  The evolution of the Buneman and two-stream instabilities driven by a cold
dilute mildly relativistic electron beam is studied as a function of the
ion\'\s charge-to-mass ratio. The growth rates of both instabilities are
comparable for the selected parameters if the charge-to-mass ratio of protons
is used and the Buneman instability outgrows the two-stream instability for a
larger ratio. Particle-in-cell (PIC) simulations show that both instabilities
grow independently during their linear growth phase. The much lower saturation
amplitude of the Buneman instability implies that it saturates first even if
the linear growth rates of both instabilities are equal. The electron phase
space holes it drives coalesce. Their spatial size increases in time and they
start interacting with the two-stream mode, which triggers the onset of
electrostatic turbulence. A reduced charge-to-mass ratio results in stronger
turbulence and ion heating and in an increased energy loss of the relativistic
electron beam compared to that in a simulation with the correct ratio.
