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Comparative modeling of the coupling properties and the power spectra with different plasma densities and different phasing were performed for 2.45 GHz and 4.6 GHz waves respectively by using the ALOHA code. The modeling results show that the electron density for optimum coupling is ∼3 × 1017/m3 and ∼10 × 1017/m3 for 2.45 GHz and 4.6 GHz LH waves respectively. For both launchers the power directivity can be high as 76% and reflection coefficient less than 1% around the optimum density. When the density increases above the optimum value, the wave coupling becomes poor and simultaneously the power directivity decreases for both waves, hence reducing the current drive efficiency. However, the coupling property shows a lower sensitivity to plasma density for 4.6 GHz wave. By adjusting the phasing between main waveguides, the parallel refractive index (N//) can be varied from 1.7 to 2.5 for 4.6 GHz wave and from 1.9 to 2.9 for 2.45 GHz. It is found that the power directivity shows weak dependence on the phase difference between main waveguides for both of the waves. Comparison between ALOHA and experimental LH coupling results on EAST are presented. The power spectra with some modules powered off is also calculated. The results show that some lobes arise from left and right sides of the first main lobe for both of 2.45 GHz and 4.6 GHz waves.

Synergistic effects between two frequencies of lower hybrid (LH) waves—operating at 2.45 and 4.6 GHz—were observed in experiment on EAST for the first time. At low density ( n e , lin ≈ 2.0 × 1 0 19 m − 3), simultaneous injection of a 65/35 mix of 2.45/4.6 GHz power achieved an lower hybrid current drive efficiency that was 25% higher than what should be expected from the linear combination of the two sources. The experiment was interpreted with time-dependent simulations, using the equilibrium and transport solver TRANSP, coupled with the ray-tracing code GENRAY and the Fokker–Planck solver CQL3D. For each discharge, profiles of current and hard x-rays from simulation and measurement agree within uncertainties. An examination of the electron distribution function indicates that the LH synergy is supported by the increased width of the LH resonance plateau in the simultaneous injection case compared to independent injection.

An eight-channel magnetic probe diagnostic system has been designed and installed adjacent to the 4.6 GHz lower hybrid (LH) grill antenna in the low-field side of the Experimental Advanced Superconducting Tokamak (EAST) in order to study the n∥ evolution of LH waves in the first pass from the launcher to the core plasma. The magnetic probes are separated by 6.6 mm, which allows measurement of the dominant parallel refractive index n∥ up to n∥ = 5 for 4.6 GHz LH waves. The magnetic probes are designed to be sensitive to the magnetic field component perpendicular to the background magnetic field with a slit on the casing that encloses the probe. The intermediate frequency stage, which consists of two mixing stages, down-coverts the frequency of the measured wave signals at 4.6 GHz to 20 MHz. A bench test demonstrates the phase stability of the magnetic probe diagnostic system. By evaluating the phase variation of the measured signals along the background magnetic field, the dominant n∥ of the LH wave in the scrape-off layer has been deduced during the 2019 experimental campaign. In the low density plasma, the measured dominant n∥ of the LH waves is about 2.1, corresponding to the main peak 2.04 of the launched n∥ spectrum. n∥ deduced by the least-squares linear fit method remains near this value in the low density plasma with a high spatial correlation magnitude of 0.9. With an eight-channel probe system, a wave-number spectrum has also been deduced, which has a peak near to the measured dominant n∥.

In order to achieve steady state operation in experimental advanced superconducting tokamak (EAST), a 6 MW/4.6 GHz lower hybrid current drive system with twenty-four 250 kW/4.6 GHz high power klystron amplifiers (model VKC7849A) has been developed. A 4.6 GHz lower hybrid wave (LHW) power control system (4.6 GHz LPCS) with continuous wave mode has been set up, which was used to control the LHW power as needed. This paper introduces the architecture and software design of LPCS system, including microwave preamplifier subsystem, power measurement subsystem, power control computers, and so on. Proportional integral differential (PID) closed-loop control time is less than 100 μs due to the use of high-speed acquisition cards and user-programmable Field Programmable Gate Arrays. So far, high power CW operation and power control experiments with PID feedback mode by plasma parameters (beta, loop voltage, and so on) were performed in detail. Results show that 4.6 GHz LPCS is stable and reliable, suggesting that it meets the needs of EAST.In order to achieve steady state operation in experimental advanced superconducting tokamak (EAST), a 6 MW/4.6 GHz lower hybrid current drive system with twenty-four 250 kW/4.6 GHz high power klystron amplifiers (model VKC7849A) has been developed. A 4.6 GHz lower hybrid wave (LHW) power control system (4.6 GHz LPCS) with continuous wave mode has been set up, which was used to control the LHW power as needed. This paper introduces the architecture and software design of LPCS system, including microwave preamplifier subsystem, power measurement subsystem, power control computers, and so on. Proportional integral differential (PID) closed-loop control time is less than 100 μs due to the use of high-speed acquisition cards and user-programmable Field Programmable Gate Arrays. So far, high power CW operation and power control experiments with PID feedback mode by plasma parameters (beta, loop voltage, and so on) were performed in detail. Results show that 4.6 GHz LPCS is stable and reliable, suggesting that i...

The magnetic fluctuations of geodesic acoustic modes (GAMs) have been investigated using a Doppler backscattering system and Mirnov probes during ion cyclotron resonance heating and lower hybrid wave heating in the edge plasma in the experimental advanced superconducting tokamak, and the magnetic component of the GAM was observed. The frequency of the GAM has a clear relationship with the edge electron temperature, and the mode numbers of the poloidal component of the magnetic structure of the GAM are approximately m = 2 ( sin ( 2 θ ) ) and n = 0. A more detailed investigation shows that the maximum values of the amplitude of the poloidal magnetic field fluctuations and poloidal electric field fluctuations are approximately 10 − 6 T and 200 V/m, respectively.

The lower hybrid current drive (LHCD) plays an important role in the long pulse and high performance experiments on the EAST Tokamak, using LHW systems with frequencies of 2.45 GHz and 4.6 GHz. With the initial parallel refractive index mostly peaking at n∥∼1.8−2.1, it is difficult to have on-axis LHCD due to the accessibility conditions which prevents the LHW from propagating into the plasma core. However, according to the investigation of HXR diagnostics, LHW did provide an on-axis current drive in an EAST experiment. To interpret this on-axis LHCD phenomenon, various physics effects that give rise to variations in parallel wave numbers, thus influencing the propagation and the absorption of the LHW, have been taken into account, such as toroidal effects, full wave effects with focusing and diffraction, and scattering effects due to density fluctuations. Numerical tools including ray-tracing and full wave codes that contain one or two of the above physics effects have been used to model the heating and current drive of the LHW on EAST. Linear Landau damping and quasi-linear Landau damping have been compared by using the coupling between ray-tracing and the Fokker-Planck codes. Convergence studies have been performed for the full wave modeling, the maximum poloidal modes in the full wave have been scanned, and the results are compared with each other. The factors that characterize the amplitude, the radial thickness, and the location of the density fluctuations have been studied and scanned. The measurement of HXR diagnostics has been compared with synthetic diagnostics by reconstructing the HXR data from both the standard GENRAY/CQL3D LHCD modelling and the modeling taking the scattering effects into account.

An improved confinement through internal transport barrier formation with lower hybrid current drive (LHCD) is obtained on the Hefei Tokamak-7 [J. K. Xie et al., Proceedings of the 16th International Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 1, p. 685]. The experimental energy confinement time increases from 14.6 ms (Ohmic discharge phase) to 24.5 ms (LHCD phase), which is consistent with the value predicted by the high confinement mode scaling law of ITER93 (edge localized mode free). The confinement factor H (H=τE/τEITER89P) up to 1.42 is obtained during the improved confinement phase. The analysis shows that the modification of the plasma current profile and E×B are two possible reasons for the confinement improvement.

EAST has been equipped with two high power lower hybrid current drive (LHCD) systems with operating frequencies of 2.45 GHz and 4.6 GHz. Comparative LHCD experiments with the two different frequencies were performed in the same conditions of plasma for the first time. It was found that current drive (CD) efficiency and plasma heating effect are much better for 4.6 GHz LH waves than for the one with 2.45 GHz. High confinement mode (H-mode) discharges with 4.6 GHz LHCD as the sole auxiliary heating source have been obtained in EAST and the confinement is higher with respect to that produced previously by 2.45 GHz. A combination of ray-tracing and Fokker-Planck calculations by using the C3PO/LUKE codes was performed in order to explain the different experimental observations between the two waves. In addition, the frequency spectral broadening of the two LH wave operating frequencies was surveyed by using a radio frequency probe. Published by AIP Publishing.

More economical fusion reactors might be enabled through the cyclic operation of lower hybrid current drive. The first stage of cyclic operation would be to ramp up the plasma current with lower hybrid waves alone in low-density plasma. Such a current ramp-up was carried out successfully on the EAST tokamak. The plasma current was ramped up with a time-averaged rate of 18 kA/s with lower hybrid (LH) power. The average conversion efficiency Pel/PLH was about 3%. Over a transient phase, faster ramp-up was obtained. These experiments feature a separate measurement of the L/R time at the time of current ramp up.

Lower hybrid wave (LHW)-plasma coupling experiments in HT-7 [J. K. Xie and HT-7 Group, Proceedings of the 16th International Conference on Fusion Energy, Montreal, 1996 (IAEA, Trieste, 1997), Vol. 1, p. 685] were carried out by means of puffing gas (CD4 and D2) just around the antenna. Both experiments show that wave-plasma coupling is improved by the gas puffing. The maximum distance between the plasma and the antenna is limited to about 8 cm due to the plasma disruption. The variation in the lined averaged density in the different channels gives a possible evidence of the mechanism of the ionization of neutral gas. The effect of the gas flow rate on the wave-plasma coupling shows that an optimized gas flow rate is necessary for good coupling, being consistent with simulation through Brambilla theory qualitatively. Experiments with puffing D2 show that the improved coupling results from the global density increase and the local gas puffing. Langmuir probe measurements indicate that the gas puffing effect...