Density distributions of (a) electrons and (c) ions in the case of λ = 1 μm. Density distributions of (b) electrons and (e) ions in the case of λ = 4 μm. Spectra of proton beam for (d) λ = 1 μm and (f) λ = 4 μm. Here the same a = 50 is used. The time step is t = 40T.