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Abstract
Simulations of ionization-induced injection in a laser driven plasma wakefield show that high-quality electron injectors in the 50–200 MeV range can be achieved in a gas cell with a tailored density profile. Using the PIC code Warp with parameters close to existing experimental conditions, we show that the concentration of N2 in a hydrogen plasma with a tailored density profile is an efficient parameter to tune electron beam properties through the control of the interplay between beam loading effects and varying accelerating field in the density profile. For a given laser plasma configuration, with moderate normalized laser amplitude, a0=1.6 and maximum electron plasma density, ne0=4×1018 cm-3, the optimum concentration results in a robust configuration to generate electrons at 150 MeV with a rms energy spread of 4% and a spectral charge density of 1.8 pC/MeV.
Highlights
Laser wakefield acceleration (LWFA) relies on an underdense plasma to transfer the energy from a laser beam to a trailing bunch of electrons, either injected internally or externally
This profile results from modeling the gas distribution inside an in-house gas cell used to conduct LWFA experiments, so-called ELISA [36], which stands for electron injector for compact staged high energy accelerator
Note that there is a slight decrease of the gradient of a0 between 2.1 and 2.5 mm, the corresponding a0 values are 1.5 and 2.0, which are the threshold values for ionization processes N5þ → N6þ and N6þ → N7þ respectively. This indicates that the delay between the starting time for the injection of the 6th and the 7th electrons will be slightly lengthened. Using these new laser parameters, the properties of the high energy electron bunch are as follows: average energy hEi 1⁄4 144ðþ7%Þ; charge Q1⁄443pCðþ17%Þ; rms energy spread ΔErms=hEi 1⁄4 3.5ð−29%Þ; normalized emittance εnx 1⁄4 0.8 mm mradð−29%Þ, εny 1⁄4 2.0ðþ3%Þ; rms bunch duration σt 1⁄4 3.3 fsðþ12%Þ; rms angular divergence θx 1⁄4 2.1 mradð−24%Þ, θy 1⁄4 3.6 mradðþ15%Þ; rms apparent transverse size σx 1⁄4 1.4 μmð−13%Þ, σy 1⁄4 2 μmð−16%Þ; average charge density hdQ=dEi 1⁄4 2.13 pC=MeVðþ54%Þ; average linear charge density Q=σz 1⁄4 43.4 pC=μmðþ5%Þ
Summary
Laser wakefield acceleration (LWFA) relies on an underdense plasma to transfer the energy from a laser beam to a trailing bunch of electrons, either injected internally or externally. Among them are self-injection [9,10,11,12], density-transition based injection [13], shock-front injection [14,15,16,17,18], colliding pulse injection [19], and ionization-induced injection schemes [20,21,22] This last mechanism provides a large number of parameters to control the injection of electrons, resulting in a larger number of accelerated electrons at comparatively low laser intensity, and can be combined with other mechanisms for improved control. Realistic plasma density profiles instead of the ideal ones were used to allow direct comparison with the experimental results This series of systematic study provides a preliminary understanding of the influence of beam loading effects on the injection of electrons in the wakefield in the nonlinear regime, and guides the development of more generalized cases.
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