Analysis of periodic spectral gaps observed in the tuning range of free-electron lasers with a partial waveguide

A phenomenon of “spectral gaps” is observed in far-infrared free-electron lasers using partial waveguiding: the laser power falls down to zero at some particular wavelengths, whatever the beam adjustments are. We show that this effect is related to a periodic increase of the cavity losses, with a period depending on the dimensions of the waveguide. A code including diffraction effects exhibits good agreement with the measurements.

Figure 1

Schematic layout of the CLIO free-electron laser optical cavity.

Figure 2

Laser power vs centroid wavelength, for spherical mirrors cavity, using hole coupling of 2 mm (dotted line: measurement; continuous line: simulation).

Figure 3

Laser power vs centroid wavelength, for hybrid cavity: spherical for upstream and toroidal for downstream, using hole coupling of 3 mm. Bold lines are simulations.

Figure 4

Laser power vs centroid wavelength, for toroidal cavity, using hole coupling of 3 mm. Bold lines are simulations.

Figure 5

Simulation of cavity losses $L$ and hole coupling extraction rate $T_X$ as a function of wavelength, for spherical cavity.

Figure 6

(a) Numerical simulation in the case of spherical cavity: laser mode transverse profiles on the output mirror, for two different wavelengths. (b) Numerical simulation in the case of spherical cavity: laser mode transverse profiles on downstream waveguide entrance (point B in Fig. 1), for three different wavelengths.

Figure 7

Simulation, total cavity losses $L$ and hole coupling extraction rate $T_X$ as a function of wavelength, for toroidal cavity.

Figure 8

Simulation, intracavity power $P_{\mathrm{in}}$, as a function of wavelength, for toroidal cavity.

Figure 9

(a) Numerical simulation in the case of toroidal cavity: laser mode transverse profiles on the output mirror, for two different wavelengths. (b) Numerical simulation in the case of toroidal cavity: laser mode transverse profiles on downstream waveguide entrance, for two different wavelengths.

Figure 10

Results for the FELIX infrared FEL.

Figure 11

Simulation for FELIX, with mode profiles on the output mirror and at the waveguide entrance, close to $\lambda =50\mu \mathrm{m}$.

Figure 12

Simulation of output power on the U100 FEL in ELBE, at 32 MeV, with 2 mm hole coupling.

Figure 13

Energy distribution of eigenmodes on TEq and TMq in the case of CLIO, with spherical cavity, at $\lambda =40\mu \mathrm{m}$.

Figure 14

Simulation—ELBE FEL power vs wavelength and waveguide aperture $b$.