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 D α 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 D 2 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.
Experiments on lower hybrid wave(LHW)coupling were investigated in the HT-7 tokamak.Good coupling of LHW plasma has been demonstrated at different conditions in the HT-7 tokamak.Relevant results have proved that LHW-plasma coupling is affected by the phase difference between adjacent waveguides.Furthermore,the edge density around the grill and relevant coupling can be adjusted by changing the plasma line average density or the gap value between the LH grill and the last closed flux surfaces(LCFS).It is found that the coupling of LHWs becomes poor when the edge density around the LH grill is large enough in the HT-7tokamak,and that coupling remains good with a proper edge density.With increasing LHW power,it is also found that the reflection coefficients(RCs)increase due to non-linear effects under conditions of low edge recycling,but can decrease under high edge recycling.The edge density depends mainly on the competition between the ponderomotive force(PMF)and the edge recycling intensity in the HT-7 tokamak.
An additional lower hybrid wave (LHW) with a higher refractive index (N_∥) wasinvestigated in the HT-7 tokamak to bridge the spectral gap. It was found that the spectralgap between the wave and the electrons in the outer region was bridged by the additional wavewith a higher N_∥ spectrum. The results showed that the sawteeth oscillation was suppressed bylaunching the additional wave, and that the power deposition profile was moved outwards and thecurrent profile was broadened due to the application of the additional wave. Our study indicatesthat the spectral gap may be bridged by an additional wave with a higher N_∥ spectrum in theouter region.
Lower hybrid wave (LHW)-plasma coupling and lower hybrid current drive (LHCD)experiments in divertor, including single-null and double-null, and limiter configurations wereconducted systematically in EAST. A maximum power for launched LHW is 1.4 MW and theplasma current with LHCD is about 1 MA. It is indicated that the coupling is best in limiterconfiguration, then in single-null one, while worst in double-null one. Study in current driveefficiency by a least squares fit shows that there is no obvious difference in drive efficiency betweenthe double-null and the single-null cases, whereas the efficiency is a slightly lower in the limitercase. The effect of plasma density on the current drive efficiency is due to the influence of densityon impurity concentration.
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 n e av = 0.7 × 10 19 m 3.
