Existence of non-axisymmetric electrostatic potential in the plug region of GAMMA10 yields radial losses of ions in thermal barrier cell. Detailed behavior of ions in non-axisymmetric potential and its contribution to the transport in GAMMA10 is not investigated in details. In this study, by modeling the plug potential and the actual magnetic field in the end-mirror cell and estimating the amount of radial shifts of ions by following ion drift motion per bounce, the resultant mapping equation is used to analyze the orbits and diffusion of ions. Especially, the stability of orbits about the first and second order fixed points is studied and the diffusion due to chaotic orbits is estimated.