Beam transmission in a high-power collective free-electron laser

Abstract

A recent free-electron laser amplifier experiment conducted at the Massachusetts Institute of Technology [M. E. Conde and G. Bekefi, Phys. Rev. Lett. 67, 3082 (1991)] has demonstrated high-power operation without recourse to a tapered wiggler field. The experimental configuration consisted in the propagation of an intense electron beam (750 keV/300 A with a nominal axial energy spread of 1.5%) through a cylindrical waveguide in the presence of both a helical wiggler (Bw≤1.8 kG and λw=3.18 cm) and an axial guide magnetic field (B0≤12 kG). The experiment operated with the axial guide field oriented both parallel and antiparallel to the direction of the wiggler field, and the maximum efficiency was obtained for the antiparallel (i.e., reversed-field) configuration. The reversed-field case demonstrated an output power of 61 MW at 33.39 GHz for an efficiency of approximately 27%. The performance in the more usual parallel alignment of the fields was much less and peak power levels of only about 4 MW were obtained for both the weak (group I) and strong (group II) field regimes of the axial guide field. A detailed analytical characterization of this experiment has been presented in a previous work [H. P. Freund and A. K. Ganguly, IEEE Trans. Plasma Sci. PS-20, 245 (1992)] in which substantial agreement was found between the theory and the experiment for the reversed-field configuration. However, some discrepancies existed for the group I and II cases, and it was conjectured that some problem with beam transport existed for these configurations which led to an increased beam energy spread. In this paper, the question of beam transport in this experiment is analyzed. It is shown that beam transport is not a problem for the reversed-field configuration. However, substantial beam losses are found in the group I and II regimes, both in the entry taper region of the wiggler and due to high-power electromagnetic waves.