Characteristics of electron beams accelerated by parallel and antiparallel circularly polarized Laguerre-Gaussian laser pulses

A direct comparison of the properties of electron beam generated by antiparallel circularly polarized Laguerre-Gaussian (CPLG) laser pulse and parallel CPLG laser pulse has been performed with three-dimensional particle-in-cell simulations. It is known that the longitudinal field of an antiparallel CPLG laser pulse with opposite signs of spin and orbital quantum number preferentially accelerates electrons to high energy. However, a direct comparison of electron beam between the other combination of spin and orbital angular momentum, the parallel CPLG laser pulse with the same sign of spin and orbital angular quantum number, has not been conducted. While the two pulses have an identical transverse field envelope, the generated electron beam properties are different. Although the magnitude of the longitudinal field is about one order of magnitude less than that of the transverse field, it has a significant effect on beam divergence. For antiparallel CPLG laser pulse, collimated electron bunches are formed with small divergence (< 50 mrad); while for parallel CPLG laser pulse, a diverging (> 100 mrad) electron beam is formed. This difference in beam quality can indicate a field-induced acceleration in actual experiments. A few-cycle laser pulse and low-density plasma are used to rule out the effect of laser-plasma interaction. It is also shown that for antiparallel CPLG laser pulse, the maximum kinetic energy increases with the square root of incident laser power, consistent with the scaling law for field-induced acceleration.