Features of operation of a FEL with reversed guide field

Abstract

Features of the nonlinear regime of a FEL with reversed guide field operating in the millimeter wavelength range have been studied numerically by particle techniques. The FEL efficiency for both conventional and reversed guide fields for the case in which the simulating particles were on a steady state trajectory at the entrance to the interaction region was rather high. Taking into account a significant spread of the transverse electron impulses which exists in the experiment due to, for example, a nonadiabatic wiggler field increase at the entrance to the FEL interaction region results in a significant decrease of efficiency for the FEL with conventional guide field orientation while it remains practically unchanged for the FEL with reversed guide field.For the FEL parameters close to the experimental ones an analysis of electron motion shows that for reversed guide field the period and amplitude of betatron oscillations are significantly less than for the conventional guide field case. Most likely, that is the reason which leads to the substantial changing of the interaction in the two cases.