Timing Diagnostics and Coherent Harmonics from a test-FEL

Abstract: The test free-electron laser (test-FEL) at MAX-lab in Lund demonstrated for the first time circularly polarized coherent femtosecond pulses at 66 nm wavelength. A 375 MeV electron bunch was seeded by a femtosecond laser at 263 nm and coherent harmonics were extracted in an APPLE-II type elliptical undulator. A thermionic gun with a barium oxide cathode was adapted for photocathode operation, and the performance of the gun was tested. Measurements showed the production of 200 pC of electrons with a normalized emittance of 5.5 mmmrad and a quantum efficiency of 1.1 10-4. To ensure the electron bunch overlap with the 500 fs seed laser pulse and to measure the bunch length, an electro-optical spectral decoding setup was built. The bunch length of about 1 ps was measured and it was determined that the longterm drifts in timing were accelerator-related. Electro-optical spectral decoding was used for the first time for online feedback to stabilize the overlap between the laser pulse and the electron bunch. This stabilization ensured lasing of the FEL on every shot and contributed to the detection of higher linearly polarized harmonics (44 nm). This thesis presents an introduction to the processes that occur in the undulators: modulation, bunching and radiation. It describes the test-FEL setup and the emittance of an electron beam. Electro-optical spectral decoding is explained and its advantages and limitations are discussed. The feedback based on a simple controller is presented.