Shan Jiafang

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.

Investigation of LHCD Efficiency and Transformer Recharging in the EAST Tokamak

The efficiency of lower hybrid current drive (LHCD) for limiter and divertor configurations in the EAST tokamak is investigated using hot electrical conductivity theory and experimental formulas. The results indicate that the efficiency of current drive in divertor geometry is slightly higher than that in limiter one. To interpret the experimental results, the GENRAY code is applied to calculate the propagation and absorption of the lower hybrid wave (LHW) in different configurations. The numerical results show that the variation in the parallel refractive index (N//) between the two configurations is quite large. Transformer recharging experiments were also successfully conducted in EAST. By means of the Karney-Fisch method, the absorption index (α) and the upshift factor of refraction (β) for the LHW are obtained. In addition, the maximum recharging efficiency in EAST is about 4% in the divertor configuration, with a line-averaged electron density of ne_av = 0.7 × 1019 m−3.

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.

Recent Results of LHCD Experiments in EAST

Lower hybrid wave (LHW)-plasma coupling and lower hybrid current drive (LHCD) experiments in divertor, including single-null and double-null, and limiter configurations were conducted systematically in EAST. A maximum power for launched LHW is 1.4 MW and the plasma current with LHCD is about 1 MA. It is indicated that the coupling is best in limiter configuration, then in single-null one, while worst in double-null one. Study in current drive efficiency by a least squares fit shows that there is no obvious difference in drive efficiency between the double-null and the single-null cases, whereas the efficiency is a slightly lower in the limiter case. The effect of plasma density on the current drive efficiency is due to the influence of density on impurity concentration.

Investigation of lower hybrid current drive during H-mode in EAST tokamak

H-mode discharges with lower hybrid current drive (LHCD) alone are achieved in EAST divertor plasma over a wide parameter range. These H-mode discharges are characterized by a sudden drop in Dα emission and a spontaneous rise in main plasma density. Good lower hybrid (LH) coupling during H-mode is obtained by putting the plasma close to the antenna and by injecting D2 gas from a pipe near the grill mouse. The analysis of lower hybrid current drive properties shows that the LH deposition profile shifts off axis during H-mode, and current drive (CD) efficiency decreases due to the increase in density. Modeling results of H-mode discharges with a general ray tracing code GENRAY are reported.

Design of a New Water Load for S-band 750 kW Continuous Wave High Power Klystron Used in EAST Tokamak

In order to test the klystrons operated at a frequency of 3.7 GHz in a continuous wave (CW) mode, a type of water load to absorb its power up to 750 kW is presented. The distilled water sealed with an RF ceramic window is used as the absorbent. At a frequency range of 70 MHz, the VSWR (Voltage Standing Wave Ratio) is below 1.2, and the rise in temperature of water is about 30 oC at the highest power level.

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.

High Power Microwave Test System and Simulated Experiments for HT-7

In lower hybrid current drive (LHCD) experiments in HT-7, LH wave coupled to the plasma from 12 klystrons is launched by a phased multi-junction grill. High power microwave reflection or arc, which threatens the klystrons, the wave-guide ceramic windows and the vacuum of the tokamak, occur sometimes during the experiments. For the sake of safety, a high power microwave test system to perform simulations was developed. In the test experiments, the klystron and window can be well protected with an optimized microwave monitor and a PIN switch. The layout of the test system and the simulated experiments for HT-7 are described in this paper.

Experimental characteristics of a lower hybrid wave multi-junction coupler in the HT-7 tokamak

A phase-controlled lower hybrid wave (LHW) multi-junction (MJ) coupler (3(rows)×4(columns)×4 (subwaveguides)) has been developed in the HT-7 tokamak. Simulations show that it is more effective for driving plasma current than an ordinary phase-controlled LHW antenna (3(rows)× 12(columns)) (traditional coupler). The plasma–wave coupling experiments show that the reflection coefficient (RC) is below 10%, implying that the MJ grill can launch the wave into the plasma effectively. The effect of power spectrum launched by the MJ coupler on RC indicates that an optimal condition is requisite for a better coupling in the lower hybrid current drive (LHCD) experiments. Studies indicate that the drive efficiency of the MJ antenna is higher than that of the traditional one, which is mainly ascribed to the discrepancy in impurity concentration, plasma temperature, and spectrum directivity. An improved confinement with an electron internal transport barrier is obtained by LHCD. The analysis shows that the modified negative (low) magnetic shear and the change of radial electric field profile due to LHCD are possible factors responsible for the eITB formation.

Feedback Control System for Antenna Phase Difference in the LHCD Experiments

It is well accepted that lower hybrid current drive (LHCD) is the most efficient method for non-inductive current drive in fusion devices and the effect of the current drive is dependent on not only microwave power but also its grill phase shift. This paper presents a new kind of feedback control system for antenna phase difference in LHCD experiments. In this high-speed control system, a lot of new technologies and methods are incorporated. The results of the experiments show a very good agreement with the system design.