Abstract
Ion Cyclotron Range of Frequency (ICRF) heating on Large Helical Device (LHD) is characterized by its high power (up to 12MW) and by steady state operation (30 minutes). The LHD is a helical device (with a major radius of 3.9 m and a minor radius of 0.6m) with super-conducting coil windings (l=2, m=10). The main physical purpose is to investigate currentless and disruption-free steady state plasmas. Research and development for ICRF heating have been carried out in recent years. A high RF power transmission system has been developed, which consists of stub tuners, ceramic feed-through and ICRF heating loop antenna. The RF transmission system was tested and withstood 58kV for 10 seconds and 40kV for 30 minutes. The RF voltage corresponds in the case of a plasma loading resistance, 5Ω to a transmitted RF power capability of 3.4MW and 1.6MW. In addition, a pre-matching stub tuner was very effective in reducing the RF voltage. The reduction rate was confirmed to be one third, which leads to a higher ICRF injection efficiency because of reduction of RF power loss in the transmission system. Furthermore a proper procedure to effectively carry out aging of the RF antenna was found in terms of selecting the RF pulse length, repetition rate and RF voltage.
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