Long pulse high gain 35 GHz free-electron maser amplifier experiments

Abstract

Results of the design and operation of a 35 GHz high gain, long pulse free-electron maser (FEM) amplifier are presented and discussed. The electron beam used in this experiment is produced by a thermionic electron gun which was successfully operated up to 580 kV and 120 A. with a measured perveance of 0.27 μperv, in excellent agreement with the design value. The beam is compressed to a nominal radius of 4 mm and guided by a 2.35 kG axial magnetic field. A 500 G amplitude, 30 mm period SmCo permanent magnet helical wiggler with a 10 period long adiabatic introduction provides the perpendicular momentum to the electrons and ensures high quality group 1 orbits in the interaction region. The measured energy spread before injection in the wiggler is Δγ∥/γ∥ < 0.5%; the spread in the interaction region inferred from the FEM performance is Δγ∥/γ∥ ≅ 1.0% The amplifier operates in the TE11 mode, with a cylindrical waveguide radius a = 8 m. and a corresponding cutoff frequency ωc/2π = 10.98 GHz. The amplifier was voltage-tuned from 18 GHz to 40 GHz in excellent agreement with TE11 linear theory. An instantaneous low gain bandwidth of more than 12 GHz was observed. In the high gain regime, a total output power of 800 kW was observed for an input power of 2 kW, yielding 26 dB of gain. The FEM was operating at an energy of 310 keV and a transmitted current of 30 A with a corresponding electronic efficiency of 8.6%.