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K. Fang

First Name: K.

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Last Name: Fang

Full Name: K. Fang

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4 papers
title: High efficiency, multiterawatt x-ray free electron lasers
format: journal article
publisher: Physical Review Accelerators and Beams
year: 2016
4 authors: C. Emma | K. Fang | J. Wu | C. Pellegrini
abstract: In this paper we present undulator magnet tapering methods for obtaining high efficiency and multiterawatt peak powers in x-ray free electron lasers (XFELs), a key requirement for enabling 3D atomic resolution single molecule imaging and nonlinear x-ray science. The peak power and efficiency of tapered XFELs is sensitive to time dependent effects, like synchrotron sideband growth. To analyze this dependence in detail we perform a comparative numerical optimization for the undulator magnetic field tapering profile including and intentionally disabling these effects. We show that the solution for the magnetic field taper profile obtained from time independent optimization does not yield the highest extraction efficiency when time dependent effects are included. Our comparative optimization is performed for a novel undulator designed specifically to obtain TW power x-ray pulses in the shortest distance: superconducting, helical, with short period and built-in strong focusing. This design reduces the length of the breaks between modules, decreasing diffraction effects, and allows using a stronger transverse electron focusing. Both effects reduce the gain length and the overall undulator length. We determine that after a fully time dependent optimization of a 100 m long Linac coherent light source-like XFEL we can obtain a maximum efficiency of 7%, corresponding to 3.7 TW peak radiation power. Possible methods to suppress the synchrotron sidebands, and further enhance the FEL peak power, up to about 6 TW by increasing the seed power and reducing the electron beam energy spread, are also discussed.
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title: TAPERING STUDIES FOR TW LEVEL X-RAY FELS WITH A SUPERCONDUCTING UNDULATOR AND BUILT-IN FOCUSING
format: conference proceeding
conference: FEL 2015 37th
year: 2015
4 authors: C. Emma | C. Pellegrini | K. Fang | J. Wu
abstract: Tapering optimization schemes for TeraWatt (TW) level X-ray Free Electron Lasers (FELs) are critically sensitive to the length of individual undulator and break sections. Break sections can be considerably shortened if the focusing quadrupole field is superimposed on the undulator field, increasing the filling factor and the overall extraction efficiency of the tapered FEL. Furthermore, distributed focusing reduces the FODO length and allows one to use smaller beta functions, reducing particle de-trapping due to betatron motion from the radial tails of the electron beam. We present numerical calculations of the tapering optimization for such an undulator using the three dimensional time dependent code GENESIS. Time dependent simulations show that 8 keV photons can be produced with over 3 TW peak power in a 100m long undulator. We also analyze in detail the time dependent effects leading to power saturation in the taper region. The impact of the synchrotron sideband growth on particle detrapping and taper saturation is discussed. We show that the optimal taper profile obtained from time independent simulation does not yield the maximum extraction efficiency when multi-frequency effects are included. A discussion of how to incorporate these effects in a revised model is presented.
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title: RADIATION PROPERTIES OF TAPERED HARD X-RAY FREE ELECTRON LASERS
format: conference proceeding
conference: FEL 2014 36th
year: 2014
6 authors: C. Emma | C. Pellegrini | J. Wu | K. Fang | S. Chen | S. Serkez
abstract: We perform an analysis of the transverse coherence of the radiation from a TW level tapered hard X-ray Free Electron Laser (FEL). The radiation properties of the FEL are studied for a Gaussian, parabolic and uniform transverse electron beam density profile in a 200 m undulator at a resonant wavelength of 1.5 Å. Simulations performed using the 3-D FEL particle code GENESIS show that diffraction of the radiation occurs due to a reduction in optical guiding in the tapered section of the undulator. This results in an increasing transverse coherence for all three transverse electron beam profiles. We determine that for each case considered the radiation coherence area is much larger than the electron beam spot size, making coherent diffraction imaging experiments possible for TW X-ray FELs.
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title: Terawatt x-ray free-electron-laser optimization by transverse electron distribution shaping
format: journal article
publisher: Physical Review Accelerators and Beams
year: 2014
6 authors: C. Emma | J. Wu | K. Fang | S. Chen | S. Serkez | C. Pellegrini
abstract: We study the dependence of the peak power of a 1.5 Å Terawatt (TW), tapered x-ray free-electron laser (FEL) on the transverse electron density distribution. Multidimensional optimization schemes for TW hard x-ray free-electron lasers are applied to the cases of transversely uniform and parabolic electron beam distributions and compared to a Gaussian distribution. The optimizations are performed for a 200 m undulator and a resonant wavelength of λr=¼1.5Å using the fully three-dimensional FEL particle code GENESIS. The study shows that the flatter transverse electron distributions enhance optical guiding in the tapered section of the undulator and increase the maximum radiation power from a maximum of 1.56 TW for a transversely Gaussian beam to 2.26 TW for the parabolic case and 2.63 TW for the uniform case. Spectral data also shows a 30% – 70% reduction in energy deposited in the sidebands for the uniform and parabolic beams compared with a Gaussian. An analysis of the transverse coherence of the radiation shows the coherence area to be much larger than the beam spotsize for all three distributions, making coherent diffraction imaging experiments possible.
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