[en] There is a development plan of large-area RF ion source at KAERI to extract the positive ions, which can be used for the second NbTi (NbTi-2) system of KSTR, and to extract the negative ions for future fusion devices such as ITER and K-Demo. An initial large-area RF ion source consists of a driver region, including a helical antenna (6-turn copper tube with an outer diameter of 6 mm) and a discharge chamber (ceramic and/or quartz tubes with a inner diameter of 200 mm, a height of 150 mm, and a thickness of 8 mm), and an expansion region (magnetic bucket of prototype LPS in the KAERI). RF power was transferred up to 10 k W with a fixed frequency of 2 MHz through a matching circuit (auto- and manual-matching apparatus). Argon gas is commonly injected to the initial ignition of RF plasma discharge, and then hydrogen gas instead of argon gas is finally injected for the RF plasma sustainment. The uniformities of plasma density and electron temperature at the lowest area of expansion region (a distance of 300 mm from the driver region) were measured by using two electrostatic probes in the directions of short- and long-dimension of expansion region