  The radiofrequency quadrupole linear ion trap is a widely used device in
physics and chemistry. When used for trapping of large ion clouds, the presence
of anharmonic terms in the radiofrequency potential limits the total number of
stored ions. In this paper, we have studied the anharmonic content of the
trapping potential for different implementations of a quadrupole trap,
searching for the geometry best suited for the trapping of large ion clouds.
This is done by calculating the potential of a real trap using SIMION8.0,
followed by a fit, which allows us to obtain the evolution of anharmonic terms
for a large part of the inner volume of the trap.
