Experimental confirmation of the linear relation between plasma current and external vertical magnetic field in EXL-50 spherical torus energetic electron plasmas

A three-fluid equilibrium plasma with bulk plasma and energetic electrons has been observed on the Xuanlong-50(EXL-50) spherical torus, where the energetic electrons play a crucial role in sustaining the plasma current and pressure. In this study, the equilibrium of a multi-fluid plasma was investigated by analyzing the relationship between the external vertical magnetic field(B_(V)),plasma current(I_(p)), the poloidal ratio(β_(p)) and the Shafranov formula. Remarkably, our research demonstrates some validity of the Shafranov formula in the presence of multi-fluid plasma in EXL-50 spherical torus. This finding holds significant importance for future reactors as it allows for differentiation between alpha particles and background plasma. The study of multi-fluid plasma provides a significant reference value for the equilibrium reconstruction of burning plasma involving alpha particles.

Effects of sawtooth heat pulses on edge flows and turbulence in a tokamak plasma

Enhancements of edge zonal flows,radial electric fields,and turbulence are observed in electron cyclotron resonance heating-heated plasmas(Zhao et al 2013 Nucl.Fusion 53083011).In this paper,the effects of sawtooth heat pulses on flows and turbulence are presented.These experiments are performed using multiple Langmuir probe arrays in the edge plasmas of the HL-2A tokamak.The edge zonal flows,radial electric fields,and turbulence are all enhanced by sawteeth.Propagation of the zonal flow and turbulence intensities is also observed.The delay time of the maximal intensity of the electric fields,zonal flows,and turbulence with respect to the sawtooth crashes is estimated as~1 ms and comparable to that of the sawtooth-triggered intermediate phases.Not only the zonal flows but also the radial electric fields lag behind the turbulence.Furthermore,the intensities of both the zonal flows and electric fields nearly linearly increase/decrease with the increase/decrease of the turbulence intensity.A double-source predator-prey model analysis suggests that a relatively strong turbulence source may contribute to the dominant zonal flow formation during sawtooth cycles.

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.

Influence of stationary driven helical current on the m=2/n=1 resistive tearing mode

The influence of stationary driven helical current on tearing mode instability in the m=2/n=1 rational surface is explored numerically using resistive magnetohydrodynamic simulation in cylindrical geometry.The results indicate that the flip instabilities result from the sustained injection of the sufficiently strong helical current driven in the island O-point.The driven helical current induces high order harmonics of instabilities due to the delay of suppressing timing and the increase of its current intensity.With the appropriate current density values,the development of the perturbed kinetic energy can be limited and the occurrence of the flip instabilities can be delayed for a long time.The radial deviation of the current deposition can lead to poor inhibition effect,and the effect of current bias on the boundary is greater than that on the axis.

Dynamics of oscillatory plasma flows prior to the H-mode in the HL-2A tokamak

This paper discusses edge oscillatory plasma flows, geodesic acoustic mode （GAM） and limit cycle oscillations （LCOs）, which have been measured by Doppler reflectometry prior to the high confinement mode （H-mode） in the HL-2A tokamak. The complex relations between the flows and background turbulence have been analyzed. It was observed that the GAM and LCO coexist, and these two flows and turbulence have strong nonlinear interactions during the intermediate confinement phase （I-phase）. Dynamics of the shear flows and turbulence prior to the H-mode shows that the oscillatory flows quench the turbulence along with the increase of the mean E x B flow at the early stage of the I-phase, then the oscillatory flows are damped and the further increased mean flow takes over the role in turbulence suppression. The reduced turbulent transport results in the formation of a steep edge transport barrier. It suggests that the oscillatory flows can initiate the L-H transition through providing a positive feedback for the increase of the mean E × B flow strength.

Upgrade of an integrated Langmuir probe system on the closed divertor target plates in the HL-2A tokamak

A newly designed divertor Langmuir probe diagnostic system has been installed in a rare closed divertor of the HL-2A tokamak and steadily operated for the study of divertor physics involved edge-localized mode mitigation,detachment and redistribution of heat flux,etc.Two sets of probe arrays including 274 probe tips were placed at two ports(approximately 180°separated toroidally),and the spatial and temporal resolutions of this measurement system could reach6 mm and 1μs,respectively.A novel design of the ceramic isolation ring can ensure reliable electrical insulation property between the graphite tip and the copper substrate plate where plasma impurities and the dust are deposited into the gaps for a long experimental time.Meanwhile,the condition monitoring and mode conversion between single and triple probe of the probe system could be conveniently implemented via a remote-control station.The preliminary experimental result shows that the divertor Langmuir probe system is capable of measuring the high spatiotemporal parameters involved the plasma density,electron temperature,particle flux as well as heat flux during the ELMy H-mode discharges.

Magnetic-island-induced ion temperature gradient mode： Landau damping, equilibrium magnetic shear and pressure flattening effects

Characteristics of the magnetic-island-induced ion temperature gradient （MITG） mode are studied through gyrofluid simulations in the slab geometry, focusing on the effects of Landau damping, equilibrium magnetic shear （EMS）, and pressure flattening. It is shown that the magnetic island may enhance the Landau damping of the system by inducing the radial magnetic field. Moreover, the radial eigenmode numbers of most MITG poloidal harmonics are increased by the magnetic island so that the MITG mode is destabilized in the low EMS regime. In addition, the pressure profile flattening effect inside a magnetic island hardly affects the growth of the whole MITG mode, while it has different local effects near the O-point and the X-point regions. In comparison with the non-zero-order perturbations, only the quasi-linear flattening effect due to the zonal pressure is the effective component to impact the growth rate of the mode.

Effects of trapped electrons on the ion temperature gradient mode in tokamak plasmas with hollow density profiles

The ion temperature gradient(ITG)mode in the presence of impurity ions and trapped electrons(TEs)is numerically investigated in tokamak plasmas with hollow density profiles,using the gyrokinetic integral eigenmode equation.It is found that in the inverted density plasma,the increase of the ITG enhances the growth rate and frequency of the ITG,and the density gradient plays an important role in the ITG modes.For weak density gradient situations,the trapped electron effects increase the instability of the ITG,while the impurity has an obviously stabilizing effect.For the strong density gradient cases,both the impurities and trapped electrons enhance the ITG instabilities.In addition,it is shown that the growth rate of the ITG decreases with positive magnetic shear s while the real frequency increases with positive magnetic shear.The growth rate of the ITG increases with negative magnetic shear s while the real frequency decreases with negative magnetic shear.The length of the calculated mode structure in the positive and negative magnetic shear intervals is also presented.

Kinetic micro-instabilities in the presence of impurities in toroidal magnetized plasmas

The theoretical and numerical studies on kinetic micro-instabilities,including ion temperature gradient（ITG） driven modes,trapped electron modes（TEMs） in the presence of impurity ions as well as impurity modes（IMs）,induced by impurity density gradient alone,in toroidal magnetized plasmas,such as tokamak and reversed-field pinch（RFP） are reviewed briefly.The basic theory for IMs,the electrostatic instabilities in tokamak and RFP plasmas are discussed.The observations of hybrid and coexistence of the instabilities are categorized systematically.The effects of impurity ions on electromagnetic instabilities such as ITG modes,the kinetic ballooning modes（KBMs） and kinetic shear Alfvén modes induced by impurity ions in tokamak plasmas of finite β（=plasma pressure/magnetic pressure） are analyzed.The interesting topics for future investigation are suggested.

Special issue on instabilities and nonlinear phenomena in plasmas--in memory of professor Changxuan Yu

An academician of Chinese Academy of Sciences （CAS）,Professor Changxuan Yu,a long-time professor of plasma physics at the University of Science and Technology of China （USTC）,Hefei,China,passed away in Hefei on May 23,2017.Professor Yu was born on July 7,1941,in Java Island,Indonesia,and then moved back to China in 1948 with his parents.He finished his high schooleducation in Xiamen City, Fujian Province in 1959, and was admi-tted to USTC, then located in Beijing. After graduating with excellence from USTC, which had been relocated to Hefei, Anhui Province, he stayed on as a faculty until his passing away. During this period he visited the University of California, Los Angeles, USA, from 1980 to 1982, 1989 to 1991, and 2000 to 2001 for research colla- boration. He was promoted to Assistant Professor, Associate Professor, and full Professor in 1979, 1985 and 1992, respectively, and elected as a CAS Academician in 2007 based on his outstanding achievements in plasma physics and nuclear fusion science, and great contributions to education and development of these fields in China.

Sensitivity Improvements for Picosecond Ultrasonic Thickness Measurements in Gold and Tungsten Nanoscale Films

Picosecond ultrasonics,as a nondestructive and noncontact method,can be employed for nanoscale metallic film thickness measurements.The sensitivity of the system,which determines the measurement precision and practicability of this technique,is often limited by the weak intensity of the ultrasonic signal.To solve this problem,we investigate the distinct mechanisms involved in picosecond ultrasonic thickness measurement for two types of metals,namely tungsten(W)and gold(Au).For thickness measurement in W films,theory and simulation show that optimizing the pump and probe laser wavelengths,which determine the intensity and shape of the ultrasonic signal,is critical to improving measurement sensitivity,while for Au film measurements,where acoustic-induced beam distortion is dominant,the signal intensity can be optimized by selecting an appropriate aperture size and sample position.The above approaches are validated in experiments.A dual-wavelength pump-probe system is constructed based on a passively mode-locked ytterbium-doped fiber laser.The smoothing method and multipeak Gaussian fitting are employed for the extraction of ultrasonic time-of-flight.Subnanometer measurement precision is achieved in a series of W and Au films with thicknesses of 43-750 nm.This work can be applied to various high-precision,noncontact measurements of metal film thickness in the semiconductor industry.

有机微结构诱导的分级多孔g-C_(3)N_(4)光催化剂

为了保证光催化的高光吸收和电化学动力学,构建分级多孔光催化剂十分必要.类石墨相氮化碳(g-C_(3)N_(4))易于合成、理化性质稳定、稳定性好和带隙合适等优点,特别是其可调的微/纳米结构,使其成为分级多孔光催化剂一个很好的选择.然而,具有层次孔的g-C_(3)N_(4)的简易制备仍然是一个难点.本文中,我们首次用均匀的有机微结构作为软模板,用简单方法制备了分级多孔g-C_(3)N_(4).该有机微纳结构模板是在硫脲前驱体溶液中原位形成的,与g-C_(3)N_(4)相似的π共轭结构使其能够有效修饰g-C_(3)N_(4)的分子结构.结果表明,合成的g-C_(3)N_(4)具有分级的中孔和大孔,比表面积为27.34 m^(2)g^(−1),孔体积为0.18 cm^(3)g^(−1),分别是未改性g-C_(3)N_(4)的6.2倍和9.0倍.这种分级多孔结构有利于电荷/质量传输过程,使其光降解有机污染物能力增强了2.4倍.同时,此光催化剂具有良好的光稳定性,长期使用100分钟后效率仅损失20%.

Three-dimensional meta-architecture with programmable mechanical properties

Artificial metamaterials have attracted widespread attention of research communities due to their anomalous physical properties compared to those of conventional materials.In this study,we designed a three-dimensional(3D)lightweight metaarchitecture consisting of 6-connected anti-chiral honeycombs.The mechanical properties(e.g.Young’s modulus,compression strength,and Poisson’s ratio)of the proposed meta-architecture could be programmed by adjusting a series of geometric parameters,as shown through numerical simulations.Moreover,an optically sensitive polymer-based 3D meta-architecture with 6-connected anti-chiral features was constructed by the stereolithography method.Owing to the regulation of the negative Poisson’s ratio,3D meta-architecture achieved a greater ductility under compression than those of traditional truss structures while retaining a relatively high strength and low density.Compression experiments validated the excellent tunability of the mechanical properties of the proposed 3D 6-connected antichiral structure.The results suggest the promising applications of this structure in lightweight aircraft,vibration isolation,and mechanical sensors.

Recent advances in high-β_(N) experiments and magnetohydrodynamic instabilities with hybrid scenarios in the HL-2A Tokamak

Over the past several years,high-β_(N) experiments have been carried out on HL-2A.The high-β_(N) is realized using double transport barriers(DTBs)with hybrid scenarios.A stationary high-β_(N) (>2)scenario was obtained by pure neutral-beam injection(NBI)heating.Transient high performance was also achieved,corresponding to β_(N)≥3,ne/ne_(G)∼0.6,H_(98)∼1.5,f_(bs)∼30%,q_(95)∼4.0,and𝐺∼0.4.The high-β_(N) scenario was successfully modeled using integrated simulation codes,that is,the one modeling framework for integrated tasks(OMFIT).In high-𝛽𝑁plasmas,magnetohydrodynamic(MHD)instabilities are abundant,including low-frequency global MHD oscilla-tion with n=1,high-frequency coherent mode(HCM)at the edge,and neoclassical tearing mode(NTM)and Alfvénic modes in the core.In some high-β_(N) discharges,it is observed that the NTMs with m/n=3/2 limit the growth of the plasma energy and decrease β_(N).The low-n global MHD oscillation is consistent with the coupling of destabilized internal(m/n=1/1)and external(m/n=3/1 or 4/1)modes,and plays a crucial role in triggering the onset of ELMs.Achieving high-β_(N) on HL-2A suggests that core-edge interplay is key to the plasma confinement enhancement mechanism.Experiments to enhance β_(N) will contribute to future plasma operation,such as international thermonuclear experimental reactor.

Organic photocatalysts:From molecular to aggregate level

Organic semiconductors(OSCs)have the advantages of tunable molecular structures,suitable band gaps,and exceptional optoelectronic properties.The π-π stacking ability of OSCs also leads to appealing molecular stacking structure,function,and stability.So far,organic photocatalysts have engaged in homogeneous or heterogeneous photocatalysis in the form of free molecules,supported molecules,or nanostructures.Meanwhile,researches on organic photocatalysts have expanded from small organic molecules to the organic macromolecules,as well as their various nanostructures and nanocomposites including isolated zero-dimensional(0D),one-dimensional(1D),two-dimensional(2D),three-dimensional(3D)nanostructures,and their combinations.Therefore,many versatile strategies have been explored to improve photocatalytic ability and practicality either from molecular synthetic modification,crystal,or interface engineering.In this review,we first discuss the photophysical and photochemical processes of organic photocatalysts that govern the ultimate photocatalytic efficiency;we then summarize different forms of organic photocatalysts,their rational design strategies,and mechanistic pathways,as well as their applications in H_(2) evolution,CO_(2) reduction,and environmental purification,aiming to highlight the structure/property relationships;we lastly propose ongoing directions and challenges for future development of organic photocatalysts in real use.

Organic semiconductor nanostructures:optoelectronic properties,modification strategies,and photocatalytic applications

Organic semiconductors(OSCs)possess diverse chemical structures and tailored optoelectronic properties via simple chemical modifications,so increasing use of them are found in efficient visible-light photo-catalysis.However,the weak chemical bonds and the poor charge behavior(e.g.,low concentration of free charge carriers,low carrier mobility)intrinsic in them,always incur quite limited stability and efficiency.Therefore,the assembly of them into refined nanostructures or nanocomposites is usually proposed to enhance their optoelectronic properties,as well as the photocatalytic efficiency and reliability.Zero-dimensional(0D)nanoparticles are low in size and hence high specific surface area(SSA);One-dimensional(1D)nanostructures are usually arranged in an orderly long range thus leading to low surface defect density and increased carrier mobility;Two-dimensional(2D)nanostructures are particularly capa-ble of enhancing the photogenerated charge utilization because of their large reaction sites and shortened charge transport length.Furthermore,the building of heterogeneous interfaces in the nanocomposites can effectively facilitate the special charge separation.All these highlight the importance of organic nanos-tructures in improving the photocatalytic activity and stability.Therefore,organic semiconductor nanostructures(OSNs)have been increasingly used in the photocatalytic water splitting into H_(2) and O_(2),CO_(2) reduction,pollutant decomposition,disinfection,etc.In this review,we first examine the important optoelectronic properties of OSNs that govern the photocatalytic processes;we then analyze different classes of OSNs and their mechanistic pathways,with an emphasis on the structure-property relationships;we also introduced various photocatalytic applications of OSNs;we lastly propose the challenges and future outlook in real use.

Continuous measurement method and mathematical model for soil compactness

With the continuous improvement of agricultural mechanization,soil compaction becomes more and more serious.Serious soil compaction has been considered as an important negative factor affecting crop growth and yield.The measurement of soil compactness is a common method to measure the soil compaction level.In order to solve the problems of discontinuous sampling,time-consuming and poor real-time soil compactness measurement,a real-time measurement method of soil compactness based on fertilizing shovel was proposed,and the mathematical model between fertilizing shovel arm deformation and soil compactness was established.Based on the interaction mechanism between fertilizing shovel and soil,through the force analysis of fertilizing shovel,it was found that the deformation of fertilizing shovel arm was positively correlated with the sum of soil compactness(SSC)within the range of tillage depth.In order to verify the theoretical analysis results and the detection accuracy of strain gauge,the static bench test was carried out.The test results showed that the strain gauge signal for measuring the deformation of the fertilizing shovel arm was significantly correlated with the applied force.The fitting curve of the linear correlation coefficient was 0.999,the maximum detection error was 0.68 kg,and the detecting accuracy was within the tolerance of 0.57%.Through field orthogonal experiments with four working depths and four compaction levels,a mathematical model of the strain gauge signal and the SSC within the range of tillage depth was established.The experiment showed that compared with the other three depths,the linear correlation coefficient at the tillage depth of 5 cm(TD5)was the lowest,and the slope of the fitting curve was obviously different from the other three depths,so the 5 cm data were excluded when modeling.The model between mean signal value and mean SSC within the range of tillage depth was established based on the data of sampling points with tillage depths of 7.5 cm(TD7.5),10 cm(TD10),and 12.5 cm(TD12.5).The linear correlation coefficient(R^(2))of the model between mean signal value and mean SSC which eliminated 5 cm data was 0.980 and the root mean square error(RMSE)was 143.57 kPa.Compared with the linear model before averaging,the R^(2) was improved by 8.65%,and the RMSE was reduced by 52.39%.This system can realize the real-time and continuous measurement of soil compactness and provide data support for follow-up intelligent agricultural operations.