In this paper we present a novel approach to free electron laser (FEL) simulations based on the decomposition of the electromagnetic field in a finite number of radiation modes. The evolution of each mode amplitude is simply determined by energy conservation. The code is developed as an expansion of the general particle tracer framework and adds important capabilities to the suite of well-established numerical simulations already available to the FEL community. The approach is not based on the period average approximation and can handle long-wavelength waveguide FELs as it is possible to include the dispersion effects of the boundaries. Furthermore, it correctly simulates lower charge systems where both transverse and longitudinal space charge forces play a significant role in the dynamics. For free-space FEL interactions, a source dependent expansion approximation can be used to limit the number of transverse modes required to model the field profile and speed up the calculation of the system’s evolution. Three examples are studied in detail including a single pass FEL amplifier, the high efficiency TESSA266 scenario, and a THz waveguide FEL operating in the zero-slippage regime.

• Received 6 August 2020
• Accepted 23 October 2020

DOI:https://doi.org/10.1103/PhysRevAccelBeams.23.110702

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society