Abstract
This paper 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 a Penning trap, that models the ion cloud as a cold fluid. This method can not reproduce the organization of cold clouds as crystals but it allows one to scale the size of large samples with the trapping parameters and the number of trapped ions, for different linear geometries of trap.