Mechanism linking alpha-BBO pulse stacking to longitudinal energy variation

Ascertain the physical mechanism by which the use of five alpha-BBO birefringent crystals for temporal laser pulse stacking in the Argonne Wakefield Accelerator photoinjector system leads to the observed longitudinal energy variation along the electron bunch, as revealed by the reconstructed 6-dimensional phase-space distribution.

Background

In the experimental demonstration at the Argonne Wakefield Accelerator, the authors applied GPSR to reconstruct a 6D beam distribution from screen images collected during quadrupole scans with various settings of a transverse deflecting cavity and a dipole spectrometer. The reconstruction reproduced key features and predicted previously unseen measurements.

The reconstructed distribution exhibited a beamlet structure in the longitudinal current profile, believed to originate from temporal pulse stacking using alpha-BBO crystals to produce a flat-top laser drive pulse. However, the authors note that the mechanism by which this laser pulse shaping results in a longitudinal energy variation along the bunch is unclear, motivating a targeted investigation of the underlying physics in the specific AWA operating conditions.

References

We believe that this structure results from a set of 5 alpha-BBO crystals used at AWA to produce longer, flat-top laser pulse profiles by stacking 32 laser sub-pulses together , however it is unclear how this leads to variations in beam energy along the bunch.

— Efficient 6-dimensional phase space reconstruction from experimental measurements using generative machine learning (2404.10853 - Roussel et al., 16 Apr 2024) in Section: Experimental Demonstration