Direct laser acceleration with radially polarized long-wave infrared lasers

Direct laser acceleration with radially polarized lasers is an intriguing variant of laser-based particle acceleration that has the potential of offering GeV/cm-level energy gain without the need for plasma and the associated complex beam dynamics. A major limiting factor for this method is the difficulty of generating high power radially polarized beams. CO2-based long-wave infrared (LWIR) lasers are potentially a good fit as a driver for direct laser acceleration, as the polarization insensitivity of the gain medium allows a radially polarized beam to be amplified. Additionally, the larger waist sizes, Rayleigh lengths, and pulse lengths associated with the long wavelength could improve the coupling efficiency of an incoming electron beam. By comparing acceleration simulations using a near infrared laser and an LWIR laser, we show that the coupling efficiency is indeed improved by up to an order of magnitude with the longer wavelength. Furthermore, MeV-level energy gains can be achieved even with sub-TW radially polarized LWIR lasers. Thus, radially polarized LWIR lasers show significant promise as a driver of a direct laser-driven demonstration accelerator.