A free electron laser experiment on "angular steering"

A paper by Jerby (1990) has discussed a number of mechanisms whereby FEL radiation may be directed electronically into different radiation patterns. We have devised an experiment (1992) to test this concept using the Columbia microwave FEL, which amplifies radiation at 24 GHz to a level <1 MW. A 4 mm dia, electron beam (580 kV) is propagated in a guiding field of 0.8 T inside an overmoded 24 mm dia. cylindrical waveguide. A TE11 mode is grown in a 33 cm long first undulator section (period 3.36 cm), and upon entering the following undulator section (period 2.26 cm, length 40 cm), the electron bunches convert to TM11 radiation which is further amplified. The far-field pattern of the TM11 emitted power is distinct from the TE11 pattern. Numerical and experimental studies are described in this paper showing the resulting radiation pattern.