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To allow first lasing at the ELBE FEL at the end of 2003 various efforts have been made. In particular, the optical resonator including the system which controls and stabilizes its length and alignment has been installed and tested. The resonator length is controlled by means of an interferometer beam passing diagonally from one side of the upstream mirror to a retroreflector on the opposite side of the downstream mirror. To avoid temperature effects the mirror holders have been equiped with a special heating and cooling system which stabilizes the mirror temperature. Simulating maximum laser power the cavity detuning due to temperature changes has not exceeded 1micron. The alignment of the resonator axis is controlled by the overlap of two He-Ne laser beams entering the resonator from each side. To predict the outcoupled laser power and the upper wavelength limit we have modeled the IR-beam propagation in the optical resonator. In particular, the vacuum chamber in the undulator, which is 10 mm high, and the outcoupling hole affect the beam power and profile significantly; respective results will be presented.

In this paper we present a time domain analysis of a short pulse partial-waveguide FEL oscillator employing toroidal mirrors and a hole outcoupling. The use of toroidal mirrors with optimized radius of curvature helps to reduce cavity losses arising from the mismatch of the free space propagating optical field into a waveguided one. We introduce semi-analytical expressions for the calculation of the scattering matrix elements describing the loss and mode coupling mechanism as well as the amount of the extracted power from the cavity. The formulation is implemented in a time domain FEL code based on modal expansion approach. The described model is applied to a partial waveguide FEL system and simulation results are compared with measurements.

We show that the 3D simulation code GPT (General Particle Tracer) can give very detailed results on both the produced radiation and the evolution of the elctron bunch when special subsets of electrons are tracked instead of individual macro- particles. The model and two apllications are presented.

Time-resolved measurements of the transverse electric field associated with relativistic electron bunches are presented. Using an ultra-fast electro-optic sensor close to the electron beam, the longitudinal profile of the electric field could be measured with sub-picosecond time resolution and without time-reversal abiguity. Results are shown for two cases: inside the vacuum beam line in the presence of wake fields, and in air behind a Be window, effectively probing the near-field transition radiation. Especially in the later case, reconstruction of the longitudinal electron bunch shape is straightforward.

Second-order optical autocorrelation measurements are reported for a far-infrared free-electron laser. Second-harmonic generation in an 840-mu m-long CdTe crystal is used to provide the nonlinear autocorrelation signal. At wavelengths of 10.4 and 24.5 mu m, FWHM pulse durations of 220 and 500 fs, respectively, are measured. In both cases this corresponds to six cycles of the optical field. The pulse energy ranges up to 25 mu J. Evidence is presented that each pulse evolves into a train of subpulses at large values of the cavity desynchronism.

The powerful output of FELIX, the recently built Rijnhuizen free electron laser, is used for the first frequency dependent study of nonlinear optical excitation of Hg1-xCdxTe in the far infrared spectral region. Two-photon interband absorption has been investigated as a function of power and wavelength from 20 mum to 36 mum. The expected quadratic dependence of the two-photon induced conductivity on laser intensity is observed. This nonlinear effect is used to perform autocorrelation experiments vielding the length of the micropulses of the free electron laser on a femtosecond time scale.

The powerful output of FELIX, the recently built Rijnhuizen free-electron laser, is used for the first frequency dependent study of nonlinear optical excitation of Hg1-xCdxTe in the far-infrared spectral region. Two-photon interband absorption has been investigated as a function of power and wavelength from 20 to 40 mum. This nonlinear optical absorption is used to perform autocorrelation experiments yielding the length of the micropulses of the free-electron laser on a femtosecond time scale. The predicted dependence of the micropulse length on the synchronization between optical and electron pulses in the laser cavity is observed.