The ion radial loss exists in the presence of a non-axisymmetric electrostatic potential in the end-mirror cells of GAMMA10, which leads to a formation of the thermal barrier potential. The non-axisymmetric electrostatic potential can also exist in the central cell. A design for divertor magnetic field of GAMMA10 is performed, the purpose of which is first to reduce an ion radial transport in the central cell by making electrostatic potential circular and second to assure the macroscopic plasma stability of GAMMA10 without help of non-axisymmetric anchor cells which enhances a neoclassical radial transport.