Timing Diagnostics and Coherent Harmonics from a test-FEL

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.