  This tutorial introduces the dynamics of charged particles in a
radiofrequency trap in a very general manner to point out the differences
between the dynamics in a quadrupole and in a multipole trap. When dense
samples are trapped, the dynamics is modified by the Coulomb repulsion between
ions. To take into account this repulsion, we propose to use a method,
originally developed for particles in Penning trap, that model the ion cloud as
a cold fluid. This method can not reproduce the organisation of cold clouds as
crystals but it allows one to scale the size of large samples with the trapping
parameters and the number of ions trapped, for different linear geometries of
trap.
