• G. Geloni, E. Saldin, E. Schneidmiller, M.V. Yurkov
  DESY, Hamburg

Nonlinear generation of coherent harmonic radiation is an important option in the operation of a X-ray FEL facility since it broadens the spectral range of the facility itself, thus allowing for a wider scope of experimental applications. We found that up-to-date theoretical understanding of second harmonic generation is incorrect. Derivation of correct radiation characteristics will follow our criticism.