Design and performance prediction studies of a disk type magnetohydrodynamic (MHD) generator which is applied to the non equilibrium plasma generator (NPG) system have been performed. The main objective of the present study was to find out whether a sufficiently high output performance demonstration is possible with the disk generator which is planned to be used for the NPG-MHD disk pulse power demonstration. A present numerical simulation showed that the original constant height channel could provide not more than 20% of enthalpy extraction because its channel shape could not sustain the working plasma in the stable regime against ionization instability throughout the channel. We concluded that, in order to obtain much higher generator performance, a detailed design of the channel shape was necessary. Design work has also been performed based on the concept that the local electron temperature must be kept at 5000K, i.e. the plasma must be controlled to locate at the center of the stable regime. With the designed channel, enthalpy extraction of up to 40% and output power of 7.2 MW can be successfully expected under the thermal input of 18 MW. In addition, the designed channel requires no major modification of the supersonic nozzle, the inlet swirl vanes and the configuration of the magnet system.