays, Ephoton ≥5keV

Soft X-rays, Ephoton ≤1 keV

New SLAC Xray FEL: 0.25 KeV<Ephoton <25 KeV, under construction

X-ray FELs worldwide summary, 2014

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C. Pellegrini, August 7, 2014

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ConclusionsLCLS, FLASH, Fermi, SACLA are a new class of photon sources that have:

– longitudinal and transverse coherence– control of spectral properties, two colors ..– order of magnitudes larger peak and average brightness

New phenomena are being discovered as we learn to utilize their novel capabilities to explore atomic and molecular science at the fs, Å resolution.