Study of achromatic beamline design for laser-driven femtosecond electron beams

BackgroundThe electron beams produced by laser plasma acceleration have excellent quality for pulse lengths of the order of fs. Due to the existence of a strong laser field, there are difficulties in direct applications, and more applications need to transmit the electron beams to the application terminal. The energy spread leads to the generation of energy chirp of the electron beam in the transmission.PurposeThis study aims to explore the design of the beam optics to compress the pulse length and keep it on the fs scale.MethodsAn achromatic beamline consisting of bending magnets and quadrupole magnets was designed to compress the pulse length of electron beams. Critical parameters of an achromatic beamline were given by a derived formula. Transformation matrix was employed to investigate the differences of the pulse lengths in achromatic transmission and non-achromatic transmission. The pulse lengths of electron beams with different energies were scanned with different deflection angles (0.3 rad, 0.6 rad, 0.9 rad) and deflection radii (0.15 m, 0.25 m, 0.35 m) to study the influence of beamline parameters. Finally, the magnetic field gradients of the quadrupole lens were adjusted to realize the compression of electron beams with different energies in a beamline.ResultsComparing to non-achromatic transmission, the pulse lengths of electrons with the same energy and different initial divergence angles can be compressed effectively in the achromatic beamline. The larger the deflection angle or the deflection radius, the longer the pulse duration of the electron beam with higher energy (>25 MeV). By adjusting the magnetic field gradients of the quadrupole lens, the pulse lengths can be reduced from more than 100 fs to around 20 fs at higher energies.ConclusionsUsing a fixed-size achromatic beamline, combined with magnetic field strength adjustment, the pulse lengths of electron beams with different energies can be kept on the order of fs after transmission.