Progress of the bolometer diagnostic system on EAST

Measurements of the total radiated power and its spatial distribution are crucial for fusion research.On the experimental advanced superconducting tokamak(EAST),both the metal foil resistive bolometer and the absolute extreme ultraviolet(AXUV)photodiodes have been used to quantify the radiated power.This article introduces the latest improvement of the bolometer diagnostic system on EAST.It also details the successful design and installation of new divertor AXUV cameras,which are dedicated to the investigation of divertor physics.The shielding components of the bolometer detector have been refined,and the article provides a detailed exposition of the double shielding structures that have been verified as effective in microwave shielding.Additionally,the changes in the radiated power distribution in the divertor region during the plasma detachment process are measured using the divertor AXUV camera.Finally,the radiated power measured by the AXUV detector and metal foil resistive bolometer are compared,and different detector performances are presented.

Impact of T_(i)/T_(e )ratio on ion transport based on EAST H-mode plasmas

At the EAST tokamak, the ion temperature(T_(i)) is observed to be clamped around 1.25 keV in electron cyclotron resonance(ECR)-heated plasmas, even at core electron temperatures up to 10 keV(depending on the ECR heating power and the plasma density). This clamping results from the lack of direct ion heating and high levels of turbulence-driven transport. Turbulent transport analysis shows that trapped electron mode and electron temperature gradient-driven modes are the most unstable modes in the core of ECR-heated H-mode plasmas. Nevertheless, recently it was found that the T_(i)/T_(e)ratio can increase further with the fraction of the neutral beam injection(NBI) power, which leads to a higher core ion temperature(Ti0). In NBI heating-dominant H-mode plasmas, the ion temperature gradient-driven modes become the most unstable modes.Furthermore, a strong and broad internal transport barrier(ITB) can form at the plasma core in high-power NBI-heated H-mode plasmas when the T_(i)/T_(e)ratio approaches ~1, which results in steep core Teand Tiprofiles, as well as a peaked neprofile. Power balance analysis shows a weaker Teprofile stiffness after the formation of ITBs in the core plasma region, where Ticlamping is broken,and the core Tican increase further above 2 keV, which is 80% higher than the value of Ticlamping in ECR-heated plasmas. This finding proposes a possible solution to the problem of Ticlamping on EAST and demonstrates an advanced operational regime with the formation of a strong and broad ITB for future fusion plasmas dominated by electron heating.

Blob properties in I-mode and ELM-free H-mode compared to L-mode on EAST

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 Nb,sizeδb,lifetimeτb and radial velocity vr,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 Nbis smaller whileτbis larger in H-mode.The experimental blob sizeδband velocity scalings vr,b-δb 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 kH z)fluctuations.In addition,the blob detection rate is found to increase with the divertor collisionalityΛdiv,indicating a dependence of blob behavior onΛdiv.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.

SOLPS-ITER drift modeling of neon impurity seeded plasmas in EAST with favorable and unfavorable toroidal magnetic field direction

To better understand divertor detachment and asymmetry in the Experimental Advanced Superconducting Tokamak(EAST),drift modeling via the comprehensive edge plasma code SOLPS-ITER of neon impurity seeded plasmas in favorable/unfavorable toroidal magnetic field(BT)has been performed.Firstly,electrostatic potential/field(f/E)distribution has been analyzed,to make sure that f and E are correctly described and to better understand drift-driven processes.After that,drift effects on divertor detachment and asymmetry have been focused on.In accordance with the corresponding experimental observations,simulation results demonstrate that in favorable BTthe onset of detachment is highly asymmetric between the inner and outer divertors;and reversing BT can significantly decrease the magnitude of in-out asymmetry in the onset of detachment,physics reasons for which have been explored.It is found that,apart from the well-known E×B drift particle flow from one divertor to the other through the private flux region,scrape-off layer(SOL)heat flow,which is much more asymmetrically distributed between the high field side and low field side for favorable BTthan that for unfavorable B_T,is also a critical parameter affecting divertor detachment and asymmetry.During detachment,upstream pressure(P_u)reduction occurs and tends to be more dramatical in the colder side than that in the hotter side.The convective SOL heat flow,emerging due to in-out asymmetry in P_u reduction,is found to be critical for understanding divertor detachment and asymmetry observed in EAST.To better understand the calculated drastic power radiation in the core and upstream SOL,drift effects on divertor leakage/retention of neon in EAST with both BTdirections have been addressed for the first time,by analyzing profile of poloidal neon velocity and that of neon ionization source from atoms.This work can be a reference for future numeric simulations performed more closely related to experimental regimes.

Prediction of multifaceted asymmetric radiation from the edge movement in density-limit disruptive plasmas on Experimental Advanced Superconducting Tokamak using random forest

Multifaceted asymmetric radiation from the edge(MARFE) movement which can cause density limit disruption is often encountered during high density operation on many tokamaks. Therefore, identifying and predicting MARFE movement is meaningful to mitigate or avoid density limit disruption for the steady-state high-density plasma operation. A machine learning method named random forest(RF) has been used to predict the MARFE movement based on the density ramp-up experiment in the 2022’s first campaign of Experimental Advanced Superconducting Tokamak(EAST). The RF model shows that besides Greenwald fraction which is the ratio of plasma density and Greenwald density limit, dβp/dt,H98and d Wmhd/dt are relatively important parameters for MARFE-movement prediction. Applying the RF model on test discharges, the test results show that the successful alarm rate for MARFE movement causing density limit disruption reaches ~ 85% with a minimum alarm time of ~ 40 ms and mean alarm time of ~ 700 ms. At the same time, the false alarm rate for non-disruptive and non-density-limit disruptive discharges can be kept below 5%. These results provide a reference to the prediction of MARFE movement in high density plasmas, which can help the avoidance or mitigation of density limit disruption in future fusion reactors.

Line identification of extreme ultraviolet (EUV) spectra from low-Z impurity ions in EAST tokamak plasmas

Extreme ultraviolet(EUV) spectra emitted from low-Z impurity ions in the wavelength range of10–500Å were observed in Experimental Advanced Superconducting Tokamak(EAST)discharges. Several spectral lines from K-and L-shell partially ionized ions were successfully observed with sufficient spectral intensities and resolutions for helium, lithium, boron, carbon,oxygen, neon, silicon and argon using two fast-time-response EUV spectrometers of which the spectral intensities are absolutely calibrated based on the intensity comparison method between visible and EUV bremsstrahlung continua. The wavelength is carefully calibrated using wellknown spectra. The lithium, boron and silicon are individually introduced for the wall coating of the EAST vacuum vessel to suppress mainly the hydrogen and oxygen influxes from the vacuum wall, while the carbon and oxygen intrinsically exist in the plasma. The helium is frequently used as the working gas as well as the deuterium. The neon and argon are also often used for the radiation cooling of edge plasma to reduce the heat flux onto the divertor plate. The measured spectra were analyzed mainly based on the database of National Institute of Standards and Technology. As a result, spectral lines of He Ⅱ, Li Ⅱ–Ⅲ, B Ⅳ–Ⅴ, C Ⅲ–Ⅵ, O Ⅲ–Ⅷ, Ne Ⅱ–Ⅹ,Si Ⅴ–Ⅻ, and Ar Ⅹ–XVI are identified in EAST plasmas of which the central electron temperature and chord-averaged electron density range in Te0=0.6–2.8 keV and ne=(0.5–6.0)×1019 m-3, respectively. The wavelengths and transitions of EUV lines identified here are summarized and listed in a table for each impurity species as the database for EUV spectroscopy using fusion plasmas.

Experimental investigation on divertor tungsten sputtering with neon seeding in ELMy H-mode plasma in EAST tokamak

Neon(Ne)seeding is used to cool the edge plasma by radiation to protect the divertor tungsten(W)target in the Experimental Advanced Superconducting Tokamak(EAST).The W sputtering in the outer divertor target with Ne seeding is assessed by the divertor visible spectroscopy system.It is observed that the W sputtering flux initially increases with Ne concentration in the divertor despite the decreasing plasma temperature.After reaching a maximum around 25 eV,the W sputtering rate starts to decrease,presenting a suppression effect.The effect on the divertor W sputtering flux and yield due to the competition between the increase of the Ne concentration and the decrease of the plasma temperature is discussed.The results show that enough Ne seeding is essential to effectively reduce the electron temperature and thus to suppress W sputtering.Moreover,ELM suppression is observed when Ne and W impurities enter the core plasma,which could be correlated to the enhanced turbulence transport in the pedestal.

Gaussian process tomography based on Bayesian data analysis for soft x-ray and AXUV diagnostics on EAST

This work presents the Gaussian process tomography(GPT)based on Bayesian data analysis and its applications in soft x-ray(SXR)and absolute extreme ultraviolet spectroscopy(AXUV)diagnostics on experimental advanced superconducting tokamak(EAST).This is the first application of the GPT method in the AXUV diagnostic system in fusion devices.It is found that even if only horizontal detector arrays are used to reconstruct the two-dimensional(2D)distribution of SXR and AXUV emissivity fields,the GPT method performs robustly and extremely fast,which enables the GPT method to provide real-time feedback on impurity transport and fast magnetohydrodynamics(MHD)events.By reconstructing SXR emissivity in the poloidal cross section on EAST,an m/n=1/1 internal kink mode has been observed,and the plasma redistribution due to the kink mode is clearly visible in the reconstructions,where m is the poloidal mode number and n is the toroidal mode number.Sawtooth-like internal disruptions extended throughout the entire plasma core and mainly driven by the m/n=2/1 mode have been acquired.During the sawtooth-like internal disruption crash phase,the conversion from an m=2 mode to an m=1 mode is observed.Using the reconstructed AXUV emissivity field we were able to observe the process of impurity accumulated in the plasma core and the mitigation of core impurity due to neon injection in the plasma edge.The data from all other diagnostics involved in the analysis shows that the reconstructions from AXUV measurements are reliable.

Divertor detachment operation in helium plasmas with ITER-like tungsten divertor in EAST

Detachment in helium(He)discharges has been achieved in the EAST superconducting tokamak equipped with an ITER-like tungsten divertor.This paper presents the experimental observations of divertor detachment achieved by increasing the plasma density in He discharges.During density ramp-up,the particle flux shows a clear rollover,while the electron temperature around the outer strike point is decreasing simultaneously.The divertor detachment also exhibits a significant difference from that observed in comparable deuterium(D)discharges.The density threshold of detachment in the He plasma is higher than that in the D plasma for the same heating power,and increases with the heating power.Moreover,detachment assisted with neon(Ne)seeding was also performed in L-and H-mode plasmas,pointing to the direction for reducing the density threshold of detachment in He operation.However,excessive Ne seeding causes confinement degradation during the divertor detachment phase.The precise feedback control of impurity seeding will be performed in EAST to improve the compatibility of core plasma performance with divertor detachment for future high heating power operations.

Observation of coherent mode induced by a molybdenum dust on EAST

The influence of a molybdenum dust buildup on plasma edge turbulence has been studied in the EAST tokamak.The motion of the dust from the upper divertor region is detected by a fast visible CCD camera,the XUV spectrometer arrays,and the EUV spectrometer.The MoXV emission intensity sharply increases compared with the spectral lines of various ionization states of other elements,which implies that the dust particles are the molybdenum impurities.The radial distribution of Mo^(14+)ion simulated by a simplified 1 D transport model indicates that the molybdenum dust mainly deposits in the pedestal bottom region.Moreover,it is observed that the coherent mode(CM)appears atρ=0.94 after the molybdenum impurities enter the main plasma region.The influx of molybdenum impurities results in increasing pedestal electron density and decreasing pedestal electron temperature in contrast to that before the event of impurities dropping.It is also found that the electron density gradient in the pedestal increases when the ablation of the molybdenum impurities is observed in the pedestal region.The qualitative experimental results indicate that the onset of CM is likely related to the increase of the density gradient and edge collisionality in the pedestal.In comparison to the density gradient,the enhancement of CM amplitude largely depends on the increase of the edge collisionality.

Achieving temporary divertor plasma detachment with MARFE events by pellet injection in the EAST superconducting tokamak

A new pellet injection system has been equipped on the experimental advanced superconducting tokamak（EAST） in the 2012 campaign,with a pellet size of Ф 2 mm×2 mm,a frequency of1 Hz–10 Hz and velocity of 150 m s^-1–300 m s^-1.The deuterium pellet is well-known for plasma fuelling as well as for triggering the edge localized mode（ELM）.In the 2012 campaign,pellet injection experiments were successfully carried out on EAST.Temporary plasma detachment achieved by deuterium pellets has been observed in a double null（DN） divertor configuration,with multi-pellet injections at a repetition frequency of 2 Hz.The partial detachment of the outer divertors and complete detachment of the inner divertors was achieved after 35 ms of each pellet injection,which have a duration of 30–60 ms with the maximum degree of detachment（DOD） reaching 3.5 and 37,respectively.Meanwhile,the multifaceted asymmetric radiation from the edge（MARFE） phenomena was also observed at the high field side（HFS） near both the lower and upper X-points with radiation loss suddenly increased to about 15%–70%,which may be the main cause of divertor plasma detachment.The temporary detachment induced by pellet injection may act as a new way to study divertor detachment behaviors.

Influence of impurity seeding on the plasma radiation in the EAST tokamak

Plasma radiation characteristics in EAST argon（Ar） gas and neon（Ne） gas seeding experiments are studied.The radiation profiles reconstructed from the fast bolometer measurement data by tomography method are compared with the ones got from the simulation program based on corona model.And the simulation results coincide roughly with the experimental data.For Ar seeding discharges,the substantial enhanced radiations can be generally observed in the edge areas at normalized radius ρ_（pol）~0.7–0.9,while the enhanced regions are more outer for Ne seeding discharges.The influence of seeded Ar gas on the core radiation is related to the injected position.In discharges with LSN divertor configuration,the Ar ions can permeate into the core region more easily when being injected from the opposite upper divertor ports.In USN divertor configuration,the W impurity sputtered from the upper divertor target plates are observed to be an important contributor to the increase of the core radiation no matter impurity seeding from any ports.The maximum radiated power fractions f_（rad）（P_（rad）/P_（heat）） about 60%–70% have been achieved in the recent EAST experimental campaign in 2015–2016.