HARMONIC MILLIMETER RADIATION FROM A MICROWAVE FEL AMPLIFIER

In this project, an electron beam is bunched at a microwave frequency and the harmonics of this bunching drive radiation at millimeter wavel engths, using a FEL, configured as a single-pass travelling wave ampli fier. A 10 kW 24 GHz microwave input signal grows to similar to 200kW level using the lower-frequency unstable root of the waveguide FEL dis persion relation. The Columbia FEL facility operates at this frequency in the TE11 mode, using a helical undulator (1.85 cm period) and a 3 mm diameter 600 KV electron beam contained in a 8.7 mm ID cylindrical waveguide. The harmonic currents set up by the microwave are found to cause growth of harmonic power under two conditions. First, we choose the parameters of the device so that the upper frequency root correspo nds to the third harmonic, in which case we observe a small amount of third-harmonic emission in the TE11 mode, accompanied by comparable se cond harmonic. The millimeter harmonic radiation produced is coherent and phase-related to the microwave source. Second, we have found subst antial emission at the seventh harmonic, most likely from the TE72 mod e - which, in cylindrical waveguide geometry, travels at very nearly t he same wave speed as the 24 GHz TE11 power. In order to excite the se venth-harmonic radiation, the electron beam must be displaced from the system axis- similar to 2 mm in this device. The seventh-harmonic out put is potentially an attractive choice for a CW FEL which must genera te appreciable power at similar to 2 mm wavelength for plasma electron cyclotron heating since we can produce this radiation for electron be am energy as low as 400 kV. We present a theoretical model of the expe riment which predicts that if the microwave signal is strong enough to drive the FEL into saturation, the harmonic emission becomes powerful .