Shear Gradient Research Articles

Manipulating the percolated network of nanorods in polymer matrix by adding non-conductive nanospheres: A molecular dynamics simulation

A coarse-grained model is used to investigate how the added non-conductive nanospheres (NS) influence the network formation of nanorods (NR) in polymer matrix. First, an algorithm is developed to calculate the formation probability of the percolated network of NRs. Then based on it, conductive probability is obtained which presents a gradual increase with decreasing the NS size. It can be rationalized by analyzing the improved effective volume fraction and the optimized percolated network of NRs, which is induced by NS. Meanwhile, conductive probability shows an initial drop and then rises with enhancing the NR-NS interaction. This is because the connection behavior of NRs can be affected by tuning the NR-NS interaction, which can effectively influence the percolated network formation of NRs. It is noted that shear flow produces a significant influence on the percolated network, which leads to the similar connection behavior of NRs. Meanwhile, conductive probability initially rises and next goes down with increasing the NS volume fraction. NSs can be beneficent to rebuild the percolated network of NRs at the low volume fraction under the shear flow. Interestingly, the appeared phase separation between NRs and NSs breaks the percolated network down along the shear gradient direction at the high NS volume fraction. In addition, the NS volume fraction is similar at the maximum conductive probability for small NS sizes. Meanwhile, the maximum conductive probability initially rises and next declines with improving the NS size. This can be explained well by the balance between number and size of NSs.

Relative Effects of the Greenhouse Gases and Stratospheric Ozone Increases on Temperature and Circulation in the Stratosphere over the Arctic

Using a stratosphere-resolving general circulation model, the relative effects of stratospheric ozone and greenhouse gases (GHGs) increase on the temperature and circulation in the Arctic stratosphere are examined. Results show that stratospheric ozone or GHGs increase alone could result in a cooling and strengthening extratropical stratosphere during February, March and April. However, the contribution of stratospheric ozone increases alone on the cooling and strengthening Arctic stratosphere is approximately 2 fold that of the GHGs increase alone. Model simulations suggested that the larger responses of the Arctic stratosphere to the ozone increase alone are closely related to the wave fluxes in the stratosphere, rather than the wave activity in the stratosphere. In response to the ozone increase, the vertical propagation of planetary waves from the troposphere into the mid-latitude stratosphere weakens, mainly contributed by its wavenumber-1 component. The impeded planetary waves tend to result from the larger zonal wind shear and vertical gradient of the buoyancy frequency. The magnitudes of anomalies in the zonal wind shear and buoyancy frequency in response to GHGs increase alone are smaller than in response to the ozone increase, which is in accordance with the larger contribution of stratospheric ozone to the temperature and circulation in the Arctic stratosphere.

Analysis of the Optical Turbulence Model Using Meteorological Data

The model of atmosphere optical turbulence is important in the research field of laser atmospheric transmission, and plays a key role in astronomical site selection. In this paper, the single and overall statistical analysis between different outer scale models (HMNSP99 and the Dewan model) were conducted and the results show that the HMNSP99 model has better performance with the lowest bias, root mean square error, and center root mean square error. The results of the statistical analysis of three turbulence parameters revealed that there is a correlation between turbulence parameters and statistical operators, where statistical operators increase significantly when wind shear and temperature gradient respectively exceed 0.016 s−1, 0 K/m, and the outer scale is within 2.5 m. Furthermore, a new statistical outer-scale model, the WSTG model, is proposed and the results of statistical analysis present that the WSTG model is more reliable than the HMNSP99 model in reconstructing optical turbulence strength. These results acquired from this paper add substantially to our understanding of atmosphere optical turbulence and the conclusions can be applied to improve the performance of an adaptive optics system and astronomical site selection.

Effect of electron temperature anisotropy on TEM in reversed-field-pinch plasmas

For the first time in the reversed-field-pinch configuration, trapped electron mode (TEM) with anisotropies of electron temperature and its gradient is studied by solving the gyrokinetic integral eigenmode equation. Detailed numerical analyses indicate that TEM is enhanced by the anisotropy with temperature in the direction perpendicular to the magnetic field that is higher than that in the direction parallel to the magnetic field when the latter is kept constant. However, the enhancement is limited, such that TEM is weakened and even stabilized when the anisotropy is higher than a critical value, due to strong Landau damping. In comparison with the isotropic case, the lower Landau damping with the higher parallel electron temperature makes TEM instability easier to excite, which expands the TEM unstable region in the diagram of density and temperature scale lengths. In addition, it is found that the electron temperature gradient in the perpendicular direction offers a stronger driving force on TEM instability than that in the parallel direction. The overall effects of the temperature gradients of electrons and ions, magnetic shear, safety factor and density gradient on TEM in the presence of the anisotropies are presented in detail.

CARD2: Demonstrating the Combined Effects Of Shear And Surface Roughness On Thrombosis On Titanium Alloy

Study: Titanium alloys used as blood contacting materials in a variety of artificial organs are often roughened on the nano- to micrometer scale during manufacture, requiring extensive and sometimes tedious polishing. However, there is no consensus of the tolerable limits of roughness. This study investigated the combined effects of supraphysiological shear and surface roughness on platelet deposition to provide guidelines for manufacturing. Methods: Microfluidic devices with roughened Ti6Al4V surfaces were used to create shear-controlled, clinically relevant micro-scale environments to quantify platelet deposition and aggregation. Ti6Al4V surfaces were roughened using diamond and glass abrasive techniques. A fresh human blood analog comprised of hemoglobin-depleted red blood cells, for optical clarity, and platelet-rich plasma with mepacrine-labeled platelets was perfused through the microdevices at three shear rates: 1000 s-1, 2000 s-1 and 5500 s-1. Four surface roughness levels were used: mirror finish, Ra 0.3 µm (data not shown here), Ra 0.8 µm and Ra 2.1 µm. Platelet deposition was visualized using fluorescent microscopy. Adherent platelet properties such as mean fluorescence intensity, aggregate area and number were quantified from processed microscopy images. Results: For all three roughnesses < 2.1 µm Ra, platelet adhesion demonstrated sustained growth in confluent monolayers at 1000 s-1 (figs 1a, 2a), formed stable, growing platelet aggregates at 2000 s-1 (fig 1b, 2b); and formed round, fibrous platelet aggregates at 5500 s-1 with increasing intensity and density with time (figs 1c, 2c). Embolization was observed with all surfaces after an initial increase in aggregate intensity (yellow shading, fig 2c). On the Ra 2.1 µm surface, platelet aggregates formed at all three shear rates, indicative of higher shear rates and gradients as a result of the roughness. Conclusions: These findings demonstrate that surface roughness and supraphysiologic shear act synergistically to promote platelet adhesion and thromboembolism. These results suggest a threshold of surface roughness, for the shear rates studied, between 0.8 and 2.1 µm Ra that provokes development of fibrous aggregates that are unstable and embolize. Ongoing work is investigating a wider combination of roughness textures, shear rate, and material chemistry. Figure 1. Platelet adhesion on Ti6Al4V (a) monolayer at 1000s-1, mirror finish), (b) aggregates 2000s-1, Ra 0.8 µm, and (c) fibrous aggregates at 5500s-1. (Scale bar length is 100 µm) Figure 2. Mean fluorescence intensity of aggregates versus time (900 seconds). Error bars represent standard error of the mean at each timepoint.

Effects of Osteocyte Shape on Fluid Flow and Fluid Shear Stress of the Loaded Bone.

This study was conducted to better understand the specific behavior of the intraosseous fluid flow. We calculated the number and distribution of bone canaliculi around the osteocytes based on the varying shapes of osteocytes. We then used these calculated parameters and other bone microstructure data to estimate the anisotropy permeability of the lacunar-canalicular network. Poroelastic finite element models of the osteon were established, and the influence of the osteocyte shape on the fluid flow properties of osteons under an axial displacement load was analyzed. Two types of boundary conditions (BC) that might occur in physiological environments were considered on the cement line of the osteon. BC1 allows free fluid passage from the outer elastic restraint boundary, and BC2 is impermeable and allows no free fluid passage from outer displacement constrained boundary. They both have the same inner boundary conditions that allow fluid to pass through. Changes in the osteocyte shape altered the maximum value of pressure gradient (PG), pore pressure (PP), fluid velocity (FV), and fluid shear stress (FSS) relative to the reference model (spherical osteocytes). The maximum PG, PP, FV, and FSS in BC2 were nearly 100% larger than those in BC1, respectively. It is found that the BC1 was closer to the real physiological environment. The fluid flow along different directions in the elongated osteocyte model was more evident than that in other models, which may have been due to the large difference in permeability along different directions. Changes in osteocyte shape significantly affect the degrees of anisotropy of fluid flow and porous media of the osteon. The model presented in this study can accurately quantify fluid flow in the lacunar-canalicular network.

Shear strain gradient in Cu/Nb nanolaminates: Strain accommodation and chemical mixing

Disentangling the intragranular and interfacial plasticity contribution to the overall strain accommodation is crucial to understanding the microstructural evolution and mass transport upon deformation in materials with the nanoscale feature size. Here, we devise a new approach to tackle the issue by introducing shear strain gradients into Cu/Nb nanolaminates of different layer thicknesses with the shear perpendicular to the laminate interfaces. The measurement of the strain gradient and the resultant lattice disorientation enables a quantitative understanding of the intragranular and interfacial plasticity contribution. We found that intragranular slip entirely governs the deformation in the 300 nm-layer laminate and, unexpectedly, contributes ∼80% of the total plasticity in the 30 nm-layer laminate. The high intragranular plasticity in the thin laminate is attributed to the large width of confined slip planes and their remnant potential for storing dislocations. In addition, substantial forced chemical mixing is observed in the top region of the 30 nm-layer laminate where the effective layer thickness is reduced below 8 nm. The transition of deformation mechanism from confined layer slip to dislocation transmission is largely responsible for the initiation of substantial mixing. Our method and findings shed light on the deformation mechanism and deformation-induced mass transport behavior in nanostructured materials.

A study of the effect of the limiting shear gradient on the pressure recovery curve during hydraulic fracturing

The article analyzes the effect of the limiting shear gradient of viscoplastic oil on the results of interpretation of well testing by the pressure recovery curve (PRC) method following the methods of stimulation by hydraulic fracturing. The lack of mobile water in the collector was analyzed. An analysis of the effect of the limiting shear gradient on the pressure build-up curve and interpretation results was conducted. According to the results of interpretation by the pressure recovery curve method in the presence of a limiting shear gradient in oil, curves were constructed for various dependences and deviations were revealed.

Evaluation of the skin-factor effect on the operation of oil-gas producing wells with viscous-plastic oil

It is well known that in most oil and gas producing regions including Tatarstan and Bashkiria (Russian Federation), Azerbaijan, Kazakhstan, etc., a significant part of produced liquid hydrocarbons consists of oils with abnormal rheological properties, which contain paraffin, asphaltenes, and resin. Such types of oils are related to those of abnormal or non-Newtonian. The paper studies the effect of skin-factor on the recovery of oil with abnormal properties. The features of such a well product are of viscous-plastic liquid nature with an initial shear gradient. It is shown that this system has properties different from non-Newtonian fluids. It is suggested to describe the effect of skin-zone in the system of the equation via the parameter of “skin-factor”, which changes in wide diapasons depending on the nature of pollution of the skin zone. After the specification of “skin factor”, additional measures are carried out in the well on the treatment of the bottom-hole zone (acid treatment, additional perforation, etc.), which will allow regulating the productivity of the operation well.

Impact of air–sea coupling on the simulation of Indian summer monsoon using a high-resolution Regional Earth System Model over CORDEX-SA

A new high-resolution Regional Earth System Model, namely ROM, has been implemented over CORDEX-SA towards examining the impact of air–sea coupling on the Indian summer monsoon characteristics. ROM's simulated mean ISM rainfall and associated dynamical and thermodynamical processes, including the representation of northward and eastward propagating convention bands, are closer to observation than its standalone atmospheric model component (REMO), highlighting the advantage of air–sea coupling. However, the value addition of air–sea coupling varies spatially with more significant improvements over regions with large biases. Bay of Bengal and the eastern equatorial Indian Ocean are the most prominent region where the highest added value is observed with a significant reduction up to 50–500% precipitation bias. Most of the changes in precipitation over the ocean are associated with convective precipitation (CP) due to the suppression of convective activity caused by the negative feedback due to the inclusion of air–sea coupling. However, CP and large-scale precipitation (LP) improvements show east–west asymmetry over the Indian land region. The substantial LP bias reduction is noticed over the wet bias region of western central India due to its suppression, while enhanced CP over eastern central India contributed to the reduction of dry bias. An insignificant change is noticed over Tibetan Plateau, northern India, and Indo Gangetic plains. The weakening of moisture-laden low-level Somalia Jets causes the diminishing of moisture supply from the Arabian Sea (AS) towards Indian land regions resulting in suppressed precipitation, reducing wet bias, especially over western central India. The anomalous high kinetic energy over AS, wind shear, and tropospheric temperature gradient in REMO compared to observation is substantially reduced in the ROM, facilitating the favourable condition for suppressing moisture feeding and hence the wet bias over west-central India in ROM. The warmer midlatitude in ROM than REMO over eastern central India strengthens the convection, enhancing precipitation results in reducing the dry bias. Despite substantially improved ROM’performance, it still exhibits some systematic biases (wet/dry) partially associated with the persistent warm/cold SST bias and land–atmosphere interaction.

Dynamic and Thermal Effects of the Tibetan and Iranian Plateaus on the Northward-Propagating Intraseasonal Oscillation during Boreal Summer

Abstract This study investigates the dynamic and thermal effects of the Tibetan and Iranian Plateaus (TIP) on the characteristics of the northward-propagating boreal summer intraseasonal oscillation (BSISO) and reveals associated physical mechanisms. The relative importance of the Tibetan Plateau (TP) and Iranian Plateau (IP) to the northward propagation of BSISO is also detected. The findings are based on comparative analyses of the control run and six sensitivity experiments using the atmospheric model component of CAS FGOALS-f3-L (the low-resolution version of the Chinese Academy of Sciences Flexible Global Ocean–Atmosphere–Land System model, finite-volume version 3). It is demonstrated that the absence of TIP surface sensible heat or terrain restricts the occurrence of upper-level easterly and lower-level westerly anomalies, as well as moisture convergence over the Asian summer monsoon (ASM) region, which leads to a weakened easterly vertical wind shear with a narrowed range and the generation of a negative meridional specific humidity gradient in the northern Bay of Bengal. These mean-state changes are not conducive to the formation of positive barotropic vorticity together with planetary boundary layer moisture convergence to the north of the convective center, thus impeding the northward propagation of the BSISO. Results indicate that the combined TIP area, via its dynamic and thermal effects, modulates the northward-propagating BSISO by directly affecting the vertical wind shear and meridional moisture gradient in the northern Indian Ocean. By contrast, the TP plays a decisive role in the BSISO northward propagation due to the limited dynamic and thermal effects of IP on the background circulation and water vapor convergence over the ASM region.

Random Forests Assessment of the Role of Atmospheric Circulation in PM10 in an Urban Area with Complex Topography

This study presents the assessment of the quantitative influence of atmospheric circulation on the pollutant concentration in the area of Kraków, Southern Poland, for the period 2000–2020. The research has been realized with the application of different statistical parameters, synoptic meteorology tools, the Random Forests machine learning method, and multilinear regression analyses. Another aim of the research was to evaluate the types of atmospheric circulation classification methods used in studies on air pollution dispersion and to assess the possibility of their application in air quality management, including short-term PM10 daily forecasts. During the period analyzed, a significant decreasing trend of pollutants’ concentrations and varying atmospheric circulation conditions was observed. To understand the relation between PM10 concentration and meteorological conditions and their significance, the Random Forests algorithm was applied. Observations from meteorological stations, air quality measurements and ERA-5 reanalysis were used. The meteorological database was used as an input to models that were trained to predict daily PM10 concentration and its day-to-day changes. This study made it possible to distinguish the dominant circulation types with the highest probability of occurrence of poor air quality or a significant improvement in air quality conditions. Apart from the parameters whose significant influence on air quality is well established (air temperature and wind speed at the ground and air temperature gradient), the key factor was also the gradient of relative air humidity and wind shear in the lowest troposphere. Partial dependence calculated with the use of the Random Forests model made it possible to better analyze the impact of individual meteorological parameters on the PM10 daily concentration. The analysis has shown that, for areas with a diversified topography, it is crucial to use the variability of the atmospheric circulation during the day to better forecast air quality.

The effect of Bruton's tyrosine kinase inhibitor ibrutinib on atherothrombus formation under stenotic flow conditions

BackgroundBruton's kinase (Btk) is critical for collagen-triggered platelet signal transduction. The Btk inhibitor ibrutinib has been shown to selectively block platelet adhesion to atherosclerotic plaque material under laminar arterial flow. However, this has not been studied under a shear gradient, which is characteristic for atherothrombosis. ObjectiveTo determine the effect of ibrutinib treatment on in vitro thrombus formation on collagen and atherosclerotic plaque material in the absence or presence of a shear gradient. MethodsBlood was obtained from patients with chronic lymphocytic leukemia, mantle-cell lymphoma and Waldenström macroglobulinemia with and without ibrutinib treatment and perfused through a microfluidic channel with(out) 60% stenosis over Horm type I collagen or human atherosclerotic plaque homogenate. ResultsAt a constant shear rate of 1500 s−1, platelet deposition was significantly decreased in blood from haematological malignancy patients treated with ibrutinib as compared to untreated patients, on atherosclerotic plaque material but not on collagen. However, thrombus size, stability, and height, were reduced on both plaque material and collagen. An increase in shear rate up to 3900 s−1, as induced by 60% stenosis, resulted in decreased platelet deposition and thrombus parameters on plaque material but not on collagen when compared to a laminar shear of 1500 s−1. Ibrutinib treatment decreased platelet deposition and thrombus parameters even further around the stenosis. ConclusionTreatment of patients with haematological disorders with the Btk inhibitor ibrutinib reduces in vitro platelet deposition, thrombus size and contraction on human atherosclerotic plaque around a stenosis when compared to patients not receiving ibrutinib.

Enhanced elastocaloric stability in NiTi alloys under shear stress

Significant elastocaloric effects (eCE) generated from stress-induced B2↔B19’ transition under axial load in NiTi shape memory alloys have already been proved. However, the huge hysteresis of B2↔B19’ transition directly leads to severe functional fatigue, which restricts its practical application. Here, we prepared linear NiTi spring using Ni50.8Ti49.2 wire with diameter of 300 μm aiming at converting normal stress into shear stress considering the fact that martensitic transition is achieved by shear deformation. An isothermal entropy change of ∼41 J/kg·K as well as an adiabatic temperature decrease of −16.8 K have been obtained in present spring. Furthermore, no significant degradation of elastocaloric strength emerged after 254 cycles, while only ∼2.5 K temperature degradation after 1505 cycles. Such superior stability of elastocaloric effect was mainly attributed to pure shear stress condition and gradient martensitic transition (MT) on the wire cross-section during a superelasticity cycle, which effectively reduced transition actuation load and avoided adverse factors under normal stress condition. Therefore, our spring allows for not only stable and huge caloric effect but also fast cycle frequency, thus, being attractive for micro-cooling devices or heating pumps.

Programmable Anisotropic Hydrogel Composites for Soft Bioelectronics.

Fabrication of hydrogel composites embedded with aligned 1D nanoparticles has shown substantial growth over the past 5 years. Direct ink printing technology (DIW) has been used in this work to create the alignment of the 1D nanoparticles due to the shear gradient of the pseudoplastic precursor (2-hydroxyethyl methacrylate (HEMA) with thickening agents). Orderly distributed 1D particles constructing anisotropic nanostructures endow the hydrogel composite with unique mechanical, electric, or electromechanical coupling properties. Quasi-static uniaxial tensile test, electric resistivity, and piezoresistivity measurements have been conducted for investigating the mechanical, electric, and electromechanical coupling properties of the hydrogel composites, respectively. Based on the experimental results, it can be speculated that the developed printing process is able to fabricate hydrogel composites with programmable anisotropic mechanical, electric, and electromechanical properties. The products pumped out from this work have the potential of being substrates for soft devices and may have a great impact on the fields of flexible bioelectronics.

TEM in toroidal plasmas with anisotropic electron temperature

Trapped electron modes (TEMs) in tokamak plasmas with anisotropies of electron temperature and its gradient are studied by solving the gyrokinetic integral eigenmode equation. Detailed numerical analyses indicate that, in comparison with that in plasmas of isotropic electron temperature, TEMs are enhanced (weakened) by the anisotropy with temperature in the direction perpendicular to magnetic field higher (lower) than that in the direction parallel to the magnetic field when the latter is kept constant. However, the enhancement is limited such that TEMs are weakened rapidly and even stabilized when the anisotropy is higher than a critical value owing to an effective reduction of bounce movement of the trapped electrons. In addition, it is found that the gradients of perpendicular and parallel temperatures of electrons have driving and suppressing effects on the TEMs, respectively. The overall effects of the temperature gradients of electrons and ions, magnetic shear, safety factor, and density gradient on TEMs in the presence of the anisotropies are presented in detail.

Self-ordering and organization of in-line particle chain in a square microchannel

Precise determination of microfluidic behaviors is theoretically significant and has shown remarkable application prospects. This work numerically studies the self-ordering and organization of an in-line particle chain flowing through a square microchannel. The immersed boundary-lattice Boltzmann method is employed, and effects of particle Reynolds number (Rep), length fraction (⟨Lf⟩, characterizes particle concentration), and particle size are focused. Results imply a relatively complex migration of small-particle chains. Three typical states are observed, that is, the equilibrium position finally in a stabilized, fluctuated, or chaotic condition. The corresponding dynamic processes are presented. Interestingly, how interparticle spacing evolves with time shows similar regularity with the three states, corresponding to a particle chain either being evenly distributed, moving like a bouncing spring, or continuously in disordered motions. The flow field and force conditions are analyzed to clarify the mechanisms, suggesting the subtle interaction among vortex-induced repulsive force, wall-induced lift force, and shear gradient lift force is the reason behind. Based on different states, migratory patterns are categorized as Stable Pattern, Spring Pattern, and Chaotic Pattern, and an overall classification is also obtained. Moreover, effects of Rep and ⟨Lf⟩ are identified, where a rising Rep leads to an equilibrium position toward the wall and larger volatility of interparticle spacings. The dynamic characteristics are characterized by lagging, translational, and angular velocities of particles in the chain. Finally, a contrastive study of large particles is performed. The present investigation is expected to provide insight into regularities of in-line particle chains and possible applications.

Influence of Al2O3 platelets addition on ceramic slurry and local flow induced platelets alignment in ceramic mask stereolithography process

Ceramic mask stereolithography can fabricate ceramic parts with complex macrostructures and specific microstructures. Adding the two-dimensional material of alumina platelet crystal to the slurry make this technology possible to print the nacre like “brick-mortar” bionic microstructure ceramics or textured ceramics. However, how the curable ceramic slurry added with the platelets affects the ceramic mask stereolithography process, and how to directly and effectively orient the platelets in the slurry are still needed to be analyzed and solved. This investigation found that without relying on the shear force generated by the slurry scraper, the platelets in the slurry can be directly oriented by the gradient shear stress induced by the local slurry laminar flow during the positioning of the platform. With the increase of the platelets content, the viscosity of the slurries decreased. Since the platelets had fewer scattering effects on ultraviolet light, the increase of the platelets content could increase the curing thickness. In the post-treatment process, compared with the green part without platelets, the shrinkage in the X-Y direction of the green part with oriented platelets was reduced, and the shrinkage in the Z direction was increased. The 50 vol% total solid content slurry with the platelets content of 5 vol% – 20 vol% could be printed through mask stereolithography. The local flow induced alignment effect of the platelets decreased as the solid content of the platelets increased.

Depth-averaged coupling of submerged granular deformation with fluid flow: An augmented HLL scheme

Debris flow in or towards water bodies, and bed load transport in open channels manifest from the interaction of fluid with deformable granular media at varied spatiotemporal scales. The present study aims to formulate a robust two-dimensional coupled depth-averaged framework for simulation of submarine deformation of non-cohesive granular media and the corresponding effect on water and the otherwise. The unified framework consisting of conditional hyperbolic set of partial differential equations has been conceptualized as two concurrent yet independent systems interacting through source/sink terms comprising of drag force, shear and water-surface gradient. The system is solved on a regular finite volume grid through a robust modification of the Harten-Lax-Van Leer (HLL) scheme. The interaction terms are embedded into the interfacial fluxes, where additional closure employs the effect of dry-wet zone transition and flow arrest for both modules. Various aspects of the proposed framework are validated against an array of test cases ranging from landslide induced tsunami to dambreak induced scour. The last representative scenario emphasizes the potential applicability of the model for real-time extension to field-scale problems. A few one-dimensional approximations are explored in the Appendix to calculate relative velocity between interstitial fluid and granular media for computation of drag force.

Comparison of dynamical features between the fast H-L and the H-I-L transition for EAST RF-heated plasmas

In this paper, a comparison of dynamical features between the fast H-L and the H-I-L transition, which can be identified by the intermediate phase, or ‘I-phase’, has been made for radio-frequency (RF) heated deuterium plasmas in EAST. The fast H-L transition is characterized by a rapid release of stored energy during the transition transient, while the H-I-L transition exhibits a ‘soft’ H-mode termination. One important distinction between the transitions has been observed by dedicated probe measurements slightly inside the separatrix, with respect to the radial gradient of the floating potential, which corresponds to the E × B flow and/or the electron temperature gradient. The potential gradient inside the separatrix oscillates and persists during the stationary I-phase, and shows a larger amplitude than that before the fast H-L transition. The reduction of the gradient leads to the final transition to the L-mode for both the fast H-L and the H-I-L transition. These findings indicate that the mean E × B flow shear and/or edge electron temperature gradient play a critical role underlying the H-L transition physics. In addition, the back transition in EAST is found to be sensitive to magnetic configuration, where the vertical configuration, i.e., inner strike-point located at vertical target, favours access to the H-I-L transition, while the horizontal shape facilitates achievement of the fast H-L transition. The divertor recycling level normalized to electron density is higher before the fast H-L transition, as compared to that before the I-phase, which strongly suggest that the density of the recycled neutrals is an important ingredient in determining the back transition behaviour.