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Reiche, S.

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TUBAU05 One Angstrom FEL Oscillator using ERL Beams  
 
  • S. Reiche
    UCLA, Los Angeles, California
  • s. Shvydko, K.-J. Kim
    ANL, Argonne, Illinois
 
  An oscillator X-ray FEL for 1-Å is feasible with ERLs. We have studied a 1-Å FEL using electron beams extrapolated from the "high coherence" mode of the proposed Cornell ERL, using the electron energy = 7 GeV, the undulator parameter K=1.4, and period length=1.88 cm. With a 30-m undulator the small signal gain is about 20%, sufficient for "lasing" if one round trip reflectivity is greater than 90%. The gain will be higher for a higher bunch current achievable with further optimization of the gun. The peak power of the circulating optical beam at saturation is about 20 MW and its bandwidth 10-6. The increased energy spread of the electron beam due to the FEL interaction does not pose problem for the recirculation optics. Two possible schemes for optical cavity are possible. One is a cavity of regular triangle with three crystal reflectors. Another is to use a cavity consisting of two Bragg reflectors at near-backscattering configuration and a grazing incidence mirror in between. Parasitic diffraction in backscattering of a cubic crystal provides a convenient out-coupling mechanism. The fraction of parasitic diffraction can be set to a small, desired value while keeping the high reflectivity in the main diffraction by suitably orienting the crystals away from the exact backscattering geometry. The mirror serves also the important function of focusing the x-ray beam.  
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TUPPH018 Development of Ultra-short Pulse, Single Coherent Spike for SASE X-ray FELs  
 
  • P. Musumeci, C. Pellegrini, S. Reiche, J. B. Rosenzweig
    UCLA, Los Angeles, California
 
  There is a large interest in the production of high power, ultra-short, one femtoseconds or less, coherent X-ray pulses, for atomic physics and other applications. However the present design of X-ray SASE FELs leads to an X-ray pulse about 100 times longer. Several methods to reduce the bunch length to the 10-1 fs region have been proposed. These methods are based on electron bunch manipulation to cut the lasing part of the bunch to a fraction of the total length, thus reducing the X-ray pulse length. We are considering here a different method, using ultra-short, very low charge electron bunches, with a length of the order or shorter than the FEL cooperation length. In this case the X-ray pulse length after amplification in the undulator is a few times the electron bunch length. Our simulations show that in an LCLS-like case we can obtain coherent, Fourier transform limited, X-ray pulses, consisting of a single spike, with a FWHM of about 0.1 um, corresponding to about 300 as, a peak power of about 5 GW, and an intensity of about 10 uJ.