Harmonic millimeter radiation from a microwave FEL amplifier

The objective is to generate a strong millimeter-wave FEL signal in a single pass, using a coherent microwave source (24GHz) to prebunch the electron beam for harmonically-related wave growth; the millimeter wave output is thus phase-referenced to the coherent microwave source. We use the Columbia FEL facility, operating the electron beam at 600k V, 100A; undulator period= 1.85cm and 2000 (K= 0.2); electron beam diameter= 3 mm inside a 8.7 mm ID drift tube; guiding field of 8800G. Both the microwave signal (5kW input) and the harmonic millimeter signal show travelling-wave gain in the TE11 FEL waveguide modes. Obtaining power gain of ∼20 for the 24GHz input wave, we have observed appreciable radiation at the second and third harmonics and a coherent relationship of the third harmonic and fundamental. Also presented are numerical solutions of the growth of the two waves using the FEL equations with slippage. Also, a strong seventh harmonic is produced, owing to the fact that the wave speeds of the TE11 (24GHZ) and TE72 (168GHz) waves are the same, and providing that the electron beam is displaced off-axis.