Collisional relaxation of an electron beam in a plasma. Top and middle panels: the electron distribution function f(v) and the spectral energy density of Langmuir waves (here fixed at the thermal level), respectively. Each coloured line shows the distribution at a different time. Bottom panel: the time-averaged electron flux spectrum [electrons keV^-1 cm^-2 s^-1] plotted against electron energy. The dashed line shows the initial power-law flux.