Change In Density Gradient Research Articles

Investigation of fast ion effects on core turbulence in FIRE mode plasmas

Further investigation of fast ion effects on turbulence and transport in the fast ion regulated enhancement (FIRE) mode discharge (Han et al 2022 Nature 609 269–275) was performed in this work as a continuation of a previous study (Kim et al 2023 Nucl. Fusion 63 124001) that showed that the dominant turbulence suppression mechanism by fast ions is the dilution effect in the FIRE mode discharge. The current study includes (i) the impact of the fast ion relevant mode observed in the simulation of thermal energy flux, (ii) dilution effects by fast ions compared to dilution effects by other species, and (iii) fast ion effects on electron-scale turbulence. First, nonlinear gyrokinetic simulation results show that turbulence is significantly suppressed even without the fast ion relevant mode, indicating that the impact of this mode on thermal transport is not significant in this discharge. Second, further analysis on the dilution effects shows the three following results: Turbulence is not completely suppressed by the reduced main ion density fraction effect due to impurities; the reduction in energy flux can be limited by a certain impurity mode that is destabilized by a high impurity density gradient from adjusting the main ion density gradient; electrons can contribute to turbulence suppression through the main ion density gradient change, although this effect is less significant compared to other species. Third, we observe that two fast ion effects can influence the linear growth rate of the electron-scale turbulence mode. The growth rate decreases by an increase in β∗(≡(−8π/B2)dp/dr) and increases by dilution effects, suggesting that fast ion effects on electron-scale turbulence can differ depending on the operation scenario, such as the fast ion fraction. The comprehensive analysis performed in this study can enhance our understanding of fast ion physics, required for burning plasma operation in the future.

Last Interglacial subsurface warming on the Antarctic shelf triggered by reduced deep-ocean convection

The Antarctic ice-sheet could have contributed 3 to 5 m sea-level equivalent to the Last Interglacial sea-level highstand. Such an Antarctic ice-mass loss compared to pre-industrial requires a subsurface warming on the Antarctic shelf of ~ 3 °C according to ice-sheet modelling studies. Here we show that a substantial subsurface warming is simulated south of 60 °S in an equilibrium experiment of the Last Interglacial. It averages +1.2 °C at ~ 500 m depth from 70 °W to 160 °E, and it reaches +2.4 °C near the Lazarev Sea. Weaker deep-ocean convection due to reduced sea-ice formation is the primary driver of this warming. The associated changes in meridional density gradients and surface winds lead to a weakened Antarctic Circumpolar Current and strengthened Antarctic Slope Current, which further impact subsurface temperatures. A subsurface warming on the Antarctic shelf that could trigger ice-mass loss from the Antarctic ice-sheet can thus be obtained during warm periods from reduced sea-ice formation.

Analysis of the inhomogeneous LPG-air flow field in a tube containing mixed obstructions

Based on numerical simulation, this paper further investigates the flow field structure near the obstacle during the premixed gas deflagration. In the deflagration flow field, the pressure gradient variation and vortex structure intensity in the upper surface region of the obstacle are larger, indicating that the pressure gradient has a stronger effect on the vortex structure. The changes in density gradients and pressure gradients induced by the combined rectangular, flat barrier configuration will be more pronounced than in the model with only flat barriers. This change in turn acts back on the combustion field, which in turn has a strong perturbative effect on the flame.

Developing a platform for Fresnel diffractive radiography with 1 μm spatial resolution at the National Ignition Facility.

An x-ray Fresnel diffractive radiography platform was designed for use at the National Ignition Facility. It will enable measurements of micron-scale changes in the density gradients across an interface between isochorically heated warm dense matter materials, the evolution of which is driven primarily through thermal conductivity and mutual diffusion. We use 4.75keV Ti K-shell x-ray emission to heat a 1000 μm diameter plastic cylinder, with a central 30 μm diameter channel filled with liquid D2, up to 8eV. This leads to a cylindrical implosion of the liquid D2 column, compressing it to ∼2.3 g/cm3. After pressure equilibration, the location of the D2/plastic interface remains steady for several nanoseconds, which enables us to track density gradient changes across the material interface with high precision. For radiography, we use Cu He-α x rays at 8.3keV. Using a slit aperture of only 1 μm width increases the spatial coherence of the source, giving rise to significant diffraction features in the radiography signal, in addition to the refraction enhancement, which further increases its sensitivity to density scale length changes at the D2/plastic interface.

Impact of the electron density and temperature gradient on drift-wave turbulence in the Large Plasma Device

In this paper we present an experimental study of edge turbulence in the Large Plasma Device at UCLA. We utilize a scan of discharge power and prefill pressure (neutral density) to show experimentally that turbulent density fluctuations decrease with decreasing density gradient, as predicted for resistive drift-wave turbulence (RDWT). As expected for RDWT, we observe that the cross-phase between the density and potential fluctuations is close to 0. Moreover, the addition of an electron temperature gradient leads to a reduction in the amplitude of the density fluctuations, as expected for RDWT. However, counter to theoretical expectations, we find that the potential fluctuations do not follow the same trends as the density fluctuations for changes either in density gradients or the addition of a temperature gradient. The disconnect between the density and potential fluctuations is connected to changes in the parallel flows as a result of differences in the prefill pressure, i.e. neutral density. Further analysis of the density and potential fluctuation spectra show that the electron temperature gradient reduces the low frequency fluctuations up to $10 \,{\rm kHz}$ and the introduction of a temperature gradient leads to an unexpected ${\sim }{\rm \pi}$ shift of the density–potential cross-phase at ${\sim }10\,{\rm kHz}$ , while maintaining the typical resistive drift-wave cross-phase at lower frequencies. These experiments partly confirm existing knowledge on resistive drift-wave turbulence, but also introduce new observations that indicate a need for dedicated nonlinear three-dimensional turbulence simulations that include neutrals.

P05.14 Dosimetric Impact of Using the Acuros XB for Planning of SBRT of Centrally Located NSCLC

Stereotactic Body Radiation Treatment (SBRT) of lung cancer is applied to a heterogeneous density gradient. Dose modeling is highly dependent on the accuracy of the calculation algorithm. This study aims to assess the dosimetric implications calculated by Acuros External Beam (AXB) instead of the Anisotropic Analytical Algorithm (AAA) in SBRT of centrally located-Non small cell lung cancer (CL-NSCLC). Twelve IMRT plans for SBRT of CL-NSCLC were investigated retrospectively. The prescribed scheme was 50 Gy in 5 fractions every 2 days. The plans originally calculated by AAA aimed to cover 95% PTV by varying prescription isodose lines at 70 to 90%. These plans were recalculated by AXB (Dose to medium) and verified by the benchmark Monte Carlo (MC) method. Dosimetric parameters e.g. V20Gy Lungs, D95%(PTV), D98%(GTV), dose gradient (the ratio of 50% prescription isodose volume to the PTV, R50%), and doses to organs at risk (spinal cord, heart, esophagus, trachea, bronchus, and major vessels) were compared. AAA has generally overestimated the PTV dose compared to AXB and MC. The mean differences for D95%(PTV) and D98%(GTV) is 3.1% and 2.8% higher than that of AXB, respectively. The differences were significant with p-value<0.05. They were further verified by MC method, the results agreed with that of AXB. The agreement between AXB and MC in all PTV parameters were within 1.6% and the differences were not significant. No significant differences were also noted on V20Gy Lungs between the three algorithms but three cases were observed with maximum point dose to the lung tissue 1cm surrounding the PTV increased over 11% after the recalculation of AXB. Moreover, two cases were reported exceeding in heart dose and three cases were found degrading from none to minor deviation in dose gradient R50%. AXB has been investigated intensively and found to perform superiorly to AAA especially in calculating the rapid change in heterogeneous density gradient e.g. in regions of re-buildup in soft tissue after passed through lung/air. Although AXB and MC have two completely different way in solving Linear Boltzmann Transport Equation (LBTE), the agreement between AXB and MC was widely acceptable, AXB provides an accurate but faster alternative to MC method. In this study, a significant under-dosage in PTV and large dose difference in modeling tissue/air interface were observed after remodeled with AXB and MC. Since the PTV of SBRT for NSCLC is relatively small, the PTV missed in AAA may lead to a reduction in treatment outcome. Hence, AXB should be used in preference to AAA to model SBRT for centrally located NSCLC.

Edge-coherent oscillation providing nearly continuous transport during edge-localized mode mitigation by n = 1 resonant magnetic perturbation in HL-2A

An edge-coherent oscillation (ECO) with a bursting feature was observed in the steep-gradient pedestal region of H-mode plasmas in the HL-2A tokamak, where the type-I edge-localized modes (ELMs) were mitigated by application of n = 1 (n is the toroidal mode number) resonant magnetic perturbation (RMP). Utilizing a newly developed beam emission spectroscopy (BES) system, it was found that the ECO with a frequency of about 2 kHz is located at the edge pedestal region, and is excited by three-wave interaction of turbulence enhanced by the RMP field through the change of electron density gradient in the pedestal region because of the pump-out effect. The oscillation drives a significant outflow of particles as directly measured by probes, thus providing a channel for nearly continuous extra particle transport across the pedestal during ELM mitigation by RMP.

The Sensitivity of Southeast Pacific Heat Distribution to Local and Remote Changes in Ocean Properties

AbstractThe Southern Ocean features ventilation pathways that transport surface waters into the subsurface thermocline on time scales from decades to centuries, sequestering anomalies of heat and carbon away from the atmosphere and thereby regulating the rate of surface warming. Despite its importance for climate sensitivity, the factors that control the distribution of heat along these pathways are not well understood. In this study, we use an observationally constrained, physically consistent global ocean model to examine the sensitivity of heat distribution in the recently ventilated subsurface Pacific (RVP) sector of the Southern Ocean to changes in ocean temperature and salinity. First, we define the RVP using numerical passive tracer release experiments that highlight the ventilation pathways. Next, we use an ensemble of adjoint sensitivity experiments to quantify the sensitivity of the RVP heat content to changes in ocean temperature and salinity. In terms of sensitivities to surface ocean properties, we find that RVP heat content is most sensitive to anomalies along the Antarctic Circumpolar Current (ACC), upstream of the subduction hotspots. In terms of sensitivities to subsurface ocean properties, we find that RVP heat content is most sensitive to basin-scale changes in the subtropical Pacific Ocean, around the same latitudes as the RVP. Despite the localized nature of mode water subduction hotspots, changes in basin-scale density gradients are an important controlling factor on heat distribution in the southeast Pacific.

Bichromatic synthetic schlieren applied to surface wave measurements

The present paper provides an introduction to bichromatic synthetic schlieren (BiCSS) for surface measurements, a novel extension of the free surface synthetic schlieren (FS-SS) by Moisy et al. (Exp Fluids 46:1021–1036, 2009). The new technique is based on the fact that light diffraction through a medium varies with wavelength. Therefore, one may apply light at two different wavelengths to measure the change in density gradient in a medium. This paper explores the use of the difference between blue visual and near-infrared light, but the choice of wavelengths will typically depend on the application. Calibration was performed using stationary targets of plexiglass and the results show that the new BiCSS technique improves accuracy for large surface gradients, compared to the traditional FS-SS technique. In order to test the applicability of the technique in the laboratory, two sets of experiments were performed. Firstly, an experiment using phase-locked regular waves was conducted for comparing BiCSS with FS-SS, analyze the properties and give an estimation of the error. Secondly, to investigate the applicability for more complex surface patterns, a study on a vertical surface-piercing cylinder exposed to a focused wave was conducted, obtaining the complex surface characteristics. The new technique clearly reveals nonlinear wave diffraction, in addition to cross waves and parasitic capillary waves.

Tailorable Elastomeric Grating With Tunable Groove Density Gradient

This paper describes a new method for fabrication of tailorable elastomeric gratings with tunable groove density gradients based on stretching of the grating sample in a direction perpendicular to the grating line within the grating surface. The advantage of this method is that the tunable groove density gradient can be achieved by tailoring of the shape of a grating sample with uniform thickness. The groove density gradient changes as a specific function of its spatial position on the tailored surface of the elastomeric grating. The resulting tailorable elastomeric gratings can be used in the fabrication of devices such as optical position sensors and tunable optical filters.

The effect of transient density profile shaping on transport in large stellarators and heliotrons

Transport studies of pellet fuelling experiments on LHD are reported. Spatio-temporal evolutions after pellet injection into LHD discharges show cases where central density increases on the time scale of particle transport processes. Both the temperature gradient and the density gradient change during the density relaxation, the latter even in sign. The resulting thermodynamic forces influence radial electric fields—both as a driving term but also by, e.g. affecting the E r dependence of ion transport. Magnetic fluctuations have been found to be induced by pellet injection but die out with the relaxation of the pressure profile.

Negative Exponential Model Parameters and Centralization in Large Urban Areas in the U.S., 1950-2010

The change in the negative exponential density gradient has often been used as a measure of urban decentralization, with the gradient itself also being taken as a measure of the degree of centralization for comparing urban areas. Changes and levels of estimated negative exponential model parameters for 43 areas from 1950 to 2010 are compared with a “pure” measure of centralization, the centralization ratio. The proportional change in the density gradient is indeed significantly related to the change in the centralization ratio, while the relationship of the change in central density to centralization is inconsistent. However, the level of the density gradient is not significantly related to the level of centralization, except that controlling for the size of the urban area produces a significant relationship. The central density is related to the level of centralization.

A 3-dimensional DTI MRI-based model of GBM growth and response to radiation therapy.

Glioblastoma (GBM) is both the most common and the most aggressive intra-axial brain tumor, with a notoriously poor prognosis. To improve this prognosis, it is necessary to understand the dynamics of GBM growth, response to treatment and recurrence. The present study presents a mathematical diffusion-proliferation model of GBM growth and response to radiation therapy based on diffusion tensor (DTI) MRI imaging. This represents an important advance because it allows 3-dimensional tumor modeling in the anatomical context of the brain. Specifically, tumor infiltration is guided by the direction of the white matter tracts along which glioma cells infiltrate. This provides the potential to model different tumor growth patterns based on location within the brain, and to simulate the tumor's response to different radiation therapy regimens. Tumor infiltration across the corpus callosum is simulated in biologically accurate time frames. The response to radiation therapy, including changes in cell density gradients and how these compare across different radiation fractionation protocols, can be rendered. Also, the model can estimate the amount of subthreshold tumor which has extended beyond the visible MR imaging margins. When combined with the ability of being able to estimate the biological parameters of invasiveness and proliferation of a particular GBM from serial MRI scans, it is shown that the model has potential to simulate realistic tumor growth, response and recurrence patterns in individual patients. To the best of our knowledge, this is the first presentation of a DTI-based GBM growth and radiation therapy treatment model.

MIGRATION TRAPS IN DISKS AROUND SUPERMASSIVE BLACK HOLES

ABSTRACT Accretion disks around supermassive black holes (SMBHs) in active galactic nuclei (AGNs) contain stars, stellar mass black holes, and other stellar remnants, which perturb the disk gas gravitationally. The resulting density perturbations exert torques on the embedded masses causing them to migrate through the disk in a manner analogous to planets in protoplanetary disks. We determine the strength and direction of these torques using an empirical analytic description dependent on local disk gradients, applied to two different analytic, steady-state disk models of SMBH accretion disks. We find that there are radii in such disks where the gas torque changes sign, trapping migrating objects. Our analysis shows that major migration traps generally occur where the disk surface density gradient changes sign from positive to negative, around 20–300R g, where R g = 2GM/c 2 is the Schwarzschild radius. At these traps, massive objects in the AGN disk can accumulate, collide, scatter, and accrete. Intermediate mass black hole formation is likely in these disk locations, which may lead to preferential gap and cavity creation at these radii. Our model thus has significant implications for SMBH growth as well as gravitational wave source populations.

Interdecadal Baroclinic Sea Level Changes in the North Pacific Based on Historical Ocean Hydrographic Observations

Abstract Using historical ocean hydrographic observations, decadal to multidecadal sea level changes from 1951 to 2007 in the North Pacific were investigated focusing on vertical density structures. Hydrographically, the sea level changes could reflect the following: changes in the depth of the main pycnocline, density gradient changes across the pycnocline, and modification of the water mass density structure within the pycnocline. The first two processes are characterized as the first baroclinic mode. The changes in density stratification across the pycnocline are sufficiently small to maintain the vertical profile of the first baroclinic mode in this analysis period. Therefore, the first mode should represent mainly the dynamical response to the wind stress forcing. Meanwhile, changes in the composite of all modes of order greater than 1 (remaining baroclinic mode) can be attributed to water mass modifications above the pycnocline. The first baroclinic mode is associated with 40–60-yr fluctuations in the subtropical gyre and bidecadal fluctuations of the Kuroshio Extension (KE) in response to basin-scale wind stress changes. In addition to this, the remaining baroclinic mode exhibits strong variability around the recirculation region south of the KE and regions downstream of the KE, accompanied by 40–60-yr and bidecadal fluctuations, respectively. These fluctuations follow spinup/spindown of the subtropical gyre and meridional shifts of the KE shown in the first mode, respectively. A lag correlation analysis suggests that interdecadal sea level changes due to water mass density changes are a secondary consequence of changes in basin-scale wind stress forcing related to the ocean circulation changes associated with the first mode.

Intrinsic rotation generation in NSTX ohmic H-mode plasmas

Intrinsic rotation generation was observed and investigated in NSTX ohmic plasmas, by utilizing passive views of charge exchange recombination diagnostics. Focus was placed on ohmic L–H transitions to minimize the effects by other momentum exchange and sources such as the NTV torque by intrinsic error fields. Results indicated that intrinsic rotation generation in the edge is well correlated with ion temperature gradient change, compared with much weaker correlations with electron temperature gradient or density gradient change. This is consistent with a corresponding theory of residual stress, and the measured torque could be directly compared with the theoretical prediction using χi as a free parameter. However, an uncertainty on the order of diamagnetic rotation exists in many places across measurement and theory, as will be discussed in detail in this paper.

Effects of magnetic field on oscillatory structures in laser-blow-off plasma

Effects of magnetic field on the self-consistent oscillatory structures of the laser-blow-off plasma plume are studied using triple Langmuir probe. It is observed that trailing portion of floating potential profile shifted to higher value and this shift also varies with the strength of magnetic field, which is varied from 0 T to 0.15 T. Number of oscillatory structures decreases with increasing the strength of field. It is suggested that the change in density gradient in the presence of magnetic field could be the reason of reduction of oscillating structures.

STAR-FORMING GAS IN YOUNG CLUSTERS

Initial conditions for star formation in clusters are estimated for protostars whose masses follow the initial mass function (IMF) from 0.05 to 10 solar masses. Star-forming infall is assumed equally likely to stop at any moment, due to gas dispersal dominated by stellar feedback. For spherical infall, the typical initial condensation must have a steep density gradient, as in low-mass cores, surrounded by a shallower gradient, as in the clumps around cores. These properties match observed column densities in cluster-forming regions when the mean infall stopping time is 0.05 Myr and the accretion efficiency is 0.5. The infall duration increases with final protostar mass, from 0.01 to 0.3 Myr, and the mass accretion rate increases from 3 to 300 x 10^(-6) solar masses/yr. The typical spherical accretion luminosity is ~5 solar luminosities, reducing the luminosity problem to a factor ~3. The initial condensation density gradient changes from steep to shallow at radius 0.04 pc, enclosing 0.9 solar masses, with mean column density 2 x 10^(22) cm^(-2), and with effective central temperature 16 K. These initial conditions are denser and warmer than those for isolated star formation.

Delayed Advective Oscillation of the Atlantic Thermohaline Circulation

Abstract A simple dynamic model is proposed to illustrate the multidecadal oscillation of the Atlantic Ocean thermohaline circulation. The proposed oscillation relies on alternating actions of positive and negative feedbacks, which are operated by a slow adjustment of the ocean circulation and the associated time delay in the advective flux response to a change in meridional density gradient. The key element of the oscillation is the time delay, which is conceptually related to the basin-crossing time of long Rossby waves in the high-latitude North Atlantic. For a sufficiently long time delay, the solution becomes unstable in some regions of model parameter space and oscillates with a period of approximately 2 times the delay time.

Observation of E × B Flow Velocity Profile Change Using Doppler Reflectometry in HL-2A

A broadband, O-mode sweeping Doppler reflectometry designed for measuring plasma E × B flow velocity profiles is operated in HL-2A. The main feature of the Doppler reflectometry is its capability to be tuned to any selected frequency in total waveband from 26–40 GHz. This property enables us to probe several plasma layers within a short time interval during a discharge, permitting the characterization of the radial distribution of plasma fluctuations. The system allows us to extract important information about the velocity change layer, namely its spatial localization. In purely Ohmic discharge a change of the E × B flow velocity profiles has been observed in the region for 28 < r < 30 cm if only the line average density exceeds 2.2 × 1019 m−3. The density gradient change is measured in the same region, too.