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| Dissertation / PhD Thesis | PUBDB-2025-04736 |
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2025
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Please use a persistent id in citations: urn:nbn:de:gbv:18-ediss-131847
Abstract: Free-electron lasers (FELs) require high-quality electron beams with low energy spread, small emittance, and high peak current to enable coherent amplification of light. Laser-plasma accelerators (LPAs) could offer compact and cost-efficient drivers for FELs. However, the electron beams from state-of-the-art LPAs do not yet meet all FEL beam quality requirements simultaneously as they exit the plasma. Percent-level energy spread, micrometer-scale normalized emittance, and shot-to-shot fluctuations in beam quality pose significant challenges for FEL operation. In this thesis, a dedicated 25 m-long beamline was designed, constructed, and commissioned to demonstrate free-electron lasing at the LUX laser-plasma accelerator. The combination of decompression and chromatic focusing of the electron beams reduces the effective slice energy spread and removes limitations by the beam emittance. Applying both schemes increases the gain in a two meter short ideal undulator from a factor of 5 to a factor of 100 over the spontaneous undulator radiation. FEL simulations with the unaveraged code Puffin, suited for ultrashort beams and complex phase-spaces, extended for measured field profiles, predict a factor of 25 in gain in the experiment. Spectrally resolved detection of the FEL pulses increases the contrast by another order of magnitude, which should provide sufficient FEL signal for first lasing experiments at LUX. Beyond numerical validation, experiments were carried out to characterize and refine the beam transport from the plasma-source to the undulator. This included precise quadrupole positioning, determination and correction of strength errors, and undulator alignment to ensure an optimal beam trajectory and matching. These measures provide the necessary beam properties required for future FEL operation at LUX and establish a foundation for exploring compact, plasma-based light sources.
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