
 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

Abstract Experimental Advanced Superconducting Tokamak (EAST) was built to demonstrate high-power, long-pulse operations under fusion-relevant conditions, with major radius R = 1.9 m, minor radius a = 0.5 m, and design pulse length up to 1000s. It has an ITER-like D-shaped cross-section with two symmetric divertors at the top and bottom, accommodating both single null and double null divertor configurations. EAST construction was started in 2000, and its first plasma was successfully obtained in 2006. In the past 15 years, plasma-facing components, plasma heating, diagnostics, and other systems have been upgraded step by step to meet its mission on exploring of the scientific and technological bases for fusion reactors and studying the physics and engineering technology issues with long pulse steady-state operation. An advanced steady-state plasma operation scenario has been developed, and plasma parameters were greatly improved. Meanwhile, front physics on the magnetic confinement plasmas have been systemically investigated and lots of fruitful results were realized, covering transport and confinement, MHD stabilities, pedestal physics, divertor and scrap-off layer (SOL) physics, and energetic particle physics. This brief review of EAST on engineering upgrading, stand-steady operation scenario development, and plasma physics investigation would be useful for the reference on construction and operation of a superconducting tokamak, such as ITER and future fusion reactor. 

/pdf/all-superconducting-tokamak-east-20be57rx.pdf

All superconducting tokamak: EAST

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

We study the microwave absorption during electron cyclotron resonance heating (ECRH) by the extraordinary wave at second harmonic (X2 mode) in the T-10 tokamak and TJ-II stellarator in a wide range of plasma densities, and compare experiments with the classical formulas for the absorption of the injected ECR power. Empirical relations for the absorption efficiency and for the critical plasma density $${{n}_{{{\text{cr}}}}}$$ , which separates the regions with full and partial absorption of the injected ECR power, are obtained using the numerical simulation of the heat transport with the transport model of canonical profiles. It is shown that in both devices, the range of densities exists, where the absorption predicted by the classical formulas is almost full, while according with the empirical formula, only a small fraction of the power is absorbed. The obtained relations allow ones to optimize the conditions of ECRH in toroidal systems for magnetic plasma confinement. 

Absorption of Microwaves During Plasma Heating at the Second Harmonic of Electron Cyclotron Resonance in Tokamaks and Stellarators: Linear Theory and Experiment

A very high core electron temperature (T e0 ∼ 10 keV) plasma has been established and stably sustained by applying both lower hybrid wave (LHW) and on-axis electron cyclotron resonance heating (ECRH) in the Experimental Advanced Superconducting Tokamak (EAST). In this work, power balance analysis shows that the increase of ECRH power can increase the normalized T e gradient significantly at the plasma core region (ρ < 0.6), but does not change the T e profile stiffness in the low-density L-mode plasmas. This has been considered to be due to a strong synergistic effect between ECRH and LHW. Furthermore, three distinguishable stages characterized by different T e profile stiffnesses can be identified from the density ramp-up in the electron-heated plasma on EAST. A stronger T e profile stiffness at ρ = 0.3 has been observed in the Stage-II, where the LHW power deposition gradually moves away from the plasma core region, following the electron density increases. Furthermore, the formation of an internal plasma density transport barrier inside ρ ∼ 0.6, accompanied by a sudden drop in core T e and a rise in both core plasma density and ion temperature, has been observed for the first time during the transition from the Stage-II to the Stage-III when the central line-averaged plasma density reaches a threshold of 2.2 × 1019 m−3. This finding strongly affects further development of high-performance gas-fueled electron-heated plasma scenarios in EAST and suggests an advanced operational regime with a wide internal plasma density transport barrier.

https://iopscience.iop.org/article/10.1088/1741-4326/aca168/pdf

Characteristics of electron temperature profile stiffness in electron-heated plasmas on EAST

This study evaluates the functional requirements of the Super Proton-proton Collider (SPPC) beam screen in view of the CEPC-SPPC project about to be built in China and aims to prove the feasibility of using the scaled FCC-type beam screen. Moreover, the study puts forward several feasible updated schemes with extra-cooling to optimize the cryogenic efficiency under the 12.8 W/m synchrotron radiation (SR) heat load and decrease the leaked SR power to the cold bore. The power distribution calculated by Synrad+ is imported into the Ansys workbench to get the exact thermal profile. The results show that the new scheme can restrict the temperature rise within 1 K, with a 30 g/s mass flow rate of the coolants. Such a profile can save over 1.3 MW electric power per turn.

Preliminary consideration of the beam screen for the super proton-proton collider

This paper describes scalings of the H-mode confinement database of neutral beam injection (NBI)-lower hybrid wave (LHW) and electron cyclotron resonant heating-LHW plasmas in the EAST tokamak. Fundamental information of the EAST H-mode database and the details of calculation of the thermal energy confinement time in EAST are presented. The result of the scaling model τE,EASTS,H,NBI - LHW of the thermal energy confinement time in NBI-LHW plasmas with root-mean-square-error RMSE=5.5% , compared with the multi-machine scaling model τth,98y2 derived from the multi-machine database (mainly NBI heating) with RMSE=12.9% , is obtained. The scaling τE,EAST,H,NBI - LHW presents a similar dependency on plasma current and power loss and a lower dependency on plasma density and elongation, but stronger dependency on toroidal field than in τth,98y2 . The scaling τE,EAST,H,ECRH - LHW demonstrates a similar dependency on plasma current and power loss and lower dependency on plasma density, but a negative dependency with large error bars on elongation compared with τE,EAST,H,NBI - LHW in NBI-LHW plasmas. The comparison of EAST models with the more recent multi-tokamak scaling ITPA20-IL shows a similar engineering dependency on plasma current, power loss and plasma density, but opposite dependency on toroidal field.

https://iopscience.iop.org/article/10.1088/1741-4326/acb15f/pdf

The scalings of the thermal energy confinement time in EAST H-mode plasmas

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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Papers

Characteristics of electron temperature profile stiffness in electron-heated plasmas on EAST

Preliminary consideration of the beam screen for the super proton-proton collider

The scalings of the thermal energy confinement time in EAST H-mode plasmas

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