SCBF(Spherically Convergent Beam Fusion) device has been studied as a neutron source. Neutron production rate is a most important factor for the application of SCBF device and is proportional to the square of the ion current[1]. It is regarded generally that some correlations between the potential well structure and the ion current exist. In this paper, the ion current and potential distribution were calculated in a variety of grid cathode geometries using FEM-FCT method. Single potential well structure was certified inside the grid cathode. The deeper the potential well became, the higher the ion current due to the high electric field near the grid cathode became.