Closed corner divertor with B×▽B away from the divertor:a promising divertor scenario for tokamak power exhaust

A major challenge facing the steady-state operation of tokamak fusion reactors is to develop a viable divertor solution with order-of-magnitude increase in power handling capability as compared with present experience.A recently developed divertor concept for this end has been tested recently on EAST tokamak through combining the effects of a closed divertor corner and E×B drifts.The E×B drifts in the divertor move particles towards the outer divertor corner area in the scrape-off layer for B×▽B directed away from the divertor,which can significantly enhance the particle concentration there,facilitating divertor detachment.In recent EAST experiments,the effects have been demonstrated where the lowest electron temperature at the divertor plate is obtained with strike point located close to the corner in the horizontal target and with B×▽B away from the divertor.These experimental results are in reasonable agreement with SOLPS-ITER simulations including drift effects,suggesting that the new divertor concept potentially provides a promising divertor solution for long-pulse operations of future tokamak fusion reactors with much higher power fluxes.

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.

A drift-kinetic perturbed Lagrangian for low-frequency nonideal MHD applications

We find that the perturbed Lagrangian derived from the drift-kinetic equation in[Porcelli F et al 1994 Phys.Plasmas 1470]is inconsistent with the ordering for the low-frequency large-scale magnetohydrodynamic(MHD).Here,we rederive the expression for the perturbed Lagrangian within the framework of nonideal MHD using the ordering system for the low-frequency largescale MHD in a low-beta plasma.The obtained perturbed Lagrangian is consistent with Chen's gyrokinetic theory[Chen L and Zonca F 2016 Rev.Mod.Phys.88015008],where the terms related to the field curvature and gradient are small quantities of higher order and thus negligible.As the perturbed Lagrangian has been widely used in the literature to calculate the plasma nonadiabatic response in low-frequency MHD applications,this finding may have a significant impact on the understanding of the kinetic driving and dissipative mechanisms of MHD instabilities and the plasma response to electromagnetic perturbations in fusion plasmas.

Presheath formation and area design limit satellite-based Langmuir probes

In this article,the effect of the finite conductive surface area of a satellite on the use of satellite-based Langmuir probes is reviewed in light of the basic theory of asymmetric double Langmuir probes(ADLPs).Recent theoretical and experimental studies have discussed electron sheath/presheath formation and the electron Bohm criterion along with their implications for satellite-based Langmuir probes.The effects predicted by the latest theory of the electron Bohm criterion were not experimentally observed and the experimental results remain supportive of a critical area ratio(A_(L)/A_(S))_(crit)=(m_(i)/(2.3m_(e)))^(1/2)between the probe area A_(S)and the satellite area A_L as conventionally believed.A satellite-based Langmuir probe must satisfy this criterion to physically act as a single Langmuir probe.However,experimental investigations also found that high-energy electrons adversely affect(A_(L)/A_(S))_(crit)and a Langmuir probe's signal quality by giving additional electron current to A_(L).Based on these results,a number of limitations of the maximum probe area are derived when designing satellite-based Langmuir probes,with consideration of both the aim of the satellite and the plasma where the satellite-based probe works.These proposed measures are expected to only partially alleviate the effect of the inadequate satellite surface area on the application of satellite-based Langmuir probes.Using a larger satellite to carry a Langmuir probe remains the most viable means to obtain precise space plasma parameters.

Numerical simulation of ultrashort-pulse reflectometry(USPR)on EAST

The microwave reflectometer is a popular non-intrusive plasma density diagnostic instrument on tokamaks that provides centimeter and millisecond level resolution.The ultrashort-pulse reflectometer(USPR)achieves plasma density measurement by emitting a chirped wave containing a broadband signal and measuring the time of flight from different frequency components.A USPR system is currently being built on EAST(Experimental Advanced Superconducting Tokamak)to meet the needs of diagnostic of the pedestal density evolution,such as high-frequency small edge-localized modes.In order to predict the density reconstruction of the EAST USPR system,this work presents a numerical simulation study of the beam propagation of the chirped wave of extraordinary waves(X-mode)in the plasma based on Python.The electron density profile has been successfully reconstructed by the reflection signal interpretation.The small gap between the right-hand cut-off layer and the electron cyclotron resonance layer,due to the low plasma density on the plasma edge,causes unexpected leakage from the transmitting microwave beam to the pedestal and the core region.This kind of‘tunneling’effect will cause the reflected signal to have energy loss in the low-frequency band.The study also discusses the influence of the poloidal magnetic field on the reflected signal.The spatial variation of the poloidal magnetic field will lead to the conversion between extraordinary(X)waves and ordinary(O)waves,which leads to energy loss in the reflected signals.The simulation results show that the‘tunneling’effect and the O-X mode conversion effect have little effect on the EAST USPR system.Therefore,the currently designed transmit power meets the working requirements.

Realization of T_(e0)>10 keV long pulse operation over 100 s on EAST

In 2021,EAST realized a steady-state long pulse with a duration over 100 s and a core electron temperature over 10 keV.This is an integrated operation that resolves several key issues,including active control of wall conditioning,long-lasting fully noninductive current and divertor heat/particle flux.The fully noninductive current is driven by pure radio frequency(RF)waves with a lower hybrid current drive power of 2.5 MW and electron cyclotron resonance heating of 1.4 MW.This is an excellent experimental platform on the timescale of hundreds of seconds for studying multiscale instabilities,electron-dominant transport and particle recycling(plasma-wall interactions)under weak collisionality.

Superposition model for analyzing the dynamic ground subsidence in mining area of thick loose layer

The dynamic ground subsidence due to underground mining is a complicated time-dependent and ratedependent process. Based on the theory of rock rheology and probability integral method, this study developed the superposition model for the prediction and analysis of the ground dynamic subsidence in mining area of thick loose layer. The model consists of two parts(the prediction of overlying bedrock and the prediction of thick loose layer). The overlying bedrock is regarded as visco-elastic beam, of which the dynamic subsidence is predicted by the Kelvin visco-elastic rheological model. The thick loose layer is regarded as random medium and the ground dynamic subsidence is predicted by the probability integral model. At last, the two prediction models are vertically stacked in the same coordinate system, and the bedrock dynamic subsidence is regarded as a variable mining thickness input into the prediction model of ground dynamic subsidence. The prediction results obtained were compared with actual movement and deformation data from Zhao I and Zhao II mine, central China. The agreement of the prediction results with the field measurements show that the superposition model(SM) is more satisfactory and the formulae obtained are more effective than the classical single probability integral model(SPIM), and thus can be effectively used for predicting the ground dynamic subsidence in mining area of thick loose layer.

Development of a compact high-density blue core helicon plasma device under 2000 G magnetic field of ring permanent magnets

A helicon wave plasma source in a tube of ring permanent magnets(PMs)has been constructed to study the effect of the conflguration of the magnetic fleld with zero magnetic points on plasma parameters.This device also serves as an exploration platform for a simple,compact helicon wave plasma source adaptable to engineering applications.A small-diameter(26 mm)highdensity(~10^(18)m^(-3))blue core plasma is produced in~1 Pa argon by helicon RF(radiofrequency)discharge using a NagoyaⅢantenna under magnetic fleld(~2 k G)of compact ring PMs(length~204 mm).Operational parameters,i.e.RF power and neutral gas pressure are scanned and plasma density is measured by an RF compensated probe to explore the operating characteristics of the device.Iconic feature of a helicon discharge,such as blue core plasmas and E-H-W mode transitions are well observed in the device,despite the wavelength calculated using the conventional dispersion relation of a bounded whistler waves(Chen 1991 Plasma Phys.Control.Fusion 33339)is order of magnitudes longer than the length of the plasma in this device which seems to suggest that such helicon device is impossible.Surprisingly,the wavelength calculated by the unbounded whistle wave dispersion formula in turn suggests the occurrence of a half wavelength resonance.

Automated electron temperature fitting of Langmuir probeⅠ-Ⅴtrace in plasmas with multiple Maxwellian EEDFs

An algorithm for automated fitting of the effective electron temperature from a planar Langmuir probe I-V trace taken in a plasma with multiple Maxwellian electron populations is developed through MATLAB coding.The code automatically finds a fitting range suitable for analyzing the temperatures of each of the electron populations.The algorithm is used to analyzeⅠ-Ⅴtraces from both the Institute of Plasma Physics Chinese Academy of Sciences's Diagnostic Test Source device and a similar multi-dipole chamber at the University of Wisconsin-Madison.Ⅰ-Ⅴtraces reconstructed from the parameters fitted by the algorithm not only agree with the measured I-V trace but also reveal physical properties consistent with those found in previous studies.Cylindrical probe traces are also analyzed with the algorithm and it is shown that the major source of error in such attempts is the disruption of the inflection point due to both decreased signal-to-noise ratio and greater sheath expansion.It is thus recommended to use planar probes with radii much greater than the plasma Debye length when signal-to-noise ratio is poor.

Gas generation from coal: taking Jurassic coal in the Minhe Basin as an example

The gas generation features of coals at different maturities were studied by the anhydrous pyrolysis of Jurassic coal from the Minhe Basin in sealed gold tubes at 50 MPa.The gas component yields(C1,C2,C3,i-C4,n-C4,i-C5,n-C5,and CO2);theδ13C of C1,C2,C3,and CO2;and the mass of the liquid hydrocarbons(C6+)were measured.On the basis of these data,the stage changes ofδ13C1,δ13C2,δ13C3,andδ13CO2 were calculated.The diagrams ofδ13C1–δ13C2 vs ln(C1/C2)andδ13C2–δ13C1 vsδ13C3–δ13C2 were used to evaluate the gas generation features of the coal maturity stages.At the high maturity evolution stage(T>527.6°C at 2°C/h),the stage change ofδ13C1 and the CH4 yield are much higher than that of CO2,suggesting that high maturity coal could still generate methane.When T<455°C,CO2 is generated by breaking bonds between carbons and heteroatoms.The reaction between different sources of coke and water may be the reason for the complicated stage change inδ13CCO2 when the temperature was higher than 455°C.With increasing pyrolysis temperature,δ13C1–δ13C2 vs ln(C1/C2)has four evolution stages corresponding to the early stage of breaking bonds between carbon and hetero atoms,the later stage of breaking bonds between carbon and hetero atoms,the cracking of C6+and coal demethylation,and the cracking of C2–5.Theδ13C2–δ13C1 vsδ13C3–δ13C2 has three evolution stages corresponding to the breaking bonds between carbon and hetero atoms,demethylation and cracking of C6+,and cracking of C2–5.

A new model of the L–H transition and H-mode power threshold

In order to understand the mechanism of the confinement bifurcation and H-mode power threshold in magnetically confined plasma,a new dynamical model of the L-H transition based on edge instability phase transition（EIPT） has been developed.With the typical plasma parameters of the EAST tokamak,the self-consistent turbulence growth rate is analyzed using the simplest case of pressure-driven ballooning-type instability,which indicates that the L-H transition can be caused by the stabilization of the edge instability through EIPT.The weak E？×？B flow shear in L-mode is able to increase the ion inertia of the electrostatic motion by increasing the radial wave number of the tilted turbulence structures,which play an important role for accelerating the trigger process of EIPT rather than directly to suppress the turbulent transport.With the acceleration mechanism of E？×？B flow shear,fast L-H and H-L transitions are demonstrated under the control of the input heating power.Due to the simplified scrape-offlayer boundary condition applied,the ratio between the heating powers at the H-L and L-H transition respectively differs from the ratio by Nusselt number.The results of the modeling reveal a scaling of the power threshold of the L-H transition,P_（L-H）？∝？n^（0.76） B^（0.8） for deuterium plasma.It is found finite Larmor radius induces an isotope effect of the H-mode power threshold.

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.

Adaptive Sliding Mode Control for the Excavator Manipulator

An adaptive sliding mode control system for the excavator manipulator during straight line digging operation was presented in this paper.The adaptive laws derived from Lyapunov stability theorem were used to update switching gains and compensate uncertainties.The dynamic model of the excavator manipulator considering link shape and weight was established and the straight line trajectory of the bucket tooth tip of excavator was transformed into the desired joint angle trajectory for direct control.Finally,simulation results showed the superiority of the proposed control scheme compared with traditional variable structure control with sliding mode.

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.

Experimental investigation of scrape-off layer blob high density transition in L-mode plasmas on EAST

Lithium Beam Emission Spectroscopy systems in the outer midplane and divertor Langmuir probe arrays embedded in the divertor target plates,are utilized to investigate the scrape-off layer(SOL)blob transition and its relation with divertor detachment on EAST.The blob transition in the near SOL is observed during the density ramp-up phase.When the plasma density,normalized to the Greenwald density limit,exceeds a threshold of f_(GW)~0.5,the blob size and lifetime increases by 2–3 times,while the blob detection rate decreases by about 2 times.In addition,a weak density shoulder is observed in the near SOL region at the same density threshold.Further analysis indicates that the divertor detachment is highly correlated with the blob transition,and the density threshold of blob transition is consistent with that of the access to the outer divertor detachment.The potential physical mechanisms are discussed.These results could be useful for the understanding of plasma-wall interaction issues in future devices that will operate under a detached divertor and high density conditions(over the blob transition threshold).

Plasma edge density fluctuation measurements via lithium beam emission spectroscopy on EAST

Accurate and stable measurements of edge density fluctuation with high spatio-temporal resolution have been achieved by the lithium beam emission spectroscopy(Li-BES)diagnostic on experimental and advanced superconducting tokamak(EAST).The new narrower band interference filter exhibits good ability to suppress background emission signal even under strong lithium coating of the tokamak.The raw data measured by channels at different spatial locations in avalanche photo diode camera with high chopping frequency show good consistency.Based on the detected experimental data,detailed information of density profile and fluctuation structures is obtained.A clear edge coherent mode in the auto-power spectrum is observed in pedestal region,which is regarded as the dominant factor for the strong pedestal density fluctuation amplitude.The cross-power spectrum analysis further excludes the additional effects of common-mode noises and non-local perturbation,demonstrating that the detected fluctuation is only caused by local density fluctuation.The normalized radial and poloidal wave-number spectra can specify the quantitative changes of radial wavenumber(k_(r))and poloidal wavenumber(k_(θ)) during the pedestal density fluctuation phase.This new Li-BES system,which can provide more accurate experimental data,allows further studies of edge density fluctuation and complex transport process on EAST.

Comparison of natural grassy ELM behavior in favorable/unfavorable Bt in EAST

The properties of grassy edge-localized modes(ELMs)in EAST in the favorable and unfavorable Btare statistically studied.Statistical analysis indicates that there is no systematical difference in the frequencies of grassy ELMs under the two different magnetic configurations in the similar parameter spaces.The high-frequency grassy ELM(fELM>1 k Hz)in unfavorable Bt is dependent on the high poloidal betaβpand high triangularityδu,while the high-frequency grassy ELM(fELM>1k Hz)in favorable Btappears to rely on the high plasma density.A frequently occurring phenomenon in favorable Btdefined as‘clustered ELM’seems to be the most evident difference in ELM behavior between favorable and unfavorable Bt.Statistical analysis shows that larger plasma-wall outer gap,longer plasma elongation,lower low-hybrid wave heating power and electron cyclotron resonance heating power favor the occurrence of clustered grassy ELMs.Further studies indicate that the generation of clustered grassy ELMs could be correlated with the lower electron temperature in the bulk plasma.

Combined Langmuir-magnetic probe measurements of type-Ⅰ ELMy filaments in the EAST tokamak

Detailed investigations on the filamentary structures associated with the type-I edge-localized modes（ELMs） should be helpful for protecting the materials of a plasma-facing wall on a future large device.Related experiments have been carefully conducted in the Experimental Advanced Superconducting Tokamak（EAST） using combined Langmuir-magnetic probes.The experimental results indicate that the radially outward velocity of type-I ELMy filaments can be up to 1.7 kms^（-1） in the far scrape-off layer（SOL） region.It is remarkable that the electron temperature of these filaments is detected to be ~50eV,corresponding to a fraction of 1/6 to the temperature near the pedestal top,while the density ~3×10^（19）m^（-3） of these filaments could be approximate to the line-averaged density.In addition,associated magnetic fluctuations have been clearly observed at the same time,which show good agreement with the density perturbations.A localized current on the order of ~100kA could be estimated within the filaments.

Ensemble Strategy for Insider Threat Detection from User Activity Logs

In the information era,the core business and confidential information of enterprises/organizations is stored in information systems.However,certain malicious inside network users exist hidden inside the organization;these users intentionally or unintentionally misuse the privileges of the organization to obtain sensitive information from the company.The existing approaches on insider threat detection mostly focus on monitoring,detecting,and preventing any malicious behavior generated by users within an organization’s system while ignoring the imbalanced ground-truth insider threat data impact on security.To this end,to be able to detect insider threats more effectively,a data processing tool was developed to process the detected user activity to generate information-use events,and formulated a Data Adjustment(DA)strategy to adjust the weight of the minority and majority samples.Then,an efficient ensemble strategy was utilized,which applied the extreme gradient boosting(XGBoost)model combined with the DA strategy to detect anomalous behavior.The CERT dataset was used for an insider threat to evaluate our approach,which was a real-world dataset with artificially injected insider threat events.The results demonstrated that the proposed approach can effectively detect insider threats,with an accuracy rate of 99.51%and an average recall rate of 98.16%.Compared with other classifiers,the detection performance is improved by 8.76%.

Dependence of the internal inductance on the radial distance between the primary and secondary X-point surfaces in the EAST tokamak

In view of their advantage in long-pulse high-performance operations, low internal inductance scenarios are being developed and tested for the EAST and other tokamaks. Dependence of the internal inductance on several main plasma configuration parameters are statistically analyzed. It is found that the internal inductance of the plasma is closely related to the radial distance dRsep between the primary and secondary X-point surfaces on the outer mid-plane as well as the poloidal field of the last closed flux surface. Moreover, dRsep is alSO related to the distribution of the fast electrons driven by the injected lower-hybrid waves, which is partially responsible for the observed relation between dRsep and the internal inductance. The results here should therefore be helpful as a guide for future experiments on internal inductance control and long-pulse operations of the EAST and other tokamaks, as well as for detailed theoretical study of the underlying physics.