A Ka-band Ubitron

In this paper we present theoretical results and design parameters of a Ka-band Ubitron (a low voltage free-electron laser) under construction at the Naval Research Laboratory. The objective of this experiment is to demonstrate high gain and efficiency in a ubitron amplifier configuration. As an aid in selecting the experimental parameters a fully self-consistent 3-dimensional theory of the ubitron interaction has been derived. This analysis includes the transverse variations in the helical wiggler field, finite waveguide geometry, and space charge effects. The source current and charge density for the Maxwell-Poisson equations are found by perturbation about the steady-state trajectories in the combined axial and helical magnetic fields. With the assumption of a thin, monoenergetic beam, a dispersion equation is obtained and solved numerically to give the linear gain as a function of signal frequency. A confined-flow Pierce gun is being designed for the experiment. The gun will operate at 250 kV and approximately 100 amperes. The electrostatic and magnetic profiles are being adjusted to achieve a high beam quality. The wiggler magnetic field will be produced by a bifilar helix electromagnet and the guide field by solenoidal windings. The RF interaction will take place with the TE11 fundamental circular waveguide mode.