Hybrid fluid-PIC simulations of injected electron beams in laser wakefield accelerators

Laser wakefield accelerators (LWFA) have accelerated $\sim nC$ electron bunches to $\sim GeV$ energies over $cm$ scale distances, via self-trapping of plasma electrons. Self-trapping cannot be tolerated in staged LWFA modules for high-energy physics applications. The \% level energy spread of self-trapped electron bunches is an order of magnitude too large for light source applications. Both of these difficulties could be resolved via external injection of a low-emittance electron bunch into a quasilinear LWFA, for which the dimensionless laser amplitude is in the range $0.5 < a_0 < 2$. It is challenging for an electromagnetic PIC simulation to model such systems without artificial emittance growth in the beam. An improved cold, relativistic fluid model is used in the parallel VORPAL framework to simulate the electron plasma with no particle noise, while PIC is used for the beam. The importance of high-order particle shapes, current smoothing and other techniques is also discussed.