Laser Plasma Acceleration (LPA) enables to generate up to several GeV electron beam with short bunch length and high peak current within centimeters scale. However, the generated beam quality (energy spread, divergence) is not sufficient for numerous applications. In view of a Free Electron Laser application, the energy spread has to be adapted to reach the required small slice value while the beam divergence has to be controlled to avoid chromatic effects and emittance growth with strong focusing elements. We report here on the magnet based COXINEL line designed to transport an LPA electron beam from the source to the undulator, the different effects that can appear and the solutions. The beam is focused by a variable gradient quadrupole close to the source into a magnetic chicane for energy selection, followed by a set of four electro-magnet quadrupoles for proper focusing inside a cryo-ready undulator. Through gradient and position adjustment of the first triplet of quadrupoles the beam pointing and focusing can be compensated during experiment, then the chicane and the slit inside it can also be adjusted to optimize the energy selection and the steerers and electromagnetic quadrupoles are optimized to well align the electron beam with the undulator. The critical effects of the residual multipolar terms of the variable gradient quadrupoles are analyzed and effectively mitigated and the laser shot-to-shot induced electron beam variations are compensated thanks to the translation capacity of the variable quadrupoles. The experiments and the modeling are presented and they show good agreement and proper and robust transport has been experimentally achieved.
|Working group||Laser-driven electron acceleration|