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An experimental study of the X-mode absorption at the second and the third harmonic frequencies has been performed in KSTAR tokamak. The X2 EC frequency is 110 GHz and the X3 EC frequency is 170 GHz at the nominal KSTAR operating toroidal magnetic field. From the 1-D model of the synergetic effect, the X3 cold resonance should lie at low field side with the X2 cold resonance at the high field side to meet the condition which both X2 and X3 EC waves interact with the same resonant electron at the same radial position. However, 170 GHz X3 cold resonance lies at the high field side with distance of 54 mm from the X2 cold resonance position in KSTAR. This paper presents the study of the synergetic effect of X3 absorption by the X2 ECCD with a scheme of two beam target positions at the same flux surface by scanning the poloidal and toroidal beam injection angles to enhance X3 absorption even in the low temperature plasma in KSTAR. For this study, the3D relativistic ray/tracing and Fokker-Planck codes C3PO/LUKE is used for the quantitative prediction of its synergetic effect. The C3PO/LUKE codes are appropriate for X2 and X3 synergy calculations as the distribution function can be calculated with either one or both waves affecting the absorption rate of each other. This paper also introduces the future ECH system upgrade plan

Study of Synergetic Effect of X2 and X3 EC Wave in KSTAR