EAST Tokamak Research Articles

Improvement on the plasma performances via application of flowing lithium limiters in EAST tokamak

Three generation of flowing liquid lithium (FLiLi) limiters have been developed and successfully tested in EAST since 2014. It was confirmed that liquid Li could be driven by an innovative in-vessel DC electromagnetic J × B pump to form a recirculating loop. The FLiLi design was continuously upgraded, resulting in improvement of the liquid Li surface coverage (>80%), elimination of surface erosion, and the increase of heat exhaust capability. Promising results are obtained during FLiLi operations, including a reduction of impurities and recycling to improve plasma confinement, and a mitigation of ELM activity in H-mode plasmas. The FLiLi limiter program has shown basic compatibility with H-mode discharges with auxiliary heating power up to 8.3 MW. These efforts will expand to flowing liquid Li PFC in high heat flux zones, e.g. near the divertor strike point in reactors.

Development of gamma ray spectrometer with high energy and time resolutions on EAST tokamak.

A new gamma ray spectrometer with high energy and time resolutions has been developed and installed on the EAST tokamak to study fast ion and runaway electron behaviors. The spectrometer is based on a LaBr3(Ce) scintillator detector and a fully digital data acquisition system that is based on a digitizer with digital pulse processing algorithms. The energy resolution of the spectrometer is about 3.9% at 662 keV, and the spectrometer can operate stably at a counting rate as high as 1 MHz, monitored by using a light emitting diode monitoring system. The measured gamma ray spectrum is simulated based on Geant4 and unfolded with the high-resolution boosted Gold deconvolution algorithm, aiming at reconstructing the energy distribution functions of fast ions and runaway electrons.

Experimental observation of coexisting electromagnetic fluctuations correlating with the inter-ELM pedestal evolution on EAST

Correlations between the edge fluctuations and the pedestal evolution during the relatively large edge localized mode (ELM) cycles at high pedestal normalized electron collisionality (νe,ped* > 1) on the EAST tokamak are investigated. Not only the edge electrostatic coherent mode (ECM, ∼50 kHz) and the low frequency magnetic coherent mode (MCM, ∼32 kHz) but also a high frequency electromagnetic mode (HFM, >150 kHz) are observed to be coexisting between ELMs. After the ELM crash, the pedestal electron temperature recovered faster than the pedestal electron density. It is found that the saturation of the ECM coincides more with the saturation of the pedestal electron density, while the saturation of the HFM and MCM coincides more with the saturation of the pedestal electron temperature. In addition, the characteristics of the electromagnetic fluctuations (the HFM and MCM) are studied in detail: the HFM propagates in the electron diamagnetic drift direction in the laboratory frame with an average poloidal wave number of k¯θHFM≈0.17 cm−1, while the MCM propagates in the ion diamagnetic drift direction in the laboratory frame with k¯θMCM ≈ 0.12 cm−1 and the toroidal mode number of n = 1. Furthermore, both the HFM and MCM have inward average radial wave numbers of k¯RHFM≈0.13 cm−1 and k¯RMCM≈4.64 cm−1. The bispectral analysis shows that the HFM and MCM have strong nonlinear interactions. The HFM is clearly observed on both low and high field side Mirnov coils, which might suggest a feature beyond a ballooning type instability, e.g., the kinetic ballooning mode. These studies may contribute to a better understanding of the pedestal evolution.

Laser-based Faraday effect polarimetry for real-time feedback control of plasma parameters on EAST

A primary goal for ITER and prospective fusion power reactors is to achieve long-pulse, steady-state burning plasmas. Real-time control of plasma parameters such as current profile, density profile, position is necessary to extend high-performance plasma regimes to long pulse and is required for development of ITER-like discharge scenarios. A double-pass, radially-view, 11 chords, POlarimeter-INTerferometer (POINT) system has been developed and routinely operated on the EAST Tokamak, and provides important plasma current profile, plasma density profile and the vertical position information for plasma control and physics understanding. Digital Phase Detection, which provides real-time 4 μs Faraday and density output for plasma control, has been developed, thereby allowing real time current and q profile monitoring using fast equilibrium reconstruction. Besides, the Faraday effect can measure the plasma vertical position, which provides a good tool for plasma position control.

Observation of deuterium molecule emission spectra under an active feedback control of H-mod plasma divertor detachment experiment on the EAST tokamak**Project supported by the National Natural Science Foundation of China (Grant No. 11805234).

Based on neon gas puffing, an active feedback control of H-mod plasma divertor detachment experiment was successfully operated on the EAST tokamak. During the feedback control discharge, the plasma was detached by puffing neon gas and the strike point splitting phenomenon on divertor target was also clearly observed by divertor probes diagnostic. In boundary region, many neutral particle processes (atom and molecule) were happened and accompanied by their emission spectra under the detachment discharge. By studying these emission spectra, it is helpful for us to understand the role of atoms and molecules in boundary recycling, which is important for studying the physical mechanism of divertor detachment. For the Fulcher-α system (, D2 emission spectra in the range from 601 nm to 606 nm were observed, identified and fitted in the detachment experiment for the first time on the EAST, and the spectra in the Q(0–0) band ( in the Q branch of the Fulcher-α system were used for detailed analysis to acquire the boundary region temperature Te (below 5 eV), which could not be provided very well by other diagnostics on the EAST. An electronic version deuterium molecular spectral line database was established to identify the spectral lines and a multi-peak fitting program was developed to fit and analyze the observed spectra.

Experimental study of double tearing mode on EAST tokamak

The double tearing mode (DTM) in a high βN (βN > 1.5) discharge with internal transport barrier on EAST was investigated. A 15 kHz tearing mode (TM) (m ≥ 3, n = 3) appears at outer q = 2 surface at first, which is stable in the high βN phase. Then a 2 kHz TM (m = 2, n = 1) occurs at inner q = 2 surface. Soon after, high βN collapsed with the crash of ELM, and DTM formed during the collapse of βN. The positions of the two islands of the DTM are consistent with the q = 2 surface. The temperature fluctuations are the strongest between the two magnetic islands. A statistical analysis of high βN discharges operating with the reversed magnetic shear configuration in the EAST 2015–2018 campaign revealed the existence of the DTM in many discharges. During the DTM phase, all βN are reduced by 10%–30% within 0.1 s. However, there are two different βN behaviors thereafter—with and without βN recovery. Studying the physical mechanism of βN recovery during the DTM phase will benefit steady-state operation with reversed shear configurations in the future.

Penetration properties of resonant magnetic perturbation in EAST Tokamak

Our three-dimensional toroidal magnetohydrodynamics code [CLT (Ci-Liu-Ti, which means magnetohydrodynamics in Chinese)] is upgraded to CLTx in which the simulation domain extends to a scrape-off layer (SOL). Linear simulation results of resonant magnetic perturbation (RMP) for edge-localized-mode mitigation discharge 52340 in EAST Tokamak agree well with those from the MARS-F code. With the presence of the X-point and the SOL, the RMP amplitude and phase shift in the pedestal region, especially for a higher m mode, are largely affected by the resistivity in the SOL. A phase shift of π for resonant harmonics in vacuum and with a plasma response is observed, which is quite different from the π/2 shift due to the presence of screen current suggested in the previous studies. Simulation results with different resistivities suggest that the amplitude reduction and the phase shift of the resonant harmonics increase with decreasing the resistivity.

A low-frequency axisymmetric oscillation in the high-confinement mode pedestal on the EAST tokamak

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.

Density compensation and stored energy recovery in resonant magnetic perturbation suppressed edge-localized mode H-mode plasmas using pellet fueling on EAST

The fueling of plasmas using frozen pellets in steady-state quiescent H-mode plasmas with edge-localized modes (ELMs) suppressed by external magnetic perturbations (RMPs) was successfully demonstrated on the EAST tokamak in plasmas with similar shapes to those in ITER with q95 ~ 3.8, and triangularity δ ~ 0.45. During the fully suppressed-ELM phase using RMPs, the plasmas were refueled by deuterium pellets with a frequency of 5 Hz from ~40 cm above the mid-plane on the low field side. The results show that the pellet fueling would partially compensate density, which decreased due to the effect of pump-out induced by RMPs. It was interesting to discover that the stored energy of the plasma could be raised to the value from before RMP application using pellet injection, which is rarely reported in other tokamaks. As for ELMs, they reappeared with a size smaller than the ELMs before RMP application accompanied with a magnetic coherent mode termination, however, the ELMs were still mitigated to a certain extent. Meanwhile, a similar experiment using supersonic molecular beam injection fueling was also carried out, which confirmed that pellet fueling is better for raising plasma stored energy. This investigation on the synergistic effect of pellet fueling and RMPs would be useful for simultaneous control of plasma density and ELMs in high performance H-mode plasmas in future devices, such as ITER.

On a burning plasma low recycling regime with PDT = 23–26 MW, QDT = 5–7 in a JET-like tokamak

The recent invention of a continuously flowing liquid lithium system, called 24/7-FLiLi, opens the opportunity to design a plasma pumping divertor which can significantly suppress plasma edge cooling by recycling.The paper describes the expected performance of deuterium–tritium burning plasma with low recycling using a tokamak of JET size, magnetic field and plasma current as an example.So far, only the technological aspects of flowing lithium have been tested on the HT-7 and EAST tokamaks (ASIPP, Hefei, China) with no attempts to create the low recycling regime. The compatibility of 24/7-FLiLi with tokamak plasma was confirmed and some mistakes in the design were revealed, thus, requiring another fabrication approach. Now, with the proper design of the Li system, recycling as low as 0.5 seems to be achievable.The suppression of plasma cooling by recycling in combination with neutral beam injection (NBI) for core plasma fueling by energetic particles automatically leads to ‘the best possible confinement regime’, uniquely compatible with burning plasma. This paper outlines the conceptual details of the 24/7-FLiLi divertor for a JET size tokamak. The main focus is in regard to the physics of the low recycling regime as well as to the expected performance of the burning plasma. In comparison with the results of 1994 on TFTR and 1997 on JET, the projected performance is astonishing: fusion power –26 MW, fusion efficiency , tritium burn up 7.8%–8.7% with modest NBI power MW, and energy keV. This new regime could resolve the outstanding plasma physics issues related to burning plasma that are unsolvable with the currently dominant approach.

Monte Carlo orbit-following simulations including the finite Larmor radius effect based on a phase-space coordinate transform method

Monte Carlo orbit-following simulations based on a phase-space coordinate transform method are presented in this paper. Guiding-center orbits are computed by using this method, that is, using the phase-space coordinate transform and solving the gyrokinetic equations of motion. This method has been adopted in a Monte Carlo orbit-following code GYCAVA for studying the behavior of fast ions in a tokamak. The finite Larmor radius effect can be included in the guiding-center orbit computation. The cylindrical coordinates are used for avoiding singularity of the safety factor at X points of the divertor configuration. An n-point discrete sum method is used to compute the gyro-average, which is related to the finite Larmor radius effect. The GYCAVA code has been integrated with other equilibrium, magnetohydrodynamic and neutral beam injection codes for studying losses and wall loads of fast ions in the presence of perturbation fields. The GYCAVA code has been verified by tests of the orbit computation and the Monte Carlo Coulomb collision. Simulations of fast ion losses in the presence of resonant magnetic perturbations in the EAST tokamak have been performed.

Unified modeling of both resonant and non-resonant neoclassical transport under non-axisymmetric magnetic perturbations in tokamaks

A numerical model for neoclassical transport under nonaxisymmetric magnetic perturbations in low collisionality plasmas in tokamaks is developed. To take into account bounce-drift resonances and magnetic drift effects, a Fourier decomposition of the drift kinetic equation in new coordinates, rather than bounce average of it, is employed. A pitch angle scattering collisional operator is used to keep accuracy in the nonresonant regimes or resonant plateau regimes with resonant pitch near pitch space boundaries. Full toroidal geometry effects are also included to increase the accuracy in the modeling of neoclassical transport in the resonant regimes. Neoclassical transport in the most important collisionless regimes, including resonant super-banana plateau and bounce-drift resonances regimes, nonresonant 1/ν and ν−ν regimes, and the transitions between them, can be modeled simultaneously in this model by numerically solving the drift kinetic equation. By application to the neoclassical toroidal viscosity modeling in one discharge in the EAST tokamak, it is found that the bounce-drift resonances dominate the contributions near the plasma core where the plasma E→×B→ drift frequency is close to the bounce frequency, while the precessional resonance dominates the contribution near the edge pedestal top where the E→×B→ drift frequency is close to zero.

Quantitative analysis on tungsten spectra of W6+ to W45+ ions

Tungsten ion densities are determined by measuring radial profiles of line emissions in LHD. Zn-(W44+) and Cu-like (W45+) ion densities are determined from line emission at 60.9 Å and 127.0 Å, respectively. Ion densities of W24+, W25+ and W26+ are also determined from tungsten pseudo-continuum (a so-called unresolved transition array (UTA)) at 29-33 Å. Observations of magnetic dipole (M1) forbidden lines at 3377Å enable evaluation of the ion density of W27+. In EAST tokamak the tungsten concentration is estimated by a combination of the UTA and total radiation loss. In HL-2A the tungsten influx is evaluated from line emission of W6+ ions at 216.2 Å and 261.4 Å, which are measured during the tungsten reappearance phase after tungsten laser blow-off. All these results demonstrate a good approach towards quantitative tungsten analysis.

D retention behavior in the CLF-1 steel after D ion irradiation and D plasma exposure

Deuterium (D) retention behavior in a reduced activation ferritic/martensitic (RAFM) steel (CLF-1) is measured by thermal desorption spectrometry (TDS) after irradiation by D ion in an ion beam facility, exposure to D plasma in a linear ECR plasma generator, and exposure to D plasmas in the EAST tokamak separately. The D release behavior of the CLF-1 steel under the three-different D implantation condition is compared. Results indicate that D release behavior in the CLF-1 steel after the tokamak exposure experiments differs significantly from those after laboratory D ion implantation experiments or D plasma exposure experiments in a linear ECR plasma generator. This demonstrates that using low temperature plasmas in laboratory facilities alone is not sufficient in the study of hydrogen isotope retention behavior in a realistic tokamak environment. In order to simulate the plasma and material interaction in tokamak more felicitously, more work need to do to improve the linear plasma generator at present. Possible explanations have been proposed to explain the different D release behavior in the CLF-1 steel after D implantation in three different environment.

Observation of filament-like structures in ELMy H-mode plasma with a VUV imaging system developed on the EAST tokamak

On the EAST tokamak, filament-like structures have been observed in ELMy H-mode discharges with a high-speed vacuum ultraviolet (VUV) imaging system. The topos, chronos and their weight can be obtained simultaneously by performing the so-called singular value decomposition (SVD) analysis of raw VUV imaging data. The fluctuation amplitude is observed to be suppressed and enhanced gradually in the edge localized mode (ELM) crash and pedestal recovery phase in the chronos, respectively, while filament-like structures can only be found in the pedestal recovery phase on the topos. The mode structure, i.e. m/n = 36/9 (m and n are the poloidal and toroidal mode number, respectively) with = 0.95, = 0.07 ( and denote the mode location and mode width, respectively) is derived by a comparison of the synthetic images and the experimental imaging data.

Comparison of measurements with different types of divertor Langmuir probe in EAST tokamak

Divertor Langmuir probes (Div-LPs), embedded in the divertor target plates, are the important diagnostic tools to measure the plasma parameters in EAST divertor region. A series of measurement methods, i.e., single-probe, double-probe and triple-probe schemes, are applied and compared for the Div-LPs on EAST, to get the electron temperature Te and the electron density ne at the same poloidal position, respectively. It is found that the electron temperature Te and the electron density ne measured by the single-probe and the double-probe schemes are basically the same during the plasma discharge phase. Further analysis of the electron energy distribution function (EEDF) represented by the first derivative curve method shows that the electrons conform to a Maxwell distribution, which indicate that the three methods can be applied in the EAST divertor region. Besides, the domed and flush-mounted probe tips for measuring the plasma parameters are also compared, and the results show a good match. The sheath potential coefficient Λ is also calculated from the I-V characteristic curves by two ways, i.e. Λp=(Vplasma-Vf)/Te and ΛI= ln (|Ies/Iis|), and compared with the traditional value Λ=2.8, the results indicate that the ∼1.5ΛI and Λ=2.8 are available to replace the Λp under the different strength of toroidal magnetic field in EAST tokamak.

High-resolution mass separation between helium and deuterium with conventional quadrupole mass spectrometer by using varied ionization energy

A new effective and reliable method is developed to monitor deuterium and helium by a conventional quadrupole mass spectrometer RGA200. The measurement of atomic mass unit (AMU) = 4 is obtained at the ionization energies set of 40 eV and 70 eV instead of 25 eV and 70 eV on RGA200. By this method, not only the minimum deuterium pressure lower limit is widened to ∼10−7 Pa, but also the detection ratio of deuterium to helium is extended to 1–100%. Likewise, it is applied to helium measurement. The calculation error of deuterium mainly depends on the ratio of deuterium to the sum of deuterium and helium. Most of the deuterium calculation errors are modified in the range of 2%–16%. The errors of the calculation results are within the acceptable range of the experiments in the EAST tokamak. It has been successfully applied in EAST 2019 campaign, during helium discharge cleaning process. It provides a viable method which can achieve high-resolution mass separation between helium and deuterium by RGA200 using varied ionization energy.

Diagnostic mirrors for ITER: research in the frame of International Tokamak Physics Activity

Mirrors will be used as first plasma-viewing elements in optical and laser-based diagnostics in ITER. Deterioration of the mirror performance due to e.g. sputtering of the mirror surface by plasma particles or deposition of impurities will hamper the entire performance of the affected diagnostic and thus affect ITER operation. The Specialists Working Group on First Mirrors (FM SWG) in the Topical Group on Diagnostics of the International Tokamak Physics Activity (ITPA) plays an important role in finding solutions for diagnostic first mirrors.Sound progress in research and development of diagnostic mirrors in ITER was achieved since the last overview in 2009. Single crystal (SC) rhodium (Rh) mirrors became available. SC rhodium and molybdenum (Mo) mirrors survived in conditions corresponding to ~200 cleaning cycles with a negligible degradation of reflectivity. These results are important for a mirror cleaning system which is presently under development. The cleaning system is based on sputtering of contaminants by plasma. Repetitive cleaning was tested on several mirror materials. Experiments comprised contamination/cleaning cycles. The reflectivity SC Mo and Rh mirrors has changed insignificantly after 80 cycles. First in situ cleaning using radiofrequency (RF) plasma was conducted in EAST tokamak with a mock-up plate of ITER edge Thomson Scattering (ETS) with five inserted mirrors. Contaminants from the mirrors were removed. Physics of cleaning discharge was studied both experimentally and by modeling.Mirror contamination can also be mitigated by protecting diagnostic ducts. A deposition mitigation (DeMi) duct system was exposed in KSTAR. The real-time measurement of deposition in the diagnostic duct was pioneered during this experiment. Results evidenced the dominating effect of the wall conditioning and baking on contamination inside the duct. A baffled cassette with mirrors was exposed at the main wall of JET for 23,6 plasma hours. No significant degradation of reflectivity was measured on mirrors located in the ducts.Predictive modeling was further advanced. A model for the particle transport, deposition and erosion at the port-plug was used in selecting an optical layout of several ITER diagnostics. These achievements contributed to the focusing of the first mirror research thus accelerating the diagnostic development. Modeling requires more efforts. Remaining crucial issues will be in a focus of the future work of the FM SWG.

Rotation braking with n = 1 nonaxisymmetric magnetic perturbation in the EAST tokamak

The toroidal plasma rotation braking effect during the application of n = 1 static resonant magnetic perturbation is studied by momentum transport analysis in the EAST tokamak. The braking torque shows a global profile and two peaks located near the plasma core and the edge, respectively. The effect of momentum diffusion contributes significantly to the calculated torque. Simulation results with the obtained torque and momentum diffusion coefficients well reproduce the observed plasma rotation evolution. Neoclassical toroidal viscosity (NTV) torque is modeled for comparison with the experimental torque. The total integrated NTV torque is around −0.12 Nm, which is comparable to the observed braking torque (around −0.33 Nm). In the plasma edge, there is a peak in the NTV torque profile, which agrees well in amplitude with the obvious peak in the observed torque density profile. An additional peak in the NTV torque profile due to the ion bounce resonance is also located in the core region. However, the magnitude of this peak is much smaller than the observed one near the plasma core.

Interactions of plasma and guard limiter in front of lower hybrid wave antenna on EAST tokamak

The hotspots on the guard limiters of the lower hybrid wave (LHW) antenna on EAST tokamak not only cause serious damage to the guard limiters, but also strongly degrade the plasma performance due to enhanced impurity productions. In published studies, the heat flux to the limiters is assumed to be carried to limiter walls by electrons which can absorb a small amount of the launched wave energy via interactions with the lower hybrid modes of high parallel refractive index , and the effects of the sheaths formed in front of the limiter surface are ignored. In this work, the heat fluxes to the limiter surfaces are obtained consistently by conducting one-dimension particle-in-cell (PIC) simulations. For the plasma between two guard limiters, while the ions are assumed to be Maxwellian, the electrons are described by using the Fisch model, whose velocity distribution function (VDF) has a resonant plateau in the superthermal region formed by the electron and LHW interaction. Secondary electron emission (SEE) from the limiter surfaces is taken into account by considering two kinds of wall materials, carbon and tungsten. It is found that the sheath potential drop is significantly raised due to the presence of fast electrons. As a result, both ion and electron heat fluxes to the limiters are strongly enhanced compared to those in a Maxwellian plasma, which increases the wall temperature and gives rise to hotspots. The heat flux to the guard limiters increases with the width of the resonant plateau of the electron VDF (i.e. the electron energy). With the enhancement of the sheath potential drop, the physical sputtering yield from the primary and impurity ions on limiter surfaces is found to be significant, which may affect the plasma confinement and discharges.