Effect of frequency modulation on impedance matching of single ferrite in ICRH antenna system

Abstract

BackgroundIon cyclotron resonance heating (ICRH) is an important auxiliary heating method in tokamak. When fast plasma phenomena such as L-H mode transitions and edge localized modes (ELM), etc., happens, a large change in the antenna coupling impedance occurs. This causes the reflected radio frequency (RF) power of ICRH antenna system increases drastically, which is an urgent problem to be solved in ICRH experiment.PurposeThis study aims to reduce the reflected power of ICRH antenna system by an optimal design of the antenna impedance matching system.MethodsThe transmission line theory was employed to analyze the effect of frequency modulation on impedance matching of single ferrite in ICRH antenna system with varied coupling impedance in the range of 2~8 Ω. The ferrite length and the mechanical length between the antenna and single ferrite tuner (the normalized length is about 0.540) were optimized under certain conditions.ResultsSimulation results show that the reflection coefficient of the ICRH antenna system is reduced within 0~8.4% under optimized design through feedback adjustment, and the relative frequency shift is between 0.146%~0.134% whilst the system impedance matching response time is less than 1 ms.ConclusionsFrequency feedback control of single ferrite tuner, not only effectively reduces the space size of the matching network, but also enhances the impedance matching effect. The relevant research results may provide a certain reference for the design of the ICRH antenna system.

Keywords

• ion cyclotron resonance heating
• coupling impedance
• impedance matching
• single ferrite tuner
• frequency feedback