High-power millimeter-wave Bragg free-electron maser oscillator experiments

Abstract

Abstract Single-mode, high-power operation of a free-electron maser (FEM) oscillator with a Bragg cavity is reported. The beam is produced by a 0.27 μperv thermionic electron gun which was successfully operated up to 580 kV and 120 A. compressed to a nominal radius of 4 mm with a measured axial energy spread of Δγ 1 /γ 11 , transported through the interaction region by a 2.35 kG axial magnetic field, and pumped by a 500 G amplitude. 30 mm period permanent-magnet helical wiggler. A 10-period-long adiabatic introduction ensures high-quality group-I operation. The Bragg resonator is designed to provide feedback in the TE11 mode at frequencies considerably higher than cut-off ( ω/2π 2 7.5 GHz and ω c /2π=11 GHz , respectively) to avoid beam interception and allow high-power operation. The FEM oscillator generated 990 kW (±0.5 dB ) of microwave power in a single axial mode at 27.47 GHz with a beam voltage of 320 kV and a transmitted current of 30 A, yielding an efficiency of 10.3%, in good agreement with nonlinear simulations. The starting current of this mode was measured to be 4 A, and a peak efficiency of 12.5% was observed for I t = 23 A and P μ w = 920 kW . The system was operated with three of the four axial modes of the Bragg resonator with an observed nonlinear frequency pulling of about 70 MHz. Far-field radiation pattern measurements confirm the TE11 operation.