An Analytic Approach For Density Gradient Injection In Laser Wake Field Acceleration

Laser wakefield accelerators, which are capable of supporting fields in excess of 100GeV/m, have potential applications for high-energy physics and photon sources in the near future. For the past few years, there is great progress in the therotical and experimental research of laser wakefield accelerator, much of this growth is due to the rapid development of chirped-pulse amplification laser technology, which is making readily available compact sources of intense, high-power, ultrashort laser pulses. Electron injection is one of key processes in the laser wakefield accelerator scheme, so many electron injection schemes are proposed to generate smaller energy spread and sufficient electricity bunch. Especially, it is easy to achieve and control the electron injection through the plasma density gradient. However, the research of plasma density transition injection mainly remain in the experiment and simulation.The injection and acceleration of electrons in plasma density inhomogeneity region is studied on a analytic method in this paper. The main work content includes:First, according to the plasma wave’s phase speed which is variating with time at the density inhomogeneity region, the dependency relationship between the energy threshold of electron injection and the parameters of plasam density is worked out. At the plasam density inhomogeneity region, the energy threshold of electron injection decrease gradually with time. And the location where electrons inject into plasma wave is near to the accelerated region.Second, ensure the electric field at the density inhomogeneity region in the linear condition and compare with the result of particle-in-cell simulation, for the rationality of this linear approximation. As it is worked out that the oscillation of back electrons at the plasam density inhomogeneity region, we get the accurate location and time of electron’s injection, in consideration of the wakefield’s phase speed variation with time. In comparision to other theoretical analysis of density gradient injection, the kinectic progresses of the injected elctrons in detail are showed in this paper.Third, hamilton dynamic is applied to the analysis of electron’s injection at the plasam density inhomogeneity region, and we abtain the seperatix of injected electron at the plasam density inhomogeneity region. So, it is easy to explain the requirement of the electron’s continued acceleration in plasma wave.Fourth, we simulate the laser wakefield accelerator with density gradient, and track the trajectory of injected electrons. The analysis results are all confirmed to the particle-in-cell simulation.