AbstractOptimization of CT injector for LHD has been carried out and conical electrode for adiabatic CT compression is adopted in the design. Point-model of CT acceleration in a co-axial electrode is solved to optimize the electrode geometry and the power supplies. The condition to attain the large acceleration efficiency is researched and it is shown to be a function of the ratio of the electrode inductance to the external inductance. Acceleration efficiency of 34% is to be obtained with 3.2 m long conical accelerator and 40 kV–42 kJ power supply. The machine overview of a CT injector named SPICA Mk.I (SPheromak Injector using Conical Accelerator) consisting of 0.8 m conical accelerator is also given.