Empirical Optimization of Undulator Tapering at FLASH2 and Comparison   with Numerical Simulations
  In a free-electron laser equipped with variable-gap undulator modules, the technique of undulator tapering opens up the possibility to increase the radiation power beyond the initial saturation point, thus enhancing the efficiency of the laser. The effectiveness of the enhancement relies on the proper optimization of the taper profile. In this work, a multidimensional optimization approach is implemented empirically in the x-ray free-electron laser FLASH2. The empirical results are compared with numerical simulations. 
Alan Mak, Francesca Curbis, Bart Faatz, Sverker Werin
physics.acc-ph