2D hydrodynamic simulations of a variable length gas target for density down-ramp injection of electrons into a laser wakefield accelerator - Kononenko, O; Lopes, NC; Cole, JM; Kamperidis, C; Mangles, SPD; Najmudin, Z; Osterhoff, J; Poder, K; Rusby, D; Symes, DR; Warwick, J; Wood, JC; et al. (Nuclear Instruments & Methods in Physics Research Section A - Accelerators Spectrometers Detectors and Associated Equipment, 2016)