In the last few years, long-pulse H-mode plasma discharges (with small edge-localized modes and normalized beta, β N ~ 1) have been realized at the Experimental Advanced Superconducting Tokamak (EAST). This paper reports on high-β N (>1.5) discharges in the 2015 EAST campaign. The characteristics of these H-mode plasmas have been presented in a database. Analysis of the experimental limit of β N has revealed several main features of typical discharges. Firstly, efficient, stable high heating power is required. Secondly, control of impurity radiation (partly due to interaction between the plasma and the in-vessel components) is also a critical issue for the maintenance of high-β N discharges. In addition an internal transport barrier (ITB) has recently been observed in EAST, introducing further improvement in confinement surpassing H-mode plasmas. ITB dynamics is another key issue for high-β N plasmas in EAST. Each of these features is discussed in this paper. Study and improvement of these issues could be considered as the key to achieving long-pulse high-β N operation with EAST.

The internal transport barrier (ITB) has been obtained in ELMy H-mode plasmas by neutron beam injection and lower hybrid wave heating on the Experimental Advanced Superconducting Tokamak (EAST). The ITB structure has been observed in profiles of ion temperature, electron temperature, and electron density within ρ<0.5. It was also observed that the ITB formation is stepwise. Due to the ITB formation, the confinement quality H 98y2 increases from 1 to 1.1 and the normalized beta, β N , increases from 1.5 to near 2. The fishbone activity observed during the ITB phase suggests the central safety factor q(0)∼1. Transport coefficients are calculated by particle balance and power balance analysis, showing an obvious reduction after the ITB formation.

In the 2017 EAST experimental campaign, a steady-state long-pulse H-mode discharge lasting longer than 100 s has been obtained using only radio frequency heating and current drive, and the confinement quality is slightly better than standard H-mode, H98y2 ~ 1.1, with stationary peaked electron temperature profiles. Integrated modeling of one long-pulse H-mode discharge in the 2016 EAST experimental campaign has been performed with equilibrium code EFIT, and transport codes TGYRO and ONETWO under integrated modeling framework OMFIT. The plasma current is fully-noninductively driven with a combination of ~2.2 MW LHW, ~0.3 MW ECH and ~1.1 MW ICRF. Time evolution of the predicted electron and ion temperature profiles through integrated modeling agree closely with that from measurements. The plasma current (Ip ~ 0.45 MA) and electron density are kept constantly. A steady-state is achieved using integrated modeling, and the bootstrap current fraction is ~28%, the RF drive current fraction is ~72%. The predicted current density profile matches the experimental one well. Analysis shows that electron cyclotron heating (ECH) makes large contribution to the plasma confinement when heating in the core region while heating in large radius does smaller improvement, also a more peaked LHW driven current profile is got when heating in the core. Linear analysis shows that the high-k modes instability (electron temperature gradient driven modes) is suppressed in the core region where exists weak electron internal transport barriers. The trapped electron modes dominates in the low-k region, which is mainly responsible for driving the electron energy flux. It is found that the ECH heating effect is very local and not the main cause to sustained the good confinement, the peaked current density profile has the most important effect on plasma confinement improvement. Transport analysis of the long-pulse H-mode experiments on EAST will be helpful to build future experiments.

Experimental and modeling investigations on the Experimental Advanced Superconducting Tokamak (EAST) show attractive confinement and stability properties in fully non-inductive, high poloidal beta plasmas. In the 2018 EAST experimental campaign, extended operation regimes of steady-state scenario were obtained (β P ~ 1.9 & β N ~ 1.5 & H 98y 2 ~ 1.3 of using only RF heating) with a high bootstrap current fraction (f BS ~ 47%) and n e /n GW ~ 70%. The confinement quality, H 98y 2 ~ 1.3, is much better than standard H-mode, and stationary peaked electron temperature profiles and peaked current density profile when ~1 MW of ECH and ~2.6 MW of LHW are both deposited in the core region. The observed improvement in plasma confinement is much better (H 98y 2 ~ 1.3) when compared with the RF-dominant heating experiments in the EAST 2016-2017 experimental campaign (H 98y 2 ~ 1.1). Integrated modeling prediction suggests that high electron density would increase the plasma performance and bootstrap current fraction, which is consistent with the general experimental trend. Linear analysis shows that the high-k (k y > 1) modes instability (ETG) is suppressed in the core region. Also, the Shafranov shift is shown to play a role in the suppression of the electron turbulent energy transport. Besides the modeling predictions, the validation of the predicted of the effect of ECH on the plasma confinement in recent experiments was done and the experimental results were consistent with the modeling results. The validation results also suggest that when ECH is deposited in the core region in the RF heating experiments, increasing the ECH heating power from 0.5 MW to 1.0 MW does make a small improvement in the bootstrap current fraction. The high bootstrap fraction scenario realized on EAST and the investigation to achieve higher-performance plasma would help expanding the operation regime on EAST.

A strong relationship between the fishbone instability and internal transport barrier (ITB) formation has been found on the Experimental Advanced Superconducting Tokamak (EAST) in high β N ELMy H-mode discharges. ITB formation always appears after the fishbone instability, and the fishbone disappears when the ITB grows to a certain extent. Hybrid simulations with the global kinetic-magnetohydrodynamic (MHD) code M3D-K have been carried out to investigate the linear stability and non-linear dynamics of beam-driven fishbone instabilities in these shots. The simulation results show that the fishbone instability absorbs the energy of the fast ions and changes the distribution function of the fast ions, leading to the accumulation of fast ions near the ITB, which might eventually assist in the formation of the ITB. The q = 1 surface disappearance caused by the bootstrap current generated by the steep pressure gradient in the ITB region has been considered as the reason for the fishbone instability vanishing. This process has also been reproduced in simulation. However, the timescale of this change in the q profile is not sufficient under classical current diffusion times. The simulation utilizes another assumption explaining the disappearance of the fishbone instability. The density will form a barrier in the ITB region, which should broaden the distribution of the fast ions, and the broadening profile of the distribution of the fast ion mitigates the growth of the fishbone instability.

To prepare for steady-state operation of future fusion reactors (e.g. the International Thermonuclear Experimental Reactor and China Fusion Engineering Test Reactor (CFETR)), experiments on DIII-D have extended the high poloidal beta (βP) scenario to reactor-relevant edge safety factor q95 ∼ 6.0, while maintaining a large-radius internal transport barrier (ITB) using negative magnetic shear. Excellent energy confinement quality (H98y2 > 1.5) is sustained at high normalized beta (βN ∼ 3.5). This high-performance ITB state with Greenwald density fraction near 100% and qmin ≥ 3 is achieved with toroidal plasma rotation Vtor ∼ 0 at ρ ≥ 0.6. This is a key result for reactors expected to have low Vtor. At high βP (≥1.9), large Shafranov shift can stabilize turbulence leading to a high confinement state with a low pedestal and an ITB. At lower βP (<1.9), negative magnetic shear in the plasma core contributes to turbulence suppression and can compensate for reduced Shafranov shift to continue to access a large-radius ITB and excellent confinement with low Vtor, consistent with the results of gyrofluid transport simulations. These high-βP cases are characterized by weak/no Alfvén eigenmodes (a.e.) and classical fast-ion transport. At high density, the fast-ion deceleration time decreases and Δβfast is lower; these reduce a.e. drive. The reverse-shear Alfvén eigenmodes are weaker or stable because the negative magnetic shear region is located at higher radius, away from the peaked fast-ion profile. Resistive wall modes can be a limitation at simultaneous high βN, low internal inductance, and low rotation. Analysis suggests that additional off-axis external current drive could provide a more stable path at reduced q95. Based on a DIII-D high-βP plasma with large-radius ITB, two scenarios are proposed for CFETR Q = 5 steady-state operation with ∼1 GW fusion power: a lower- ( ∼ 0.66) and a higher- ( ∼ 0.75) case. Using a Landau closure model, multiple energetic particle (EP) effects on the a.e. stability are analyzed modifying the growth rate of the a.e.s triggered by the neutral-beam-injection EPs and alpha particles, although the stabilizing/destabilizing effect is weak for the cases analyzed. The stabilizing effects of the combined EP species β, energy, and density profile in CFETR need further investigation.

Extensive experiments of advanced scenario development, which contribute to the ITER hybrid operational scenario have been carried out on experimental advanced superconducting tokamak (EAST) tokamak recently with the ITER-like tungsten divertor. The β N in this operational scenario is intermediate up to 2.1 (EAST#78987, β N ∼ 2.1, I p ∼ 0.45 MA, q 95 = 3.7, B T ∼ 1.5 T, 3 MW neutron beam injection and 1 MW 4.6 GHz lower hybrid wave). In these hybrid H-mode plasmas, the internal transport barrier (ITB) has been frequently observed with central flat q profile and it is found that the fishbone mode (m/n = 1/1) can be beneficial to sustain the central flat (q(0) ∼ 1) q profile, thus a stable ITB can be obtained. In this case, better plasma performance is achieved. The formation of the ITB of the electron density is related to the fishbone activities. Energy transport analysis shows that the fishbone instabilities have a suppression on electron turbulent energy transport, while the ITB of ion temperature is due to the suppression of high-k modes (electron temperature gradient). The mechanism of turbulence suppression from fishbone instabilities in the EAST tokamak is not clear and needs more investigation. It is also observed that the power threshold for ITB formation is ≥ 3.5 MW, which is consistent with the scaling law for other tokamaks. The dimensionless parameter G   ( =   H 89 β N / q 95 2 ) obtained in the EAST reaches 0.3, but is still lower than the ITER hybrid scenario design (G ≥ 0.4) and needs more extension. Further investigation of extending the operational regimes, such as expanding the ITB foot outwards, would be important for the development of the hybrid and steady-state scenarios for next-step fusion devices like ITER and CFETR.

Sixty-five discharges have been confirmed as I-mode discharges after searching the EAST database from 2016 to 2018, and some additional I-mode discharges were obtained in 2019. The I-mode regime features no edge localized modes (ELMs), high energy confinement, and a steep temperature pedestal, while particle confinement remains at L-mode levels. The I-mode regime has been obtained over a small parameter space (B T = 2.4 -2.7 T, I p = 0.4 -0.6 MA, q 95 = 4 -6) using a configuration with B × ▽B drift away from the active X-point. Most of the discharges in EAST are dominated by lower hybrid wave (LHW) heating. In addition, some are heated with both LHW and neutral beam injection (NBI) heating, and a few discharges are heated by NBI and electron cyclotron resonance heating (ECRH). Turbulence suppression in density perturbation has been defined as signaling the onset of the L-I transition. The power threshold of the I-mode is slightly larger than that of the 2008 ITPA scaling law of H-mode. The minimum L-I power threshold varies only weakly with B T , and the power range for the I-mode increases with increasing B T . The I-mode is superior to the L-mode in terms of energy confinement but is still slightly inferior to the H-mode under similar conditions, although the energy confinement time decreases more slowly with increasing heating power than it does in the typical H-mode case. NBI has some benefit for achieving higher confinement. The I-mode is very sensitive to the density near the edge. Under a constant high heating power, a small enhancement in the density will terminate the I-mode, sometimes leading to the I-H transition.