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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.

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.

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.

Density modification in the scrape-off-layer during LHW coupling at EAST

The results presented in this paper are an extension of our recent studies (Kong et al 2013 Plasma Phys. Control. Fusion 55 065007, 065008) on lower hybrid wave (LHW) coupling. The effects of LHW power, edge recycling, edge-localized-mode (ELM) activity and gas puffing on the scrape-off-layer (SOL) density modifications will be further studied in this paper. The SOL density profile usually depends on the competition between an increment by ionization of neutrals and a depletion by the ponderomotive force (PMF) in L-mode plasma. Experimental results indicate that the SOL density with a lower edge recycling intensity could be reduced by a higher LHW power because ionization of a few neutrals cannot provide sufficient electrons to increase the SOL density and thus degrade the LHW coupling. In addition, in H-mode plasma, it is observed that the significant reduction of RC is associated with higher LHW power compared with the oscillation of RC during ELM activity, implying the priority of neutral gas ionization by LHW power. Studies also indicate that gas puffing in the GIM_e side could be more efficient in improving LHW coupling due to local ionization, inferred by the luminosities on the surface of the poloidal limiter (PL) and confirmed by SOL density measurement. The evolution of edge density with various puffing rates exhibits increments of far SOL density focused on the narrower layer several centimeters in front of the PL, possibly suggesting that the ionization of neutral gas mainly occurs in this region.

Simulated and experimental investigation on synergy of LHW and IBW in HT-7

By combining the lower hybrid current drive (LHCD) and ion Bernstein wave (IBW) heating codes, synergistic simulation of lower hybrid wave (LHW) and IBW is preliminarily done and used to investigate synergistic experiments of LHW and IBW in HT-7, offering an effective tool for analyzing LHW and IBW experiments, particularly separating the contribution of temperature and synergistic effect to driven current due to IBW heating. Results show that IBW can effectively heat plasma and play with LHW, hence modifying the LHW power deposition and plasma current profile. Compared with the LHW plasma, there is an increment in driven current in the case of the LHW + IBW plasma, due to a temperature increase and synergistic effect of LHW and IBW. Studies show that the driven current fraction of temperature contribution and synergistic effect depends significantly on the plasma parameters and the resonant layer. Compared with global IBW heating, localized IBW heating makes localized electron temperature higher, which is helpful in driving a larger off-axis current and benefits the hollow current profile, suggesting that off-axis IBW heating is preferred for a weak or negative magnetic shear plasma and for improving plasma confinement. Although the simulated results are not completely consistent with experimental measurements, they are nearly in agreement qualitatively. More work will be done later for further investigation.

Theoretical analysis of the EAST 4-strap ion cyclotron range of frequency antenna with variational theory*

A variational principle code which can calculate self-consistently currents on the conductors is used to assess the coupling characteristic of the EAST 4-strap ion cyclotron range of frequency (ICRF) antenna. Taking into account two layers of antenna conductors without lateral frame but with slab geometry, the antenna impedances as a function of frequency and the structure of RF field excited inside the plasma in various phasing cases are discussed in this paper.