Surface Fluxes Research Articles

Biogenic CO2 flux uncertainty: numerical experiments and validation over south-eastern South America

Understanding carbon dioxide (CO2) surface fluxes is essential in the context of a changing climate. In particular, agriculture significantly contributes to CO2 fluxes. Recently, some studies have focused on understanding how synoptic-scale variability modulates CO2 fluxes associated with vegetation and agriculture, finding that frontal passages and precipitation events exert a strong influence on these fluxes. This variability is particularly relevant in the Argentinean Pampas, where large CO2 fluxes associated with extensive agriculture combine with strong synoptic variability. Numerical modelling provides a valuable tool for investigating surface CO2 fluxes and their variability, despite the uncertainties associated with the model’s limitations. In this work, we investigate simulated CO2 fluxes in the Argentinean Pampas using the Weather Research and Forecasting Model (WRF) coupled with the Vegetation, Respiration and Photosynthesis Model (VPRM) over three case studies representing different synoptic-scale conditions. In addition, we estimate the uncertainty in the simulations by comparing simulated CO2 fluxes using various WRF configurations and the ERA5 reanalysis. We found that the synoptic-scale conditions have a considerable impact on the magnitude of fluxes as well as the simulation uncertainty. Uncertainties in simulated CO2 fluxes can be as high as 60%, being larger at sunrise and sunset. Also, the largest uncertainty is associated with a case with a cold frontal passage and widespread precipitation. These results allow a more accurate estimation of CO2 flux uncertainty, which is key to understanding the effects of climate change.

Characterization of the SCR-1 Stellarator Physics: Investigating Plasma Discharge, MHD Equilibrium Calculations, and O-X-B Mode Conversion Feasibility

Abstract The Stellarator de Costa Rica 1 (SCR-1) is a small modular stellarator that serves as a valuable research and training tool for plasma magnetic confinement. This study aimed to analyze the characteristics of SCR-1, including its peripheral systems, technical plasma discharge processes, and advancements in plasma characterization. In addition, this study explored a new heating mechanism and the factors that influence it. The current state of the device and plasma discharge are initially presented. Subsequently, the measurement process was utilized to determine the electronic density and plasma temperature using a single Langmuir probe and the results were compared with theoretical predictions based on the particle and energy balance. Additionally, the VMEC code was employed to calculate magnetic flux surfaces with characteristics such as a low aspect ratio, low beta parameter, negative magnetic shear, and decreasing rotational transform along magnetic flux surfaces. The Mercier criterion was employed to conduct a linear stability analysis, which identified a magnetic well that played a crucial role in the linear stability of the majority of magnetic flux surfaces. Feasibility studies of electron Bernstein waves were conducted using the IPF-FMDC full-wave code. The results obtained from the IPF-FMDC code revealed that the O-X conversion percentage reached a maximum of 63 % when considering radiation reflection in the vacuum vessel. Significant effects of plasma curvature on the O-X wave conversion and normalized electron density scale length were observed, while the change in the SCR-1 heating position did not produce a significant impact. Three damping mechanisms affecting O-X conversion were studied, and one of the principal effects was the SX-FX conversion due to steep electron density gradient. Additionally, stochastic electron heating showed a low electron field amplitude, which is important for limiting the electron Bernstein wave propagation.

Zonally Asymmetric Increase in Southern Ocean Heat Content

Abstract A zonally symmetric perspective has proven to be very useful in studying the key role of the Southern Ocean for global ocean heat uptake. Despite this, reconstructions of changes in Southern Ocean heat content ΔOHC over the past few decades have revealed substantial deviations from symmetry. Here, we investigate the zonal asymmetry of ΔOHC and the processes driving it for the period 1979–2019 using an eddy-permitting ocean model forced by atmospheric reanalysis (ERA5). We find that the model successfully reproduces the observed zonal asymmetry in ΔOHC and that it is primarily due to zonal asymmetries in changing surface fluxes. Zonal asymmetries in mean ocean circulation play an important secondary role. Factorial simulations are used to attribute the asymmetry in ΔOHC to the different surface flux components (wind stress, heat, and freshwater fluxes), revealing strong regional differences. North of the Antarctic Circumpolar Current (ACC), we find roughly equal contributions from changes in wind stress and surface heat flux. Within the ACC, all three forcings play an important role, while south of it only wind stress and freshwater flux changes are important. Our findings argue for full consideration of the three-dimensional circulation to understand heat uptake and storage in the Southern Ocean, especially when considering multidecadal time scales where natural variations are as important as the long-term warming trend.

Contrasting Southern Ocean Sea Level Responses to Surface Flux Changes in Eddy-Rich and Eddy-Parameterized Climate Model Configurations

Abstract The impact of ocean model resolution on sea level projections in the Southern Ocean is investigated using eddy-rich (ER) and eddy-parameterized configurations of the Max Planck Institute Earth System Model under the Shared Socioeconomic Pathway (SSP) 5-8.5 scenario. We employ the Flux-Anomaly Forced Model Intercomparison Project (FAFMIP) experiment—heat, stress, and freshwater perturbations—at both resolutions to pinpoint the sources of these differences. South of 55°S, we found that the changes in thermosteric and halosteric sea levels vary substantially between resolutions due to different responses to freshwater perturbations. In the eddy-parameterized model, the resulting increase in stratification suppresses the mixing of salt and heat from the Circumpolar Deep Water with surface layers. These cause differences in the response of surface fluxes and meridional transports yielding an increase in thermosteric sea levels and a decrease in halosteric sea levels. In the eddy-rich configuration, the main driver of eddy-induced warming and salinification between 40° and 44°S is wind stress perturbations. The efficiency of direct eddy effects in ER is restricted to small areas such as the Agulhas Retroflection, the Brazil–Malvinas confluence zone, the Tasman Sea, and, to some extent, the Antarctic Circumpolar Current (ACC). Contrary to expectations, ACC transport increases in the eddy-rich model while decreasing in the eddy-parameterized model under the SSP5-8.5 scenario. FAFMIP results reveal that this decrease is a result of the overcompensation of wind-induced changes by freshwater flux forcing. These results underscore the critical importance of high-resolution models for capturing the processes in sea level projections in the Southern Ocean and beyond. Significance Statement We studied how ocean model resolution affects sea level projections in the Southern Ocean using Max Planck Institute Earth System Model simulations. Higher-resolution models provide a more accurate representation of ocean circulation and its response to changing forcings. We examined how surface heat, momentum, and water fluxes, both separately and combined, shape ocean dynamics. In a strong global warming scenario, significant differences in steric sea level change were observed south of 55°S between the model that simulates eddies and the one that has their effects parameterized. The response to surface freshwater forcing is the primary cause of these differences. Our findings emphasize the critical role of ocean model resolution in accurately understanding and predicting future sea level changes, which is essential for effectively addressing our needs for adaptation.

On the universality of star formation efficiency in galaxies

We analyze high-resolution hydrodynamics simulations of an isolated disk dwarf galaxy with an explicit model for unresolved turbulence and turbulence-based star formation prescription. We examine the characteristic values of the star formation efficiency per free-fall time, ϵff, and its variations with local environment properties, such as metallicity, UV flux, and surface density. We show that the star formation efficiency per free-fall time in ≈10 pc star-forming regions of the simulated disks has values in the range ϵff≈0.01−0.1, similar to observational estimates, with no trend with metallicity and only a weak trend with the UV flux. Likewise, estimated using projected patches of 500 pc size does not vary with metallicity and shows only a weak trend with average UV flux and gas surface density. The characteristic values of ϵff≈0.01−0.1 arise naturally in the simulations via the combined effect of dynamical gas compression and ensuing stellar feedback that injects thermal and turbulent energy. The compression and feedback regulate the virial parameter, αvir, in star-forming regions, limiting it to αvir≈3−10. Turbulence plays an important role in the universality of ϵff because turbulent energy and its dissipation are not sensitive to metallicity and UV flux that affect thermal energy. Our results indicate that the universality of observational estimates of ϵff can be plausibly explained by the turbulence-driven and feedback-regulated properties of star-forming regions.

Upper-ocean processes and sea ice production in the southeastern Amundsen Sea Polynya in austral autumn

Abstract The mixed layer of polynyas is vital for local climate as it determines the exchange of properties and energy between ocean, sea ice and atmosphere. However, its evolution is poorly understood, as it is controlled by complex interactions among these components, yet highly under-sampled, especially outside summer. Here we present a two-month, high vertical resolution, full-depth hydrographic dataset from the southeastern Amundsen Sea Polynya in austral autumn (mid-February to mid-April 2014) collected by a recovered seal tag. This novel dataset quantifies the changes in upper ocean temperature and salinity stratification in this previously unobserved season. Our seal-tag measurements reveal that the mixed layer experiences deepening, salinification, and intense heat loss through surface fluxes. Heat and salt budgets suggest a sea ice formation rate of ∼3 cm per day. We use a one-dimensional model to reproduce the mixed-layer evolution and further identify key controls on its characteristics. Our experiments with a range of reduced or amplified air-sea fluxes show that heat loss to the atmosphere and related sea ice formation are the principal determinants of stratification evolution. Additionally, our modelling demonstrates that horizontal advection is required to fully explain the mixed-layer evolution, underlining the importance of the ice-covered neighbouring region for determining sea ice formation rates in the Amundsen Sea polynya. Our findings suggest that the potential overestimation of sea ice production by satellite-based methods, due to the absence of oceanic heat flux, could be offset by horizontal advection inhibiting mixed layer deepening and sustaining sea ice formation.

Vacuum vessel design with lofted toroidal surfaces for a QHS configuration

A vacuum vessel in between the hot plasma and the magnetic coils is crucial for a fusion reactor. In this work, a tool is presented to perform the initial design of such a vessel automatically. Starting from the last closed flux surface, an algorithm is developed which lofts this manifold out as much as possible. At the same time, the algorithm makes sure that no intersection with the coils takes place. In the presented work, this algorithm is applied to the HSX stellarator and it has been verified that the resulting vessel wall can be built from an engineering perspective. Using field line tracing, the strikepoints on the vessel wall indicate the initial locations for a divertor.

In pursuit of precise Ca II H&K chromospheric surface fluxes

The emission lines of the Ca II H&K doublet present one of the most important channels of radiative cooling for the chromospheres of cool stars. Although most other line emissions (Mg II h&k and numerous iron lines) populate the far-UV and require a very competitive space-born observing time, the Ca II H&K lines in the optical UV are easily accessible by ground-based spectroscopy with relatively cheap instrumentation, and the observational data are plentiful. At the same time, in order to include realistic mechanical and magnetic heating, advanced chromospheric models now require radiative cooling losses in absolute terms and therefore call for a precise surface flux scale, which could be provided by matching a photospheric Ca II H&K line profile computed by photospheric models. However, a major obstacle here is the significant ambiguity in parameter space in the face of a very sensitive dependence of cool stellar optical UV surface fluxes on the effective temperature. Consequently, we have developed a rigorous method by which precise physical parameters, most notably the effective temperature, are first determined by ISPEC tools working on high signal-to-noise spectra and based on a suitable line list and reference continuum. However, crosstalk and navigation of multiple local chi-square minima (best solutions) in parameter space must be considered. Only with the optimal set of parameters is a single PHOENIX model calculated, which defines the spectral surface flux scale in the Ca II H&K line region. In this paper, we discuss the results of the absolute measurements of Ca II K line fluxes for 32 representative cool stars and the accuracy of this approach. Finally, we show a comparison of the Ca II K chromospheric fluxes with line widths, and a relation of chromospheric flux with effective temperature and gravity following the logarithmic form log FCa II = α′ log g + β′ log Teff + C′.

Seepage and stability analysis of hydraulically anisotropic unsaturated infinite slopes under steady infiltration

An analytical model is derived for predicting the flow field and stability of an unsaturated infinite slope subjected to steady infiltration. The proposed model is novel because it accounts for the hydraulic anisotropy of unsaturated soil. The governing equation for steady-state seepage in an infinite slope is established in terms of matric suction under a constant surface flux boundary condition. On the basis of the available experimental findings on the hydraulic anisotropy behavior of unsaturated soils, the relative hydraulic conductivity for a soil under unsaturated conditions with respect to the soil at saturation is postulated to be a direction-independent scalar. This postulation simplifies the governing equation to a form that is directly solvable via the relative hydraulic conductivity and the saturated hydraulic conductivity tensor. To enable sophisticated applications, an exponential law and a power law that are well established in the unsaturated soil literature are used to relate the relative hydraulic conductivity to the matric suction and the effective degree of saturation, respectively. Closed-form solutions are derived for the matric suction, the flow net (potential function and stream function), and the effective degree of saturation. Analytical solutions are also derived for the soil unit weight and overburden stress. These solutions are incorporated into the unsaturated infinite slope stability formula constructed on a suction stress-based effective stress failure criterion. Hydraulic anisotropy has been shown to directly affect the flow field and the change in matric suction, which, in turn, drastically affects the slope safety factor against shallow landslides. This finding demonstrates that neglecting hydraulic anisotropy can cause a considerable overestimation of the safety factor, resulting in an unsafe slope stability prediction. The proposed model is useful for preliminary evaluation of the long-term stability of unsaturated slopes during wet periods and the antecedent slope conditions for shallow landslide initiation under transient infiltration.

Soft x-ray tomography using L1 regularization for MHD modes with limited sight lines in JT-60SA.

Soft x-ray (SX) tomography is a useful diagnostic in fusion research, and a multi-channel SX diagnostic will be installed in JT-60SA, the largest elongated tokamak in the world. However, in the SX diagnostic of JT-60SA, plasmas will be only viewed from the low field side and the upper side of plasmas; the sight lines are limited, which would be common in future devices as well as JT-60SA. This kind of limited sight lines is not preferred for SX tomography to investigate the spatial structure of magnetohydrodynamics (MHD) modes because inadequate information of plasmas makes artifacts in the reconstructed SX profiles. One of the solutions to reduce the artifacts is to employ L1 regularization, which gives the essential and sparse contributions [Kaptanoglu etal., Phys. Plasmas 30, 033906 (2023)]. In this study, as a first topic, the applicability of L1 regularization to reduce the artifacts in SX tomography with limited sight lines is investigated with traditional L2 regularization for a high beta scenario of JT-60SA where MHD modes would occur. Here, as a series of basis functions, the Fourier-Bessel series (FBS) is employed because FBS has the poloidal Fourier modes explicitly. A disadvantage of FBS is that the accurate equilibrium inside the last closed flux surface (LCFS) is needed; interior measurement such as the motional Stark effect measurement is required, which is not always available during a whole discharge. The second topic of this study is to investigate other appropriate basis functions to study the spatial structure of MHD modes in elongated tokamak plasmas. Here, we introduce Saito's Laplacian eigenfunction (LEF). Saito's LEF can be calculated if LCFS is given and the LEF is expected to show the explicit poloidal Fourier mode. Because the calculation of LCFS with magnetic measurements is a basic task of plasma operations, Saito's LEF may be used anytime. Our investigation showed that L1 regularization can strongly improve the SX tomography with the traditional L2 regularization having FBS/LEF and would be effective against other tomographic problems in fusion devices.

An asymptotic Grad–Shafranov equation for quasisymmetric stellarators

A first-order model is derived for quasisymmetric stellarators where the vacuum field due to coils is dominant, but plasma-current-induced terms are not negligible and can contribute to magnetic differential equations, with $\beta$ of the order of the ratio induced to vacuum fields. Under these assumptions, it is proven that the aspect ratio must be large and a simple expression can be obtained for the lowest-order vacuum field. The first-order correction, which involves both vacuum and current-driven fields, is governed by a Grad–Shafranov equation and the requirement that flux surfaces exist. These two equations are not always consistent, and so this model is generally overconstrained, but special solutions exist that satisfy both equations simultaneously. One family of such solutions is the set of first-order near-axis solutions. Thus, the first-order near-axis model is a subset of the model presented here. Several other solutions outside the scope of the near-axis model are also found. A case study comparing one such solution to a VMEC-generated solution shows good agreement.

Investigation on the density shoulder near the separatrix through monostatic reflectometry in EAST tokamak

Abstract The density shoulder represents a universal physical phenomenon that is closely related to the particle and energy transports occurring within the scrape-off layer (SOL) region of tokamak devices. A novel method has been developed to identify the density shoulder through the analysis of the bump structure in the time delay spectra from monostatic microwave reflectometry in EAST tokamak, obviating the need for density profile reconstruction. The density shoulder in EAST is characterized by a number of distinctive features. The density shoulder is mostly situated at a distance of 0–3 cm from the last closed flux surface, with a width of a few centimeters. No significant correlation is observed between its occurrence and the auxiliary heating or the confinement state. In the case of H-mode with quasi-coherent mode (QCM), a significant and positive correlation is observed between the density shoulder amplitude and QCM intensity. In the case of grassy-edge localized mode (ELM)-like H-mode, a density shoulder is also observed during the inter-ELM stage. Furthermore, as supersonic molecular beam injection (SMBI) deposits occur within the range of ρ = 0.9 ∼ 1, the density shoulder is also enhanced during the SMBI fueling process. Moreover, it appears that the neutral pressure has a more pronounced impact on the overall offset of the density profile than the strength of the density shoulder. These results collectively indicate that the outward particle transport from the pedestal to the SOL region plays a crucial role in the evolution of the density shoulder.

Improved erosion estimates for the STEP divertor

Net tungsten erosion and deposition profiles are simulated for the outer and inner lower divertor of STEP, a spherical prototype fusion reactor targeting ∼ 2040. In this contribution, previous modelling work [1] has been extended by studying the influence of various input parameters. Main aim is to analyse the influence of several modelling improvements on the erosion/deposition characteristics at the divertor targets. This comprises the consideration of an electron density decay according to the Boltzmann relation within the sheath region as well as a certain tungsten influx to the divertor originating from main wall erosion. Also, for the inner divertor improved background plasma parameters are applied taking into account a gradient along the flux surfaces compared to formerly constant electron temperature and density and ion temperature along the flux surfaces. ERO simulations have been performed for one selected plasma case with Ar seeding, both for the outer (with peak target Te ∼ 25 eV) and inner lower divertor (with peak target Te ∼ 3 eV).The simulated tungsten erosion and redeposition profiles do not significantly change for the cases studied if the Boltzmann-related decay of electron density within the sheath is considered. The assumption of tungsten within the background plasma can notably alter the overall erosion/redeposition behaviour at a background tungsten concentration of 1/10 with respect to the Ar divertor concentration. The inner divertor target plate shows net deposition everywhere, while at the outer one the net erosion zone becomes smaller and the maximum net erosion peak reduces by a factor of two. Lastly, the improved background plasma for the inner divertor has moderate effects, for instance the overall tungsten redeposition at the target plate increases from 88.7% to 94.4%.

Characterizing the Impacts of 2024 Total Solar Eclipse Using New York State Mesonet Data.

On 8 April 2024, a rare total solar eclipse (TSE) passed over western New York State (NYS), the first since 1925 and the last one until 2079. The NYS Mesonet (NYSM) consisting of 126 weather stations with 55 on the totality path provides unprecedented surface, profile, and flux data and camera images during the TSE. Here we use NYSM observations to characterize the TSE's impacts at the surface, in the planetary boundary layer (PBL), and on surface fluxes and CO2 concentrations. The TSE-induced peak surface cooling occurs 17min after the totality and is 2.8°C on average with a maximum of 6.8°C. It results in night-like surface inversion, calm winds, and reduced vertical motion and mixing, leading to the shallowing of the PBL and its moistening. Surface sensible, latent and ground heat fluxes all decrease whereas near-surface CO2 concentration rises as photosynthesis slows down.

Origins of Extreme CAPE Around the World

AbstractSevere convection, responsible for hazards such as tornadoes, flash floods, and hail, is usually preceded by abundant convective available potential energy (CAPE). In this work, we use a Lagrangian approach to study the buildup of anomalously large values of CAPE from 2012 to 2013 in various regions. Nearly all extreme values of CAPE arise from surface fluxes underneath a layer of convective inhibition (the CIN layer) over several diurnal cycles, but the origin of the CIN layer and the diurnal cycle of surface fluxes differ around the world. In some regions, such as North America and Europe, the air above the boundary layer must be much warmer than usual to form this CIN layer, whereas in other regions, especially the Middle East and central Africa, a CIN layer is common. Additionally, high CAPE occurrences that are over land (those in the Americas, Europe, Africa, and Southeast Asia) tend to lose their CIN layers before the time of maximum CAPE due to large diurnal cycles of sensible heating, whereas those that occur over coastal waters (in the Middle East, Northern Australia, South Asia, and the Mediterranean) usually retain substantial convective inhibition. Uniquely, CAPE in Southeast Australia often builds up due to cooling aloft rather than to boundary layer warming. These results show that one hoping to understand or predict CAPE patterns must understand a variety of mechanisms acting in different regions.

Quantifying the Role of Ocean Dynamics in SST Variability across GCMs and Observations

Abstract Midlatitude SSTs forced by mesoscale oceanic processes can affect the large-scale atmosphere, pointing to the ocean’s crucial role outside the tropics. Previous studies have shown oceanic mesoscale processes’ effect on global and regional climate variability. This study quantifies the local contribution of ocean dynamics to mixed-layer temperature across the globe by directly estimating the ocean heat flux divergence resolved by state-of-the-art ocean reanalysis, eddy-resolving, and eddy-parameterized versions of two U.S. national climate models and indirectly from air–sea flux satellite-based estimates. Our results show that the eddy-resolving climate simulations resolve mixed-layer temperature variances that are larger and closer to those inferred from observations than both their eddy-parameterized counterparts and ECCO over much of the extratropics. The observations and the eddy-resolving models indicate a more significant role of ocean dynamics in the mixed-layer temperature variability than the surface fluxes over most extratropics compared to their eddy-parameterized versions. A frequency domain analysis shows that the better-resolved ocean mesoscale and thermal gradients enhance the variance over a time scale from 2 months to 30 years. Results show agreement in the ocean’s contribution among satellite-based estimates, ocean reanalysis products, and ocean eddy-resolving simulations. At the same time, differences emerge for ECCO and the eddy-parameterized models, suggesting that surface fluxes account for a larger fraction of the mixed-layer temperature variability in most of the extratropics.

The effect of vertical displacements on the runaway electron avalanche in ITER mitigated disruptions

Abstract In order to prepare the operation of ITER, it is important to estimate the amplitude of the runaway electron (RE) current that could appear following disruptions at large plasma currents. Theoretically, the avalanche gain of RE population on a toroidal flux surface (TFS) is an exponential function of the change in the poloidal magnetic flux Δψ on that TFS. However, once a TFS is open, REs on it escape rapidly and can no longer proliferate. It is expected that the vertical motion of the plasma during a disruption could lead to the opening of TFSs as the plasma cross-section is pushed into the wall. The influence of this vertical motion on the RE avalanche gain was rarely considered in previous studies. This study investigates the RE generation in ITER major disruptions and hot vertical displacement events mitigated by the injection of neon and hydrogen, using an axisymmetric 2D model on the JOREK code. Compared to those with a stationary plasma, our simulations show a smaller Δψ on a given TFS by the time it becomes open as a result of the vertical motion. The maximum potential avalanche gain is thus reduced from 10 16 − 10 20 without considering the vertical motion to 10 9 − 10 12 , depending on the plasma dynamics during the disruption. In addition, the RE current produced by a typical nuclear seed (∼ 10 mA) at ITER is investigated.

Studies of beam ion confinement to enhance plasma performance on EAST

A key physics issue for achieving steady-state high-performance plasmas on EAST tokamak is to decrease beam-ion losses to improve plasma confinement during neutral beam injections (NBIs). To decrease the beam losses, previous counter-I p NBI injections are upgraded to co-I p injections. Analysis shows that due to the reversed direction of drift across the flux surfaces caused by the pitch angle, the beam prompt loss fraction decreases from about 49% to 3% after the upgrade. Moreover, because of the change of entire beam path, beam shine-through (ST) loss fraction for counter-I p tangential and counter-I p perpendicular injections is reversed to co-I p tangential and co-I p perpendicular injections, respectively. Due to the change in the initial trapped-confined beam ion fraction caused by the peaked pitch profiles, the losses induced by toroidal ripple field are also reversed after the upgrade. To further improve the beam-ion confinement under the present NBI layout, the amplitudes of toroidal field are increased from 1.75 to 2.20 T. Result shows that, due to the smaller orbit width and peaked pitch angle profile, the beam prompt loss power is lower with higher toroidal field. Due to the synergy of higher initial trapped-confined beam ion fraction and narrower Goldston-White-Boozer (GWB) boundary, the loss induced by ripple diffusion is higher with higher toroidal field. The combined effect of beam ST loss, prompt loss and ripple loss, contributes to the increase in beam ion density. The decrease in beam loss power enhances beam heating efficiency, especially the fraction of beam heating ions. Finally, comparison between simulation and measurement by 235U fission chamber (FC) indicates that the increase in neutron rate is mainly contributed by improvement of beam-ion confinement. This study can provide potential support for beam operation and high-T i experiment on EAST tokamak.

Impact of Surface Turbulent Fluxes on the Formation of Roll Vortices in a Mediterranean Windstorm

AbstractRoll vortices can have a significant influence on the exchanges of momentum, sensible heat and moisture throughout the boundary layer. They have been shown to reinforce air‐sea interactions under strong wind conditions. This raises the question of how air‐sea exchanges can, in turn, affect the roll vortices. However, representing surface turbulent fluxes during extreme wind conditions is a major challenge in numerical weather prediction and can lead to large uncertainties in surface wind speed. The sensitivity of rolls to different representations of surface fluxes is investigated using large eddy simulations. The study focuses on Mediterranean windstorm Adrian, where rolls are responsible for the transport of strong winds to the surface. Considering the effects of sea spray increases surface heat fluxes and significantly influences the roll morphology. The rolls become narrower and extend over a greater height, while the downward transport of momentum is intensified and results in higher wind speed near the surface. Roll vortices vanish within a few minutes in the absence of momentum fluxes, which maintain the wind shear necessary for their organization. They also quickly weaken without sensible heat fluxes, which sustain the near‐neutral stratification required for their development. In contrast, latent heat fluxes play a minor role. These findings emphasize the necessity of precisely representing the processes occurring at the air‐sea interface, which do not only affect the thermodynamic surface conditions but also the vertical transport of momentum within the windstorm.

ECW assisted plasma startup with low toroidal electric field and full metal wall in EAST superconducting tokamak

Abstract Recent experimental results from the EAST superconducting tokamak with a full metal wall (without Li-coating) provide strong support for the ITER startup scheme with electron cyclotron wave (ECW) assistance. Detailed investigations were performed on the impact of different magnetic field geometries, including quadrupolar field configuration (QFC), wide null field configuration (NFC), and trapped particle configuration (TPC). All three configurations enabled the breakdown of neutral gas with ECW pre-ionization. However, in most discharges with QFC, the absence of closed flux surfaces after the breakdown was associated with unsuccessful plasma startups. Conversely, both NFC and TPC achieved successful plasma startups under normal conditions, with TPC demonstrating higher robustness against high impurity levels compared to NFC. Experiments found that the minimum ECW power required for a successful TPC startup is approximately 0.48 MW. Plasma startup failed if the prefill gas pressure exceeded optimal levels for a given ECW power (≥0.48 MW), with the critical gas pressure linearly dependent on the ECW power. Additionally, higher ECW power was required with increased impurity levels. With ECW assistance, the toroidal electric field at R=1.85 m in EAST routine operations (<0.15 V/m) is significantly lower than ITER's maximum value (0.3 V/m), suggesting the possibility of reducing flux consumption and lowering demands on coils and power supplies for ITER.