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The X-ray Free-electron Laser: Exploring Matter at the angstrom-femtosecond Space and Time Scales

The X-ray Free-electron Laser: Exploring Matter at the angstrom-femtosecond Space and Time Scales. C. Pellegrini UCLA/SLAC. Why x-ray free electron lasers?.

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The X-ray Free-electron Laser: Exploring Matter at the angstrom-femtosecond Space and Time Scales

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  1. The X-ray Free-electron Laser: Exploring Matter at the angstrom-femtosecond Space and Time Scales C. Pellegrini UCLA/SLAC

  2. 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. 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.

  3. Early work on X-ray lasers 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. Ted Maiman (25 years after first Ruby laser) Scientists at LLNL used a nuclear weapon to drive an X-ray laser in the Dauphin experiment, apparently with success, in 1980.

  4. TheX-ray free-electron laser (X-ray FEL) a user facility • 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 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.

  5. X-ray FELs and other light sources The jump by 9 orders of magnitude obtained at LCLS in 2009 is a remarkable event. Plot from J. Ullrich, A. Rudenko, R. Moshammer, Ann. Rev. Phys. Chem. 63, 635 (2012) Brilliance, also called brightness, is a measure of the coherence of the photon beam. Improved longitudinal coherence will further increase the brilliance.

  6. Injector Linac(1 km) X-ray Transport (200 m) Undulator (130 m) Near Experiment Hall LLNL UCLA Far Experiment Hall Linac Coherent Light Source at SLAC A new era in x-ray sources and science 1.5-15 Å (14-4.3 GeV)

  7. X-ray FEL physics: One electron of energy E=mc2γ Undulator with NUperiods and magnetic field on axis BU. The electron has a sinusoidal trajectory around the axis. Each electron emits a wave train with NUperiods Line width For a case like that of LCLS:

  8. Superposition of wave trains emitted by many, Ne, electrons Synchrotron radiation sources Disordered state, single electron wave trains superimpose with random phases. Intensity ~ Ne X-ray FEL Ordered 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

  9. SASE: a beam self-organization effect. 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. Evolution of power and longitudinal beam density along the undulator from spontaneous radiation to FEL amplified radiation. λ Random Well bunched

  10. 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 4to 0.12 nm. Emma P. et al. ScienceTheX-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

  11. X-ray FELs worldwide summary, 2014 Hard X-rays, Ephoton ≥5keV Soft X-rays, Ephoton ≤1 keV New SLAC Xray FEL: 0.25 KeV<Ephoton <25 KeV, under construction

  12. Conclusions • LCLS, 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.

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