Experimental Observation of Self-Organised Mode-Locked Emission in a W-Band Free-Electron Maser

Marks, H.S. and Khorosh, M. (2026) Experimental Observation of Self-Organised Mode-Locked Emission in a W-Band Free-Electron Maser. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1086: 171339. ISSN 0168-9002

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Abstract

Experimental observation is reported of passive, self-organised mode-locking in a high-power free-electron maser (FEM) oscillator operating near 103 GHz, using a ∼1.36 MeV, 1.15 A energy-recovered electron beam. Macropulses of 10 μs duration were recorded in which mode-locking is established from the onset of measurable radiation and persists throughout the pulse. Power-detector and heterodyne measurements show a regular train of kW-level spikes with a spacing of 10.4 ns, consistent with the cavity free spectral range. A modal analysis of the intermediate-frequency signal reveals strong phase coherence across more than sixteen longitudinal modes. A distinct sideband appears with a spacing that remains stable to within 0.2 MHz even though the carrier frequency drifts by several megahertz between pulses due to variations in beam energy. This behaviour contradicts the power-dependent scaling expected from synchrotron-driven FEL sidebands and instead indicates a cavity-anchored coupling mechanism. The results demonstrate that robust passive mode-locking can arise naturally in mm-wave FEM oscillators without external modulation.