Liquid stub tuner for ion cyclotron heating

Abstract

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 component for high power transmission systems. Test results have quantitatively demonstrated that it can be used at high rf voltage: 61 kV for 10 s and 50 kV for 30 min. Furthermore, the liquid surface can be shifted under high rf voltage without breakdown, which suggests that it can be employed as a feedback control impedance matching tool to keep reflected rf power at a low level with regard to a temporal variation of plasma loading resistance.

Commissioning the ICRF system and an ICRF assisted discharge cleaning at the KSTAR

“…The nominal size of the main coaxial transmission line is 9-3/16 and it is pressurized as 2 atm. The impedance matching between the antenna including the RDL and the transmitter is done by adjusting the liquid level of the two liquid stub-tuners [5] whose matching conditions are checked with a network analyzer. Presently it is tuned at 30 MHz but it could be A high voltage standing wave is generated in ICRF system because currents are circulating in the resonant loops.…”

Control, data acquisition and remote participation for steady-state operation in LHD

“…Feedback control of the liquid impedance matching system [4,5] is used for reducing the reflected power. Reflection of RF is a severe problem for the steady state operation.…”

Current phase control test based on real-time measurement of impedance matrix of ICRF antennas

“…The power is fed from upper and lower ports of the LHD. The antennas are connected to impedance matching devices consisting of three liquid stub tuners [3,4]. By changing the phase difference between the currents on the straps, one can control the wave number parallel to the magnetic field, which was not controllable with the former antennas [5].…”