[en] LHCD system of 2.45 GHz in EAST has been updated to 4MW in last campaign. Aimed at high confinement (H-mode) plasma in EAST, the LHW-plasma coupling and current drive experiments were continued. Experiments of local gas puffing near LHW antenna shows that gas puffing from electron side is better to improve LHW-plasma coupling than that from ion side. LHCD experiments at high density are also performed, demonstrating that the decrease of current efficiency at high density may be related to the parametric decay instability (PDI) effect. Lithiation and local gas puffing near LHW antenna are utilized so as to sustain H-mode plasma. H-mode plasma is obtained by LHCD with a wide range of parameters: Ip=0.4∼0.8MA, B_t=1.35∼1.81T, n_e=1.5∼2.5x10"1"9 m"-"3, P_L_H_W>=0.5MW. LHW power deposition and driven current profile with C3PO/LUKE are calculated with the experimental parameters, showing that central and large driven current seems not a necessary condition for the H-mode plasma. H-mode is reproduced with CRONOS. Long pulse plasmas, >400s L mode fully driven by LHCD and >30s H-mode with LHCD and ICRF, have been achieved and demonstrated in EAST. (author)

[en] A 4.6 GHz lower hybrid current drive (LHCD) system has been firstly commissioned in EAST in 2014 campaign. Effect of turbulence on plasma-coupling and the comparison between 2.45 GHz and 4.6 GHz are compared, suggesting a better coupling for 4.6 GHz LHW. Studies show that compared to H-mode, the plasma-wave coupling indicated by the mean reflection coefficient is better in the case of L-mode plasma, suggesting the lower density in the case of H-mode. Compared to 2.45 GHz LHW, 4.6 GHz LHW has a better capability on current drive, plasma heating, confinement improvement, modifying current profile, possibly due to less parametric instability in this case. (author)

[en] Lower hybrid current drive (LHCD) is an important heating system for long pulse plasma with high performance in EAST. The effect of LH (lower hybrid) wave frequency on LHCD characteristics has been studied on EAST for the first time with two different frequencies (2.45 and 4.6 GHz), showing that higher frequency improves penetration of the RF power into the plasma core, leading to a better effect on plasma characteristics. The improvement in LHCD is mainly ascribed to a reduction in parametric instability (PI) and a lesser extent collisional absorption (CA) in the edge region with the 4.6 GHz wave, demonstrating the role and mitigation of parasitic effects of edge plasma. These results are encouraging that LHCD is essential for current profile control in reactor grade plasmas. (author)

[en] Lower hybrid current drive (LHCD) is an important heating system for long pulse plasma with high performance in EAST. The effect of LHCD on current profile was summarized, including edge parameters, LH frequency, LH spectrum and plasma density. The role of parasitic effects of edge plasma on LHCD is studied and it can be mitigated by increasing source frequency or lowering recycling. Lower recycling and higher LH frequency are preferred for LHCD at higher density. Effect of LH spectrum and plasma density on LHCD suggests that the optimization is possible methods to control current profile. Results are encouraging that LHCD is essential for current profile control in reactor grade plasmas. (author)

[en] Full text: A 2 MW 2.45 GHz lower hybrid wave (LHW) system has been installed and run in EAST tokamak, in which LHW coupling and lower hybrid current drive (LHCD) experiments have been performed in both divertor configuration and limiter plasma with parameters of a plasma current of 250 kA and a central line averaged density of 1.0 - 1.3 x 10¹⁹ m^-3, suggesting that present LHW system is effective to couple LHW into plasma and drive plasma current with low reflection coefficient (RC) of ∼ 10%. Results show that RC firstly decreases with increasing distance between plasma and LHW grill, and then increases with the distance, in agreement with the simulation through Brambilla theory. Studies indicate that with the same plasma parameters, the best coupling is obtained in the limiter case, followed by the single null, and the last one is the double null configuration. The main reason for this is the different magnetic connection length. Current drive efficiencies investigated by a least squares fit show that there is no obvious difference in the drive efficiency (∼ 0.73 x 10¹⁹ Am^-2W^-1) between the double null and the single null cases, whereas the efficiency is a little small in the limiter configuration (0.58 x 10¹⁹ Am^-2W^-1), in agreement with the ray tracing code simulation by LUKE/C3PO. The different current efficiency can be explained by the power spectrum up-shift factor estimated from a power flow process in different plasma configurations. There is little dependence of drive efficiency on LHW power spectrum. The possible reason is that the temperature is low and LHW is absorbed after multi-pass propagation. (author)

[en] LHW-plasma coupling and lower hybrid current drive (LHCD) experiments in both divertor configuration and limiter plasma have been performed systematically in EAST. In normal experiment, LHW can be coupled into plasma with RC less than 10% and the current drive efficiency is about 0.6 - 0.9 x 10¹⁹ Am²W^-1. The maximum of incident LHW power is up to 1.4 MW and the obtained plasma current with LHCD is about 830 kA. Studies indicate that the best coupling is obtained in the limiter case, followed by the single null, and the last one is the double null configuration. Current drive efficiency investigated by a least squares fit shows that there is no obvious difference in the drive efficiency between the double null and the single null cases, whereas the efficiency is a little small in the limiter configuration. Current drive efficiency is affected by plasma density, mainly due to influence of density on impurity concentration. (author)

[en] In addition to 2.45 GHz/4 MW lower hybrid current drive (LHCD) system, a 4.6 GHz LHCD system has been firstly commissioned in EAST in 2014 campaign. First LHCD results with 4.6 GHz show that LHW can be coupled to plasma with low reflection coefficient, drive plasma current, modify plasma current profile, and heat plasma effectively. Also, CD efficiency and current profile depend on the launched wave spectrum, suggesting the possibility of controlling current profile by changing phase difference. Repeatable H-mode plasma is obtained by either 4.6 GHz LHCD system alone, or together with the 2.45 GHz LHCD system, the NBI (neutral beam injection) system. (author)

[en] The edge plasma transport around the last closed flux surface was investigated using Langmuir probes in a lower hybrid current drive (LHCD) experiment in the Hefei Tokamak-7 [J. K. Xie et al., in Proceedings of the 16th International Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 1, p. 685]. Fluctuations of plasma density, plasma temperature, and poloidal electric field in the boundary plasma are reduced, hence the electrostastic turbulence-induced particle flux and heat flux both significantly decrease after the application of LHCD. The suppression of density fluctuation is more sensitive to the lower hybrid wave (LHW) than that of temperature fluctuation, which is in agreement with the model proposed by Ware in the presence of flow shear [A. S. Ware et al., Phys. Plasmas 5, 173 (1998)]. The difference between turbulent convective and conductive fluxes depends on such a discrepancy between normalized density fluctuation and temperature fluctuation. It is speculated that the reduced turbulence-induced transport is possibly attributable to a sheared flow resulting from the varying radial electric field, which is formed possibly due to the loss of the energetic electron generated by LHW. In addition, studies show that the effect of magnetohydrodynamic activity, possibly modified by LHCD, on plasma transport could be another possible candidate for the reduced electrostatic turbulent flux

[en] A high confinement plasma produced by using lower hybrid current drive (LHCD) has been obtained on the HT-7 superconducting tokamak. An internal transport barrier in the core plasma was formed. The energy confinement time increases from 14.6 (OH phase) to 24.5 ms (LHCD phase), which is near the value of the ITER93 ELM free scaling. The H₈₉-factor was increased from 0.78 (OH phase) to 1.42 (LHCD phase). The experimental results were in good agreement with the simulations calculated with a ray tracing code and a 2- dimensional Fokker-Planck equation. The edge plasma characteristics around the last closed flux surface were investigated using Langmuir probes. Turbulence and transport of the edge plasma were suppressed significantly by the lower hybrid wave. Studies show that the high confinement plasma is mainly attributable to a sheared flow resulting from the varying radial electric field. (author)

[en] The research of EAST program is mostly focused on the development of high performance steady state scenario with ITER-like poloidal configuration and RF-dominated heating schemes. With the enhanced ITER-relevant auxiliary heating and current drive systems, the plasma profile control by coupling/integration of various combinations has been investigated, including lower hybrid current drive (LHCD), electron cyclotron resonance heating (ECRH) and ion cyclotron resonance heating (ICRH). The 12 MW ICRH system has been installed on EAST. Heating and confinement studies using the Hydrogen Minority Heating scheme have been investigated. One of the importance challenges for EAST is coupling higher power into the core plasma, experiments including changing plasma position, electron density, local gas puffing and antenna phasing scanning were performed to improve ICRF coupling efficiency on EAST. Results show that local gas injection and reducing the k|| can improve the coupling efficiency directly. By means of the 4.6 GHz and 2.45 GHz LHCD systems, H-mode can be obtained and sustained at relatively high density, even up to n_e ∼ 4.5*10¹⁹/m³, where a current drive effect is still observed. Meanwhile, effect of source frequency (2.45 GHz and 4.6 GHz) on LHCD characteristic has been studied on EAST, showing that higher frequency improves penetration of the coupled LH (lower hybrid) power into the plasma core and leads to a better effect on plasma characteristics. Studies demonstrate the role of parasitic effects of edge plasma in LHCD and the mitigation by increasing source frequency. Experiments of effect of LH spectrum and plasma density on plasma characteristics are performed, suggesting the possibility of plasma control for high performance. The development of a 4 MW ECRH system is in progress for the purpose of plasma heating and MHD control. The built ECRH system with 1 MW source power has been successfully put into use on EAST in 2015. H-mode discharges with L-H transition triggered by ECRH injection were obtained and its effects on the electron temperature, particle confinement and the core MHD stabilities were observed. By further exploring and optimizing the RF combination for the sole RF heating and current drive regime, fully non-inductive H-mode discharges with V_loop ∼ 0 V has progressed steadily in the 2016 campaign. The overview of the significant progress of RF dominated experiments is presented in this paper. (authors)