Efficiency optimization in a FEL with fields' nonadiabatic tapering

Amplification of an electromagnetic wave in free electron lasers with a reversed guide field and right-hand polarized wiggler field is investigated both analytically and numerically. An effect of electron bunch trapping by the high frequency electromagnetic field is used for efficiency optimization. On the basis of motion stability criteria a possibility of bunches trapping by FEL parameters nonadiabatic (experimentally realizable) tapering is shown. The stability analysis of electron motion is based on Lyapunov theory for autonomy systems. A particle simulation is carried out for FEL parameters close to the experimental ones (relativistic factor γ=4.75, wiggler field strength Bw= 2.8 kG, guide field strength Bo= -1.4 kG, operation wavelength λ=6.2 mm) for the case of wiggler field tapering. Theoretically predicted rule of wiggler field tapering corresponding to FEL efficiency of 55% is approximated by stepped functions. For the experimentally realizable tapering it is found that FEL efficiency can be over 40%