PIC simulations of mid-infrared radiation from a laser wakefield accelerator.

The formation of a plasma ``bubble'' during Laser Wakefield Acceleration (LWFA) results in a co-propagating refractive index (electron density) gradient that produces time dependent frequency shifts in the driving laser pulse$^{\mathrm{1,2}}$. High-resolution spectral measurements of mid-infrared radiation extending to 2.5 microns during LWFA in the bubble regime were obtained on the HERCULES laser system at the University of Michigan. Particle-in-cell (PIC) simulations with OSIRIS show radiation extending briefly up to 50 microns in this regime. The PIC simulations indicate that the slow-moving long-wavelength radiation, which slips backward relative to the driving laser pulse, can interact the accelerated electron bunch. The interaction blue-shifts and scatters the long-wavelength radiation while decreasing the energy of the electron beam. These results suggest that measurements of side-scattered radiation may serve as a diagnostics of electron dynamics and bunch formation. $^{\mathrm{1}}$J. D. Ludwig, P.-E. Masson-Laborde, S. H\"{u}ller, et al. Phys. Plasmas \textbf{25}, 053108 (2018) $^{\mathrm{2}}$Zan Nie, Chih-Hao Pai, Jianfei Hua, et al. Nature Photonics vol. \textbf{12}, 489--494 (2018)