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We have newly designed, fabricated, and installed a decoupler and a 3 dB hybrid coupler for load-resilient operation of the KSTAR ICRF. The new ICRF TL system for load resilient operation has been successfully performed in 2012 plasma experiments. In L-mode plasmas, the ratio between two powers measured at the isolated port of the hybrid coupler and reflected from two resonant loops becomes approximately 1. Furthermore, a load resilient transmission line (TL) system results in stable ICRF transmitter operation and power transmission in ELMy discharges. The maximum ICRF transmitted power is approximately 500 kW and pulse length is 3 s. With power transmission at higher than 500 kW, breakdowns frequently occurred in the vacuum feedthrough. After the campaign, we discovered that surfaces of alumina, coaxial inner and outer conductor of the vacuum feedthrough were severely damaged by the arcs. We removed the silver coat from 8 feedthroughs and repaired the damaged surfaces. In order to study a higher standoff capability, we tested the repaired feedthroughs in the vacuum chamber with a high power RF test stand. [ABSTRACT FROM AUTHOR]

A 170 GHz Electron Cyclotron Heating and Current Drive (ECH/CD) system on KSTAR is designed to launch total 2.4 MW of power for up to 300 sec into the plasma. At present the first 1 MW ECH/CD system is under installation and commissioning for 2011 KSTAR campaign. The 170 GHz, 1 MW, 300 sec gyrotron and the matching optics unit (MOU) will be provided from JAEA under collaboration between NFRI and JAEA. The transmission line consists of MOU and 70 m long 63.5 mm ID corrugated waveguides with the eight miter bends. The 1 MW, 10 sec launcher is developed based on the existing two-mirror front-end launcher in collaboration with Princeton Plasma Physics Laboratory and Pohang University of Science and Technology, and is installed on the low field side in the KSTAR equatorial plane. The mirror pivot is located at 30 cm below from the equatorial plane. 3.6 MVA power supply system is manufactured and now is under commissioning to meet the triode gun operation of JAEA gyrotron. The power supply consists of 66 kV/55 A cathode power supply, mode-anode system, and 50 kV/160 mA body power supply. In this paper, the current status of KSTAR 170 GHz, 1 MW ECH/CD system will be presented as well as the experimental plan utilizing 170 GHz new ECH/CD system. [ABSTRACT FROM AUTHOR]

Theoretical 5 GHz lower hybrid current drive (LHCD) efficiency using power spectrum given by 0-D Brambilla code and Lower Hybrid Simulation Code (LSC) have been studied for KSTAR. In LSC simulation, RF-driven current and current drive efficiency has been found to be deeply dependent on the profiles of the plasma density and temperature as well as on current profile in order to obtain hollow current profile favorable for advance tokamak operation mode and steady state operation. The peaked density and broad temperature profile control has been found to be efficient in current drive with maximum RF-driven current larger than 400 kA/MW with very high efficiency when the peak plasma density is ranged from 0.2 to 2.0×1020 m-3, and the peak electron temperature range of 2-20 keV together with toroidal field 2-3.5 T and Ip = 0.5-2 MA. The on-/off-axis current profile controllability is also investigated through parametric scan, and small negative magnetic shear is seen at the narrow region of the off-axis for very high temperature regime and for high BT and Ip. In order to achieve the same for lower temperature regime Ip has to be lower and also for higher LH-power compromising with CD efficiency in this case. [ABSTRACT FROM AUTHOR]

The electron cyclotron heating (ECH)-assisted startup was successful in the Korea Superconducting Tokamak Advanced Research (KSTAR) first plasma campaign completed in June, 2008. It was observed that the second harmonic EC wave of 0.35 MW was sufficient to achieve breakdown in the ECH pre-ionization phase, to allow burn through, and to sustain the plasma during the current ramp with a low loop voltage of 2.0 V. This corresponds to a toroidal electric field of 0.24 Vm-1 at the innermost vacuum vessel wall (R = 1.3 m). Since there is no feedback control of the plasma radial position in the initial phase of the KSTAR first plasma campaign, wall contact caused the plasma current fall to zero soon after the ECH beam was turned off. Extending pulse duration of the ECH power to 190 ms allowed the plasma current to rise up to more than 100 kA with a ramp-up rate of 0.8 MA/s and the pulse duration of 210 ms. Later in the first plasma campaign, the plasma was sustained up to 865 ms with the help of additional heating of 350-ms long ECH beam and with the help of the plasma radial position feedback control. The plasma current in the pre-ionization phase was observed and it is considered to be pressure-driven Pfirsch-Schlüter current. [ABSTRACT FROM AUTHOR]