Within the framework of plane-wave angular spectrum analysis of the electromagnetic field structure, a solution valid for tightly focused radially polarized few-cycle laser pulses propagating in vacuum is presented. The resulting field distribution is significantly different from that based on the paraxial approximation for pulses with either small or large beam diameters. We compare the electron accelerations obtained with the two solutions and find that the energy gain obtained with our new solution is usually much larger than that with the paraxial approximation solution.
A wakefield driven by a short intense laser pulse in a perpendicularly magnetized underdense plasma is studied analytically and numerically for both weakly relativistic and highly relativistic situations. Owing to the DC magnetic field, a transverse component of the electric fields associated with the wakefield appears, while the longitudinal wave is not greatly affected by the magnetic field up to 22 Tesla. Moreover, the scaling law of the transverse field versus the longitudinal field is derived. One-dimensional particle-in-cell simulation results confirm the analytical results. Wakefield transmission through the plasma-vacuum boundary, where electromagnetic emission into vacuum occurs, is also investigated numerically. These results are useful for the generation of terahertz radiation and the diagnosis of laser wakefields.
By one-dimensional Vlasov-Poisson simulation, the critical initial state marking the transition between the Landau scenario, in which the electric fields definitively damped to zero and the O＇NEIL scenario, in which the Landau damping is stopped after a certain damping stage, is studied. It is found that the critical initial amplitude e＊ can only exist when the product of the wave number （k~） and the electron thermal velocity （vth） is moderate, that is, 0.2 〈 k^vth 〈 0.7. Otherwise, no critical initial amplitude is found. The value c＊ increases with the increase in km for a fixed Vth, and also increases with the increase in Vth for a fixed kin. When kmVth is fixed, the value s＊ also changes with the wave number and the electron thermal velocity, even though the damping rate and the oscillation frequency are the same in this case.
