Harmonic millimeter radiation from a microwave free-electron-laser amplifier

Abstract

Using a free-electron laser (FEL) configured as a traveling-wave amplifier, we have caused the bunching produced by the amplification of a coherent microwave source to drive appreciable power at the harmonics. A 10-kW 24-GHz microwave input signal grows to the $\ensuremath{\sim}200\mathrm{kW}$ level using the lower frequency unstable root of the waveguide FEL dispersion relation. The FEL operates in the TE11 mode, using a helical undulator (1.85-cm period) and a 3-mm-diam 600-kV electron beam contained in an 8.7-mm-i.d. cylindrical waveguide. The harmonic currents set up by the microwave are found to cause growth of harmonic power under two conditions. First, if the design is such that the upper frequency root corresponds to the third harmonic, we see small amounts of third- and second-harmonic power, coherent with the source. Second, we have found kW emission of the seventh harmonic, most likely from the TE72 mode, which travels at the same speed as the 24-GHz wave. In order to excite the seventh-harmonic radiation, the electron beam must be displaced from the axis of the guide by $\ensuremath{\sim}2\mathrm{mm}.$ In both cases, no harmonic power is produced without gain at the fundamental. We present a one-dimensional theoretical model of the experiment, and use the numerical results to interpret our findings. The model predicts that if the microwave signal is strong enough to drive the FEL into saturation, the harmonic radiation should become powerful.