A 35 GHz free electron laser amplifier operating with high guide magnetic field

Summary form only given, as follows. A free electron laser (FEL) has been operated as an amplifier at 35 GHz. The strong axial guide magnetic field (12 kG) places the electrons into the so-called group II orbits. In the regime of interest the expected efficiency is high because the space charge wave is unstable. The mildly relativistic beam (1 MeV) is generated by a Marx capacitor bank. The field-emission cathode yields about 360 A, with a beam radius of 2.54 mm. The bifilar helical wiggler has a period of 3.18 cm and provides a magnetic field of 1 kG. The FEL amplifies the TE₁₁ mode in a circular wave-guide with a radius of 5.1 mm. The high-efficiency below-grazing regime, in which the beam resonance line falls slightly below the vacuum waveguide dispersion curve, is being investigated. Preliminary data show a gain of 30 dB/m. About 13 kW has been injected from a magnetron and an output of about 20 MW has been obtained, corresponding to an efficiency of approximately 6%