Lasing experiments at TEUFEL

The waveguide structure of the resonator of the TEUFEL system has a significant impact on the resonance condition for lasing. The phase velocity of the modes in the waveguide shifts the resonance wavelength to larger values. Although this effect is known in Raman type FELs, it is less known that this mechanism has a severe impact on the resonance condition for Compton-type FELs. Due to this effect, various modes, that differ significantly in wavelength, can be resonant simultaneously. This paper shows that the mode spacing between adjacent modes can be as large as 5%, whereas, the gain bandwidth is not more than 1%.