a, Coordinate space distribution of accelerated electrons, with various colours representing the various energies. b, Beam current (blue) and slice energy spread (red) within the beam. RES, relative energy spread. c, Normalized slice emittance of the beam in the horizontal (enx, red) and vertical (eny, blue) directions. Each slice has a length of 31.25 nm, which was set as the grid size in the particle-in-cell simulation.