VUV imaging of type-ⅠELM filamentary structures and their temporal characteristics on EAST

In the H-mode experiments conducted on the Experimental Advanced Superconducting Tokamak(EAST),fluctuations induced by the so-called edge localized modes(ELMs)are captured by a high-speed vacuum ultraviolet(VUV)imaging system.Clear field line-aligned filamentary structures are analyzed in this work.Ion transport induced by ELM filaments in the scrape-off layer(SOL)under different discharge conditions is analyzed by comparing the VUV signals with the divertor probe signals.It is found that convective transport along open field lines towards the divertor target dominates the parallel ion particle transport mechanism during ELMs.The toroidal mode number of the filamentary structure derived from the VUV images increases with the electron density pedestal height.The analysis of the toroidal distribution characteristics during ELM bursts reveals toroidal asymmetry.The influence of resonance magnetic perturbation(RMP)on the ELM size is also analyzed using VUV imaging data.When the phase difference of the coil changes periodically,the widths of the filaments change as well.Additionally,the temporal evolution of the ELMs on the VUV signals provides rise time and decay time for each single ELM event,and the results indicate a negative correlation trend between these two times.

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.

Characteristics of divertor heat flux distribution with an island divertor configuration on the J-TEXT tokamak

On J-TEXT,the temporal evolution of heat flux distribution on the high-field side(HFS)divertor plate has been measured by an infrared(IR)camera during the plasma operation with an island divertor configuration.In experiments,the island divertor configuration is an edge magnetic island chain structure surrounded by stochastic layers,which can be induced by resonant magnetic perturbations(RMPs).The experimental results show that the heat flux distribution on the HFS target plate depends significantly on the edge magnetic topology.Furthermore,the impact of hydrogen fueling using supersonic molecular beam injection(SMBI)on the divertor heat flux distributions is studied on J-TEXT with an island divertor configuration.It has been observed that power detachment can be achieved when the radiation front approaches the last closed flux surface(LCFS)after each SMBI pulse.This result may provide a method of access for divertor detachment on a fusion device with a three-dimensional(3D)boundary magnetic structure.

Application of three-dimensional MHD equilibrium calculation coupled with plasma response to island divertor experiments on J-TEXT

Three-dimensional(3D) equilibrium calculations, including the plasma rotation shielding effect to resonant magnetic perturbations(RMPs) produced by the island divertor(ID) coils, were carried out using the HINT and MARS-F codes on J-TEXT. Validation of 3D equilibrium calculations with experimental observations demonstrates that the shielding effect will prevent the penetration of the edge m/n = 3/1 mode component when the ID coil current is 4 k A, while change the size of magnetic islands once the current exceeds the penetration threshold. This indicates that equilibrium calculations including the plasma rotation shielding effect to RMPs can lead to better agreements with experimental observations compared to the vacuum approximation method. Additionally, the magnetic topology at the boundary undergoes changes,impacting the interaction between the plasma and the target plate. These results may be important in understanding RMP effects on edge transport and magnetohydrodynamic(MHD)instability control, as well as divertor heat and particle flux distribution control.

Improved training framework in a neural network model for disruption prediction and its application on EXL-50

A neural network model with a classical annotation method has been used on the EXL-50tokamak to predict impending disruption.However,the results revealed issues of overfitting and overconfidence in predictions caused by inaccurate labeling.To mitigate these issues,an improved training framework has been proposed.In this approach,soft labels from previous training serve as teachers to supervise the further learning process;this has lead to a significant improvement in predictive model performance.Notably,this enhancement is primarily attributed to the coupling effect of the soft labels and correction mechanism.This improved training framework introduces an instance-specific label smoothing method,which reflects a more nuanced model assessment on the likelihood of a disruption.It presents a possible solution to effectively address the challenges associated with accurate labeling across different machines.

Characteristics of the SOL ion-to-electron temperature ratio on the J-TEXT tokamak with different plasma configurations

Accurate measurement of the average plasma parameters in the edge region,including the temperature and density of electrons and ions,is critical for understanding the characteristics of the scrape-off layer(SOL) and divertor plasma transport in magnetically confined fusion research.On the J-TEXT tokamak,a multi-channel retarding field analyzer(RFA) probe has been developed to study average plasma parameters in the edge region under various poloidal divertor and island divertor configurations.The edge radial profile of the ion-to-electron temperature ratio,τ_(i/e),has been determined,which gradually decreases as the SOL ion self-collisionality,v_(SOL)*,increases.This is broadly consistent with what has been observed previously from various tokamak experiments.However,the comparison of experimental results under different configurations shows that in the poloidal divertor configuration,even under the same v_(SOL)*,τ_(i/e) in the SOL region becomes smaller as the distance from the X-point to the target plate increases.In the island divertor configuration,τ_(i/e) near the O-point is higher than that near the X-point at the same v_(SOL)*,and both are higher than those in the limiter configuration.These results suggest that the magnetic configuration plays a critical role in the energy distributions between electrons and ions at the plasma boundary.

J-TEXT achievements in turbulence and transport in support of future device/reactor

Following the reconstruction of the TEXT tokamak at Huazhong University of Science and Technology in China, renamed as J-TEXT, a plethora of experimental and theoretical investigations has been conducted to elucidate the intricacies of turbulent transport within the tokamak configuration. These endeavors encompass not only the J-TEXT device's experimental advancements but also delve into critical issues pertinent to the optimization of future fusion devices and reactors. The research includes topics on the suppression of turbulence, flow drive and damping, density limit, non-local transport, intrinsic toroidal flow, turbulence and flow with magnetic islands, turbulent transport in the stochastic layer, and turbulence and zonal flow with energetic particles or helium ash. Several important achievements have been made in the last few years, which will be further elaborated upon in this comprehensive review.

Impact of carbon disclosure on debt financing costs

Creditors,such as banks,often use disclosed environmental information to assess a company’s environmental risk and ensure the safety of debt funds.Consequently,carbon disclosures have become an important consideration for creditors when making investments.This study explores the relationship between carbon disclosure and debt financing costs using data on listed companies from 2008 to 2019.The results show that carbon disclosure can reduce the debt financing costs of enterprises,and that this influence is more significant for private companies than for state-owned enterprises.Instrumental variables and Propensity Score Matching(PSM)were used to evaluate the robustness of negative relationships.Furthermore,carbon disclosure has a more significant impact on debt costs with less environmental supervision pressure,weak residents’environmental awareness,and weak product market competition.These findings provide guidance for companies’carbon information disclosure and support the establishment of official carbon disclosure standards.

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.

An application of the shortest path algorithm for the identification of weak MHD mode

The identification of magnetohydrodynamic(MHD)modes is a crucial issue in the control of magnetically confined plasmas.This paper proposes a novel method for identifying the evolution of MHD modes from a signal with a low signal-to-noise ratio.The proposed method generates a weighted directed graph from the time-frequency spectrum and calculates the evolution of the mode frequency by solving the shortest path.This method addresses the limitations posed by the lack of data channels and the disturbance of noise in the estimation of mode frequency and yields much better results compared to traditional methods.It is demonstrated that,using this method,the evolution of an unlocked tearing mode was more accurately calculated on the J-TEXT tokamak.This method remains feasible even with a low signal-to-noise ratio of 0.5,as shown by its uncertainty.Furthermore,with appropriate parameters,this method can be applied to not only signals with MHD modes,but also to general signals with continuous modes.

Overview of the J-TEXT progress on RMP and disruption physics

The J-TEXT tokamak has been operated for ten years since its first plasma obtained at the end of 2007. The diagnostics development and main modulation systems, i.e. resonant magnetic perturbation （RMP） systems and massive gas injection （MGI） systems, will be introduced in this paper. Supported by these efforts, J-TEXT has contributed to research on several topics, especially on RMP physics and disruption mitigation. Both experimental and theoretical research show that RMP could lock, suppress or excite the tearing modes, depending on the RMP amplitude, frequency difference between RMP and rational surface rotation, and initial stabilities. The plasma rotation, particle transport and operation region are influenced by the RMP. Utilizing the MGI valves, disruptions have been mitigated with pure He, pure Ne, and a mixture of He and Ar （9：1）. A significant runaway current plateau could be generated with moderate amounts of Ar injection. The RMP has been shown to suppress the generation of runaway current during disruptions.

Toroidal component of velocity for geodesic acoustic modes in the edge plasmas of the J-TEXT tokamak

The toroidal component of the velocity for geodesic acoustic modes(GAMs)is first demonstrated.Multiple Langmuir probe arrays set up near the top tokamak of the J-TEXT were utilized for this study.A significant peak at the GAM frequency is observed in Mach number fluctuations.The toroidal velocity for the GAMs is estimated as 10–100 ms-1 and increases with the poloidal velocity.The ratio of toroidal component to the poloidal one of the velocity is mainly located in the interval between 0.3 and 1.0.With higher safety factors q,the ratio almost does not change with decreasing the safety factor,whereas it goes up sharply at low q.The coherencies between poloidal electric fields and Mach number fluctuations in turbulence frequency bands are also evaluated,and are higher than those between radial electric fields and Mach number fluctuations.

Overview of runaway current suppression and dissipation on J-TEXT tokamak

The main works on disruption mitigation including suppression and mitigation of runaway current on the J-TEXT tokamak are summarized in this paper.Two strategies for the mitigation of runaway electron(RE) beams are applied in experiments.The first strategy enables the REs to be completely suppressed by means of supersonic molecular beam injection and resonant magnetic perturbation which can enhance RE loss,magnetic energy transfer which can reduce the electric field,and secondary massive gas injection(MGI) which can increase the collisional damping.For the second strategy,the runaway current is allowed to form but should be dissipated or soft landed within tolerance.It is observed that the runaway current can be significantly dissipated by MGI,and the dissipation rate increases with the injected impurity particle number and eventually stabilizes at 28 MA s^(-1).The dissipation rate of the runaway current can be up to 3 MA s^(-1)by ohmic field.Shattered pellet injection has been chosen as the main disruption mitigation method,which has the capability of injecting material deeper into the plasma for higher density assimilation when compared to MGI.Moreover,simulation works show that the RE seeds in the plasma are strongly influenced under different phases and sizes of 2/1 mode locked islands during thermal quench.The robust runaway suppression and runaway current dissipation provide an important insight on the disruption mitigation for future large tokamaks.

Realization of minute-long steady-state H-mode discharges on EAST

In the 2016 EAST experimental campaign,a steady-state long-pulse H-mode discharge with an ITER-like tungsten divertor lasting longer than one minute has been obtained using only RF heating and current drive,through an integrated control of the wall conditioning,plasma configuration,divertor heat flux,particle exhaust,impurity management,and effective coupling of multiple RF heating and current drive sources at high injected power.The plasma current（Ip - 0.45 MA） was fully-noninductively driven（Vloop 〈 0.0 V） by a combination of-2.5 MW LHW,-0.4 MW ECH and -0.8 MW ICRF.This result demonstrates the progress of physics and technology studies on EAST,and will benefit the physics basis for steady state operation of ITER and CFETR.

Magnetic field topology modeling under resonant magnetic perturbations on EAST

In order to understand the mechanism by which the resonant magnetic perturbation (RMP)mitigates or suppresses the edge-localized mode (ELM),the topological study of the edge magnetic field in ELM mitigation or suppression phase is a critical issue.To model the three-dimensional magnetic field topology superposed RMP on Experimental Advanced Superconducting Tokamak,a numerical model using the field line tracing method for both vacuum and ideal plasma response approximations is proposed.Using the numerical model,the topological change and the penetration depth of the stochastic field lines in the edge magnetic field are studied in an RMP experiment.Comparing profiles of minimum9 on edge stochastic field lines and the particle flux pattem,the ideal plasma response changes the field line penetration depth while remaining similar profile relative to vacuum approximation.To mitigate and suppress ELM strongly,the deep penetration of RMP fields and topological changes of the edge magnetic field is a key from our modeling.

Study of the spectrum effect on the threshold of resonant magnetic perturbation penetration on J-TEXT

The spectrum effect on the penetration of resonant magnetic perturbation(RMP) is studied with upgraded in-vessel RMP coils on J-TEXT.The poloidal spectrum of the RMP field,especially the amplitudes of 2/1 and 3/1 components,can be varied by the phase difference between the upper and lower coil rows,ΔΦ=Φ_(top)-Φ_(bottom),where Φ_(top)and Φ_(bottom)are the toroidal phases of the n=1 field of each coil row.The type of RMP penetration is found to be related to ΔΦ,including the RMP penetration of either 2/1 or 3/1 RMP and the successive penetrations of 3/1 RMP followed by the 2/1 RMP.For cases with penetration of only one RMP component,the penetration thresholds measured by the corresponding resonant component are close for variousΔΦ.However,the 2/1 RMP penetration threshold is significantly reduced if the 3/1 locked island is formed in advance.The changes in the rotation profile due to 3/1 locked island formation could partially contribute to the reduction of the 2/1 thresholds.

Effect of edge magnetic island on carbon screening in the J-TEXT tokamak

The effect of externally applied resonant magnetic perturbation(RMP)on carbon impurity behavior is investigated in the J-TEXT tokamak.It is found that the m/n=3/1 islands have an impurity screening effect,which becomes obvious while the edge magnetic island is generated via RMP field penetration.The impurity screening effect shows a dependence on the RMP phase with the field penetration,which is strongest if the O point of the magnetic island is near the low-field-side(LFS)limiter plate.By combining a methane injection experimental study and STRAHL impurity transport analysis,we found that the variation of the impurity transport dominates the impurity screening effect.The impurity diffusion at the inner plasma region(r/a<0.8)is enhanced with a significant increase in outward convection velocity at the edge region in the case of the magnetic island’s O point near the LFS limiter plate.The impurity transport coefficient varies by a much lower level for the case with the magnetic island’s X point near the LFS limiter plate.The interaction of the magnetic island and the LFS limiter plate is thought to contribute to the impurity transport variation with the dependence on the RMP phase.A possible reason is the interaction between the magnetic island and the LFS limiter.

Spectroscopic studies of fuel recycling and impurity behaviors in the divertor region of Wendelstein 7-X

The first divertor operation phase (OP1.2a) was carried out on Wendelstein 7-X in the second half of 2017. Fuel recycling and impurity behaviors in the divertor region were investigated by employing a newly built ultraviolet–visible–near infrared overview spectroscopy system. The characteristic spectral lines of the working gases (hydrogen and helium), intrinsic impurities (carbon, oxygen and iron), and seeded impurities (neon and nitrogen) were identified and analyzed. The divertor electron temperature and density were measured using He I (667.8, 706.5, and 728.1 nm) line intensity ratios. The Hα(656.3 nm), He I (587.6 nm), C II (514.5 nm), and O I (777.2 nm) emissions were investigated over a wide range of operating conditions. The results showed that fuel and impurity emissions in the divertor region exhibit a strong dependence on magnetic topology and plasma conditions. The levels of Hα, He I, C II, and O I emissions are all reduced moving from the standard configuration to the high mirror configuration, and even further reduced for the high iota configuration, which is associated with decreasing connection length in these island divertor configurations. The H/He influx ratio shows that the plasma is a mixture of helium and hydrogen. The neutral and impurity influxes from the divertor target tend to increase with increasing divertor electron temperature.

Study of the fast electron behavior in electron cyclotron current driven plasma on J-TEXT

In J-TEXT tokamak,fast electron bremsstrahlung diagnostic with 9 chords equipped with multichannel analyzer enables detailed studies of the generation and transport of fast electrons.The spatial profiles and energy spectrum of the fast electrons have been measured in two ECCD cases with either on-axis or off-axis injection,and the profiles processed by Abel-inversion are consistent with the calculated power deposition locations.Moreover,it is observed that the energy of fast electrons increases rapidly after turning off the ECCD,which may be attributed to the acceleration by the recovered loop voltage at low electron density.

Realization of divertor configuration discharge in J-TEXT tokamak

To extend the operation region of the Joint-Texas Experimental tokamak(J-TEXT) to the divertor configuration and even the H-mode,the divertor configuration discharge has been realized for the first time in the J-TEXT tokamak.Along with the establishment of a power supply for the divertor configuration,the construction of relevant diagnostics,and the installation of the divertor target on the high-field side,divertor discharge has been tested.Through the equilibrium calculation and position stability analysis,the control strategy has evolved to be more stable.High-density experiments and auxiliary heating experiments have been carried out on the divertor configuration.The special midplane single-null(MSN) divertor configuration is shown to be more stable than the limiter configuration in the density limit condition and can reach a higher density in the experiment.In the ECRH experiment,the power injection enhances the electron temperature and density,while more heat outflux is loaded on the divertor target tiles and causes more intensive recycling and impurity release.The future plan for the divertor configuration operation in the J-TEXT tokamak is also included.