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The edge coherent mode (ECM) is considered a highly attractive pedestal mode because it extends the duration of edge-localized modes and increases particle and impurity transport without significantly affecting energy transport. Moreover, it operates compatibly with high-performance plasma discharges. The ECM can also be detected using Langmuir probes on the divertor target plate, indicating that it extends from the pedestal region into the SOL and resulting in a connection to the divertor target plate via magnetic field lines. In this work, the distribution of ECM on divertor target plates is investigated by analyzing 215 upper single null discharges on the EAST tokamak. The coherence analysis of plasma fluctuations between the electron cyclotron emission signal in the pedestal region of the outer midplane and the ion saturation current measured by Langmuir probes in the divertor region reveals that the ECM is hardly detected by the divertor probe close to the outer strike point but can be observed at far SOL. This finding indicates the presence of an ECM quiescent region near the SOL on the divertor plate, and the extent of the quiescent region in the poloidal flux coordinate (Δ) has been statistically analyzed. A pronounced relationship between Δ and triangularity ( δ ) has been observed, that is Δ increasing with δ . Further analysis reveals that this relationship can be attributed to the average magnetic shear in the SOL. This result is consistent with the physical picture, which states that strong magnetic shear close to X -point significantly squeezes the cross-section of flux tubes down to scales dominated by collisions.

In the pedestal region of the Experimental Advanced Superconducting Tokamak (EAST) during high confinement mode plasma operations with radio-frequency heating, two distinct fluctuations are observed: high-frequency fluctuations (HFFs) and edge harmonic oscillation-like (EHO-like) modes. The HFFs are characterized by intermittent fluctuations with a broadband frequency range of $1{-}3{\text{ MHz}}$ and a poloidal wave number ( ${k_\theta }$ ) greater than $0.9{\text{ c}}{{\text{m}}^{ - 1}}$ . On the other hand, the EHO-like mode exhibits characteristics similar to magnetohydrodynamics (MHD)-like modes with n = 1−5 and lower poloidal wave numbers ( ${{\text{k}}_{{\theta }}} \unicode{x2A7D} 0.12{\text{ c}}{{\text{m}}^{ - 1}}$ ). During the pedestal establishing phase following the L–H transition, a significant concurrent presence of HFF and EHO-like modes in high-density pedestal regions has been noted. In this phase, the EHO-like mode not only modulates the amplitude of the HFF but also engages in nonlinear interactions. The occurrence of EHO-like mode and HFF is associated with particle transport toward the divertor, though it is notably less than that caused by edge coherent modes. During the inter-edge localized mode (ELM) period, a significant decrease in the ${D_\alpha }$ baseline is observed whenever the low frequency fluctuation (LFF) weakens and the HFF grows, prior to each large ELM. One possible explanation is that the rapid increase of $E \times B$ shear stabilizes the LFF and destabilizes the HFF, which lowers the pedestal transport and enables the further growth of the pedestal until the onset of the ELM.

The attainment of a reliable equilibrium is a critical aspect of tokamak experiments and physics analysis. A common method for establishing a reliable equilibrium current involves reconstructing it from indirect measurements, such as those obtained from polarimeter-interferometers (POINT) and motional Stark effect (MSE) systems. However, uncertainties still exist in the reconstruction results. For the equilibrium reconstruction on the EAST tokamak, which is based on the POINT system, the primary sources of uncertainty are the limited scope of measurements and the sensitivity of the reconstruction process. This paper proposes an enhanced approach that utilizes current simulation as a constraint to maintain consistency between the initial equilibrium and the simulated results. The radio frequency waves driven current is identified as a particularly influential component due to its interaction with the q profiles of the equilibrium and the deposition region of the waves. Two specific discharges are presented to illustrate how a new equilibrium can be achieved, which enhances consistency between the equilibrium and the simulated current, taking into account the dependencies of various components.

This paper reports the recent observation of a weakly coherent mode (WCM) within a conventional reflectometer on EAST and successfully determines its poloidal wavenumber range. During the transition from the L-mode to I-mode, the line-averaged density remains nearly unchanged while a significant change is observed in the electron cyclotron emission (ECE) signals at the boundary. The difference between the signals for the two channels at the edge increased, coinciding with the appearance of the WCM and a simultaneous rise in the boundary electron temperature. Further investigation unveiled the modulating role of edge temperature ring oscillation (ETRO) (Liu et al 2020 Nucl. Fusion 60 126016) on high-frequency density fluctuations. Statistical results unveil an inverse relationship between the centeral frequency of the WCM and q _95 . Simulation results provide additional insights, demonstrating that the simulated ‘WCM’ in the density fluctuations aligns with experimental data in terms of center frequency. Additionally, the radial distribution of the simulated ‘WCM’ closely corresponds to regions with the strongest electron temperature gradients. Finally, through a cross-correlation analysis of the simulated fluctuations, the following phase relationship for the wavenumber range of ‘WCM’ was observed: $\alpha_{\widetilde{T}_e} \gt \alpha_{\widetilde{n}_i}\sim\alpha_{\widetilde{\phi}} \gt \alpha_{\widetilde{T}_i}$ .

The neutral energy spectrums with an energy range of 20–3000 eV was measured by the low energy neutral particle analyzer (LENPA) in the 2021 EAST summer campaign. The proportion of the neutral particles below 1000 eV measured by LENPA is more than 85 %. In the ohmic discharges, the integrated neutral flux increases with increasing the line-averaged density. However, the neural particles with higher energy (1000–3000 eV) are insensitive to the line-averaged density. In addition, the neutral flux in the energy range of 20–1000 eV of the discharges with supersonic molecular beam injection (SMBI) was lower than those without SMBI, which was different from neutrals in the range of 1000–3000 eV. The neutral flux increases with higher auxiliary heating power in all energy range. Compared to the ohmic discharges, the mean energy of neutral particles in auxiliary heated discharges is increased significantly.

By combining the X-mode polarized lower and upper cut-off reflections obtained from the density profile reflectometer, we have successfully attained a comprehensive density profile spanning from the edge to the core region in pellet injection (PI) experiments on EAST. During the homogenization process after PI on EAST, an innovative method was introduced to quantify the local density increase. This approach employed the distinctive ‘dual-reflection’ phenomenon observed in the EAST microwave reflectometer, encompassing measurements of both the X-mode lower and upper cut-off frequencies. Furthermore, experimental investigations were carried out on EAST to comprehensively explore the parallel and poloidal expansion of the high-density pellet cloud. Notably, this study marks the first instance of measuring expansion velocities of pellet materials in both parallel and poloidal directions on EAST. A comparative analysis was performed initially between these experimental measurements and simulation results obtained from the HPI2 code, marking a pivotal stride towards enhancing its applicability in EAST.

The edge turbulence characteristics and the induced radial transport have been investigated in edge localized mode (ELM) suppression by using the $n = $ 4 resonant magnetic perturbation coils on EAST, with ${q_{95}} = $ 3.6 and the electron collisionality $\nu _{\text{e}}^* \approx $ 0.5. During ELM suppression, the edge turbulence is enhanced dramatically, as measured by the reciprocating probe and the poloidal correlation reflectometry. In the near SOL, the low frequency turbulence (250 kHz), which could be beneficial to form the observed turbulent transport. The estimated upstream cross-field particle flux is consistent with the total particle flux deposited on divertor targets, demonstrating that the enhanced radial turbulent particle transport is an important mechanism for particle exhaust in ELM suppression.

Experiments in EAST have concentrated on studying the internal transport barrier (ITB) regime in high normalized beta () discharges, where the study of the compatibility between ITB dynamics and divertor plasmas is an important step for future steady-state and high-performance plasma operations. In this work, the characteristics of the divertor particle flux and their responses to ITB dynamics have been studied in high- discharges. In order to describe the characteristics, the ITB duration is divided into two phases: phase I (i.e. ITB formation) and phase II (i.e. ITB degradation), according to the variation of plasma stored energy. In phase I, the particle flux near the inner strike point (SP) is continuously enhanced during the inter-edge-localized mode (ELM) phase in both the lower single-null and upper single-null configurations. The total particle flux in the scrape-off layer (SOL) region reveals a similar trend with an increase of the flux near the SP. However, in the private flux region (PFR) the total particle flux shows a reduction. Meanwhile, a movement of the SP away from the divertor corner is also observed during the ITB formation. In phase II, the particle flux near the inner SP, the total particle flux in the inner SOL and PFR region recover to their initial level before ITB formation, respectively. Additionally, the particle decay length is obviously reduced in phase I and then gradually enhanced in phase II. The continuous variation of the particle flux at the inner divertor target is in accordance with a compression of the magnetic flux surfaces due to ITB formation, which increases the gradient of electron density in the edge region. It is indicative that the ITB has a feasible impact on the behavior of divertor plasmas by means of increasing the Shafranov shift during the inter-ELM phase. [ABSTRACT FROM AUTHOR]

A low-frequency (~1 kHz) oscillation (LFO) in the pedestal region emerged during the ELM-free phase after the L–H transition on the EAST tokamak. Its characteristics were studied. This oscillation has a magnetic component with a toroidal mode number (n) of 0, indicating an axisymmetric mode, while its poloidal magnetic structure demonstrates an m  =  1 standing wave with in–out asymmetry. The density fluctuation shows no phase difference radially, indicating that it is not a radial traveling pulse. Before this axisymmetric oscillation, pedestal density fluctuation was dominated by low-frequency (LF) fluctuation with a frequency of less than 400 kHz. During the phase of axisymmetric oscillation, this LF fluctuation was modulated by this axisymmetric oscillation and thus experienced ‘suppressed’ and ‘recovery’ phases alternately. As a result, both pedestal density and the particle flux onto the divertor were modulated. During the ‘suppressed’ phase of the LF fluctuation, a new high-frequency (HF) fluctuation appeared, with a frequency larger than 500 kHz. The result demonstrates complex turbulence behavior in pedestal region. [ABSTRACT FROM AUTHOR]

A high frequency (HF) density fluctuation, ranging from 100 kHz to 400 kHz, is locally destabilized with an m/n  =  1/1 internal kink mode before a sawtooth crash in the experimental advanced superconducting tokamak (EAST). The location is just around the region with a high electron pressure gradient due to the flux compression from the 1/1 mode. And this HF fluctuation can be triggered at both the low field side and high field side. This fluctuation has a poloidal wave number (kθ) from 3 to 5 cm−1, corresponding to a poloidal mode number (m) of 36–60. A threshold of the local electron pressure gradient (|∇p e|) exists for the stimulation of the HF fluctuation, and its amplitude increases with |∇p e|. The results could be beneficial for the understanding of a sawtooth crash, and also physics in 3D equilibrium. [ABSTRACT FROM AUTHOR]

A cross-scale interaction between a kink instability with toroidal mode number (n) of 1 and background microturbulence (mainly a coherent mode with n  =  12–17) has been observed in the H-mode edge pedestal region in the EAST tokamak. The n  =  1 kink mode with a frequency ~3.5 kHz is localized in the pedestal region. The amplitude of pedestal microturbulence is evidently modulated at the frequency of the n  =  1 mode. This amplitude modulation is mainly for the fluctuation component with frequency less than 100 kHz, while no modulation by the n  =  1 mode is observed for the component with frequency larger than 100 kHz. This implies that the observed amplitude modulation by the n  =  1 mode is not due to the Doppler shift effect. A further analysis shows that the amplitude modulation of microturbulence could be attributed to modulation of the local temperature gradient by the n  =  1 kink mode. In addition, bicoherence analysis indicates that there exists a possible direct nonlinear interaction between background microturbulence and the n  =  1 kink instability. A preliminary analysis shows that this cross-scale interaction has no evident effect on the pedestal structure. [ABSTRACT FROM AUTHOR]