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  • Open Access

Startup regime of high-efficiency tapering-enhanced FEL oscillator

Margarit Asatrian* and Wolfgang Hillert

Eugenio Ferrari

Andrew Fisher and Pietro Musumeci†

  • Universität Hamburg, Institute of Experimental Physics, Luruper Chaussee 149, D-22761 Hamburg, Germany

  • Department of Physics and Astronomy, UCLA, Los Angeles, California 90095, USA

  • *Contact author: margarit.asatrian@desy.de
  • †Contact author: musumeci@physics.ucla.edu
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Phys. Rev. Accel. Beams 29, 020702 – Published 9 February, 2026

DOI: https://doi.org/10.1103/zgmq-8xkd

Abstract

In this paper, we present a design of a high-efficiency high-gain free-electron laser oscillator based on the use of a strongly tapered undulator for extracting energy from high-brightness electron beams. We provide an analytical model of the setup followed by numerical simulations for lasing at the wavelength of 13.5 nm. We discuss the optimization of the system in steady state and the conditions necessary for the pass-per-pass buildup of the power from shot noise level. We propose the use of fast phase shifters as a way to accelerate the buildup. Finally, we present time-dependent simulations of the oscillator and discuss the role of spectral filtering. The optimized working point yields a total energy conversion efficiency from the electron beam to output radiation above 1% at the wavelength of 13.5 nm.

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Phys. Rev. Accel. Beams 29, 020702– Published 9 February, 2026
  • Received 11 June 2025
  • Accepted 3 December 2025
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