  When a charged particle moves through a plasma at a speed much higher than
the thermal velocity of the plasma, it is subjected to the force of the
electrostatic field induced in the plasma by itself and loses its energy. This
process is well-known as the stopping power of a plasma. In this paper we show
that the same process works in particle-in-cell (PIC) simulations as well and
the energy loss rate of fast particles due to this process is mainly determined
by the number of plasma electrons contained in the electron skin depth volume.
However, since there are generally very few particles in that volume in PIC
simulations compared with real plasmas, the energy loss effect can be
exaggerated significantly and can affect the results. Therefore, especially for
the simulations that investigate the particle acceleration processes, the
number of particles used in the simulations should be chosen large enough to
avoid this artificial energy loss.
