Powerful free-electron masers with novel Bragg resonators

High-efficiency Ka-band FEM-oscillator was elaborated during the last few years in collaboration between JINR (Dubna) and IAP RAS (N.Novgorod). The induction linac LIU-3000 (JINR), which generates a 0.8 MeV / 200 A / 250 ns electron beam with a repetition rate of 1 Hz, drives the FEM-oscillator. Transverse velocity in the magnetically guided beam is pumped in a helical wiggler of 6 cm period. The main advantages of the JINR-IAP FEM is the use of a reversed guide magnetic field, which provides high-quality beam formation in the tapered wiggler section with a low sensitivity to the initial beam spread, alongside with Bragg resonator having a step of phase of corrugation [2], which possesses high electrodynamical mode selection. As a result, stabile single-mode operation with high electron efficiency was achieved in the FEM. At the present stage the FEM generates 20 MW / 200 ns pulses at 30 GHz with the spectrum width of 6 - 7 MHz, which is close to the theoretical limit (Fig.2). Stability of the FEM radiation parameters (frequency, output power, pulse shape, etc.) was demonstrated over a sequence of ~ 10⁵ pulses.