Abstract
The infrared free-electron laser (FEL) offers a large tunability since the FEL gain remains high throughout the infrared spectral range, and the reflectivity of metal mirrors remains also close to unity. The main limitation comes from the diffraction of the optical beam due to the finite size of the vacuum chamber of the undulator. A solution is to use this chamber as a waveguide by adapting the radius of curvature of the cavity mirrors to this regime. Then, as has been shown before, a minimum appears in the spectrum that can be produced by the FEL. We discuss the physical mechanism of this particular regime and compare it to experiments using vacuum chambers of different transverse sizes. A good agreement is found with results of simulations and with a simple analytical formula.
Highlights
Infrared free-electron lasers (IRFELs) can operate in a very large spectral range
The amplifying medium of the FEL is a high energy electron beam, which has to circulate in a magnetic periodic structure (“undulator”) and in a vacuum chamber
The optical beam passes through a waveguide inside the undulator and in free space elsewhere
Summary
Infrared free-electron lasers (IRFELs) can operate in a very large spectral range. This is due to the combination of wideband optical gain and metallic mirrors. If one wants to produce an FEL with a large tunability, extending from near to far infrared, one is led to use an intermediate case where the beam propagation is “free space” propagating in the near infrared and becoming progressively guided in the waveguide/vacuum chamber as the wavelength increases. The optical beam passes through a waveguide inside the undulator and in free space elsewhere (where it can be diffracted by the finite size of the dipole gaps) We tested both spherical and toroidal mirrors. We always observe a gap in FEL power located at the same position, when sweeping its wavelength across the far-infrared region This was correctly simulated with a numerical method taking into account the propagation effects [7].
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