A comparison of various schemes of beam conditioning for FEL

It is proposed in this report to employ different devices from the one used in [A.M. Sessler, D.H. Whittum and L.-H. Yu, Phys. Rev. Lett. 68 (1992) 309], namely, a combination of two identical plane undulators with the second shifted a quarter-period along the common axis and rotated 90° around the axis relative to the first or a helical undulator. The proposed scheme [V.A. Papadichev, Proc. 16th Int. FEL Conf., Stanford University, Stanford, USA, August 21–26, 1994, Nucl. Instr. and Meth. A 358 (1995) ABS 79] using a helical undulator combined with uniform magnetic field is analysed. The method of exciting the necessary profile of transverse cyclotron velocity in the beam is analysed in more detail.