Regional Gradient Research Articles

Deep learning for pockmark detection: Implications for quantitative seafloor characterization

Occurring globally, pockmarks are seafloor depressions associated with seabed fluid escape. Pockmark ubiquity and morphologic heterogeneity result in an irregular seafloor that can be difficult to quantitatively describe. To address this challenge, we test the hypothesis that deep-learning based object detection and segmentation can be used to develop data-driven models for pockmark identification and characterization. This study describes the development, testing, and deployment of eight separate deep learning-based pockmark detection models using publicly available, gridded bathymetric data from the Belfast Bay, Maine, USA, Blue Hill Bay, Maine, USA, and Passamaquoddy Bay, New Brunswick, Canada estuarine pockmark fields. The models tested include three types of convolutional neural network architectures, as well as a generative adversarial network. We find that the data-driven models consistently resolve pockmarks from the background seafloor, allowing for quick and accurate delineation of pockmarks in a variety of seabed habitats. With these delineations we examine and compare the morphology of the muddy estuarine pockmark fields. We then compare these morphometric results to pockmark fields in two distinct settings, the sandy German Bight and the Aquitaine continental slope. We find that in all the pockmark fields a power law relationship, generally, exists between pockmark area and pockmark depth, though this relationship deteriorates with the smallest pockmarks, suggesting that there may be a minimum size needed for geomorphic stability. These results show that the training data and trained models developed here can be applied for quick detection and characterization of pockmarks where other high-resolution bathymetry is available, demonstrating the value of data-driven detection models for characterizing morphologically complex seafloors. Last, the morphologic characteristics of pockmarks identified in this study will aid future studies in relating pockmark size to environmental characteristics like seabed sediment texture and regional gradient.

Microstructures and mechanical properties of Invar/MnCu functionally graded material fabricated by directed energy deposition

Directed energy deposition (DED) is an ideal approach for the direct fabrication of metallic functionally graded materials (FGMs) and has attracted great interest during the past decade. However, researches about DED-fabricated FGMs are mainly focused on structural metals, such as iron/nickel, and titanium/aluminum material systems. It is significant to develop new systems and give useful insight for a wider application field of FGMs. Invar and MnCu alloys are important materials for thermal stability and vibration suppression in high-precision equipment. In this study, the crack-free FGM from 100% Invar to 100% MnCu with a 10 wt% linear increment was prepared by DED. In the gradient region, grains were refined (size from 46 μm to 20 μm) with the increasing content of MnCu, owing to the combination of columnar to equiaxed transition (CET) and recrystallization. The ultimate tensile strength and elongation of Invar/MnCu FGM are 359 MP and 5.7%, respectively. The deformation behavior of FGM shows a “cask effect”, most of the plastic deformation is concentrated at the 90%Invar-10%MnCu position, which is the part with the lowest hardness value. This study will provide a useful reference to aid in the selection of materials for the designing of new FGMs and extend the application field.

Infrared Weak and Small Target Detection Based on Top-Hat Filtering and Multi-Feature Fuzzy Decision-Making

Infrared weak and small target detection in a complex background has always been a research hotspot in the fields of area defense and long-range precision strikes. Among them, the single-frame infrared weak and small target detection technology is even more difficult to study due to factors such as lack of target motion information, complex background, and low signal-to-noise ratio. Aiming at the problem of a high false alarm rate in infrared weak and small target detection caused by the complex background edges and noise interference in infrared images, this paper proposes an infrared weak and small target detection algorithm based on top-hat filtering and multi-feature fuzzy decision-making. The algorithm first uses the multi-structural element top-hat operator to filter the original image and then obtains the suspected target area through adaptive threshold segmentation; secondly, it uses image feature algorithms, such as central pixel contrast, regional gradient, and directional gradient, to extract the feature information of the suspected target at multiple scales, and the fuzzy decision method is used for multi-feature fusion to achieve the final target detection. Finally, the performance of the proposed algorithm and several existing comparison algorithms are compared using the measured infrared sequence image data of five different scenarios. The results show that the proposed algorithm has obvious advantages in various performance indicators compared with the existing algorithms for infrared image sequences in different interference scenarios, especially for complex background types, and has a lower performance under the condition of ensuring the same detection rate and false alarm rate and in meeting the real-time requirements of the algorithm.

Lessons from a regional analysis of forest recovery trajectories in West Africa

In West Africa, very poorly documented are the recovery trajectories of secondary forests, and even less is known about the origin of the observed variability in recovery rates. To understand the relative importance of local and regional environmental conditions on these trajectories, we inventoried all trees larger than 2.5 cm DBH on 236 plots (0.2 ha), aged from 0 to 45 years plus controls, on eight chronosequences representing the typical regional North-South climatic gradient of West Africa. In a hierarchical Bayesian framework, we modelled recovery trajectories of biodiversity, aboveground biomass and floristic composition and tested the influence of variability in local (plot history, landscape context, remnant trees) and regional (climate and soil) conditions on recovery rates. Our results show that (a) diversity recovers faster than composition and biomass, (b) among the local variables, the number of remnant trees has a positive impact on recovery rates while the duration of agricultural cultivation has a negative impact, and (c) among the regional variables, the high seasonality of precipitation and climate, typical of the dry forests of the northern West African forest zone, leads to faster secondary successions. Our simulation approaches have indicated that poor regional conditions can be counterbalanced by adequate local conditions and vice versa, which argues strongly in favour of a diagnosis that integrates these two aspects in the choice of more or less active technical itineraries for forest restoration.

Validating TG119 VMAT Commissioning on a Volumetric Phantom

<h3>Purpose/Objective(s)</h3> The present work adapts AAPM TG119 on a volumetric detector array and evaluates its feasibility for VMAT commissioning for two different Treatment Planning Systems (TPS). <h3>Materials/Methods</h3> TG119 describes a group of test plans for standard structure sets following prescription and dose constraints objectives. These plans were calculated and evaluated using both Elekta Monaco V5.0 (Montecarlo) and Varian Eclipse v15.6 (AAA algorithm) TPS. TG119 structure sets were inserted on virtual CT images for volumetric Quality Assurance (QA) phantom. A set of plans calculated on both TPS were delivered on a 6MV photon beam on an Elekta Synergy Linac. A set of plans was generated only on Eclipse for a Varian Halcyon Linac with 6MV Flattening Filter Free beam. Generated plans were delivered, dose distribution was concurrently measured with Ion Chamber (IC) point doses using a cavity plug holder inserted on the QA phantom. Dose constraints and goals comparison between TPS was executed. Gamma analysis was performed for 3%3mm and 3%2mm global gamma index and Confidence Limits (CL) according to the TG119 definition of Normal quantile were calculated. Measured and planned point doses on relevant positions were compared. <h3>Results</h3> TG119 goals were mostly satisfied except for some critical situations: PTV D99 goal on head & neck plan calculated on Eclipse TPS and Superior Target D10 goal on Multitarget Plan for Monaco TPS calculation. Gamma Analysis for 3%3mm range of passing rate was 96.8% to 100% while CLs range between 4.1% and 4.7% for a Normal quantile for Monaco TPS. For Eclipse TPS range of passing rate achieved was among 97.2% and 99.9%, CLs range was between 3.2% and 3.1%. For Eclipse plans delivered on Halcyon, the passing rate range was among 98.6% and 99.8%, CLs between 1.6% and 1.7%. 3%2mm global gamma index analysis shows a range of passing rate of 93.5% and 100% for Monaco TPS while CLs range between 4.8% and 8.3%. For Elekta plans on Eclipse TPS range of passing rate was 95.6% and 98.8%, CLs range was between 5.1% and 4.5%. The gamma passing rate range for Halcyon plans was between 94.5% and 99.3 and CLs between 3.4% and 6.0%. IC measurements for Synergy Linac showed an average dose difference of 0.84% between measured and calculated plans on Monaco, range among -3.15% and 7.01%, the greatest differences were found on C-Shape plan for high gradient regions measurements. For Eclipse TPS, the average dose difference was 1.28%, range between -1.04% and 10.26%, the highest difference was found in high gradient region C-Shape plan. For Halcyon plans, IC measurements showed an average dose difference of 0.74%, ranging between -0.27% and 3.38%. <h3>Conclusion</h3> CLs agreed with TG119 reference values even when stricter requirements were used. New criteria are more appropriate for modern treatments, but TG119 methodology remains a valid procedure for VMAT. The methodology described for commissioning VMAT technique on a volumetric QA phantom shows to be robust and consistent for various TPS and lilacs.

Local-scale spatial variability in firn properties in Southwest Greenland

The increasing mass loss from the Greenland ice sheet is becoming growingly important to present-day global sea level rise. Meltwater export to the ocean may be amplified by near-surface ice layers in firn (i.e., snow that has survived at least one melt season) which prevent infiltration. Such ice layers have been documented at various sites in southwest Greenland, but little is known about the local-scale (less than 1 km) variability in firn and ice properties. Here we investigate the local variability in firn density and ice content (ice layer thickness fraction) by analyzing 45 shallow firn cores retrieved from 15 sites in southwest Greenland between 2012 and 2019. The core dataset contains groups of cores that were collected at the same site (1–632 m apart from each other) and in the same year (here referred to as same site/year cores). The two-sided Mann-Whitney U-tests suggest that most same site/year cores do not have statistically significant differences in median density or ice content over the upper 0–4.5, 4.5–9, and 9–13.5 m sections below the winter snow layer. That said, the same site/year cores still exhibit substantial density and ice content differences as well as variable ice layer stratigraphy (especially the distribution of centimeters-thick ice lenses) not explainable by measurement uncertainties. The local-scale variability in density and ice content (calculated from the same site/year cores) over the three 4.5-m sections is 4% and 19–29%, respectively. The variability is comparable to the regional gradient in our study area within an elevation difference of up to ∼200 m. The local variability is non-negligible particularly at mid- to high-elevation sites (&amp;gt;2,000 m a.s.l.), where firn structure changes such as ice layer formation are more recent phenomena compared to lower elevations. This study confirms that single cores can provide representative bulk statistics such as mean density and ice content of one site. However, these mean values mask substantial local variability which can compromise regional comparisons. Therefore, collection of same site/year cores and consideration of local variability are necessary for an in-depth analysis of regional variability.

Subsurface structural characterization as deduced from potential field data—West Beni Suef, Western Desert, Egypt

The goal of present study is determining subsurface structural elements by analyzing geomagnetic data sets aiming to examine their impacts on geological surface structures characterizing the western region in Beni-Suef Governorate, Western Desert, Egypt. A land-magnetic survey is conducted by two exceedingly sensitive proton magnetometers to measure the total intensity of geomagnetic field at the region under investigation. The measurements of regional gradients and time variation were applied to get the required reduction of daily variation. The regional expansions of subsurface structural units were obtained after applying the reduction to north magnetic pole (RTP) method to the corrected magnetic data acquired from land-magnetic survey along with Bouguer maps (1:100,000 scale and 1-mGal contouring interval). Regional-residual technique was applied depending on the power spectrum analysis. Moreover, the edge detection approach is conducted to depict the structures and subsurface hidden anomalies. Different handling processes were applied for acquired land magnetic datasets such as Euler deconvolution and trend analysis. The results of current study revealed that four significant structural forces across the N-S, NW–SE, NE-SW, and E-W directions affected the region. Estimating the basement depth was accomplished using a number of spectral analysis techniques. The values of estimated depths are used for creating a basement relief map. Moreover, the achieved results show that depths to the basement at the region under study vary between 2.3 and 4.7 km.

Bacterial diversity changes in response to an altitudinal gradient in arid and semi-arid regions and their effects on crops growth

The microbiome of soil has a fundamental role in maintaining the health of soil and plants. While the diversity of microbes is one of the most important factors in the environment, little is known about the effects of elevation on the microbiome and the impact of the affected microbiome on plants. The main goal of this study is to expand our knowledge of what happens to the soil bacterial community along an altitudinal gradient and investigate their possibly different impacts on plant growth. Bacteria from soils at various altitudes have been isolated, characterized, and identified by Matrix-Assisted Laser Desorption/Ionization-Time of Flight Mass Spectrometry (MALDI-TOF MS) to determine the effects of an elevational gradient on the microbiome and plant growth. Furthermore, their effects have been investigated by isolates assessment on maize, wheat, and canola. Based on our results, higher altitude results in a higher diversity of the microbiome and lower bacteria biomass. Bacillus cereus is found in abundance in arid and semi-arid samples. Interestingly, enhanced diversity in higher altitudes shows similarity in response to environmental stress and tolerates these factors well. Furthermore, the inoculation of these bacteria could enhance the overall growth of plants. We prove that bacterial communities could change their biomass and diversity in response to altitude changes. These indicate evolutionary pressure as these bacteria could tolerate stress factors well and have a better relationship with plants.

Gravimetric contributions to the study of the collapsed structures in northeast Tadla Plain:structural and hydrogeological implications

This study was realized in the north-east of the plain of Tadla, in the region of Beni Mellal-khenifra, Morocco. The main aim of this research is to establish the geometry and hydrogeology characteristics of the region's aquifer system, based on the results of analysis and interpretation of deep-drilling data, and on gravimetric and electrical survey measurements. First, the regional gradient was determined, then the initial data was extracted, and based on the extracted data, we created the gravity map of the Bouguer, where the anomalies are clearer. The examination of the Bouguer anomaly's gravity map reveals the existence of a regional gradient. These anomalies are typically found in outcrop areas of the basement and differences in thickness of the sediment cover in the study area. The Electrical survey measurements, and the interpretation of the deep-drilling data, confirm the results of gravimetry survey, after applying different techniques such as horizontal gradiometer and upward extension, on the Gravimetric map. The founded results allowed us to draw up a structural map showing the system faults, which is accountable for the study area structure. The structural map is a very useful tool to guide future hydrogeological researches in the plain of Tadla.

Highly Accelerated Stress Testing of Proton Exchange Fuel Cell Membranes for Heavy Duty Transportation Applications

According to the US DOE’s multi-year Fuel Cell research, development, and demonstration plan, a 30,000 h lifetime, approximately four times that of the light-duty vehicles (LDV), is required for Heavy Duty Vehicle (HDV) applications [1]. Thus, highly accelerated stress tests are necessary in order to assess the viability to novel membrane concepts within a reasonable amount of time. Previously we have reported the development of a highly accelerated stress test (HAST) to generate local stressful conditions that are representative of those in automotive fuel cell stacks [2]. Using a 50-cm2 cell cycled between 0.05 and 1.2 A/cm2 with a low inlet RH in the co-flow configuration, the HAST creates a distribution of combined mechanical/chemical stressors in the membrane with the maximum chemical stress occurring near the gas inlets and the maximum mechanical stress near the outlets. Conducting HASTs using a current distribution measurement tool and a shorting/crossover diagnostic method to track the state of health of a state-of-the-art membrane containing both a mechanical support and a cerium-containing chemical stabilizing additive, the result shows that the membrane location with the most severe thinning coincides with that of the deepest membrane hydration cycling. Upon examination of the cerium redistribution patterns after the test, it was found that the severe humidity cycling generated by the HAST condition near the outlet region not only generated the highest membrane mechanical stress but also resulted in the strongest water flux, which may cause local depletion of the cerium added as chemical stabilizer.In this study, we have focused on determining the root cause of membrane failure in the region of hydration cycling. Two hypotheses were investigated. The first is that chemical degradation can be accelerated in the presence of mechanical stress. The chemical degradation leads to membrane thinning which, in turn, leads to fewer sulfonic acid sites in the membrane available for Ce cation exchange. Two synergistic pathways for membrane degradation by combined mechanical and chemical stressors have been summarized by Kusoglu and Weber [3]. Ex-situ hydrogen peroxide vapor cell tests conducted with a pressure bias across the membrane to create a tensile stress were conducted to see if mechanical stress has an impact on membrane chemical degradation rate [4].The second hypothesis is that local depletion of the mobile Ce stabilizer in regions of high humidity gradients can lead to chemical degradation in those regions. We have shown previously that humidity gradients can cause cations to move with convective forces from wetter to drier regions within a perfluoro-sulfonic acid (PFSA) membrane [5]. To study this hypothesis, we conducted two types of tests. First, we repeated the HAST tests of the state-of-the-art membranes and stopped the tests prior to the onset on membrane thinning to see whether Ce depletion occurs before or after the onset of thinning. We also ran HAST tests on membranes which contained no Ce stabilizer to see if membrane failure still occurred in the region of strongest humidity cycles. In the absence of stabilizer, we expect chemical degradation to occur faster towards the cell inlets where, on average, the MEA is driest.We will show that the HAST test significantly accelerates membrane failure by increasing temperature and frequency of local deep humidity cycles without changing the primary failure mode or location of a state-of-the-art membrane. This work suggests that these HAST tests can be used to accelerate the development of membranes for HD fuel cell applications. DOE H2 Heavy Duty Truck Targets; https://www.hydrogen.energy.gov/pdfs/19006_hydrogen_class8_long_haul_truck_targets.pdf Y.-H. Lai et. al, J. Electrochem. Soc., 165 (6) (2018) F3100-F3103Kusoglu and Weber, J. Phys. Chem. Lett., 6, 4547 (2015)Y.-H. Lai et. al, ECS s 2021; https://iopscience.iop.org/article/10.1149/MA2021-02381128mtgabs F.D. Coms et. al, ECS s 2021; https://iopscience.iop.org/article/10.1149/MA2021-02381130mtgabs

Drivers of landscape evolution in eastern Tibet

The eastern margin of the Tibetan Plateau is characterized by two end-member morphologies: 2–3-km-deep canyons that are juxtaposed against extensive high elevation, low relief landscapes. Past researchers have highlighted topographic evidence for mobile drainage divides in this region. Active thrusting is thought to drive river gorge incision originating from the east, but within the eastern Tibetan plateau, there is no evidence for focussed thrusting, and no significant post-Cenozoic shortening structures are evident at the surface upstream of the Three Rivers regions. In this region, researchers have attributed the evident landscape transience to different mechanisms, including upward propagation of an incision signal, regional uplift, and local uplift caused by strike-slip motion. Here, we quantify topographic metrics to document the geomorphic response to the proposed tectonic forcing. Specifically, we quantify channel steepness and identify knickpoints to assess evidence for landscape transience and potential accelerated incision triggered in eastern Tibet. We find that ksn and slope systematically increase downstream along the Tongtianhe River (TR) which bisects the study area from NW to SE and is part of the Yangtze River system, but not along adjacent main stem rivers with similar orientation, therefore ruling out the effect of a regional uplift gradient. The spatial distribution of knickpoints does not cluster along or around fault lines, which we interpret to mean that local faulting is not likely a significant factor causing the systematic variations in the topographic metrics. We find that tributaries of the TR exhibit large non-lithological channel convexities, and that the elevation of these knickpoints above the tributaries' mouth increases towards the SE, in the TR's downstream direction. This observation can be best explained by a transient signal propagating upstream along the TR. An additional large non-lithological knickpoint along the TR, as well as a strong local drainage divide disequilibrium between the TR's tributaries and adjacent basins, suggest that waves of incision, potentially set by regional uplift, are propagating up the TR but not up the adjacent major river systems (the Mekong and eastern branch of Yangtze rivers). We propose these waves of incision are the first-order driving force for river arrangements in eastern Tibet. Accordingly, we reconstruct the evolution processes of landscapes in eastern Tibet.

Experimental study of the solitary wave induced groundwater hydrodynamics

Coastal groundwater hydrodynamics are affected by the swash uprush and backwash processes. In this study, laboratory experiments were carried out to investigate the solitary wave induced groundwater hydrodynamics over a coarse-grained sand beach. Pore water pressures at 27 cross-shore positions (covering both the subaqueous and subaerial areas) along 3 different beach depths were measured, and the synchronized shoreline position as well as the local water surface elevations (WSE) were also recorded. Temporal variation of the total water head (TWH) was found almost hydrostatically controlled by the instantaneous WSE in the subaqueous area. In the capillary truncated zone, the capillary effect contributes to the non-hysteresis phenomenon between the TWH and WSE. In other subaerial area, time-varying features of the TWH and WSE show clear differences in terms of their magnitudes and phases. Regarding the experimental observations, four representative regions could be identified along the cross-shore direction, i.e., Region I corresponding to the initially subaqueous area, Region II in between the still water shoreline and the top of capillary fringe (saturated beach), Region III in between the upper boundary of Region II and the maximum swash run-up, and Region IV landward beyond the Region III. With respect to the cross-shore distribution characteristics of the maximum TWH and the maximum WSE, as well as the time instants when the maximum TWH and the maximum WSE occur, different groundwater hydrodynamics were observed during the swash event in these four regions. In addition, spatiotemporal characteristics of the TWH were scrutinized and discussed. Both the vertical and horizontal TWH gradients in different regions were considered to reveal the sophisticated groundwater hydrodynamics. Finally, the groundwater movement pattern and its percolation circulation under the solitary wave swash were also specified. • Solitary wave induced groundwater hydrodynamics are investigated through detailed laboratory experiments. • Four regions are identified regarding the spatial characteristics of the maximum total water head and its occurrence time. • Hydrostatic effect, capillary effect and the wetting front are significant for the swash groundwater hydrodynamics. • Horizontal groundwater seepage flow is evident, even larger than the vertical one, during the entire swash process. • Two groundwater circulations are observed in the subaqueous and subaerial areas during the swash cycle.

Evaluation and Comparison of Dosimetric Characteristics of Semiflex®3D and Microdiamond in Relative Dosimetry under 6 and 15 MV Photon Beams in Small Fields

Background: In modern radiotherapy techniques, the frequently small and non-uniformed fields can increase treatment efficiency due to their highly conformal dose distribution. Particular features including lack of Lateral Charge Particle Equilibrium (LCPE) lead to detectors with high resolution since any error in obtained dosimetric data could cause patient mistreatments.Objective: This study aims to evaluate and compare two small detectors (Semiflex®3D and microdiamond) dosimetric characteristics in small field relative dosimetry.Material and Methods: In this experimental study, the dosimetric properties of Semiflex®3D and microdiamond were assessed under 6 and 15 MV photon beams. The linearity and stability of the detector’s response and dose rate were measured. Square-field sizes ranging from 0.6×0.6 - 5×5 cm2 were used for obtaining percentage depth dose curves (PDDs) and in-plane profiles. The angular and temperature dependence of both detectors’ responses were also studied.Results: The detector response shows good stability, no deviation from linearity, and low dose rate dependence (≤1.6%). PDDs and in-plan profiles of both detectors are in good agreement and no significant difference was observed except for the high dose gradient regions (P-value≤0.017). Both detectors demonstrated low angular dependence (<0.3%) with temperature dependence lower than 1% for both detectors.Conclusion: The results indicate both investigated detectors were well performed in small field relative dosimetry and for measuring penumbra, it is better to use microdiamond detector.

Gyrokinetic simulations of turbulence and zonal flows driven by steep profile gradients using a delta-f approach with an evolving background Maxwellian

Long global gyrokinetic turbulence simulations are particularly challenging in situations where the system deviates strongly from its initial state and when fluctuation levels are high, for example, in strong gradient regions. For particle-in-cell simulations, statistical sampling noise accumulation from large marker weights due to large deviations from the control variate of a delta-f scheme makes such simulations often impractical. An adaptive control variate in the form of a flux-surface-averaged Maxwellian with a time-dependent temperature profile is introduced in an attempt to alleviate the former problem. Under simplified collisionless physics, this adaptive delta-f scheme is shown to reduce noise accumulation in the zonal flows and the simulated heat flux in a quasi-steady turbulent state. The method also avoids the collapse of the signal-to-noise ratio, which occurs in the standard non-adaptive scheme, and, therefore, allows one to reach numerically converged results even with lower marker numbers.

Comparisons of soil organic carbon enrichment and loss in sediments among red soil, black soil, and loess in China

Water erosion could cause wide and serious soil organic carbon (SOC) loss, but differences in SOC loss and enrichment in sediments among red soil, black soil, and loess in China have received less attention. This study investigates the transport of sediments and generation regulation of runoffs during the erosion process by collecting data from indoor or outdoor artificial simulated rainfall experiments and selecting typical regional rainfall intensity and slope gradient for bare cultivate soil slopes as well as 5–8 m length and 1.5–2 m width runoff plots or soil pans. Then, the change in SOC loss for the three widely distributed and seriously eroded soils, from south to north in China, is clarified. Results show that the stable value and growth rate of soil and SOC loss rates followed the following order: black soil < red soil < loess. The SOC loss rate of loess was more sensitive to rainfall intensity and slope gradient than those of the two other soils. The SOC enrichment ratio (ERocs) of the sediments of the red soil and loess soil is higher than that of the black soil, and this difference increases as the soil loss rate decreases. ERocs generally has a negative exponential relationship with soil loss, but it has a negative logarithmic relationship with soil loss for the loess soil with high aggregate and clay contents. SOC and clay content determine the SOC enrichment in sediments for different soils. In addition, this study provides recommendations for improving SOC dynamic models for soil under water erosion.

Global gyro-kinetic study of magnetic shaping effects on linear trapped electron mode instability in normal/reversed magnetic shear plasmas

A linear δf version of the gyro-kinetic Vlasov code GKNET (Gyro-Kinetic Numerical Experiment of Tokamak) is extended to the global cylindrical coordinates (R, φ, Z) and includes the kinetic electron response, to study the shaping effect on linear drift-wave instabilities in Tokamak plasmas. Cross-verifications are performed between three GKNET versions that use different electron models, i.e. the adiabatic electron model, hybrid electron model (only trapped electrons are calculated kinetically) and full-kinetic electron model (all electrons are calculated kinetically). A destabilizing effect of non-adiabatic passing electrons is observed in the full-kinetic case for the ion temperature gradient (ITG) mode in the short wavelength region and the trapped electron mode (TEM). The ion-to-electron mass ratio and the electron–electron collisionality have weak impacts on the destabilization of ITG/TEM. Utilizing magnetohydrodynamic equilibria designed with normal and reversed magnetic shear (NS and RS), the characteristics of linear TEMs and the corresponding impact of plasma shaping are studied based on the GKNET code with full-kinetic electrons. Due to the change in temperature/density gradient and magnetic shear either measured locally in the low field side (LFS) or averaged over the flux surface, plasma shaping shows different impacts on linear TEMs in NS and RS configurations. For the elongation κ, the increase in κ always stabilizes linear TEMs due to the reduction in the effective profile gradient over the flux surface. For the triangularity δ, in the NS case, the change in δ shows weak dependence on TEM linear growth rates due to the balance between variations of profile gradients and magnetic shear in the LFS. In the RS case, local magnetic shear in the peak gradient region is nearly zero. Thus, the variation in local profile gradient plays a dominant role on the linear growth in a plasma shaping scan. Consequently, the negative triangularity (δ < 0) has a destabilizing effect on the linear TEMs in the RS configuration mainly due to the upshift in local profile gradient in the LFS.

Plant species composition and diversity along successional gradients in arid and semi-arid regions of China

Research into species composition and diversity in forest systems with successional stages can provide a scientific basis for optimal allocation and regulation of community structure. However, it is unclear how succession in arid and semi-arid forests changes the composition and diversity of species. The present study analyzed how species composition and diversity within successional gradients in arid and semi-arid areas of China evolved based on a spatially substituted temporal approach and identified the key factors influencing community composition and species diversity. Study findings have shown that species composition and life forms differ greatly between and among successional stages. In contrast, species richness is significantly lower in late-successional stages than in mid-successional stages. In the α-diversity, Shannon-Wiener diversity and Margalef richness reached an apex in the middle of the succession. At the same time, Pielou evenness and Simpson dominance did not change significantly over time. Except for Margalef richness, the α-diversity of different life forms throughout the successional gradient was consistent. The results of the β-diversity analysis indicate that the species turnover process has occurred during succession. It is evident that communities located at adjacent successional stages have high similarities in species composition. However, the loss or renewal of species was more advanced in the early and late stages, and the differences between communities were more evident. The main reasons for this can be attributed to differences in microhabitats (precipitation and temperature) and competition among species in various successional stages. It has been demonstrated that species diversity changes considerably in dry forest ecosystems as the ecosystem progresses through different stages of development. Effective conservation measures are needed to allow new species to establish themselves.

Plasma fireball-mediated ion implantation for nonvolatile memory application

Extraordinary performance of nonvolatile memories based on resistive switching is expected to fulfill the requirements of next-generation data-intensive technologies. However, due to the separate growth of functional layer and electrodes in the device, resistive random access memories still suffer from their implementation at a commercial scale. Herein, depending upon the polarity, a single device is used to grow Ti thin film followed by plasma fireball-mediated oxygen ion implantation to transform the top surface of Ti film into a functional TiOx layer to fabricate the device. The formation of TiOx layer at the near-surface region and oxygen-concentration gradient with depth in the implanted film is demonstrated. The current–voltage characteristics of the device analyzed at the nanoscale show forming-free bipolar resistive switching, which is further confirmed by the twofold erase-write process. At 5 × 1016 ions cm−2 fluence, resistive switching occurs at higher voltages, while at 5 × 1017 ions cm−2 fluence, it occurs at lower voltages. As an application, the rapid fabrication of periodic arrays of squared memory cells of different sizes is demonstrated. We propose a simple and cost-effective one-step technique to fabricate resistive switching-based nanoscale nonvolatile memory devices, which can be utilized for the production of high-density commercial devices at a large scale.

Climatic controls on stable carbon and nitrogen isotope compositions of temperate grasslands in northern China

The natural abundances of stable carbon (C) and nitrogen (N) isotopes (δ13C and δ15N) are extensively used to indicate the C and N biogeochemical cycles at large spatial scales. However, the spatial patterns of δ13C and δ15N in plant-soil systems of grasslands in northern China and their main driving factors across regional climatic gradient are still not well understood. We measured plant and soil δ13C and δ15N compositions as well as their associated environmental factors across 2000 km climatic gradient (-0.2 to 9 °C; 152 to 502 mm) in grasslands of northern China. The soil δ13C and δ15N values in surface were lower than those in bottom for temperate typical steppe but had no significant differences for temperate meadow steppe and temperate desert steppe. Soil δ13C values declined with increasing soil organic carbon (SOC) but increased as mean annual temperature (MAT). These changes were attributed to the microbial decomposition rate. The δ15N values in soil and plant were negatively correlated with MAT and mean annual precipitation (MAP), which were mainly related to the low soil organic matter mineralization rate and the shift of dominant species from C4 to C3. Our results indicate the spatial patterns and different influencing factors on δ13C and δ15N values along the climatic gradient in grasslands of northern China. The findings will provide scientific references for future research on the C and N biogeochemical cycles of temperate grasslands.

Precipitation controls the annual net primary productivity in sown pastures across a precipitation gradient in dry subtropical Chaco

Exotic pasture species are increasingly replacing native vegetation in subtropical areas because of their potential to increase forage yields. However, there is a scarce analysis of the relationship between forage yield and mean annual precipitation across regional gradients. We analyzed at a regional scale the temporal relationship between the aboveground net primary productivity (ANPP) and the precipitation for different sown grasses for several growing seasons across Dry Chaco, Argentina. We compiled biomass harvest data for seven cultivars of Megathyrsus maximus and Cenchrus ciliaris at eight different sites across a gradient of mean annual precipitation (MAP; 200 mm–1100 mm). We found positive relationships between ANPP and precipitation for most of the cultivars. The strong relationship between annual ANPP and MAP found implies that the high temporal variability in MAP translates into ANPP variability. The regional slope between annual ANPP and MAP was 11.12 kg DM.ha−1.year−1.mm−1. The ratio between the temporal and the spatial models ranged from 0.26 to 1.35. Precipitation use efficiency ranged between 3.9 and 24.1 kg DM.ha−1.year−1.mm−1. These models are robust enough to estimate the ANPP of specific pastures from MAP data. This represents a breakthrough, in grazing systems planning and defining conservation policies for natural resources.