HARMONIC MILLIMETER RADIATION FROM A MICROWAVE FREE-ELECTRON-LASER AMPLIFIER

Using a free-electron laser (FEL) configured as a traveling-wave ampli fier, we have caused the bunching produced by the amplification of a c oherent microwave source to drive appreciable power at the harmonics. A 10-kW 24-GHz microwave input signal grows to the similar to 200 kW l evel using the lower frequency unstable root of the waveguide FEL disp ersion relation. The FEL operates in the TE11 mode, using a helical un dulator (1.85-cm period) and a 3-mm-diam 600-kV electron beam containe d in an 8.7-mm-i.d. cylindrical waveguide. The harmonic currents set u p by the microwave are found to cause growth of harmonic power under t wo conditions. First, if the design is such that the upper frequency r oot corresponds to the third harmonic, we see small amounts of third-a nd second-harmonic power, coherent with the source. Second we have fou nd 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 exci te the seventh-harmonic radiation, the electron beam must be displaced from the axis of the guide by similar to 2 mm. In both cases, no harm onic power is produced without gain at the fundamental. We present a o ne-dimensional theoretical model of the experiment, and use the numeri cal results to interpret our findings. The model predicts that if the microwave signal is strong enough to drive the FEL into saturation, th e harmonic radiation should become powerful.