Angular Distribution of Nonlinear Harmonic Generation in Helical   Undulators: a comparison between experiments and theory
  Using harmonic emission from circularly polarized undulator is a procedure that is normally employed on synchrotron beamlines in order to extend the covered spectral range. A similar capability is likewise beneficial for next generation free-electron lasers. In this paper, we perform a first quantitative experimental analysis of the angular distribution of free-electron laser harmonic emission from helical undulators. Experimental results are compared to those obtained by means of a theoretical model based on the paraxial solution of Maxwell's equations. 
E. Allaria, G. De Ninno, G. Geloni, C. Spezzani
physics.optics