Energy and frequency measurements on the Twente Raman free-electron laser

Results of energy and frequency measurements on the Twente Raman FEL are presented and discussed. A highest energy of 230 mJ was measured using a joule meter. The spectrum was determined using waveguide cutoff filters. These measurements generally showed a broad spectrum containing two or more peaks. Far from magneto-resonance this indicates that the radiation field grows at the upper as well as at the lower interaction of the slow space-charge wave with the electromagnetic waveguide modes. Near magneto-resonance, the analysis of the spectra shows the presence of the cyclotron instability together with the FEL instability. The two instabilities are resonant with different waveguide modes. The latter was confirmed by a measurement of the radial profile of the emitted radiation for a single setting of the guide and undulator field.