Yitao Yang

Experimental investigations of LHW?plasma coupling and current drive related to achieving H-mode plasmas in EAST

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.

Experimental observations of plasma edge magnetic field response to resonant magnetic perturbation on the TEXTOR Tokamak

The plasma response to a resonant magnetic perturbation (RMP) is experimentally measured on the TEXTOR tokamak using a fast movable magnetic probe. It is shown that, due to the plasma response, the magnetic field significantly differs from the vacuum field. Clear linear and non-linear responses to RMP are observed by varying the intensity of these perturbations or the plasma edge safety factor. Both the radial profile evolution and the dependence of plasma response on the edge safety factor show a stronger resonant effect when the RMP rotates at 5 kHz in the counter-current direction.

Investigations of LHW-plasma coupling and current drive at high density related to H-mode experiments in EAST

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 (Bt) show that the density near the antenna is affected by both the radial electric field induced by plasma without a LHW (Er_plasma) in the scrape off layer (SOL), and the radial electric field induced by LHW power (Er_LH) near the grill. Investigations indicate that Er × Bt in the SOL leads to a different effect of configuration on the LHW-plasma coupling and Er_LH × Bt accounts for the asymmetric density behaviour in the SOL observed in the experiments, where Er 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.

Investigation of lower hybrid wave coupling and the related effects of ion cyclotron range of frequencies in EAST

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 (q95) and a large edge recycling (Dα) 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.

Operational region and sawteeth oscillation in the EAST tokamak

The first plasma discharges were successfully achieved on the experimental advanced superconducting tokamak (EAST) in 2006. The sawteeth behaviours were observed by means of soft x-ray diagnostics and ECE signals in the EAST. The displacement and radius of the q = 1 surface was studied and compared with the result of equilibrium calculation. The density sawtooth oscillation was also observed by the HCN laser interferometer diagnostics. The structure of the EAST operational region was studied in detail. Plasma performance was obviously improved by the boronization and wall conditioning. It was observed that lower qa and a wider stable operating region is extended by the GDC boronization.

Study and optimization of lower hybrid wave coupling in the experimental advanced superconducting (EAST) tokamak

The results presented in this paper are an extension of our recent (Kong et al 2012 Plasma Phys. Control. Fusion 54 105003) studies on lower hybrid wave (LHW) coupling. By optimizing the shape of the LH grill, the misalignment between the poloidal limiter (PL) and the LH grill is nearly eliminated and the coupling of LHW is improved, especially on the top row, although some discrepancies are still present in the case with low edge density. Density modifications both by LHW and ion cyclotron range of frequency (ICRF) power are studied in EAST. Experimental results show that the edge density modification in front of the LH grill during LHW power depends mainly on the competition between ponderomotive force (PMF) and the ionization of neutral gas, provided by gas puffing and edge recycling. However, the local edge density during ICRF power can be reduced rapidly. Furthermore, such a modification is more obvious with higher ICRF power and the relevant mechanism of density modification by ICRF power can be related to RF sheaths. In addition, another analogous effect of RF sheaths on the coupling of LHW is also investigated, i.e. the density convection induced by Er???B drift. The changes in LHW coupling associated with different ICRF antennas are discussed and it is shown that in some cases the coupling on the lower rows of the LH grill is improved possibly due to magnetic connection between ICRF antennas and the LH grill. The local coupling of LHW can be improved by gas puffing from gas introduction modules (GIM) on both sides of the launcher, but it is difficult to judge which one is more beneficial due to errors in measurements. Experimental results with gas (D2) injection during ICRF power clearly show that the coupling of LHW on the upper rows will be first improved by gas injection on the electron side and the coupling on the lower rows will be effectively improved by gas injection on the ion side. The results are consistent with the mapping of field lines.

Neutron flux in lower hybrid current drive plasmas on EAST

Diagnostics application of fusion reaction measurements on the Experimental Advanced Superconducting Tokamak (EAST) has been proved by fusion neutrons during deuterium plasma discharges. In lower hybrid current drive plasmas, fusion neutrons are dominant mainly due to a sufficiently low loop voltage, which suppresses the photo-neutrons caused by runaway electrons. For fixed plasma current and lower hybrid wave (LHW) power, the fusion neutron flux depends on ion density as , stronger than a power of two, caused by an increased ion temperature, which is due to an increased electron and ion energy exchange by more frequent collisions at higher ion densities. The fusion neutron flux depends on , at fixed LHW power and main ion density, which is mainly due to ion temperature affected by ohmic power and energy confinement time. Finally, the unchanged fusion neutron flux with LHW power indicates little variation in the main ion temperature, which is consistent with direct ion temperature measurements. This is explained by the fact that an increased energy transfer to ions by collisions is almost compensated for by less frequent collisions and decreased energy confinement time at higher LHW powers.

Suppression of MHD instabilities by ion Bernstein wave (IBW) heating in the HT-7 tokamak

In the HT-7 tokamak, the m = 2/n = 1 tearing mode can be effectively suppressed by the ion Bernstein wave (IBW) when the location of power deposition is near the q = 2 rational surface. Off-axis electron heating and electron density broadening have been observed. In the meantime, the particle confinement is improved with an increase in electron density and a drop in Dα emission. The induced large electron density gradients and electron pressure gradients can be spatially correlated with the IBW deposition profile via theoretical calculation. It is suggested that off-axis IBW heating modified the electron pressure profile and the current density profile can be redistributed, thus resulting in the suppression of the magnetohydrodynamics (MHD) instability. It provides an integrated way for both the stabilization of tearing modes and controlling pressure profiles.

Transport modeling of L- and H-mode discharges with LHCD on EAST

High-confinement (H-mode) discharges with lower hybrid current drive (LHCD) as the only heating source are obtained on EAST. In this paper, an empirical transport model of mixed Bohm/gyro-Bohm for electron and ion heat transport was first calibrated against a database of 3 L-mode shots on EAST. The electron and ion temperature profiles are well reproduced in the predictive modeling with the calibrated model coupled to the suite of codes CRONOS. CRONOS calculations with experimental profiles are also performed for electron power balance analysis. In addition, the time evolutions of LHCD are calculated by the C3PO/LUKE code involving current diffusion, and the results are compared with experimental observations.

On the effects of magnetic perturbations on fast ion losses studied at TEXTOR

One criterion for the ignition of a fusion plasma is sufficient fast ion confinement. A key aspect in that context is the maintainability of good fast ion confinement in the presence of non-axisymmetric fields, such as those found in stellarators and during the application of resonant magnetic perturbations (RMPs) in tokamaks. This paper focuses on the influence of RMPs on the fast ion losses, studied at the Tokamak experiment for technology-oriented research (TEXTOR), a medium-sized device. TEXTOR is equipped with a flexible perturbation coil system, the dynamic ergodic divertor, allowing for the application of static or rotating perturbation fields. A rotating directional probe is used for the radially resolved fast ion loss measurements. The results achieved are presented and discussed for the poloidal/toroidal perturbation modes m/n = 3/1 and 6/2, with static and rotating fields.