KSTAR ICRF Research Articles

Resonant loop system for the KSTAR ICRF current drive

The phased current distribution at current straps for the KSTAR ICRF antenna causes a power imbalance at each strap owing to the mutual couplings between current straps. In order to mitigate the effect of coupling, a decoupler connecting two phased feeding lines are designed based on both a lumped element antenna model and a distributed transmission line model. Though the decoupler parameter is dependent on the loading resistance, which depends on plasma condition, an analysis shows that the decoupling is effective in the wide range of loading resistance assuming the low variation of mutual inductance between straps. A circuit analysis also shows that the RF characteristics of a complex RF transmission system are matched well for the asymmetric antenna current spectrum aiming for a non-inductive current drive of KSTAR. The calibration result of decoupler after installation is also discussed.

커플링이 고려된 KSTAR ICRF 안테나의 8포트 전송선 회로 모델링 및 측정 결과 비교

KSTAR ICRF 시스템에서 안테나 전류띠 간 커플링에 의한 전류띠의 전압, 전류 분포 변화와 전류띠 간 전력 전달에 의한 공명현상 그리고 전송선상의 이상 전압 분포 등을 예측하거나 분석하는 것은 그것의 안정적이고 신뢰성 있는 운전을 위해 매우 중요하다. 본 연구에서는 이러한 전류띠 간 커플링에 의한 현상들을 이해하기 위해 ICRF 안테나에서 측정한 S-parameter를 커플링이 고려된 8포트 전송선회로 모델에 적용하여 전류띠의 전송선 회로 모델을 완성하였다. 완성된 전송선 회로모델의 자체유도계수, 상호유도계수, 전기용량성 등은 전류띠의 유한한 길이로 인하여 2D 모델의 값보다 작은 것으로 나타났다. 커플링이 고려된 전류띠의 전송선 회로모델은 공명루프와 결합되어 있는 KSTAR ICRF 시스템의 운전에 활용될 것이다. It is very important to predict and analyze the change of voltage and current distribution of current strap, abnormal voltage distribution of transmission line and resonance phenomenon by coupling between current straps for more stable operation of ICRF system. In this study, to understand those phenomena by coupling, 8-port coupled transmission line model is completed by appling S-parameter measured in the prototype KSTAR ICRF antenna to the model. The determined self-inductance, mutual-inductance and capacitance of antenna straps are shown to be lower than that calculated from 2D approximate model due to finite length of strap. The coupled transmission line model of current strap will be utilized to the operation of ICRF system of KSTAR in the future.

RF Discharge in the KSTAR ICRF Antenna

RF Discharge in the KSTAR ICRF Antenna

Development of the liquid matching components for the KSTAR ICRF system

A liquid stub tuner has been developed which is a method for a liquid matching system for transmitting a MW level of RF power continuously in the KSTAR ICRF heating and current drive system. It is made of a 228.6 mm coaxial transmission line, and it uses the difference between the RF wavelength in a liquid and that in a gas due to their different dielectric constant. The level of the liquid can be changed from 0 to 2.8 m for each liquid stub tuner, which corresponds to a 2.0 m mechanical length variation at 30 MHz. DC high voltage tests of various conditioned silicon oils such as used silicon oil, fresh silicon oil, and vacuum-pumped silicon oil were accomplished. Vacuum-pumped silicon oil showed an improved high voltage characteristic when compared with the used silicon oil and fresh silicon oil. The DC high voltage tests showed that the high voltage characteristic of the silicon oils was worse after a breakdown had occurred. High RF test for the liquid stub tuner was performed with vacuum-pumped silicon oil to ascertain its’ stand-off voltage. A stand-off voltage of 37.5 kV (average) was achieved for the liquid stub tuner with vacuum-pumped silicon oil during a 300 s operation.

Water-cooling effects on the high-voltage and long-pulse capabilities of the KSTAR ICRF antenna

A new ICRF antenna originating from the prototype antenna was constructed for the KSTAR tokamak in 2002. The performance of the antenna was experimentally estimated at the RF test stand without a plasma. Recently three series of RF tests were performed at a frequency of 30 MHz; without any cooling, with a water-cooling for only the antenna, and with a water-cooling of the antenna and the transmission line connected to the antenna. In the tests, a half of the current strap was connected to a RF source via a matching circuit with the other half one connected to an open terminated coaxial line, and the other three straps were shorted at the input ports. During the RF pulse, the temperatures at several positions of the antenna cavity wall were measured by embedded thermocouples and the temperature profile of the front face of the antenna was measured by an IR camera. The line voltage, forward and reflected powers, and the RFTC pressure were also measured. The water-cooled antenna showed several enhanced performances in a comparison with the non-cooled case, and the standoff voltage was significantly increased. By utilizing a water-cooling of the antenna and the transmission line, we achieved a standoff voltage of 41.3 kVp for a pulse length of 300 s, and we could extend the pulse length up to 600 s at a maximum voltage of 35.0 kVp without encountering any problems, which considerably exceeds the design requirements.

Progress of the KSTAR ICRF components development for long pulse operation

Abstract The ICRF heating and current drive system for the KSTAR tokamak is being developed to support long pulse, high β, advanced tokamak physics experiments. Key ICRF components such as a water-cooled, high power density (∼10 MW/m2) antenna, a vacuum feedthrough for 1 MW of RF power transmission, and the tuning and matching components for slow/fast matching, are developed. High voltage characteristics and long pulse operation capability are investigated in KAERI test stand. Test results show that they have high voltage stand-off characteristics with the peak voltage over 30 kV and they can be used for long pulse (300 s) high RF power transmission.

Design of a Dummy Load for the 2 MW HF Transmitter

A 2 MW transmitter with a frequency ranging from 30 to 60 MHz is used for the KSTAR ICRF heating source and it is necessary to equip it with a dummy load which has a low VSWR above the second or third harmonic frequency for testing the 2 MW transmitter. The resistive film type is well known as a non-inductive dummy load, however it can't be used above 300 kW because of problem at the surface adhesion between the ceramic substrate and the resistive film or sputtered carbon which imposes a limit on the power density of the film resistor. So, above 1 MW the most promising candidate is the soda water dummy load where the soda water plays a role in both the rf power absorbing medium and the cooling water. In this work, the electrical and mechanical design requirements of the 2 MW soda dummy load are introduced and RF characteristics are discussed using a lumped circuit model and a numerical analysis.