
 Since the 2021 IAEA FEC, EAST experiments have been carried out in support of high-performance steady-state operation and physics understanding for ITER and CFETR. The first demonstration of reproducible 403 s long-pulse steady-state H-mode plasma with H98y2>1.3, βP~2.5, βN~1.6, ne/nGW~0.7 and a good control of impurity and heat exhaust have been achieved on EAST using the pure radio frequency (RF) power heating and current drive. In support of ITER and CFETR long pulse operation, a thousand-second time scale (~1056 s) fully non-inductive plasma has been achieved. The EAST operational regime of high βP has been significantly extended (H98y2>1.3, βP~4.0, βN~2.4 and ne/nGW~1.0) using RF and neutral beam (NB). The full edge localized mode suppression using the n = 4 resonant magnetic perturbations (RMP) has been achieved in ITER-like standard type-I ELMy H-mode plasmas with q95 ≈ 3.1 on EAST, extrapolating favorably to the ITER baseline scenario. The sustained large ELM control and stable partial detachment have been achieved with Ne seeding. The plasma-beta effect for error field penetration is investigated with n = 1 RMP. Breakdown and plasma initiation at low toroidal electric fields (<0.3 V/m) with EC pre-ionization is developed. The transport and control of high-Z impurity of tungsten is investigated.

Overview of recent experimental results on the EAST tokamak


 Ion temperature (Ti) profiles are commonly observed to increase in peaking, leading to higher central Ti, after impurity seeding in EAST H-mode plasma. Argon can be more efficient to raise Ti than neon. Toroidal rotation can also be enhanced in scenarios with NBI heating. More significant increased toroidal rotation is brought by seeding argon than seeding neon. Turbulence is experimentally observed to be suppressed. Extensive modelling by quasilinear gyrokinetic code QuaLiKiz is performed to explain above observations. It is found that the enhanced Ti can be always explained by the turbulence stabilization. But the mechanism of turbulence stabilization is related to heating methods and the seeding impurity species. In the pure RF (ECRH+LHW) heating scenarios, where only TEM exists, argon can stabilize TEM more significantly than neon due to its higher charge and heavier mass. In the scenarios with increasing NBI power, ion heat flux can be dominated by ITG, thus the enhanced Ti is mainly attributed to ITG stabilization. In these cases, except argon’s ability to more efficiently stabilize TEM, more evident increased toroidal rotation brought by argon seeding can be also beneficial to stabilize turbulences.

Mechanism of enhanced ion temperature by impurity seeding in EAST H-mode plasma

The internal transport barrier (ITB) which is related to the fishbone instability has been observed on the Experimental Advanced Superconducting Tokamak (EAST) in ELMy H-mode discharges. An interpretation of the formation of the ITB on EAST tokamak is provided, based on both analytical and numerical calculations. The fishbone instability induces the redistribution of fast ions and leads to the accumulation of fast ions in a local region where the ITB is going to appear. Correspondingly, the gradients of fast ions are enhanced, where the ion temperature gradient (ITG) mode exists. Fast ions can interact with the ITG mode through the dilution, Shafranov shift and wave-particle resonance mechanisms. It is found that the ITG mode is stabilized by fast ions and the stabilizing effects are determined mainly by the density, temperature and their gradients of fast ions. The enhanced density and temperature gradients of fast ions lead to a stronger stabilizing effect on ITG mode. Compared with the stabilizing effect before the appearance of fishbone instability, the stabilization on ITG mode is enhanced after the fishbone instability, which is beneficial to the formation of the ITB.

The internal transport barrier formation on EAST tokamak during the fishbone instability

Abstract Following the reconstruction of the TEXT tokamak in China at Huazhong University of Science and Technology, renamed as J-TEXT, a plethora of experimental and theoretical investigations have been conducted to elucidate the intricacies of turbulent transport within the tokamak configuration. These endeavors encompass not only the J-TEXT device's experimental advancements but also delve into critical issues pertinent to the optimization of future fusion devices and reactors. The researches include the topics about the suppression of turbulence, flow drive and damping, density limit, nonlocal transport, intrinsic toroidal flow, turbulence and flow with magnetic island, turbulent transport in stochastic layer, turbulence and zonal flow with energetic particles or helium ash. Several important achievements have been got in the last few years, which will be further elaborated upon in this comprehensive review. 

J-TEXT achievements in turbulence and transport in support of future device/reactor

The linear instabilities and nonlinear transport driven by the ion temperature gradient (ITG) instability are numerically investigated in deuterium–tritium plasma in the CFETR hybrid scenario by using the NLT code. In both linear and nonlinear simulations, effects of the tritium fraction ɛT and the temperature ratio of deuterium and tritium τDT = TD/TT are studied, with TD and TT being the temperature of deuterium and tritium, respectively. Results from linear simulations illustrate that the ITG instability can be well stabilized as ɛT increases. When ɛT = 0.5, the maximum growth rate occurs at around τDT = 1.5. During the nonlinear simulations, the anomalous particle and energy flux in deuterium–tritium plasma are analyzed. For τDT = 1.0, it is found that the tritium (deuterium) particle flux is inward (outward) and the largest inward tritium particle flux appears at ɛT = 0.5. The total ion energy flux is found to be insensitive to ɛT. In the case with ɛT = 0.5, as τDT decreases from 3.0 to 0.5, the particle flux for tritium (deuterium) changes from the outward (inward) direction to the inward (outward) direction. The quasilinear analysis clarifies that the particle flux driven by the temperature gradient is the key part in determining the direction of the particle flux. Besides, the largest and the smallest energy flux appear at around τDT = 1.5 and 0.5, respectively. It is indicated that better energy confinement and better particle confinement for tritium could be realized by choosing smaller τDT (or higher TT).

Gyrokinetic simulations of ion temperature gradient instability in deuterium–tritium plasma in the CFETR hybrid scenario

Improved confinement caused by Ne injection is investigated in EAST. The safety factor q profile and electron-scale turbulence are studied with a polarimeter-interferometer system and CO2 laser collective scattering system in a series of Ne injection experiments in EAST. Ne with a suitable injection rate can lead to negative magnetic shear in the plasma core, which promotes electron-scale turbulence suppression. The continuous injection of Ne under the reversed shear condition further reduces the electron-scale turbulence intensity. Core electron temperature increase and confinement improvement are observed. QuaLiKiz is applied to model the influence of an impurity and reversed q on turbulence. The combination of stabilization effects of negative magnetic shear and Ne on turbulence is much stronger than the impurity’s own effect, which could explain the experimentally observed electron temperature (T e) increase and turbulence suppression. Besides, the relationship between pedestal structure induced by Ne injection and lower hybrid wave (LHW)-driven current deposition is discussed. These results provide a new perspective to improve the confinement in the case of Ne injection, i.e. to adjust the Ne injection to modify the ideal plasma equilibrium.

Observation of electron-scale turbulence suppression under weak magnetic shear with neon seeding in EAST plasma

Avoiding the plasma instability, especially the disruption instability, is an important problem for the stable operation of tokamak. Large-scale instabilities driven by free energy evolve nonlinearly and lead to the disruption. The microscale turbulence is highly sensitive to the change of free energy. The paper show the electron-scale turbulence evolution in the pre-precursor phase of TMs included disruption with the CO2 laser coherent scattering system in EAST. In the pre-precursor phase of disruption, it is observed that the characteristics of turbulence (e.g. intensity, spatial correlation) have obviously changed for more than 30 ms. In addition, before TM ( n=1 ) included major disruption, the spatial-correlation of turbulence in two regions ( ρ = 0–0.4 and ρ = 0.4–0.8) increase obviously, while opposite turbulence spatial-correlation evolution was observed before TM ( n=1 ) included minor disruption. The warning time for disruption with microscale turbulence is competitive while 30 ms for ITER. According to the experimental results in EAST, it may provide a new experimental evidence for the method improvement of predicting disruption.

Evolution of turbulence characteristics in the pre-precursor phase of tearing mode included disruption in the core of EAST

Spontaneous evolution of density peaking factor in TEM turbulence-dominated H-mode plasma on the EAST Tokamak

Improved confinement at the low q 95 < 3.5 operation regime with fishbone instability compared to sawtooth oscillation has been observed and investigated on the EAST under the dominant electron heating condition with a tungsten divertor. The formation of an internal transport barrier in the ion thermal channel strongly correlates to the excitation of the fishbone, accompanied by reduced particle outward transport in the core region identified by a central peaked density profile. Current density distribution is found to change from a monotonic shape with q 0 < 1 during sawtooth oscillation to a central flat structure, magnetic shear s ∼ 0 at ρ < 0.4, with the fishbone instability at where the higher off-axis bootstrap current fraction might play a critical role. Linear gyrokinetic simulation by NLT code was carried out to investigate the turbulent transport characteristics, which is qualitatively in good agreement with experimental measurements from CO2 laser collective scattering diagnostics. The electron-scale trapped electron mode that dominates the turbulent transport during sawtooth is found to be stabilized with the fishbone at identical heating power and plasma configurations.

Progress on physics understanding of improved confinement with fishbone instability at low q 95 < 3.5 operation regime in EAST

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Observation of electron-scale turbulence suppression under weak magnetic shear with neon seeding in EAST plasma

Evolution of turbulence characteristics in the pre-precursor phase of tearing mode included disruption in the core of EAST

Spontaneous evolution of density peaking factor in TEM turbulence-dominated H-mode plasma on the EAST Tokamak

Progress on physics understanding of improved confinement with fishbone instability at low q 95 < 3.5 operation regime in EAST