The radio frequency (RF) driven ion source is a promising solution to the neutral beam injectors of ITER and future fusion reactor. Because the demanded RF power is so high ( > 50 kW for one driver), a copper-made Faraday screen is installed inside the low dielectric tube to protect it against the plasma heat load. However, the Faraday screen may decrease the RF power transfer efficiency due to electromagnetic shielding and induced eddy current. Especially during the long-pulse experiments of plasma generation, the high heat load on the Faraday screen limits the increase of RF input power. Hence, a design of the water-cooled tube is proposed to avoid using the Faraday screen. The water-cooled tube should be able to suffer the plasma heat load by itself. Hence, the cooling circuit of the tube is designed and estimated through a fluid-thermal analysis. The water inlets with tangential injection direction are located inside the metal flanges. Such a design can induce a swirling flow around the tube, which can cause a more uniform temperature distribution and a lower peak temperature on the tube. In order to control the stress along the joints between the metal and the ceramic, the Kovar alloy (an iron-nickel constant expansion alloy) is chosen for the metal flanges, which has a similar thermal expansion coefficient with the ceramic. The thermo-structural analysis indicates a tolerable thermal stress on the ceramic layers of the designed tube under a uniform heat load of 30 kW.