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Experimental observation of self-organised mode-locked emission in a W-band free-electron maser.

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Title: Experimental observation of self-organised mode-locked emission in a W-band free-electron maser.
Authors: Marks, H.S.1 (AUTHOR) h.marks@lancaster.ac.uk, Khorosh, M.2 (AUTHOR)
Source: Nuclear Instruments & Methods in Physics Research Section A. Jun2026, Vol. 1086, pN.PAG-N.PAG. 1p.
Subjects: Mode-locked lasers, Microwave oscillators, Electron beams, Cavity resonators, Millimeter waves, Coherence (Physics)
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. [ABSTRACT FROM AUTHOR]
Copyright of Nuclear Instruments & Methods in Physics Research Section A is the property of Elsevier B.V. and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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DbLabel: Engineering Source
An: 191840023
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  Data: Experimental observation of self-organised mode-locked emission in a W-band free-electron maser.
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  Data: <searchLink fieldCode="AR" term="%22Marks%2C+H%2ES%2E%22">Marks, H.S.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<i> h.marks@lancaster.ac.uk</i><br /><searchLink fieldCode="AR" term="%22Khorosh%2C+M%2E%22">Khorosh, M.</searchLink><relatesTo>2</relatesTo> (AUTHOR)
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  Data: <searchLink fieldCode="JN" term="%22Nuclear+Instruments+%26+Methods+in+Physics+Research+Section+A%22">Nuclear Instruments & Methods in Physics Research Section A</searchLink>. Jun2026, Vol. 1086, pN.PAG-N.PAG. 1p.
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  Data: <searchLink fieldCode="DE" term="%22Mode-locked+lasers%22">Mode-locked lasers</searchLink><br /><searchLink fieldCode="DE" term="%22Microwave+oscillators%22">Microwave oscillators</searchLink><br /><searchLink fieldCode="DE" term="%22Electron+beams%22">Electron beams</searchLink><br /><searchLink fieldCode="DE" term="%22Cavity+resonators%22">Cavity resonators</searchLink><br /><searchLink fieldCode="DE" term="%22Millimeter+waves%22">Millimeter waves</searchLink><br /><searchLink fieldCode="DE" term="%22Coherence+%28Physics%29%22">Coherence (Physics)</searchLink>
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: 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. [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of Nuclear Instruments & Methods in Physics Research Section A is the property of Elsevier B.V. and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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RecordInfo BibRecord:
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      – Type: doi
        Value: 10.1016/j.nima.2026.171339
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      – Code: eng
        Text: English
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        PageCount: 1
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    Subjects:
      – SubjectFull: Mode-locked lasers
        Type: general
      – SubjectFull: Microwave oscillators
        Type: general
      – SubjectFull: Electron beams
        Type: general
      – SubjectFull: Cavity resonators
        Type: general
      – SubjectFull: Millimeter waves
        Type: general
      – SubjectFull: Coherence (Physics)
        Type: general
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      – TitleFull: Experimental observation of self-organised mode-locked emission in a W-band free-electron maser.
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              M: 06
              Text: Jun2026
              Type: published
              Y: 2026
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