T. Seki

Liquid stub tuner for ion cyclotron heating

Ion cyclotron range of frequency (ICRF) heating on the large helical device (LHD) is characterized by high power (up to 12 MW) and steady state operation (30 min). The LHD is a helical device (with a major radius of 3.9 m and a minor radius of 0.6 m) with superconducting coil windings (l=2, m=10). The main purpose of physical research is to investigate currentless and disruption-free plasma. Research and development for steady state ICRF heating has been carried out in recent years: A high rf power transmission system consisting of stub tuners, a ceramic feedthrough, and an ion cyclotron heating loop antenna has been developed. In addition, steady state operation of a rf oscillator has been achieved at a power higher than 1 MW. A liquid stub tuner has been proposed as an innovation. The liquid stub tuner makes use of the difference between the rf wavelengths in liquid and in gas due to the different relative dielectric constants. The liquid stub tuner has been experimentally proved to be a reliable rf com...

Liquid impedance matching system for ion cyclotron heating

Ion cyclotron heating has been established as one of the heating schemes in nuclear fusion research and its use in steady state plasma heating in various devices is being considered. The optimal technology for steady state ion cyclotron range of frequency heating has not been firmly established. This article reports on the liquid stub tuner which was newly developed in research and development activities on the large helical device. It demonstrated high performance in real use in experiments. Two different impedance-matching systems based on the liquid stub tuner are studied: one is a triple liquid stub tuner system and the other is a single stub tuner system with a liquid phase shifter. The characteristics of the two systems are compared from the points of view of how wide a frequency range is covered, and how great the reduction of the voltage in the transmission line is.

Liquid impedance matching system for Ion Cyclotron heating

Ion Cyclotron Range of Frequency (ICRF) heating on the Large Helical Device (LHD) is characterized by high power (up to 12MW) and by steady-state operation (30 minutes). Research and development for ICRF heating have been carried out in recent years. A newly developed liquid stub tuner has demonstrated highly reliable performance as a stub tuner; it has withstood 63kV for 10 seconds and 50kV for 30 minutes. The liquid surface level could be shifted under high RF voltage without breakdown. A liquid impedance matching system has been designed and fabricated for ICRF heating on the LHD. This system consists of a liquid stub tuner and a liquid phase shifter. The liquid phase shifter was constructed by connecting two liquid stub tuners in a U-shaped configuration. An impedance matching can be acquired in a wide frequency range, i.e., 25–95MHz by selecting the length of 4m for the liquid stub tuner. At some frequencies, it was a problem that the RF voltage at the phase shifter became higher than that between th...

Initial ICRF heating experiments on the LHD

The final goal of Ion Cyclotron Range of Frequency (ICRF) heating on the Large Helical Device (LHD) is characterized by its high power (up to 12MW) and by steady state operation (30 minutes). Initial ICRF heating experiments were carried out using a pair of loop antemas in the 2nd experimental campaign in 1998. The ICRF heating power was applied to an ECH-produced plasma at an RF power level of 300 kW for 0.2 seconds. An applied frequency of f=25.6u200aMHz was selected. A cyclotron resonance layer of hydrogen ions was located at the half minor radius during operation at B=1.5u200aT. A mode conversion layer of a He plasma with a minority of hydrogen ions was located between the magnetic axis and the last closed magnetic flux. The plasma stored energy was observed to increase to twice that of the ECH plasma (PECH=300u200akW). The plasma stored energy of the ECH target plasma was 11-13 kJ at an average electron density of ne=8–9×1018u200am−3 and a central electron temperature of Te0=400u200aeV. The plasma stored energy increase...