Intrapulse frequency stability of a magnetron frequency-locked through a wave reflected from an accelerating cavity

A laboratory-size Free Electron Laser (FEL) driven by a classical S-band microtron fed by a 2.5 MW magnetron generates terahertz radiation tunable in a wide range. The FEL provides output pulse power of ∼50 W in the wavelength range of 100–300 μm with a pulse duration of 2–4 μs. The FEL parameters are available due to stabilization of the accelerated beam current and the magnetron frequency. The latter is stabilized by the frequency locking in the magnetron through a wave reflected from the microtron accelerating cavity, which also serves as an external stabilizing resonator, providing the RF reference signal for the magnetron. The simple RF stabilizing scheme provides the r.m.s. magnetron frequency instability, δf≈10–13 kHz relative to the reference frequency at the frequency pulling bandwidth of 0.43–0.85 MHz and at the initial power of the reflected wave ≈30 kW. The developed stabilization provided stable and reliable operation of the FEL for more than 10 years. Measured and simulated results are described in this paper. ► Stability of a magnetron frequency-locked with an accelerating cavity was analyzed. ► Magnetron was considered as a forced oscillator coupled with a stabilizing cavity. ► Wave reflected from the cavity provided frequency pulling in the magnetron. ► Simulation of the magnetron-accelerating cavity system coincides with measurements. ► Measurements and simulation show excellent intrapulse stability of the magnetron.