Conceptual design of a laser-plasma accelerator driven free-electron laser demonstration experiment

Up to now, short-wavelength free-electron lasers (FEL) have been
systems on the scale of hundreds of meters up to multiple
kilometers. Due to the advancements in laser-plasma acceleration in
the recent years, these accelerators have become a promising
candidate for driving a fifth-generation synchrotron light source –
a lab-scale free-electron laser. So far, demonstration experiments
have been hindered by the broad energy spread typical for this type
of accelerator. This thesis addresses the most important challenges
of the conceptual design for a first lab-scale FEL demonstration
experiment using analytical considerations as well as simulations.
The broad energy spread reduces the FEL performance directly by
weakening the microbunching and indirectly via chromatic emittance
growth, caused by the focusing system. Both issues can be mitigated
by decompressing the electron bunch in a magnetic chicane,
resulting in a sorting by energies. This reduces the local energy
spread as well as the local chromatic emittance growth and also
lowers performance degradations caused by the short bunch length.
Moreover, the energy dependent focus position leads to a focus
motion within the bunch, which can be synchronized with the
radiation pulse, maximizing the current density in the interaction
region. This concept is termed chromatic focus matching. A
comparison shows the advantages of the longitudinal decompression
concept compared to the alternative approach of transverse
dispersion. When using typical laser-plasma based electron bunches,
coherent synchrotron radiation and space-charge contribute in equal
measure to the emittance growth during decompression. It is shown
that a chicane for this purpose must not be as weak and long as
affordable to reduce coherent synchrotron radiation, but that an
intermediate length is required. Furthermore, the interplay of the
individual concepts and components is assessed in a start-to-end
simulation, confirming the feasibility of the envisioned
experiment. Moreover, the setup tolerances for a first
demonstration experiment are determined, confirming the general
practicability. The revealed challenges, besides the energy spread,
especially concern the source stability and the precision of the
beam optics setup.