Transverse ion loss in the thermal barrier region of the GAMMA10 tandem mirror is investigated with the mapping equations of ion drift orbits. The effects of a non-axisymmetric electrostatic potential in the thermal barrier are taken into account. The local stability of orbits and its diffusion are calculated numerically and these are compared with the results of A.B. Rechester and R.B. White [1]. It is found that there are two kinds of the transverse ion transport. One is chaotic ion orbits

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... to unstable ion drift, which cause cross-field ion diffusion. Another is the enhanced effects of ion radial step sizes on the transverse diffusion, because banana-like ion drift orbits appear due to the non-axisymmetric electrostatic potential formation.