A compact FEL upconverter of coherent radiation

The objective is to generate a powerful millimeter-wave FEL signal in a single pass, using a coherent microwave source (24 GHz) to prebunch the electron beam for a harmonically-related wave (72 GHz); one would obtain a millimeter wave source which is phase-referenced to a coherent laboratory source. We use the Columbia FEL facility, operating the electron beam at 600 kV, 100 A; undulator period = 1.85 cm and 200 G (K = 0.2); electron beam diameter = 3 mm inside an 8.5 mm ID drift tube; guiding field of 8800 G. Under these conditions, both the microwave signal (5 kW input) and the millimeter signal will show travelling-wave gain in the TE11 mode. We report initial experimental results for the millimeter wave spectrum and find an overall power gain of ∼20 for the 24 GHz input wave. Also presented are numerical solutions of the wave growth using the FEL equations with slippage.