Particle-in-cell simulations of quasi-phase matched direct laser electron acceleration in density-modulated plasma waveguides

Abstract

TaiwanQuasi-phase matched direct laser acceleration (DLA) of electrons can be realized with guided, radially polarized laser pulses in density-modulated plasma waveguides. A 3-D particle-in-cell model has been developed to describe the interactions among the laser field, injected electrons, and the background plasma in the DLA process. Simulations have been conducted to study the scheme in which seed electron bunches with moderate energies are injected into a plasma waveguide and the DLA is performed by use of relatively low-power (0.5-2 TW) laser pulses. Selected bunch injection delays with respect to the laser pulse, bunch lengths, and bunch transverse sizes have been studied in a series of simulations of DLA in a plasma waveguide. The results show that the injection delay is important for controlling the final transverse properties of short electron bunches, but it also affects the final energy gain. With a long injected bunch length, the enhanced ion-focusing force helps to collimate the electrons and a relatively small final emittance can be obtained. DLA efficiency is reduced when a bunch with a greater transverse size is injected; in addition, micro-bunching is clearly observed due to the focusing and defocusing of electrons by the radially directed Lorentz force. DLA should be performed with a moderate laser power to maintain favorable bunch transverse properties, while the waveguide length can be extended to obtain a higher maximum energy gain, with the commensurate increase of laser pulse duration and energy.

Maximal charge injection of a uniform separated electron pulse train in a drift space

“…[3] and τ p ≅ 1 ½fs in Ref. [2]. Both of the corresponding normalized pulse lengths are X DSCL ≡ τ p T DSCL ≅ 10 −5 ≪ 1, which is consistent with the assumption of our model.…”

“…As an example, the recent DLA [2] enables the generation of an electron bunch train at 80 MeV. In the relativistic regime, Eq.…”

“…According to DLA simulations [2], we choose the gap spacing d ¼ 5 ½mm, time interval Δ t ¼ 3 ½fs, the relativistic injected kinetic energy K ¼ 80 ½MeV and N max ¼ 36 to verify whether the criteria f ¼ 0.1 for the convergence Δ f to Δ t remains valid. In this setting, there are about 10 microbunches in a train generated by DLA, thus we can consider the given charge density σ ¼ fσ 36 with N ¼ 10 injection and numerically solve Eq.…”

“…[3] are used as an example to determine the maximum charge density per pulse while the pulse interval almost remains constant. By including relativistic effect, the model developed in the study is still valid for devices with the electron beam energy up to 80 MeV [2].…”

“…Consecutive electron pulses or multipulse comb beams, which are generated by comb-like laser pulse illuminated photocathode [1] or direct laser electron accelerator (DLA) [2], have wide applications, such as terahertz radiation sources [3], free electron laser (FEL) [4], high harmonics generation (HHG) processes [5] and four-dimensional ultrafast electron microscopy (UEM) [6]. The pre-bunched charge sources have the potential to greatly enhance the efficiency or power of the devices, currently attracting the attention of many scientists.…”