Aimed at high-confinement (H-mode) plasmas in the Experimental Advanced Superconducting Tokamak (EAST), the effect of local gas puffing from electron and ion sides of a lower hybrid wave (LHW) antenna on LHW–plasma coupling and high-density experiments with lower hybrid current drive (LHCD) are investigated in EAST. Experimental results show that gas puffing from the electron side is more favourable to improve coupling compared with gas puffing from the ion side. Investigations indicate that LHW–plasma coupling without gas puffing is affected by the density near the LHW grill (grill density), hence leading to multi-transition of low–high–low (L–H–L) confinement, with a correspondingly periodic characteristic behaviour in the plasma radiation. High-density experiments with LHCD suggest that strong lithiation gives a significant improvement on current drive efficiency in the higher density region than 2 × 1019 m−3. Studies indicate that the sharp decrease in current drive efficiency is mainly correlated with parametric decay instability. Using lithium coating and gas puffing from the electron side of the LHW antenna, an H-mode plasma is obtained by LHCD in a wide range of parameters, whether LHW is deposited inside the half-minor radius or not, implying that a central and large driven current is not a necessary condition for the H-mode plasma. H-mode is investigated with CRONOS.

Using a 2 MW 2.45 GHz lower hybrid wave (LHW) system installed in experimental advanced superconducting tokamak, we have systematically carried out LHW-plasma coupling and lower hybrid current drive experiments in both divertor (double null and lower single null) and limiter plasma configuration with plasma current (I p ) $ 250 kA and central line averaged density (n e ) $ 1.0-1.3 Â 10 19 m À3 recently. Results show that the reflection coefficient (RC) first is flat up to some distance between plasma and LHW grill, and then increases with the distance. Studies indicate that with the same plasma parameters, the best coupling is obtained in the limiter case (with plasma leaning on the inner wall), followed by the lower single null, and the one with the worst coupling is the double null configuration, explained by different magnetic connection length. The RCs in the different poloidal rows show that they have different coupling characteristics, possibly due to local magnetic connection length. Current drive efficiency has been investigated by a least squares fit with N peak == ¼ 2:1, where N peak == is the peak value of parallel refractive index of the launched wave. Results show that there is no obvious difference in the current drive efficiency between double null and lower single null cases, whereas the efficiency is somewhat small in the limiter configuration. This is in agreement with the ray tracing=Fokker-Planck code simulation by LUKE=C3PO and can be interpreted by the power spectrum up-shift factor in different plasma configurations. A transformer recharge is realized with $0.8 MW LHW power and the energy conversion efficiency from LHW to poloidal field energy is about 2%.

Two important issues in achieving lower hybrid current drive (LHCD) high confinement plasma in EAST are to improve lower hybrid wave (LHW)-plasma coupling and to drive the plasma current at a high density. Studies in different configurations with different directions of toroidal magnetic field (B t) show that the density near the antenna is affected by both the radial electric field induced by plasma without a LHW (E r_plasma) in the scrape off layer (SOL), and the radial electric field induced by LHW power (E r_LH) near the grill. Investigations indicate that E r   ×  B t in the SOL leads to a different effect of configuration on the LHW-plasma coupling and E r_LH   ×  B t accounts for the asymmetric density behaviour in the SOL observed in the experiments, where E r is the total radial electric field in the SOL. Modelling of parametric instability (PI), collisional absorption (CA) and scattering from density fluctuations (SDF) in the edge region, performed considering the parameters of high density LHCD experiments in EAST, has shown that these mechanisms could be responsible for the low current drive (CD) efficiency at high density. Radiofrequency probe spectra, useful for documenting PI occurrence, show sidebands whose amplitude in the case of the lithiated vacuum chamber is smaller than in the case of poor lithiation, consistently with growth rates from PI modeling of the respective reference discharges. Since strong lithiation is also expected to diminish the parasitic effect on the LHCD of the remaining possible mechanisms, this appears to be a useful method for improving LHCD efficiency at a high density.

Effect of gas puffing from electron-side and ion-side on lower hybrid wave (LHW)-plasma is investigated in experimental advanced superconductive tokamak for the first time. Experimental results with different gas flow rates show that electron density at the grill is higher in the case of gas puffing from electron-side; consequently, a lower reflection coefficient is observed, suggesting better effect of puffing from electron-side on LHW-plasma. The difference in edge density between electron-and ion-side cases suggests that local ionization of puffed gas plays a dominant role in affecting the density at the grill due to different movement direction of ionized electrons and that part of gas has been locally ionized near the gas pipe before diffusing into the grill region. Such difference could be enlarged and important in ITER due to the improvement of plasma parameters and LHW power. V C 2013 AIP Publishing LLC. [http://dx.

Effective coupling for lower hybrid waves (LHWs) is achieved by adjusting the launcher position and optimizing the plasma configuration in L-mode in EAST. It is found that, compared with other divertor shapes, the plasma with double null shows better coupling performance at the same position of lower hybrid (LH) grill, especially in the case of a large safety factor near the separatrix (q 95 ) and a large edge recycling (D α ) intensity. The ion cyclotron range of frequency (ICRF) power has a significant impact on LH wave coupling when the ICRF antenna is magnetically connected to the LH grill. The asymmetry effects in the poloidal direction on reflection coefficients are obtained with a low edge density during ICRF power application. The origin of such a relevant asymmetry with ICRF is different from LHWs. Results not only suggest that ICRF power could modify the density in the local scrape-off layer (SOL), but also indicate that density convection in the SOL could be easily obtained with a low edge density. One promising alternative for eliminating the negative impact on LHW coupling induced by ICRF is gas (D2) injection both from the electronic side and ionic side in EAST.

Recent ion cyclotron range of frequency (ICRF) experiments combined with lower hybrid wave (LHW) on EAST show that LHW coupling can be strongly modified when the LHW launcher is connected magnetically to a powered ICRF antenna. Using Langmuir probes, investigation of radio frequency (RF)-enhanced potential and local plasma parameters under an applied ICRF pulse is carried out on EAST. When the ICRF antenna connected to the probe is powered, localized high positive peaks appear on the floating potential. The dependence of floating potential modifications on various parameters is investigated. These initial results are shown to be consistent with RF sheath physics. A new four-strap antenna is modelled with the high frequency structural simulator (HFSS) code for evaluation of the parallel electrical fields (E ) and sheath potentials. It is found that the E fields with the (0, 0, π , π ) phase are much higher than those with other phases. Only the (0, π, π , 0) phasing has a significant reduction potential compared with the original two-strap antenna at the dipole phase.

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.

韦 维) a)b) , Ding Bo-Jiang(丁伯江) a) † , Zhang Xin-Jun(张新军) a) , Wang Xiao-Jie(王晓洁) a) , Li Miao-Hui(李妙辉) a) , Kong Er-Hua(孔二华) a) , and Zhang Lei(张 磊) a) a)