Yanping Zhao

Design and Test of the Fast Ferrite Tuner for ICRF Heating in EAST

Abstract Ion cyclotron range of frequency (ICRF) heating has been used in tokamaks as one of the most successful auxiliary heating tools and has been adopted in EAST. To ensure the steady operation of the ICRF heating system in EAST, the research and development of the fast ferrite tuner (FFT), which aimed to achieve real-time impedance matching of transmitter to antenna, has been carried out. The design and analysis of the FFT is an iterative process where multiple parameters have to be taken into account. The dimensions of the FFT should be chosen as a compromise between relative equivalent electrical length and high-power performance by using the finite element method and numerous computer simulations. The first prototype aimed at achieving a response time of milliseconds and operation with a peak power of 300 kW, which will inform us about the radio-frequency and the high-power performance of such a ferrite tuner. The bench test results have demonstrated that the FFT with a tuning speed of [approximately]200 ms is faster than the traditional methods, and it can be one of the candidates for the real-time impedance matching of the ICRF heating system in EAST. The high-power performance of the FFT should be tested in the EAST 2012 spring campaign. To be fit for the real-time impedance matching for ICRF heating experiments, development of a new prototype, which aims at a response time of 0.5 ms, an insertion loss of <1%, and operation with a peak power of 1.5 MW, is in progress.

Development of the High Radio Frequency Power Amplifiers for ICRF Heating in EAST

Abstract High radio frequency (rf) power amplifiers were designed as a part of research and development of an ion cyclotron range of frequency (ICRF) system that aimed at long-pulse operation at the megawatt level in a frequency range of 25 to 70 MHz. A study on the high-power amplifiers for ICRF heating in Experimental Advanced Superconducting Tokamak (EAST) is presented. To realize the design with a compact structure, a double coaxial cavity was employed as the output circuit of the final power amplifier (FPA) for tuning and matching, and the strip line was adopted for the input impedance matching circuit of the drive power amplifier (DPA). A double-stub tuner matching network with a variable-length U-link was used to obtain the impedance matching between the DPA and the FPA. To ensure the stable operation of the amplifiers, a grounded-grid configuration was chosen, and precautions were taken to suppress all parasitic oscillations of the anode output circuit. The rf power amplifiers performed successfully in stable operation at the megawatt level at each integer frequency from 25 to 70 MHz during the tests, and a rf power of 1.5 MW was achieved in a matching dummy load. The test results show a good agreement with the calculated values. The amplifiers operated reliably in long-pulse mode in EAST, and the total rf power of [approximately]1.8 MW was injected into plasmas in EAST ICRF heating experiments in the 2010 autumn campaign.

Pre-Stub Tuner for Reduction of Radio-Frequency Voltage Along Transmission Line in EAST-ICRF System

The ion cyclotron range of frequency (ICRF) heating system of the Experimental Advanced Superconducting Tokamak (EAST) is characterized by high radio-frequency (RF) power up to 12 MW and wide frequency range over 25 to 70 MHz. A high RF power transmission system composed of a liquid impedance matching device, ceramic feedthrough, decoupler, and ICRF heating antenna with four straps has been in operation for some years. In a high-power ICRF experiment, one issue that needs to be solved is the high RF voltage on the coaxial transmission line between the ICRF antenna and the impedance matching device, which is caused by low antenna loading resistance compared to the characteristic impedance of the transmission line. A stub tuner is employed to reduce the RF voltage in the EAST ICRF power transmission system. Two methods to reduce RF voltage using short-circuited and open-circuited stub tuners are introduced in detail. The optimized position and length of the stub tuner are analyzed and calculated to achieve a smaller voltage reduction ratio (VRR) on the transmission line. The test with the stub tuner to reduce the RF voltage of the transmission line is carried out, and a RF VRR of ∼0.57 is achieved. The RF voltage on the transmission line is significantly reduced, and the capability of the transmission power is obviously improved. Ohmic losses caused by the surface resistance of the conductor of the coaxial transmission line are also decreased, and the probability of breakdown within the transmission line is reduced under high RF power operation.

High-Power Fast-Response Ferrite Tuner for ICRF Impedance Matching in EAST

Ion cyclotron resonance frequency (ICRF) heating is one of the traditional auxiliary heating methods adopted in the Experimental Advanced Superconducting Tokamak (EAST). The radio-frequency (rf) source consisting of eight transmitters has been fabricated since 2012 and has a working frequency of 24 to 70 MHz. It has a maximum total power of ∼12 MW. However, the power injection into plasma has been restricted by the variable antenna load, which is sensitive to the scrape-off-layer boundary condition and the gradient distribution of plasma density. Triple liquid stub tuners, which have been employed for ICRF impedance matching, cannot cope with such rapid variations because of the low response speed. In previous research, a 300-kW ferrite tuner (FT) was developed and tested, but it was not good enough to meet the requirements of real-time impedance matching. Research on a high-power fast-response FT with maximum power of 1.5 MW was carried out to achieve real-time tuning to trace the load variations of the antenna. The design parameters of the FT were determined according to the experimental data of the antenna load in EAST. The ferrite material, rf circuit, and magnet system of the FT were discussed to satisfy the design goals. The test results showed good performance of response time, differential phase shift, and insertion loss, which was extremely significant for the high-power, real-time operation of an impedance matching network based on FTs.

Theoretical analysis of the EAST 4-strap ion cyclotron range of frequency antenna with variational theory*

A variational principle code which can calculate self-consistently currents on the conductors is used to assess the coupling characteristic of the EAST 4-strap ion cyclotron range of frequency (ICRF) antenna. Taking into account two layers of antenna conductors without lateral frame but with slab geometry, the antenna impedances as a function of frequency and the structure of RF field excited inside the plasma in various phasing cases are discussed in this paper.