Evolution Of Density Profile Research Articles

Simulating unsteady flows on a superconducting quantum processor

Recent advancements of quantum technologies have triggered tremendous interest in exploring practical quantum advantage. The simulation of fluid dynamics, a highly challenging problem in classical physics but vital for practical applications, emerges as a potential direction. Here, we report an experiment on the digital simulation of unsteady flows with a superconducting quantum processor. The quantum algorithm is based on the Hamiltonian simulation using the hydrodynamic formulation of the Schrödinger equation. With the median fidelities of 99.97% and 99.67% for parallel single- and two-qubit gates respectively, we simulate the dynamics of a two-dimensional (2D) compressible diverging flow and a 2D decaying vortex with ten qubits. Note that the former case is an inviscid potential flow, and the latter one is an artificial vortical flow with an external body force. The experimental results well capture the temporal evolution of averaged density and momentum profiles, and qualitatively reproduce spatial flow fields with moderate noises. This work demonstrates the potential of quantum computing in simulating more complex flows, such as turbulence, for practical applications.

Study of the evolution of density and particle transport during density relaxation phase after pellet fueling on the EAST tokamak

Abstract The impact of pellet injection depth on particle transport characteristics and density profile evolution during the post-pellet density relaxation phase is investigated on the experimental advanced superconducting tokamak. Experiments were conducted under similar background plasma conditions and the results showed that variations in pellet deposition depths led to differences in density profiles despite comparable core fueling efficiencies. By computing the effective particle transport coefficients immediately after the pellet injection, it is found that the differences in density profiles are mainly due to significantly different transport velocities in the inner region of ρ < 0.4. The potential influence of pellets on electrostatic turbulence in this region is investigated using the trapped gyro Landau fluid model and CO 2 laser collective scattering diagnostics. The simulation suggests that the increase in transport coefficients following pellet injection is attributed to the destabilization of lower-k modes rather than the higher-k modes within the diagnostic measurement range.

Particle transport in reduced turbulence neutral beam heated discharges at Wendelstein 7-X

A spontaneous reduction in anomalous particle transport in the plasma core is seen experimentally in reproducible, purely neutral beam heated plasma phases at Wendelstein 7-X (W7-X). Heating and fueling the plasma exclusively with the neutral beam injection system for several seconds leads to continuously peaking plasma density profiles with strong gradients inside mid minor radius. A significant acceleration of the density peaking occurs after a certain onset time and is examined with a detailed particle transport analysis in several discharges. By invoking the particle continuity equation, the total experimental radial electron flux is deduced from the time evolution of the electron density profile and the radially resolved particle sources. Subtracting the modeled neoclassical particle flux contribution gives the anomalous particle flux. Exploiting the evolving plasma conditions, anomalous diffusion and convection coefficients are computed from the flux variation with density and density gradients. In several discharges a significant and consistent change of the anomalous transport coefficients is seen when crossing a specific normalized density gradient length.

L-H Transition Initiated by Limit Cycle Oscillation Transition in the Globus-M Tokamak

The intermediate mode between the ohmic, or low confinement (L-mode), and the increased confinement (H-mode) regimes, or the so called I-phase, which is characterized by the existence of zonal flows in the form of limit cycle oscillations (LCOs), was observed on the Globus-M tokamak. Depending on the LCO frequency, the I-phase resulted in either a transition to H-mode or back to L-mode. The possibility of L-I-H transition initiation induced by LCOs and the effect of LCO frequency were studied by means of numerical modeling of the density profile evolution, taking into account turbulence suppression by the inhomogeneous radial electric field. The modeling results show that lower LCO frequency could be a factor facilitating the L-H transition, whereas higher frequency LCOs are more likely to cause the backward transition to L-mode. The results are in qualitative agreement with the results of the studies of geodesic acoustic mode (GAM)–initiated L-H transition in the TUMAN-3M tokamak, where a lower GAM frequency was found to be beneficial for L-H transition initiation.

Direct laser acceleration: A model for the electron injection from the walls of a cylindrical guiding structure.

We use analytical methods and particle-in-cell simulation to investigate the origin of electrons accelerated by the process of direct laser acceleration driven by high-power laser pulses in preformed narrow cylindrical plasma channels. The simulation shows that the majority of accelerated electrons are originally located along the interface between the channel wall and the channel interior. The analytical model based on the electron hydrodynamics illustrates the underlying physical mechanism of the release of electrons from the channel wall when irradiated by an intense laser, the subsequent electron dynamics, and the corresponding evolution of the channel density profile. The quantitative predictions of the total charge of released electrons and the average electron density inside the channel are validated by comparison with the simulation results.

Simulation of the time-dependent propagation of leak-triggered helium density gradients in cryogenic and nonisothermal accelerator vacuum systems

A novel simulation code for the calculation of the time-dependent evolution of helium density profiles in tubular-shaped beam vacuum systems of particle accelerators is presented. The code called TransVac was written in the statistics programming language R using the noncommercial development software RStudio and is based on an analytical approach. In contrast to earlier simulation tools based on analytical computational methods, the new code does not only master the profile calculation in conventional vacuum systems operated at room temperature, but also in fully cryogenic and in nonisothermal systems composed of cryogenic and warm sections. Data of helium adsorption isotherms are used to calculate gas densities profiles in cold vacuum systems with the cryosorption-based wall-pumping effect. The article discusses how the simulation code works and which mathematical algorithm is used. Comparisons between experimental and theoretical data confirm that the software developed provides sufficiently reliable predictions on the propagation behavior of leak-triggered helium pressure waves in cryogenic and room-temperature vacuum systems as well.

On learning latent dynamics of the AUG plasma state

In this work, we demonstrate the utility of state representation learning applied to modeling the time evolution of electron density and temperature profiles at ASDEX-Upgrade (AUG). The proposed model is a deep neural network, which learns to map the high dimensional profile observations to a lower dimensional state. The mapped states, alongside the original profile's corresponding machine parameters, are used to learn a forward model to propagate the state in time. We show that this approach is able to predict AUG discharges using only a selected set of machine parameters. The state is then further conditioned to encode information about the confinement regime, which yields a simple baseline linear classifier, while still retaining the information needed to predict the evolution of profiles. We, then, discuss the potential use cases and limitations of state representation learning algorithms applied to fusion devices.

Collisional Shaping of Nuclear Star Cluster Density Profiles

A supermassive black hole surrounded by a dense, nuclear star cluster resides at the center of many galaxies. In this dense environment, high-velocity collisions frequently occur between stars. About 10% of the stars within the Milky Way’s nuclear star cluster collide with other stars before evolving off the main sequence. Collisions preferentially affect tightly bound stars, which orbit most quickly and pass through regions of the highest stellar density. Over time, collisions therefore shape the bulk properties of the nuclear star cluster. We examine the effect of collisions on the cluster’s stellar density profile. We show that collisions produce a turning point in the density profile, which can be determined analytically. Varying the initial density profile and collision model, we characterize the evolution of the stellar density profile over 10 Gyr. We find that old, initially cuspy populations exhibit a break around 0.1 pc in their density profile, while shallow density profiles retain their initial shape outside of 0.01 pc. The initial density profile is always preserved outside of a few tenths of a parsec irrespective of initial conditions. We generalize this model to an arbitrary galactic nucleus and show that the location of the collisional break can be simply estimated from the nuclear properties. Lastly, we comment on the implications of collisions for the luminosity and color of stars in the collisionally shaped inner cluster.

Characterization of plasma-discharge Capillaries for Plasma-based Particle Acceleration

Novel particle accelerators based on plasma technology allow a drastic reduction in size, due to the high accelerating field established inside plasmas, which are created and confined by specific devices. Plasma Wakefield Acceleration experiments are performed at the SPARC_LAB test facility (Laboratori Nazionali di Frascati - INFN) by using gas-filled capillaries, in which the plasma formation is achieved by ionizing hydrogen gas through high voltage pulses. In this work, the characterization of gas-filled plasma-discharge capillaries is presented. Several geometrical configurations are tested, including capillaries with different channel shapes and arrangement of inlets positions for the gas injection. Such configurations are designed in order to enhance the uniformity of the plasma density distribution along the plasma channel, which is necessary to improve particle beam acceleration. Plasma sources are characterized by means of the spectroscopic technique based on the Stark broadening method, which allows to measure the evolution of the plasma density profile along the channel. In addition, the CFD software OpenFoam is used to simulate the dynamics of the neutral gas during the filling of the capillary.

Core transport modelling of the DTT full power scenario using different fuelling strategies

A theory-based integrated modelling work of plasma response to deuterium fuelling in the new Divertor Tokamak Test facility (DTT) is performed, using the 1.5D transport code JETTO with the quasi-linear anomalous transport model QuaLiKiz for the core region. The full power DTT scenario E1 is investigated. It is characterised by 28.8 MW of Electron Cyclotron Resonance Heating, 10 MW of Neutral Beam Injection and 6 MW of Ion Cyclotron Resonance Heating to the plasma. Plasma density and temperature profile evolution is calculated up to the separatrix using two different fuelling methods, gas puffing and pellet injection, and two different seeding gases, argon and neon. To sustain the desired pedestal density level with gas puffing a big amount of neutral flux at the separatrix is needed. The feasibility limits of the pumping system are exceeded, regardless of the type of impurity introduced, thus making the use of pellets mandatory. The simulations performed with pellet injection as fuelling method predict that the pedestal density is well sustained with realistic parameters foreseen for the DTT pellet injector. Strong dependence of the core density on the electron cyclotron resonance (ECR) power deposition profile is found. Trapped Electron Modes dominance, low outward flux and strongly hollow density in the inner core region are foreseen with central peaked ECR power deposition profile. Ion Temperature Gradient modes dominance, inward flux and robust density sustainment on the whole radial interval are predicted for spread ECR power deposition, though with central density close to the ECR cut-off limit and with peaked impurity densities. An intermediate deposition extension is found to sustain the whole density profile and to obtain flatter core densities, as previously predicted for the reference full power DTT scenario by fixed pedestal simulations. The ECR deposition is negligibly modified by refraction changes both during a single pellet cycle and after several pellet cycles, indicating full compatibility between the ECR system and the pellet injection system.

Combined analysis of laser interferometer and microwave reflectometer for a consistent electron density profile on HL-2A

In order to acquire a consistent and accurate electron density profile for HL-2A tokamak, a combined analysis of the laser interferometer and microwave reflectometer based on the mapping of density profiles to a common equilibrium magnetic coordinate has been proposed. In this approach, the edge density profile measured locally by microwave reflectometer is taken as a supplementary boundary condition in the procedure of inverting line-integrated data from interferometer diagnostic. The density profile inside that boundary is reconstructed by a Gaussian Process Tomography (GPT) method [1] within the Bayesian framework, by which the uncertainty of the result can be quantified via the confidence interval of a posterior probability. In the simulation benchmark, two representative cases of density distribution with different profile shapes, e.g. peaked and flat, are tested and the factors that may affect the accuracy of the results are investigated, providing a useful reference for practical applications. The application of this approach to actual data proves its ability to resolve the dynamic evolution of density profiles. Specifically, the effect of Electron Cyclotron Resonance Heating (ECRH) on electron particle transport observed in HL-2A experiments has been confirmed. The combined analysis approach developed is expected to provide a robust means to study the confinement performance and particle transport on HL-2A, with the focus on the core region.

Influence of Curing Agents Molecular Structures on Interfacial Characteristics of Graphene/Epoxy Nanocomposites: A Molecular Dynamics Framework

AbstractThis paper presents a comprehensive molecular dynamics study on the effects of the stoichiometric ratio of epoxy:hardener, hardener's linear and cyclic structure, and number of aromatic rings on the interfacial characteristics of graphene/epoxy nanocomposite. The van der Waals gap and polymer peak density as a function of the type of the hardener is calculated by analyzing the local mass density profile. Additionally, steered molecular dynamics are used to conduct normal pull‐out of graphene to study the effect of the mentioned features of hardeners on the interfacial mechanical properties of nanocomposites, including traction force, separation distance, and distribution quality of reacted epoxide rings in the epoxy. Influence of the hardeners on the damage mechanism and its initiation point are also studied by analyzing the evolution of local mass density profile during the normal pull‐out simulation. It is seen that stoichiometric ratio and geometrical structure of the hardeners affect the interfacial strength. It is also revealed that the hardener type can change the epoxy damage initiation point. The damage occurs in the interphase region for a higher stoichiometric ratio or cyclic structure of hardener. In comparison, for hardener's lower stoichiometric ratio and non‐cyclic structure, failure begins in the epoxy near graphene layers.

Development of Thomson Scattering Measurement System for Long Duration Discharges on the QUEST Spherical Tokamak

The Thomson scattering control system has been modified to measure the time evolution of the electron density and temperature profiles during long duration discharges in the QUEST spherical tokamak. The system consists of a signal generator and a control circuit. The former accepts a QUEST main trigger and provides multiple triggers, each of which starts a short-term (e.g. 15 s) measurement. The latter provides triggers to synchronize the oscilloscope and laser oscillation during a short-term measurement. The system was used in 1000 s long duration discharges in QUEST, and the temporal evolutions of density and temperature profiles were obtained successfully. It was found the profiles are stationary after about 300 s.

Extreme UV spectroscopy measurements and analysis for tungsten density studies in the WEST tokamak

The VUV emission of tungsten in WEST is measured by an absolutely calibrated grazing incidence spectrometer of the Schwob–Fraenkel type, which can scan the lower half of the plasma. We have analysed the detected spectral lines in the range 120–140 Å and compared their behaviour with calculations and published information. We obtained an unambiguous identification of four intense and well-resolved spectral lines emitted by W42+-W45+ close to the magnetic axis in the analysed experiments. The measured spectral-line brightnesses are used to assess the Tungsten density in the emission region. In the case of a scanning line of sight, we investigate the possibility to calculate the Tungsten density profile from the angular brightness profiles. In a case of a fixed line of sight, we deduce from the measurements the core Tungsten density profile evolution during a radiative collapse.

Transport simulations of pre-thermal quench shattered pellet injection in ITER: code verification and assessment of key trends

The material assimilation of the shattered pellet injection (SPI) in the pre-thermal quench phase in ITER is studied numerically by means of one-dimensional (1D) transport simulations. Such a simplified 1D approach is useful to perform extensive and systematic studies of key engineering parameters to optimize the ITER Disruption Mitigation System. The simulation results are compared with two-dimensional (2D) axisymmetric simulations by the nonlinear magnetohydrodynamic code JOREK for 5 neon/95 hydrogen SPI in the 15 MA hydrogen L-mode discharge to clarify the characteristics of SPI assimilation that can be analyzed within the range of the 1D model. Reasonable agreement between the 1D SPI simulation by the INDEX code and the 2D simulations by the JOREK code is found for total ablation rates, the radiation power, and the density and temperature profile evolution. The key process that was studied with the transport code is the onset of the radiative cold front that destabilizes the plasma current profiles. The injection parameters for the neon mixed hydrogen SPI are widely scanned to identify the timescale for the radiative cold front to form. Depending on the relative velocity of the cold front and the SPI fragments, the plasma cooling process can differ significantly. SPI with high injection velocities and large shard sizes results even in an inside-out plasma cooling that leads to hollow temperature profiles in the simulations reported here.

Generation of bottle beam using low-density channel in air

Cylindrical density depressions generated by femtosecond laser pulses filamenting in air for different energy depositions are investigated numerically, by using a set of hydrodynamic equations. The evolution of density profile is calculated for different temperature elevations. The results indicate that the gas density hole is getting shallower and wider with the increasing temperature elevations. A simulation of the propagation inside low-density channel implies a new way to generate a type of bottle beam.

Dark stars powered by self-interacting dark matter

Dark matter annihilation might power the first luminous stars in the Universe. These types of stars, known as dark stars, could form in $(10^6\mathrm{-}10^8)\,M_\odot$ protohalos at redshifts $z \sim 20$, and they could be much more luminous and larger in size than ordinary stars powered by nuclear fusion. We investigate the formation of dark stars in the self-interacting dark matter (SIDM) scenario. We present a concrete particle physics model of SIDM that can simultaneously give rise to the observed dark matter density, satisfy constraints from astrophysical and terrestrial searches, and address the various small-scale problems of collisionless dark matter via the self-interactions. In this model, the power from dark matter annihilation is deposited in the baryonic gas in environments where dark stars could form. We further study the evolution of SIDM density profiles in the protohalos at $z \sim 20$. As the baryon cloud collapses due to the various cooling processes, the deepening gravitational potential can speed up gravothermal evolution of the SIDM halo, yielding sufficiently high dark matter densities for dark stars to form. We find that SIDM-powered dark stars can have similar properties, such as their luminosity and size, as dark stars predicted in collisionless dark matter models.

Self-gravitating clusters of Fermi-Dirac gas with planar, cylindrical, or spherical symmetry: Evolution of density profiles with temperature.

We calculate density profiles for self-gravitating clusters of an ideal Fermi-Dirac gas with nonrelativistic energy-momentum relation and macroscopic mass at thermal equilibrium. Our study includes clusters with planar symmetry in dimensions D=1,2,3, clusters with cylindrical symmetry in D=2,3, and clusters with spherical symmetry in D=3. Wall confinement is imposed where needed for stability against escape. The length scale and energy scale in use render all results independent of total mass and prove adequate at all temperatures. We present exact analytic expressions for (fully degenerate) T=0 density profiles in four of the six combinations of symmetry and dimensionality. Our numerical results for T>0 describe the emergence, upon quasistatic cooling, of a core with incipient degeneracy surrounded by a more dilute halo. The equilibrium macrostates are found to depend more strongly on the cluster symmetry than on the space dimensionality. We demonstrate the mechanical and thermal stability of spherical clusters with coexisting phases.

Drivers of Low Salinity Effect in Carbonate Reservoirs Using Molecular Dynamic Simulation

• Molecular dynamic simulation of calcite-oil-brine system reveals that monovalent ions are settled at calcite-brine interface. • The time-dependent evolution of the mass density profile demonstrates oil detachment from the calcite mineral. • The extension of the charge density profile coincides to the detachment of oil from the calcite surface. • The MD results show that an electrical double layer (EDL) is formed at calcite-brine interface. • The extension of the charge density profile occure ofter the settling of the monovalent ions at calcite-brine interface. Low salinity water (LSW) flooding has yielded promising results in improving the enhanced oil recovery (EOR) techniques in sandstone and carbonate reservoirs. Despite various studies and advances, the main mechanism behind the oil displacement in the rock-brine-oil system is unclear. To get a better understanding of the governing mechanisms, we used the molecular dynamic (MD) method to simulate a system including calcite-brine-oil under two different scenarios. At T=300 K and T=360 K each scenario is completed during 40 ns using a canonical ensemble. The results show that all mono/divalent ions are hydrated initially. Furthermore, Na + , Cl − and H 2 O molecules reside close to the calcite surface and their positions show little change at different time steps. However, the position of divalent ions is dynamic depending on the simulation time steps. Ca 2+ , which is initially detected at 2.35 Å above the solid surface at T=300 K and T=360 K, migrates away by time. On the other hand, Mg 2+ and SO 4 2− which are invisible up to t = 16 ns even at the radius of 10 Å, move gradually toward the surface with time and consolidate their position after oil displacement. Namely, prior to oil displacement, a shift in Na + , Cl − and Ca 2+ locations is noticed, and the trend of double electric layer (EDL) is extended, which might be the cause of oil displacement. The findings of this study are applicable in the improvement of EOR technology, water resource treatment and contaminant transfer from geological systems, dealing with solid solutions such as ocean-soil systems.

Evolution equations dynamical system of the Lemaître–Tolman–Bondi metric containing coupled dark energy

In this paper, we consider inhomogeneous spherically symmetric models based on the Lemaître–Tolman–Bondi (LTB) metric, assuming as its source an interactive mixture of ordinary baryonic matter, cold dark matter and dark energy with a coupling term proportional to the addition of energy densities of both dark fluids. We reduce Einstein’s field equations to a first-order seven-dimensional autonomous dynamical system of evolution equations and algebraic constraints. We study in detail the evolution of the energy density and spatial curvature profiles along the phase–space by means of two subspace projections: a three-dimensional projection associated with the solutions of the Friedman–Lemaître–Robertson–Walker metric (invariant subspace) and a four-dimensional projection describing the evolution of the inhomogeneous fluctuations. We also classify and study the critical points of the system in comparison with previous work on similar sources, as well as solving numerically the equations for initial energy density and curvature profiles that lead to a spherical bounce whose collapsing time we estimate appropriately.