During BOUT++ simulations for edge-localized modes (ELMs), the electric potential is generally calculated using the flute-ordering one-dimensional Laplace solver. However, it is not valid for the toroidal axisymmetric (i.e., toroidal mode number n = 0) component, leading to the limitation on evolving n = 0 electric field and parallel current. Recently, to evolve the n = 0 electric field and plasma current, Seto et al. [Phys. Plasmas 26, 052507 (2019)] have adopted the two-dimensional Laplace solver to calculate the n = 0 electric potential and indicate the ELM evolution can be significantly affected. In this work, based on the EAST upper single null equilibrium, ELM simulation using BOUT++ six-field two-fluid model with n = 0 electric field and parallel current evolution is performed. Compared to the case with fixed n = 0 parallel current (similar to 5%) and net-drift-flow-free n = 0 electric field (similar to 7%), the simulated ELM size is significantly reduced (similar to 2%) and more consistent with the small ELM observed in experiment. Further analysis indicates that the reduction of simulated ELM size is mainly because: (1) the decrease in the n = 0 parallel current density during the nonlinear phase leads to the reduction of instability drive, causing smaller initial crash; (2) the relatively strong radial electric field shear suppresses the turbulence transport.

We report the first application of a nonlinear correlation algorithm based on phase-space reconstruction to investigate the nonlinear dynamics of the edge coherent mode (ECM) on the EAST tokamak. By adopting a unified reconstructed phase-space scale for parameter selection, we perform quantitative comparison and analysis of ECM nonlinear dynamical characteristics across different physical regimes. Key findings include: (1) distinct evolution of nonlinear directionality during the ECM establishment following the L-H transition as compared to that during the inter-ELM recovery phase; (2) the nonlinear correlation coefficient decreases before the ECM amplitude does, acting as a “precursor” to the ECM decay, while the linear correlation coefficient remains nearly constant; (3) higher-frequency, narrower-band ECMs exhibit stronger nonlinear correlation and a more pronounced directionality bias, whereas lower-frequency, broader-band ECMs show weaker coupling and balanced directionality. These findings provide a new nonlinear physical perspective for understanding the saturation mechanism of the ECM.

The alkali beam emission spectroscopy diagnostic is an active spectroscopic diagnostic method that can measure edge electron density profile and density fluctuation. Due to the limitations of the previous lithium beam emission spectroscopy (Li-BES) (Zoletnik et al 2018 Rev. Sci. Instrum. 89 063503) installed on the Experimental Advanced Superconducting Tokamak (EAST), providing edge electron density profile and density fluctuation, its upgrade to a sodium beam emission spectroscopy (Na-BES) is now in process. Here, we report a performance prediction of upgrading the existing Li-BES diagnostic detecting the Li-I(2p-2s) line emission to the ongoing Na-BES diagnostic detecting the Na-I(3p-3s) line emission on EAST. This is done by calculating the occupation numbers of different atomic states of sodium and lithium resulting from collisions with the bulk plasma particles using a collisional-radiative model. During the calculations, the beam energy and the electron density profile (starting from an experimental measurement in an EAST H-mode discharge) are respectively varied. Calculation results demonstrate that, in contrast to Li(2p) state, the radial extension of Na(3p) occupation number distribution is shallower (still appropriate for pedestal measurements) and less influenced by the change in either beam energy or electron density profile. Moreover, although the occupation numbers of Na(3p) are approximately half those of Li(2p), under identical beam current conditions, the lower velocity of the sodium particles with a much higher density results in a higher total population of Na(3p) than that of Li(2p). Considering the significantly higher spontaneous transition coefficient of Na(3p) compared to Li(2p), along with the improved optical system, the signal intensity of Na-BES with a 50% pixel size is predicted to be 2-6 times higher than that of Li-BES in the edge region of EAST. When we further consider a much lower background impurity emission close to the Na-I line wavelength, our calculation suggests that a significantly improved system performance of such a Li-BES to Na-BES upgrade may be accomplished on EAST.

The compression effect of compact torus(CT) plasma in gradient magnetic field is investigated in this work. A magnetic compression platform is built which can generate a variable axial magnetic field(1.0, 0.8, 0.6, 0.4, 0.2, 0.1 T) with different coil currents(146.0, 116.8, 87.6, 58.4,29.2, 14.6 A). Five magnetic pickup coils are arranged which are used to display the time-spatial revolution of CT plasma during the penetration process, and they are located at the positions of 0, 200, 400, 600, and 800 mm in the axial direction, respectively. These experiments are carried out on the Experimental Advanced Superconducting Tokamak(EAST)-Compact Torus Injector(CTI) system, and the results show that the velocity of CT plasma was reduced significantly in places with large magnetic field gradients, as well as the full width at half maximum of the CT plasma. In addition, the magnetic field strength increases initially and then rapidly decreases during the compression process, or even reverses at the end, which may be related to the squeezing of the magnetic field lines and reconnection of the magnetic platform.

In the EAST tokamak, the divertor particle flux profiles in L- and H-mode plasmas with modulated lower hybrid current drive (LHCD) power in the upper single null configuration are analyzed using a theoretical model profile by a nonlinear least square fitting method. The fitting analysis successfully derives time evolutions of the strike-line positions as well as the effective widths of the scrape-off-layer (SOL) and private flux region for the particle flux, distinguishing respective effects of the LHCD power and edge localized modes (ELMs). For the studies, power deposition area of the LHCD power is estimated by using extreme ultraviolet radiation signals from the SOL. When an appreciable amount of the LHCD power is deposited in the SOL region, a significant expansion of the divertor particle flux profiles on the upper-outboard side by the LHCD power input is observed in an ELMy H-mode plasma. The upstream electron density profile measured is also considerably expanded by the LHCD power. The observed divertor particle flux profile is qualitatively explained by the additional expansion of the upstream density profile due to the LHCD effects. However, the particle flux profiles on the upper-inboard side do not show noticeable changes. The divertor strike-line positions on both sides and the loop voltage do not show any responses to the LHCD power. On the other hand, when the LHCD power is dominantly deposited in the core region of the L-mode plasma, the SOL particle widths are not simply increased. The SOL widths and the divertor strike-line positions on both sides are clearly modulated by the LHCD power modulation. In the H-mode plasma, low frequency density fluctuations less than similar to 50 kHz in the SOL having low poloidal wavenumber less than 2cm-1 are noticeably suppressed in the SOL during the LHCD power-on phase. This fact suggests that the observed expansion of the divertor particle flux profiles is not caused by enhanced SOL turbulence. ExB drifts by electrical biasing of the SOL flux tube realized by appreciable LHCD power deposition in the SOL are proposed as a likely candidate mechanism for the interpretation of the experimental observations.

The transition from small edge-localized modes (ELMs) to large ELMs has been repetitively observed in minute-scale long-pulse high-confinement mode (H-mode) discharges during the 2017 EAST campaign. The appearance of large ELMs is found to be strongly correlated with the decrease in separatrix density due to the gradual decrease in fuel recycling during long-pulse H-mode operations (LPHOs). By the numerical scanning of separatrix density with a fixed temperature profile, it has been found that the dependence of ELM instability on separatrix density is related to the competition between the ion diamagnetic stabilizing effect and destabilizing effect of pressure gradient and current density in the pedestal region. This sheds light on a comprehensive understanding of the different roles of separatrix density in ELM instability observed during EAST experiments. With a high separatrix density, the ideal ballooning mode can be destabilized near the separatrix, which is thought to help achieve small ELMs in EAST LPHOs. During the 2021 EAST campaign, an experiment of large ELM control was performed through actively changing fuel recycling by moving the strike point location on the lower tungsten divertor target plate. It has been demonstrated that the mitigation of large ELMs is strongly correlated with the significant increase in separatrix density, which is thought to be attributed to a higher ionization source in the scrape-off layer (SOL) region by SOLPS-ITER simulation. The high ionization source in the SOL region is believed to provide a strong fueling effect near the separatrix and thus raise the local density, which is considered an important reason for triggering ballooning instabilities near the separatrix and achieving small ELMs.

Since the last IAEA-FEC in 2021, significant progress on the development of long pulse steady state scenario and its related key physics and technologies have been achieved, including the reproducible 403 s long-pulse steady-state H-mode plasma with pure radio frequency (RF) power heating. A thousand-second time scale (similar to 1056 s) fully non-inductive plasma with high injected energy up to 1.73 GJ has also been achieved. The EAST operational regime of high beta(P) has been significantly extended (H-98y2 > 1.3, beta(P) similar to 4.0, beta(N) similar to 2.4 and n(e)/n(GW) similar to 1.0) using RF and neutral beam injection (NBI). The full edge localized mode suppression using the n = 4 resonant magnetic perturbations has been achieved in ITER-like standard type-I ELMy H-mode plasmas with q(95) approximate to 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 underlying physics of plasma-beta effect for error field penetration, where toroidal effect dominates, is disclosed by comparing the results in cylindrical theory and MARS-Q simulation in EAST. Breakdown and plasma initiation at low toroidal electric fields (<0.3 V m(-1)) with EC pre-ionization is developed. A beneficial role on the lower hybrid wave injection to control the tungsten concentration in the NBI discharge is observed for the first time in EAST suggesting a potential way toward steady-state H-mode NBI operation.

To better study the magnetic fluctuations in the high-field side of EAST tokamak, a new high-field side magnetic probe array (HFS-MPA) has been developed on EAST recently. The HFS-MPA consists of 12 identical three-dimensional (3D) magnetic probes, which are mounted on the HFS wall with carefully designed arrangement. The HFS-MPA magnetic probes bring additional toroidal magnetic fluctuation measurements compared with the HFS regular magnetic probes which can only provide poloidal and radial magnetic fluctuation measurements. The upper limits of frequency and toroidal mode number (n) measurements of the magnetic fluctuations have been improved by comparing HFS-MPA with the HFS regular magnetic probes, i.e., 650 kHz vs 100 kHz and n = 23 vs n = 1. In developing the HFS-MPA diagnostic system, many practical challenges have been overcome and many special designs have been developed. These will be mentioned in the main subsystem description of the HFS-MPA diagnostic in this paper, which might be useful for developing new magnetic probe diagnostics in the future on EAST or other magnetically confined fusion devices. The calibration of the effective area and frequency response of the HFS-MPA is also described. The preliminary application in studying the frequency and propagation characteristics of the magnetic fluctuations with HFS-MPA compared with EAST regular magnetic probes shows that the HFS-MPA is well developed for plasma physics studies.

The blob properties in I-mode and ELM-free H-mode plasmas compared to L-mode have been investigated on the EAST tokamak, including the blob detection rate N b , size δ b , lifetime τ b and radial velocity v r , b . The blob properties in L-mode and I-mode are similar, and those in ELM-free H-mode are different to them. The blob N b is smaller while τ b is larger in H-mode. The experimental blob size δ b and velocity scalings v r , b − δ b show a good agreement with the theoretical models. The variation in blob properties during the L-I and H-L transitions, and their relations to the scrape-off layer (SOL) density, edge and SOL turbulence, and SOL collisionality are discussed. The suppression of the edge (inside the last closed flux surface) turbulence is not reflected in the blob behavior, while the blob detection rate shows a correlation with the SOL density and its low-frequency (3–50 kHz) fluctuations. In addition, the blob detection rate is found to increase with the divertor collisionality Λ div , indicating a dependence of blob behavior on Λ div . The differences in blob detection rates among the three operating regimes might be due to their different SOL densities and collisionalities. The investigation contributes to understanding the influences of edge and SOL plasma parameters on the blob behavior.

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 ν e ∗ ≈ 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 (<30 kHz) has a large fluctuation level and propagates along the ion diamagnetic drift direction with a speed of 0.35 km s −1 in the plasma frame; an n = 1 electromagnetic mode around 120 kHz with a small k θ (∼0.15 cm −1 ) appears when the ELM is suppressed; weak broadband turbulence between 40–120 kHz propagates in the electron diamagnetic drift direction with a velocity of 3.4 km s −1 in the plasma frame. During the ELM suppression, the radial turbulent particle flux, calculated in both the time and frequency domains, is much higher (can be up to five times) than that in the inter-ELM phase. Furthermore, the low frequency turbulence (<30 kHz) dominates the cross-field particle transport. The 120 kHz electromagnetic mode also contributes to outward particle flux, which is relatively small. A set of CGYRO simulations are performed to illustrate the nature of the 120 kHz electromagnetic mode and the low frequency turbulence, suggesting that the former is the micro-tearing mode and the latter is the ion temperature gradient mode. The bispectral analysis suggests a strong three-wave coupling between the low frequency and high 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.

The Experimental Advanced Superconducting Tokamak (EAST) reflectometry is a Frequency-Modulated Continuous-Wave monostatic system with the transmission lines similar to the ITER reflectometer design. One of the most significant and common problems for reflectometry to reconstruct the density profile is the determination of initialization, i.e. zero density position ( R start ), which could be determined by the extraordinary mode (X-mode) reflectometry. The main source of noise still comes from the wave scattering of plasmas despite the sweeping period of around 10 microseconds. It is found that to reduce the random noise of R start , averaging on 25 periods is the optimal solution. During the L-mode discharges with only lower hybrid wave (LHW) heating, R start would move outwards and its fluctuation and the high frequency components of turbulence around R start would be obviously increased when the 2.45 GHz system is switched on, while no obvious change is observed in the 4.6 GHz case. These phenomena are consistent with that 2.45 GHz LHW has less current drive ability than 4.6 GHz LHW on EAST. During ELMy H-mode, the peak of R start is consistent with Deuterium signal and the maximum displacement is about 3 cm. The comparison of density profiles from reflectometry and Lithium beam emission spectroscopy (Li-BES) suggests that setting the density at R start to (4 ± 2)× 10 17 m -3 is much better than zero, while this value is somewhat empirical and closely related to the amplitude threshold for determining the first probing frequency corresponding to R start .

Compact torus（CT） injection is a highly promising technique for the central fueling of future reactor-grade fusion devices since it features extremely high injection velocity and relatively high plasma mass. Recently, a CT injector for the EAST tokamak, EAST-CTI, was developed and platform-tested. In the first round of experiments conducted with low parameter settings, the maximum velocity and mass of the CT plasma were 150 km·s -1 and 90 μg, respectively.However, the parameters obtained by EAST-CTI were still very low and were far from the requirements of a device such as EAST that has a strong magnetic field. In future, we plan to solve the spark problem that EAST-CTI currently encounters（that mainly hinders the further development of experiments） through engineering methods, and use greater power to obtain a more stable and suitable CT plasma for EAST.

To upgrade the Li-BES to Na-BES on Experimental Advanced Superconducting Tokamak (EAST), a high-brightness and stable sodium (Na) ion source is developed, and the performance of Na ion beam optics is simulated by the COMSOL Multiphysics. The porous tungsten disk with 70% porosity and filled with albite Na 2 O·Al 2 O 3 ·6SiO 2 is successfully developed as the Na ion source. The extracted current density of the Na-beam increases with the extraction voltage, and its maximum current can reach 9 mA with the ion source diameter of 20 mm. The lifetime of the Na ion source is evaluated to be 63 mAh, which is much longer than that of the Li ion source. The simulation shows that the preferable distances among the ion source (or anode electrode), extractor and puller to get the well-focused beam are x puller - x extractor ∼ 0.06 m and x extractor - x ion source ∼ 0.02 m. Further simulation with these optimized distances determines the extraction voltage and ion source size for good beam performance. In addition, the simulation shows the beam divergence angle decreases by more than 40% by optimizing the Einzel voltage to 5 kV, suggesting an excellent focusing property of the Einzel lens. These works provide the basis and the design references for the Na-BES development on EAST.

A novel electromagnetic probe array（EMPA） diagnostic, which consists of a magnetic probe array and an electrostatic probe array, has recently been developed on EAST. The EMPA is fixed near the first wall at horizontal port P. The magnetic probe array of the EMPA consists of 24 identical magnetic probes, each of them capable of measuring toroidal, poloidal and radial magnetic fluctuations simultaneously, providing additional toroidal magnetic fluctuation measurements compared with the regular magnetic probes on EAST. With a higher sampling rate and self-resonant frequency, the EMPA magnetic probes can provide higher frequency magnetic fluctuation measurements. The magnetic probe array of the EMPA is composed of two parallel layers of magnetic probes with a radial distance of 63 mm, and each layer of magnetic probes is arranged in four poloidal rows and three toroidal columns. The compact arrangement of the EMPA magnetic probe array largely improves the toroidal mode number measurement ability from-8≤ n≤ 8 to-112≤ n≤ 112, and also improves the high poloidal wave number measurement ability of magnetic fluctuations compared with the regular high frequency magnetic probes on EAST. The electrostatic probe array of the EMPA consists of two sets of four-tip probes and a single-tip probe array with three poloidal rows and four toroidal columns. It complements the electrostatic parameter measurements behind the main limiter and near the first wall in EAST. The engineering details of the EMPA diagnostic, including the mechanical system, the electrical system, the acquisition and control system, and the effective area calibration, are presented. The preliminary applications of the EMPA in L-mode and H-mode discharges on EAST have demonstrated that the EMPA works well for providing information on the magnetic and electrostatic fluctuations and can contribute to deeper physical analysis in future EAST experiments.

A general phenomenon that the edge localized modes (ELMs) can be effectively mitigated with the enhanced coherent modes (CMs) has been observed on EAST. For this phenomenon, the experimental statistical analysis and electromagnetic (EM) simulations have been performed. There is a threshold value of the CM intensity in the experiments, which plays a key role in ELMs mitigation. Through the ELITE and conventional BOUT++ analysis, we found that when the insignificant ELM and enhanced CM co-exist, the pedestal is located in unstable P–B region and the ELM is relatively large. The simulation results only using the experimental profiles without considering other factors cannot reproduce the no significant ELM experiment. The CM enhances the edge turbulence, which can control ELMs. Therefore, the effects of CM are considered to explain the ELM mitigation. Modifying the three-field reduced model in BOUT++, an imposed perturbation is added as the CM. The simulation results indicate that: without the CM, the ELM size belongs to the relative large ELM region; after considering the CM, the ELM is mitigated and the energy loss is reduced by about 44.5%. Analysis shows that the CM enhances the three-wave nonlinear interactions in the pedestal and reduces the phase coherence time (PCT) between the pressure and potential, which lead the perturbation to tend to be ‘multiple-mode’ coupling. The competition of free energy between the multiple modes leads to the lack of obvious filament structures and the decreased energy loss. The above reveals that there is a competitive relationship between turbulence and ELMs, and the CM-enhanced turbulence can effectively reduce ELM energy loss. In addition, through the parameter scanning, there is a threshold of the amplitude A, which is consistent with the statistical results in the experiments.

The gas puff imaging (GPI) system on EAST was developed in 2012 and upgraded in 2021. A new relay optical system, consisting of a front reflecting prism, a series of lenses and a filter, is developed for the GPI diagnostic. At the end of the relay optical system, the rays are focused on a thin image surface, which is captured by the sensor of a high-speed camera. In contrast with the previous optical system of GPI in which a coherent glass fiber bundle is used to transmit the image from the end of a telescope inside the vacuum vessel to the outside, the new relay optical system has much lower light loss, i.e., the emission intensity on the image plane of the new GPI is at least 15 times higher than the previous one. In consequence, the temporal resolution of GPI diagnostic on EAST can be raised significantly. The analysis of the optical design denotes that the imaging quality is high enough to ensure a spatial resolution of 2 mm on the objective plane. In the spring experimental campaign of 2021, the upgraded GPI system was commissioned in EAST. Clear poloidal and radial propagations of the edge fluctuations are measured directly by GPI with a high sampling rate of 530 kHz. The poloidal and radial velocities of the edge fluctuations are derived by the time-delay cross-correlation method, with the radial velocity propagating outward, and the poloidal velocity propagating in the ion-diamagnetic drift direction in the SOL and in the electron-diamagnetic drift direction inside the LCFS.

Edge localized modes (ELMs) are completely suppressed by ion cyclotron resonant heating (ICRH) during H-mode discharge, which is first observed in the Experimental Advanced Superconducting Tokamak (EAST). The electromagnetic turbulence model in the BOUT++ framework is applied to the simulations, with the aim of obtaining the key physics mechanisms of ELM suppression by ICRH. The simulations, for the EAST experiment with no ELMs after ICRH, reveal that the radio frequency (RF) sheath plays a key role in the suppression of ELMs. The simulation results show that: (a) using the flow-balanced electric field, the ELM sizes are about 3.4% before ICRH, and 2.1% after ICRH. Although the smaller ELM is obtained with ICRH, it still belongs to relatively large ELMs; (b) instead of the flow-balance condition, the radial electric field E r is calculated using the RF sheath model, and the ELM size is about 0.36% with ICRH, much closer to the small/no ELM regime. After considering the effect of ICRH, the larger E × B shear rate in the pedestal and scrape-off layer induced by the RF sheath reduces the linear growth rate, and enhances the nonlinear mode coupling. Furthermore, we find that there is an effective range of the sheath potential, in which the ELM can be well suppressed, but beyond this range the ELM may be only mitigated rather than suppressed. This can reasonably explain why the probability of ELM suppression by ICRH appearing in the experiments is low.

Compact torus injection is considered as a high promising approach to realize central fueling in the future tokamak device. Recently, a compact torus injection system has been developed for the Experimental Advanced Superconducting Tokamak, and the preliminary results have been carried out. In the typical discharges of the early stage, the velocity, electron density and particles number of the CT can reach 56.0 km/s, 8.73*10^20 m^(-3) and 2.4*10^18 (for helium), respectively. A continuous increase in CT density during acceleration was observed in the experiment, which may be due to the plasma ionized in the formation region may carry part of the neutral gas into the acceleration region, and these neutral gases will be ionized again. In addition, a significant plasma rotation is observed during the formation process which is introduced by the E*B drift. In this paper, we present the detailed system setup and the preliminary platform test results, hoping to provide some basis for the exploration of the CT technique medium-sized superconducting tokamak device in the future

In this paper, a compact torus (CT) injector system is designed for experimental advanced superconducting tokamak (EAST). The injector system consists of a coaxial electrode and four pulsed power systems and generates self-organized CT with high density and velocity. In addition, the performance of EAST-CTI is evaluated using a series of diagnostics systems. The experimental results show that the velocity, electron density, and number of particles of the CT are 56.0 km/s, 8.73 × 1020 m-3, and 2.4 × 1018, respectively. The detailed description and experimental results are presented in this paper.

A newly developed limiter Langmuir probe (LLP) array has been applied to the scrape-off layer (SOL) plasma measurement in EAST tokamak. The LLP system consists of two poloidal arrays located at opposite sides of the low field side guard limiter, and each poloidal array has 26 Langmuir probe channels with a spatial resolution of 35 mm in the vertical direction. The probe tip is polished to match the local graphite tile plane and avoid the leading edge and the corresponding high heat load. The probe can be assembled easily and has an excellent electric insulating property. Langmuir probes have three operation modes, i.e., floating potential mode, ion saturation current mode, and swept Langmuir probe mode. The LLP array could be operated in different modes according to the experimental purpose. The LLP array has been successfully applied to the EAST experiment in 2021, and the three-dimensional plasma structure and the plasma parameters (electron density and temperature) in the SOL have been measured.