The radiation from SASE FEL has always limited value of the degree of transverse coherence. When transverse
size of the electron beam significantly exceeds diffraction limit, the mode competition effect does not provide the
selection of the ground mode, and spatial coherence degrades due to contribution of the higher order transverse
modes. It is important that the most strong higher modes are azimythally non-symmetric which leads to
fluctuations of the spot size and of the pointing stability of the photon beam. These fluctuations are fundamental
and originate from the shot noise in the electron beam. The effect of the pointing instability becomes more
pronouncing for shorter wavelengths. We analyze in detail the case of optimized SASE FEL and derive universal
dependencies applicable to all operating and planned x-ray FELs. It is shown that x-ray FELs driven by low
energy electron beams will exhibit poor spatial coherence and bad pointing stability.
E. A. Schneidmiller and M. V. Yurkov, "Transverse coherence and pointing stability of the radiation from x-ray free electron lasers," Proc. SPIE 10237, Advances in X-ray Free-Electron Lasers Instrumentation IV, 102370I (Presented at SPIE Optics + Optoelectronics: April 27, 2017; Published: 14 June 2017); https://doi.org/10.1117/12.2265567.
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