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KSTAR Plasma Current Ramp-up Assisted by 5 GHz Lower-hybrid Current Drive

KSTAR Plasma Current Ramp-up Assisted by 5 GHz Lower-hybrid Current Drive
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Magnetic flux (volt-sec) consumption during the plasma current ramp-up phase is one of the critical issues in a fully superconducting tokamak. Lower-hybrid current drive (LHCD) has been used to drive off-axis non-inductive current and heat the electron with high current drive efficiency in existing tokamak devices. The LHCD system is under consideration to be used in the operation phase of the International Thermonuclear Experimental Reactor (ITER). For this reason, 5 GHz, 500 kW LHCD system has been developed for Korean Superconducting Tokamak Advanced Research (KSTAR) for the experimental research of the LH wave coupling and the magnetic flux saving during the plasma current ramp-up phase in KSTAR. To model the operation scenario of LHCD experiments one needs to implement the theory based on three physical processes
LH wave-plasma coupling, the ray-tracing and the power absorption, and the current drive. For coupling calculations computer codes, Brambilla and ALOHA, provide a detailed spectrum based on experimental data. The LH wave propagation, deposition and current drive are calculated by the ray-tracing code C3PO and Fokker-Planck solver LUKE. Finally, the METIS code generates the time evolving plasma parameters and current profile with the input data empirically generated from the real experiments (for e.g., this thesis used (KSTAR shot #5952 and #6184). From the self-consistent simulation, the flux-saving effects by the LH wave during the plasma current ramp-up phase on KSTAR are predicted. The flux-saving effects are investigated with changes of LH power and the switch-on time of LH wave. The early LH application is effective to reduce the flux consumption. The fractions of inductive and resistive terms in total flux-saving is calculated to be approximately 0.08 Wb (19%) and about 0.39 Wb (81%) in #6184 case, respectively. For KSTAR 2012 campaign, an initial 500 kW LHCD system has been developed and successfully commissioned. The prototype of 5 GHz, 500 kW continuous-wave (CW) klystron as the RF source is successfully tested at 304 kW for 800 s, 460 kW for 20 s, and recently 500 kW for 600 s. The 4-way splitter based fully active waveguide grill launcher has been designed using HFSS program, fabricated and installed in KSTAR tokamak. This thesis describes the study result of the computational prediction of flux-saving effect by 5 GHz LHCD system in KSTAR and the commissioning results of initial 500 kW KSTAR LHCD system.
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