Laser Acceleration of Protons with an Optically Shaped, Near-Critical Hydrogen Gas Target - Chen, Y-H; Helle, M; Ting, A; Gordon, D; Dover, N; Ettlinger, O; Najmudin, Z; Polyanskiy, M; Pogorelsky, I; Babzien, M (AMER INST PHYSICS, 2016)