Reduction algorithms for simulation of laser wakefield acceleration

We present results from simulations, using the 2D particle-in-cell (PIC) code EPOCH, in which we analyse the suitability of low transverse field resolution for the study of laser wakefield acceleration (LWFA). The simulations use a tailored density profile to stimulate injection, maintaining a constant post-injection density. We also examine the applicability of reducing simulations post-injection, both through algorithmic nearest-neighbour (NN) particle coalescing, and through a combination of variation in the number of particles-per-cell (PPC) with field interpolation.
For low transverse field resolution, we demonstrate that both the peak energy injected and the resultant beam emittance converge rapidly, when the overall domain size is held constant, with little variation beyond 1.2 cells-per-wavelength (CPW) in the transverse direction. Results also show that whilst the proposed NN particle coalescing method maintains the injected electron beam, this degrades sufficiently that it does not improve accuracy over low transverse resolution, whilst being more computationally expensive.
In the case of variable PPC, we show that this method can exactly preserve the electron beam profile for a PPC decrease from 32 to 1, whilst offering an order-of-magnitude speedup in post-reduction computation. Through the incorporation of field interpolation, we demonstrate minimal degrading in the simulated electron beam profile for a 20-fold decrease in the number of transverse simulation cells post-injection, with a corresponding computational speedup of factor 90.