Zhao Lianmin

A 2450 MHz/2 MW Lower Hybrid Current Drive System for EAST

A 2 MW-2.45 GHz lower hybrid current drive (LHCD) system was designed, fabricated and installed successfully on EAST in 2008 to investigate high performance regimes and achieve a steady-state operation. The system is composed of 20 continuous wave (CW) klystron amplifiers, 4 sets of high voltage power supply, 20 standard rectangle waveguide (American National Standard, WR430) transmission lines with a length of 30 m to 40 m for each, and a multi-junction grill containing 160 active (in groups of 8) and 25 passive sub-waveguides arranged in 5 rows and 4 columns. Fixed phase shifters in sub-waveguides and electronically-controlled digital phase shifters driving each klystron are used to control the radiated power spectrum of the grill. By means of LHCD, a plasma current of 150 kA to 500 kA at a central plasma density of 1 × 1013 cm−3 to 2 × 1013 cm−3 and a toroidal magnetic field of 2.0 T to 2.5 T was achieved. Both a fully non-inductive plasma current of 250 kA lasting up to 23 s and the ramp up were achieved. These results successfully demonstrated the capability of the LHCD system. The system is described in detail and the primary experimental results are presented.

Design and Test of an Antenna Module Mock-Up for the EAST 4.6 GHz LHCD Launcher

The launcher of the 4.6 GHz lower hybrid current drive (LHCD) system for the Experimental Advanced Superconducting Tokamak (EAST) consists of 24 antenna modules, each composed of an array of 3 (row) × 8 (column) waveguides. In order to verify the radio frequency (RF) design and the feasibility of the manufacturing process, a mock-up of the module has been fabricated and measured. The measured return losses of all three input waveguides are less than −25 dB at a frequency of 4.6 GHz. The transmission coefficients from the input waveguide to the output waveguides are −9.13 ± 0.2 dB and the insertion loss is 0.104 dB. These good results mean that the design method of the antenna module can be used for the new 4.6 GHz launcher on EAST. The detailed design of the multi-junction antenna module and its initial test result are described in this paper.

Design of a TE10–TE30 Rectangular Mode Converter for 4.6 GHz LHCD Launcher in the Experimental Advanced Superconducting Tokamak

A compact rectangular TE10–TE30 mode converter is developed for the lower hybrid current drive (LHCD) launcher on the Experimental Advanced Superconducting Tokamak (EAST) at 4.6 GHz. The converter with periodic width perturbation aims to divide the microwave power into three sub-waveguides in the poloidal direction. We present the design and numerical calculation of the mode converter. Calculations are performed on the ripple wall converter by codes based on numerical solving the coupled-mode differential equations and on the simulation of the High Frequency Structure Simulator (HFSS) package. The resulting conversion efficiency from TE10 mode to TE30 mode exceeds 95% within the bandwidth from 4.56 GHz to 4.64 GHz, and the return loss of the oversized transducer can be considerably decreased to 0.068% by means of a capacitive button embedded in the E-plane of the waveguide.

Study of Bridging of the Spectral Gap in the Lower Hybrid Wave Current Drive in the HT-7 Tokamak

An additional lower hybrid wave (LHW) with a higher refractive index (N//) was investigated in the HT-7 tokamak to bridge the spectral gap. It was found that the spectral gap between the wave and the electrons in the outer region was bridged by the additional wave with a higher N// spectrum. The results showed that the sawteeth oscillation was suppressed by launching the additional wave, and that the power deposition profile was moved outwards and the current profile was broadened due to the application of the additional wave. Our study indicates that the spectral gap may be bridged by an additional wave with a higher N// spectrum in the outer region.

Experimental Investigation of Lower Hybrid Wave Coupling in HT-7

Experiments on lower hybrid wave (LHW) coupling were investigated in the HT-7 tokamak. Good coupling of LHW plasma has been demonstrated at different conditions in the HT-7 tokamak. Relevant results have proved that LHW-plasma coupling is affected by the phase difference between adjacent waveguides. Furthermore, the edge density around the grill and relevant coupling can be adjusted by changing the plasma line average density or the gap value between the LH grill and the last closed flux surfaces (LCFS). It is found that the coupling of LHWs becomes poor when the edge density around the LH grill is large enough in the HT-7 tokamak, and that coupling remains good with a proper edge density. With increasing LHW power, it is also found that the reflection coefficients (RCs) increase due to non-linear effects under conditions of low edge recycling, but can decrease under high edge recycling. The edge density depends mainly on the competition between the ponderomotive force (PMF) and the edge recycling intensity in the HT-7 tokamak.