Maximum Density Gradient Research Articles

Impact of divertor neutral pressure on confinement degradation of advanced tokamak scenarios at ASDEX Upgrade

Over previous campaigns, an intense experimental program on advanced tokamak (AT) scenarios, has been carried out at the ASDEX Upgrade tokamak with full-tungsten wall. These discharges have been executed shortly after the boronization of the first wall to reduce the density and the impurity influx. The confinement level of such AT discharges was found to vary considerably, even when discharges with similar, if not identical, engineering parameters were carried out. This work investigates the causes of such confinement variations. Among all plasma quantities analyzed, confinement quality of AT scenarios correlates best with divertor neutral pressure, highlighting the key role of edge and scrape-off layer physics in determining global plasma confinement. In particular, it is found that the main cause of confinement degradation is the reduction of pedestal stability, which is in turn caused by the outward shift of the maximum density gradient position typically observed when the divertor neutral pressure increases. Owing to the low density of AT discharges under analysis, the movement of the maximum density gradient position can be caused entirely by changes in deuterium outgassing from the wall, which is strongly influenced by the boron layer deposited on the plasma-facing components and by the deuterium wall inventory. Finally, the predictive capability of confinement quality with the integrated model IMEP [Luda et al., Nucl. Fusion 60, 036023 (2020)] is tested on these discharges and shows promising results.

Global climatological data of ocean thermohaline parameters derived from WOA18

This is a global ocean climatological dataset of 17 thermohaline parameters such as isothermal layer (ITL) depth (hT), mixed layer (ML) depth (hD), thermocline gradient (GT), pycnocline gradient (GD) determined from temperature (T) and salinity (S) profiles of the National Centers for Environmental Information (NCEI) world ocean atlas 2018 (WOA18) using the double gradient method along with the identity-index (i-index) showing the quality of the determination. With the identified (hT, hD) and (GT, GD), other parameters such as ITL heat content (HITL), mixed layer fresh-water content (FML), maximum thermocline gradient (GTmax), thermocline depth (hth), temperature at thermocline depth (Tth), maximum density gradient GDmax), pycnocline depth (hpyc), density at pycnocline depth (ρpyc), and (hT − hD) (barrier layer if positive or compensated layer if negative). The dataset is located at the NOAA/NCEI website (https://doi.org/10.25921/j3v2-jy50). It provides useful background information for ocean mixed layer dynamics, air-sea interaction, climatological studies. This paper is the only document for the dataset.

Dynamics of the pedestal in the recovery phase in EAST type-I ELM plasmas

In the Experimental Advanced Superconducting Tokamak high-confinement mode plasmas, a low-frequency n = 1 magnetic coherent mode with frequency f = 20–50 kHz and a high-frequency mode with electromagnetic characteristics (HFEM, f ∼ 280 kHz) have been found between type-I edge localized modes (ELMs). Both the HFEM and the n = 1 mode are located in the pedestal region, but their radial locations seem to be somewhat different. It seems from the present data that the HFEM is closer to the maximum density gradient region, while the n = 1 mode may be closer to the separatrix. The experimental results demonstrate that the electron temperature recovers more rapidly than the pedestal density, and the n = 1 mode is excited in the pedestal after an ELM collapse. With the increase in the pedestal density, the HFEM appears and becomes dominant, while the amplitude of the n = 1 mode decreases significantly. The observations indicate that the HFEM may suppress the amplitude of the n = 1 mode. In the pre-ELM phase, the pedestal electron density and temperature are saturated, the characteristics of the HFEM show a significant change (a much broader frequency spectrum and reduced mode amplitude), and the n = 1 mode recovers again. Analysis using a wavelet bispectrum reveals that a nonlinear coupling between the n = 1 mode and the high-frequency magnetic fluctuations exists in the pre-ELM phase. The relations between the nonlinear mode coupling, the reappearance of the n = 1 mode and the ELM crash are discussed.

Experimental characterization and analysis of supercritical jet dynamics by phase-shifting interferometer system

The dynamics of trans-/super-critical jet significantly influence system behaviors for applications in spray and combustion, power conversion and chemical reactions. In this study, a pixelated phase-shifting interferometer with high spatial and temporal resolution has been established to investigate the quantitative characteristics of the sub-/trans-/super-critical jets. Thermodynamic features of the jets have been revealed through the transient quantitative field data. Theoretical analysis through experimental results on density and its gradient data has been elaborated. Boundary fractal analysis and radial diffusion process are examined with parameters of mixing layer thickness and spreading rate. The radial distance of the maximum density gradient decreases with time, which indicates jet transition from liquid to supercritical state as approaching the dense core. The fractal dimension of the edge varies from 1.30 to 1.38 for liquid-into-supercritical jet, showing that the initial “liquid” dominated feature and gradually transform process to a supercritical state from the mixing layer.

The Impact of Radial and Non‐Radial IMF on the Earth's Magnetopause Size, Shape, and Dawn‐Dusk Asymmetry From Global 3D Kinetic Simulations

AbstractThe boundary between the solar wind and the Earth's magnetosphere, the magnetopause (MP), is highly dynamic. Its location and shape depend on SW dynamic pressure and interplanetary magnetic field (IMF) orientation. We use a 3D kinetic Particle‐In‐Cell code (IAPIC) to simulate an event observed by THEMIS spacecraft on July 16, 2007. We investigate the impact of radial ( = ) and non‐radial ( = ) IMF on the shape and size of Earth's MP for a dipole tilt of using maximum density gradient and pressure balance methods. Using the model as a reference (MP at 10.3 ), we find that for non‐radial IMF the MP expands by 1.4 and 1.7 along the Sun‐Earth (OX) and tilted magnetic equatorial (Tilt) axes, respectively, and it expands by 0.5 and 1.6 for radial IMF along the same respective axes. When the effect of backstreaming ions is removed from the bulk flow, the expansion ranges are 1.0 and 1.3 and 0.2, and 1.2, respectively. It is found that the percentage of backstreaming to bulk flow ions are 16.5% and 20% for radial and non‐radial IMF. We also show that when the backstreaming ions are not identified, up to 40% of the observed expansion that is due to backstreaming particles can be inadvertently attributed to a change in the SW upstream properties. Finally, we quantified the temperature anisotropy in the magnetosheath, and observe a strong dawn‐dusk asymmetry in the MP location, being more extended on the duskside than on the dawnside.

A kinetic approach to studying low-frequency molecular fluctuations in a one-dimensional shock

Low-frequency molecular fluctuations in the translational nonequilibrium zone of one-dimensional strong shock waves are characterized for the first time in a kinetic collisional framework in the Mach number range 2≤M≤10. Our analysis draws upon the well-known bimodal nature of the probability density function (PDF) of gas particles in the shock, as opposed to their Maxwellian distribution in the freestream, the latter exhibiting two orders of magnitude higher dominant frequencies than the former. Inside the (finite-thickness) shock region, the strong correlation between perturbations in the bimodal PDF and fluctuations in the normal stress suggests introducing a novel two-bin model to describe the reduced-order dynamics of a large number of collision interactions of gas particles. Our model correctly predicts two orders of magnitude differences in fluctuation frequencies in the shock vs those in the freestream and is consistent with the small-amplitude fluctuations obtained from the highly resolved direct simulation Monte Carlo computations of the same configuration. The variation of low-frequency fluctuations with changes in the conditions upstream of the shock revealed that these fluctuations can be described by a Strouhal number, based on the bulk velocity upstream of the shock and the shock-thickness based on the maximum density gradient inside the shock, that remains practically independent of Mach number in the range examined.

Stability of a dust cloud in the radio frequency low-pressure gas discharge

We analyze stability of a three-dimensional cloud of the dust particles in the low-pressure radio frequency discharge under microgravity conditions. The parameters of such complex plasma are assumed to conform to the recently developed ionization equation of state that includes the effects of the ion–atom collisions on the particle charge and of the ion–particle collisions on the ion mean free path. Propagation of the particle number density perturbation in a cloud is treated using the fluid approach. The equation for dust cloud dynamics allows for the electric and ion drag forces that are not compensated in a nonstationary state and yields the wave equation for the dust acoustic waves with either a positive or negative effective friction coefficient. The resulting dispersion relation defines a condition of the instability onset, which is written for the maximum ambipolar field and dust number density gradient. Analysis of the available experimental data reveals a satisfactory correspondence with the theory. In particular, it is demonstrated that the maximum dust density gradient in the cloud increases with the increase in the particle diameter and in the pressure of plasma-forming gas.

Interannual Change in Mode Waters: Case of the Black Sea

AbstractMore than 6,000 profiles from profiling floats in the Black Sea over the 2005–2020 period were used to study the ventilation of this basin and the mixing pathways along isopycnals. The layer of the minimum potential vorticity (PV), the Black Sea pycnostad, approximately follows the core of the cold intermediate layer, similar to the case of oceanic mode waters. However, unlike in the ocean, the horizontal patterns of PV are shaped by cyclonic gyre circulation. There is a principle difference in the probability distribution of the thermohaline properties presented in geopotential coordinates from those presented in density coordinates. In the latter case, several mixing pathways, which are not known from previous studies, dominate the ocean states. These formed after three intermittent events of cold water formation. The density ratio decreased three times during the last 15 years, revealing the decreasing role of temperature in the vertical layering of the Black Sea halocline. The basin‐wide distribution of PV above σθ = 16, which is where the maximum vertical density gradient appears, is opposite to the distribution below this depth. This finding suggests a complex change in the mesoscale dynamics in different layers. Comparisons of observations with data from the Copernicus Black Sea operational model demonstrate that the mixing parameterizations of models need further improvements.

Measurement of the inner structure of turbidity currents by ultrasound velocity profiling

The inner velocity structure and particle concentration profile of opaque turbidity currents were measured simultaneously by ultrasound velocity profilers. Currents consisting of a quartz particle suspension were generated by using the lock-exchange method in a flume to experimentally reproduce the quasi-steady state of a turbidity current. A pair of ultrasound transducers captured the horizontal and vertical velocities from Doppler frequencies, and the particle concentration profile was extracted from the echo amplitude. The data obtained were analyzed in terms of momentum conservation according to the two-fluid model. We found that: i) the viscous and Reynolds shear stresses balance in the top half of the current; and ii) the lower border of the stress balancing appears around the depth of the maximum vertical density gradient. These findings indicate that the reduction of flow resistance inside the body region of the turbidity current is maintained downstream, which enables the current to transport particles over a long distance.

A Subtidal Box Model Based on the Longitudinal Anomaly of Potential Energy for Narrow Estuaries. An Application to the Guadalquivir River Estuary (SW Spain)

AbstractThe objective of the present study is to demonstrate the informative capacity of the longitudinal anomaly of potential energy (LAPE) in the analysis of the magnitude and spatiotemporal variability of estuarine processes. For this purpose, a LAPE balance equation is formulated. The LAPE integrates and varies with the vertical and longitudinal density distribution. The formulation is applied on a subtidal scale to each box or stretch of the Guadalquivir River estuary, a narrow, highly turbid, weakly stratified, and strongly anthropized estuary. Data recorded by a large network of monitoring stations in 2008 and 2009 are used to quantify advective transports as well as the transports associated with longitudinal dispersion and vertical turbulent mixing in different hydraulic regimes. In low‐river flow conditions, (river flows Q<40 m3s−1), the magnitude of LAPE transports decreases upstream and varies locally, depending on neap‐spring tidal cycles. The direction of the net LAPE transport creates convergence zones that are particularly consistent with maximum levels of estuarine turbidity. During high‐river flows (Q>400 m3s−1), this convergence disappears and the maximum longitudinal density gradient moves towards the mouth. More specifically, tidal pumping ‐induced LAPE governs during these conditions and manages to compensate the sum of the mean nontidal and dispersive and differential advective LAPE transports. However, during the post‐riverflood period, the mechanisms controlling recovery downstream from the mouth are the longitudinal dispersive and differential advective LAPE transports. Furthermore, the convergence zone reappears with a longitudinal gradient of the net LAPE transport that is even greater than in low‐river flow conditions.

Instability of a shear layer between multicomponent fluids at supercritical pressure

The temporal instability of a thin shear layer lying between streams of two components of fluids has been studied. The effects of density profile of the layer on the instability behavior were mainly considered. The detailed density profile was obtained through Linear Gradient Theory. The eigenvalue problem was calculated, and the temporal instability curves were obtained for the thermodynamic parameters, e.g. pressure and temperature. The results show that, increase of pressure leads to the increase of the maximum growth rate. However, increasing pressure has opposite effects on the disturbances with small and large wave length. The increase of temperature causes the decrease of disturbance growth rate. The instability behavior of the shear layers was determined mainly by the interval between the inflections of the velocity and density profiles, and the maximum density gradient. The total effects, determined by coupling density stratification, and interval between the inflections of the velocity and density profiles, were quite distinct for different ranges of temperature and pressure.

Stability, intermittency and universal Thorpe length distribution in a laboratory turbulent stratified shear flow

Stratified shear flows occur in many geophysical contexts, from oceanic overflows and river estuaries to wind-driven thermocline layers. We explore a turbulent wall-bounded shear flow of lighter miscible fluid into a quiescent fluid of higher density with a range of Richardson numbers$0.05\lesssim Ri\lesssim 1$. In order to find a stability parameter that allows close comparison with linear theory and with idealized experiments and numerics, we investigate different definitions of$Ri$. We find that a gradient Richardson number defined on fluid interface sections where there is no overturning at or adjacent to the maximum density gradient position provides an excellent stability parameter, which captures the Miles–Howard linear stability criterion. For small$Ri$the flow exhibits robust Kelvin–Helmholtz instability, whereas for larger$Ri$interfacial overturning is more intermittent with less frequent Kelvin–Helmholtz events and emerging Holmboe wave instability consistent with a thicker velocity layer compared with the density layer. We compute the perturbed fraction of interface as a quantitative measure of the flow intermittency, which is approximately 1 for the smallest$Ri$but decreases rapidly as$Ri$increases, consistent with linear theory. For the perturbed regions, we use the Thorpe scale to characterize the overturning properties of these flows. The probability distribution of the non-zero Thorpe length yields a universal exponential form, suggesting that much of the overturning results from increasingly intermittent Kelvin–Helmholtz instability events. The distribution of turbulent kinetic energy, conditioned on the intermittency fraction, has a similar form, suggesting an explanation for the universal scaling collapse of the Thorpe length distribution.

Retinal ganglion cell topography and spatial resolving power in the oriental fire-bellied toad Bombina orientalis.

The vertebrate visual system is determined by two main factors, a species' lifestyle and phylogenetic legacy. Studying the visual system in outgroup lineages may shed some light on the balance of these factors within a certain radiation. We studied the topography of retinal ganglion cells (RGCs) in the retina of the oriental fire-bellied toad Bombina orientalis. These toads belong to the ancient superfamily Discoglossoidea, a sister group to all extant Anura except for two small families. RGCs were retrogradely labeled with tetramethylrhodamine- dextran amine (TMR-DA) and examined in retinal wholemounts. RGCs occurred all over the retina except for the far periphery. Their total number was [Formula: see text] ([Formula: see text], [Formula: see text]). They comprised 73-77% of all cells in the ganglion cell layer. The spatial density of GCs increased gradually from the dorsal and ventral retinal periphery toward the equator to form a weak visual streak and a moderately pronounced area centralis. The minimum density was [Formula: see text], and the maximum, [Formula: see text]. The maximum density gradient was [Formula: see text]. The spatial resolution was minimum in the dorsal and ventral periphery ([Formula: see text] and [Formula: see text] cycles per degree in water and air, respectively). Intermediate values of spatial resolving power were found within the visual streak ([Formula: see text] and [Formula: see text] cycles per degree) and reached a peak in area centralis ([Formula: see text] and [Formula: see text] cycles per degree). This is sufficient for efficient prey location and capture. The relatively high RGC density and the presence of specialized retinal regions in oriental fire-bellied toads are consistent with their highly visual behavior. A brief review comparing the phylogeny and ecology of this with other anuran species suggests that the main factor shaping the RGC distribution in Anura is phylogenetic legacy; the environmental pressure results mainly in adjusting the maximum spatial density of RGCs (and hence the visual acuity) to meet the species' needs.

Stand density, tree social status and water stress influence allocation in height and diameter growth of Quercus petraea (Liebl.).

Even-aged forest stands are competitive communities where competition for light gives advantages to tall individuals, thereby inducing a race for height. These same individuals must however balance this competitive advantage with height-related mechanical and hydraulic risks. These phenomena may induce variations in height-diameter growth relationships, with primary dependences on stand density and tree social status as proxies for competition pressure and access to light, and on availability of local environmental resources, including water. We aimed to investigate the effects of stand density, tree social status and water stress on the individual height-circumference growth allocation (Δh-Δc), in even-aged stands of Quercus petraea Liebl. (sessile oak). Within-stand Δc was used as surrogate for tree social status. We used an original long-term experimental plot network, set up in the species production area in France, and designed to explore stand dynamics on a maximum density gradient. Growth allocation was modelled statistically by relating the shape of the Δh-Δc relationship to stand density, stand age and water deficit. The shape of the Δh-Δc relationship shifted from linear with a moderate slope in open-grown stands to concave saturating with an initial steep slope in closed stands. Maximum height growth was found to follow a typical mono-modal response to stand age. In open-grown stands, increasing summer soil water deficit was found to decrease height growth relative to radial growth, suggesting hydraulic constraints on height growth. A similar pattern was found in closed stands, the magnitude of the effect however lowering from suppressed to dominant trees. We highlight the high phenotypic plasticity of growth in sessile oak trees that further adapt their allocation scheme to their environment. Stand density and tree social status were major drivers of growth allocation variations, while water stress had a detrimental effect on height in the Δh-Δc allocation.

Distribution of hypoxia and pycnocline off the Changjiang Estuary, China

Abstract The distributions of hypoxia and the pycnocline off the Changjiang Estuary were investigated by making several field observations from June 2 to 11, from July 18 to 23, from August 20 to 30, from October 3 to 13, 2006, and from August 27 to September 3, 2009. The observations from July 18 to 23, 2006, mainly focused on analyzing the relationship between hypoxia and the extension of the river plume and vertical stratification. In July, the Changjiang diluted water (CDW) was influenced by the easterly typhoon winds, causing it to extend northward rather than northeastward. By using the maximum vertical density gradient as a stratification intensity index, we found that the area of low ( 2.0 kg/m4), which indicated that the summer pycnocline can effectively block vertical DO exchange and maintain hypoxia near the bottom. The observed hypoxic area was 500 km2, which was much smaller than the hypoxic areas observed in previous studies, and occurred because of the enhanced mixing that resulted from Typhoon Bill. During the observation period of August 20–30, 2006, the maximum density gradient was weaker due to distinct low river discharge. No hypoxia was observed in the eastern and southeastern sea off the Changjiang Estuary where hypoxia often occurs. However, hypoxia occurred over a large area of 15,400 km2 in the northern observation domain where hypoxia rarely occurs. During June 2–11 and October 3–13, 2006, the maximum density gradient was weaker, and the area with low DO was smaller than in July 2006. This finding resulted from relatively low river discharge and weaker solar heating. Consequently, no hypoxia occurred in the bottom layer. The area of low DO was similar to that of the maximum vertical density gradient. From August 27 to September 3, 2009, high river discharge and strong solar heating produced a larger and more intense pycnocline. The hypoxic area reached 3735 km2 and was very similar to the area of the pycnocline, which was greater than 3.0 kg/m4. The seasonal variations of the pycnocline were consistent with those of hypoxia, and the pycnocline played an important role in preserving hypoxic conditions. The seasonal influences of biogeochemical process on hypoxia in 2006 were discussed. The residual current speeds at the bottom were small and favorable for maintaining hypoxia during the summer.

The island‐scale internal wave climate of Moorea, French Polynesia

Analysis of five‐year records of temperatures and currents collected at Moorea reveal strong internal wave activity at predominantly semi‐diurnal frequencies impacting reef slopes at depths ≥30 m around the entire island. Temperature changes of 1.5°C to 3°C are accompanied by surges of upward and onshore flow and vertical shear in onshore currents. Superimposed on annual temperature changes of approximately 3°C, internal wave activity is high from Oct–May and markedly lower from Jun–Sep. The offshore pycnocline is broadly distributed with continuous stratification to at least 500 m depth, and a subsurface fluorescence maximum above the strong nutricline at approximately 200 m. Minimum buoyancy periods range from 4.8 to 6 min, with the maximum density gradient occurring at 50 to 60 m depth in summer and deepening to approximately 150 to 200 m in winter. The bottom slope angle around all of Moorea is super‐critical relative to the vertical stratification angle suggesting that energy propagating into shallow water is only a portion of total incident internal wave energy. Vertical gradient Richardson numbers indicate dominance by density stability relative to current shear with relatively limited diapycnal mixing. Coherence and lagged cross‐correlation of semi‐diurnal temperature variation indicate complex patterns of inter‐site arrival of internal waves and no clear coherence or lagged correlation relationships among island sides. Semi‐diurnal and high frequency internal wave packets likely arrive on Moorea from a combination of local and distant sources and may have important impacts for nutrient and particle fluxes in deep reef environments.

Electron tomography and STEM investigations of the structure of multilayer amorphous and nanocrystalline alloys of CoP-CoNiP, CoW-CoNiW systems under external action

The structure of amorphous and nanocrystalline alloys of the CoP-CoNiP and CoW-CoNiW systems under the external action is investigated by means of high-resolution transmission electron microscopy and electron tomography. It is revealed that the crystalline phase is predominantly nucleated in the regions with the maximum density gradient. Algorithms for image filtering and reconstruction for a graphical processor unit are implemented.

한반도 근해의 상대적 위치에너지 편차 변화를 이용한 성층화의 특성과 계절별 변화에 대한 연구

In this paper, we have proposed a method for quantization of the stratification strength in the sea water and analysing the distributions of the maximum stratification depths calculated by the method at the seas near the Korean peninsular. For calculating the stratification strength, modified and applied the potential energy anomaly formular which was suggested by Simpson in 1977. The data had been collected by NFRDI from 1971 to 2008 were used to determine the maximum vertical density gradient depth and the relative potential energy anomaly at that depth. In the East Sea, the stratification depth has become deepened about 20m in February and April since 1971. In Yellow-South Sea, the maximum density gradient depth has been deepened about 10m only in December during the same period and the difference of the stratification depth between summer and winter has been enlarged. These trends of variation of stratification strength and depth near the Korean peninsular should be investigated more carefully and continuously. And the results of these studies could be adopted for the more efficient operation of underwater weapon and detection systems.

Effects of thermal stratification and mixing on reservoir water quality

In this study, the effect of thermal stratification on water quality in a reservoir has been investigated by field observations and statistical analysis. During the summer period, when stratification is evident, field observations indicate that the observed dissolved oxygen concentrations drop well below the standard limit of 5 mg l−1 at the thermocline, leading to the development of anoxia. The reasons for variations in the dissolved oxygen concentrations were investigated. Variations of air temperature and other meteorological factors and lateral flows from side arms of the lake were found to be responsible for the increase of dissolved oxygen concentrations. It was also observed that turbidity peaked mostly in the thermocline region, closely related to the location of the maximum density gradient and thus low turbulence stabilizing the sediments in the vertical water column. Relatively cold sediment-laden water flowing into the lake after rain events also resulted in increased turbidity at the bottom of the lake. Nondimensional analysis widely used in the literature was used to identify the strength of the stratification, but this analysis alone was found insufficient to describe the evolution of dissolved oxygen and turbidity in the water column. Thus correlation of these parameters was investigated by multivariate analysis. Fall (partial mixing), summer (no mixing), and winter (well mixed) models describe the correlation structures between the independent variables (meteorological parameters) and the dependent variables (water-quality parameters). Statistical analysis results indicate that air temperature, one day lagged wind speed, and low humidity affected variation of water-quality parameters.

Formation and propagation of Turbulent Structures in Drift‐Wave Turbulence

AbstractNumerical simulation results of weakly‐developed drift‐wave turbulence using the gyro‐fluid code GEM3 [B. D. Scott, Plasma Phys. Controlled Fusion, 45, A385 (2003)] is analyzed. The formation of turbulent structures in the maximum density gradient region is observed, which is causally connected with the primary drift‐wave instability. Turbulent structures peel‐off quasi‐coherent drift modes and propagate radially outwards into the plasma edge where they appear as intermittent density bursts. It is demonstrated that the radial propagation of the turbulent structures is due to convection in the self‐consistent potential perturbation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)