Abstract
Conventional free electron laser (FEL) oscillators minimize the optical mode volume around the electron beam in the undulator by making the resonator Rayleigh length about one third to one half of the undulator length. This maximizes gain and beam-mode coupling. In compact configurations of high-power infrared FELs or moderate power UV FELs, the resulting optical intensity can damage the resonator mirrors. To increase the spot size and thereby reduce the optical intensity at the mirrors below the damage threshold, a shorter Rayleigh length can be used, but the FEL interaction is significantly altered. We model this interaction using a coordinate system that expands with the rapidly diffracting optical mode from the ends of the undulator to the mirrors. Simulations show that the interaction of the strongly focused optical mode with a narrow electron beam inside the undulator distorts the optical wave front so it is no longer in the fundamental Gaussian mode. The simulations are used to study how mode distortion affects the single-pass gain in weak fields, and the steady-state extraction in strong fields.
Highlights
It has been suggested that a free electron laser (FEL) oscillator can optimize the electronoptical coupling by minimizing the optical mode volume around the smaller relativistic electron beam
When the application requires moderate to high power at infrared or shorter optical wavelengths, the conventional FEL oscillator design leads to high intensity at the mirrors and possible mirror damage
We look at the single-pass extraction, defined as the output optical power divided by the input electron beam power, as various FEL parameters are modified
Summary
It has been suggested that a free electron laser (FEL) oscillator can optimize the electronoptical coupling by minimizing the optical mode volume around the smaller relativistic electron beam. The origin of the idea was stated in Madey’s initial paper inventing the FEL concept [1], and has lead to the common practice of designing the FEL’s optical resonator so that its Rayleigh length Z0 is about one third to one half of the undulator length L. When the application requires moderate to high power at infrared or shorter optical wavelengths, the conventional FEL oscillator design leads to high intensity at the mirrors and possible mirror damage. Assuming the electron beam and optical mode each have a radius of about 1 mm in the undulator, the commonly used design criteria would lead to a limitation in output power of 150 W in the infrared and only 3 W in the UV. The UV FEL is a particular problem since the longer Rayleigh length Z0 associated with the shorter UV wavelength increases mirror intensity further by decreasing the mirror spot size.
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