Xu Handong

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.

Development and Preliminary Commissioning Results of a Long Pulse 140 GHz ECRH System on EAST Tokamak (Invited)

A long pulse electron cyclotron resonance heating (ECRH) system has been developed to meet the requirements of steady-state operation for the EAST superconducting tokamak, and the first EC wave was successfully injected into plasma during the 2015 spring campaign. The system is mainly composed of four 140 GHz gyrotron systems, 4 ITER-Like transmission lines, 4 independent channel launchers and corresponding power supplies, a water cooling, control & inter-lock system etc. Each gyrotron is expected to deliver a maximum power of 1 MW and be operated at 100-1000 s pulse lengths. The No.1 and No.2 gyrotron systems have been installed. In the initial commissioning, a series of parameters of 1 MW 1 s, 900 kW 10 s, 800 kW 95 s and 650 kW 753 s have been demonstrated successfully on the No.1 gyrotron system based on calorimetric dummy load measurements. Significant plasma heating and MHD instability suppression effects were observed in EAST experiments. In addition, high confinement (H-mode) discharges triggered by ECRH were obtained.

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.

Ohmic Radio-Frequency Synergy Current Drive and Transformer Recharging Experiments in the HT-7 Tokamak

Lower hybrid current drive (LHCD) experiments for investigating the interaction between lower hybrid (LH) wave and residual dc electric field were performed in extensive plasma parameter ranges in the HT-7 superconducting tokamak. The experimental results are well fitted to the Karney–Fisch theory on the efficiency of LH waves energy converted to poloidal magnetic field energy. The fraction of absorbed LH power is about 0.75 for the HT-7 machine, and the upshift of the LH-wave parallel refraction index during LHCD experiments have been derived by the optimizing fitting parameters. The LH wave is also used for the transformer recharging when the plasma current is maintained unchanged. The highest efficiency about 7% has been achieved in HT-7 machine.

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.

Effects of Plasma Density and Toroidal Magnetic Field on Lower Hybrid Current Drive Efficiency on HT-7 Tokamak

Lower hybrid current drive experiments on the HT-7 device have been carried out by scanning the following parameters: central line averaged electron density (ne = 0.6 - 2.0 × 1019 m-3) and toroidal magnetic field (Bt = 1.62 - 2.0 T). The dependence of current drive efficiency on these parameters has been studied and the experimental curves of current drive efficiency as a function of ne and Bt have also been obtained. From these experimental results, it can be seen that current drive efficiency rises with the increase of toroidal magnetic field. As plasma density increases, the current drive efficiency first increases to a certain value, then gradually decreases, that is, there exists an optimized density regime where a better drive efficiency can be obtained. The analysis shows that the current drive efficiency is mainly affected by wave accessibility and impurity concentration, and the competition of these two factors determines the current drive efficiency.

Experimental Results of MHD Suppression with Modulated LHCD on the HT-7 Tokamak

Modulation of lower hybrid current drive was used successfully to suppress MHD activity. This was achieved in discharges with MHD m = 2 tearing modes during the discharge conditions Ip = 110 kA, Bt = 1.75 T, e0 similar 1.1 × 1013 cm-3. The delivering time of LHCD pulse is less then 30 μs. The amplitude, interval and the period of LHCD modulation pulse can be adjusted very conveniently. The modulation LHCD can be delivered very fast at any time during the discharge. The modulation LHCD period was always much shorter than the plasma resistive time (τη ~ 100 ms). So the profile of plasma current is changed much faster than the plasma resistive time. The different forms of LHCD modulating can be proved.

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.