Top to bottom: electron distribution function f(v); the spectral energy density of Langmuir waves W(k); in-source harmonic radio brightness temperature T_T(k) (for k/k_De = 0.0225 to 0.0235, corresponding to ω/ω_pe = 2 to 2.08); spectral energy density of ion-sound waves W_S(k); in-source fundamental radio brightness temperature T_T(k) (for k/k_De = 0. to 0.01, corresponding to ω/ω_pe = 1 to 1.1) for a collisionally relaxing electron beam in homogeneous plasma (left column) and in inhomogeneous plasma with τ_D ≃ 0.24 s (right column). Each coloured line shows the distribution at a different time, as shown in the colour bar.