Relative Density Fluctuations Research Articles

Effects of plasma resistivity in FELTOR simulations of three-dimensional full-F gyro-fluid turbulence

A full-F, isothermal, electromagnetic, gyro-fluid model is used to simulate plasma turbulence in a COMPASS-sized, diverted tokamak. A parameter scan covering three orders of magnitude of plasma resistivity and two values for the ion to electron temperature ratio with otherwise fixed parameters is setup and analysed. Two transport regimes for high and low plasma resistivities are revealed. Beyond a critical resistivity the mass and energy confinement reduces with increasing resistivity. Further, for high plasma resistivity the direction of parallel acceleration is swapped compared to low resistivity.Three-dimensional visualisations using ray tracing techniques are displayed and discussed. The field-alignment of turbulent fluctuations in density and parallel current becomes evident. Relative density fluctuation amplitudes increase from below 1% in the core to 15% in the edge and up to 40% in the scrape-off layer.Finally, the integration of exact conservation laws over the closed field line region allows for an identification of numerical errors within the simulations. The electron force balance and energy conservation show relative errors on the order of 10−3 while the particle conservation and ion momentum balance show errors on the order of 10−2.All simulations are performed with a new version of the FELTOR code, which is fully parallelized on GPUs. Each simulation covers a couple of milliseconds of turbulence.

Statistical analysis of magnetic divertor configuration influence on H-mode transitions

DIII-D plasmas are compared for two upper divertor configurations: with the outer strike point on the small angle slot (SAS) divertor target and with the outer strike point on the horizontal divertor target (HT). Scanning the vertical distance between the magnetic null point and the divertor target over a range 0.10–0.16 m is shown to increase the threshold power, Pth , and edge plasma power, PLoss , for the low-to-high confinement (L–H) and H–L transitions respectively, by up to a factor of 1.4. The X-point height scans were performed at three L-mode core plasma line average electron densities, n¯e= 1.2, 2.2 and 3.6 ×1019m−3 , to investigate the density dependence of divertor magnetic configuration influence on Pth . The X-point height, Zx-pt , was further extended across the range 0.16–0.22 m with the more open HT divertor configuration, for which a clear decrease in Pth with increasing Zx-pt is observed. The dependence of Pth on divertor magnetic geometry is further investigated using a time-dependent probability density function (PDF) model and information geometry to elucidate the roles played by pedestal plasma turbulence and perpendicular velocity flows. The degree of stochasticity of the plasma turbulence is observed to be sensitive to the plasma heating rate. The calculated square of the information rate shows changes in the relative density fluctuations and perpendicular velocity PDFs begin 2–5 ms prior to the L–H transition for three plasmas; providing a crucial measurement of the dynamic timescale of external transport barrier formation. Additionally, both information length and rate provide potential predictors of the L–H transition for these plasmas.

Enhancing Triangulation of Interplanetary Type III Bursts through Wavevector Correction

Interplanetary Type III bursts, generated by relativistic electron beams at solar flare reconnection sites, are explored through an investigation of 152 instances observed by the Solar Terrestrial Relations Observatory mission. This study reveals that the absolute values of the wavevector deviations from the Sun–spacecraft line are statistically 3.72 and 2.10 larger than predicted by the density model, assuming fundamental and harmonic emission, respectively. Through Monte Carlo simulations, we examine the impact of scattering by density inhomogeneities on the apparent locations of radio emissions in the interplanetary medium. The findings indicate that relative density fluctuations of 0.40 can account for the observed angular shift, a conclusion supported by the multiple flux-tube solar wind model, which confirms the presence of such magnitude of relative perpendicular density fluctuations in the solar wind. We propose a wavevector correction that incorporates this effect to enhance the triangulation of interplanetary Type III bursts, demonstrating that radio triangulation, with this correction, can reliably track electron beams in the interplanetary medium.

Study of neutron density fluctuation and neutron-proton correlation in Au+Au collisions using PYTHIA8/Angantyr* *Supported in part by the Natural Science Foundation of Henan Province, China (212300410386), the Key Research Projects of Henan Higher Education Institutions (20A140024), the Scientific Research Foundation of Hubei University of Education for Talent Introduction (ESRC20220028, ESRC20230002), the NSFC (12005114) and NSFC key Grant (12061141008), and the

Utilizing the PYTHIA8 Angantyr model, which incorporates the multiple-parton interaction (MPI) based color reconnection (CR) mechanism, we study the relative neutron density fluctuation and neutron-proton correlation in Au+Au collisions at = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV. In this study, we not only delve into the dependence of these two remarkable observations on rapidity, centrality, and energy, but also analyze their interplay with the MPI and CR. Our results show that the light nuclei yield ratio of protons, deuterons, and tritons, expressed by the elegant expression , remains unchanged even as the rapidity coverage and collision centrality increase. Interestingly, we also reveal that the effect of CR is entirely dependent on the presence of the MPI; CR has no impact on the yield ratio if the MPI is off. Our findings further demonstrate that the light nuclei yield ratio experiences a slight increase with increasing collision energy, as predicted by the PYTHIA8 Angantyr model; however, it cannot describe the non-monotonic trend observed by the STAR experiment. Based on the Angantyr model simulation results, it is essential not to overlook the correlation between neutron and proton fluctuations. The Angantyr model is a good baseline for studying collisions in the absence of a quark-gluon plasma system, given its lack of flow and jet quenching.

Particle pump-out induced by trapped electron mode turbulence in electron cyclotron heated plasmas on XuanLong-50 spherical torus

Particle pump-out effects induced by low-frequency (<200 kHz) density fluctuations were observed in solely electron cyclotron wave (ECW)-heated plasmas on the spherical torus XuanLong-50 (EXL-50) without a central solenoid. The intensity of the relative density fluctuations increases with increasing ECW power and decays when the ECW is turned off while sustaining the plasma current. The electron densities are maintained relatively high and steady when the density fluctuations are completely absent, indicating that the outward transport of electrons is dominated by the particle pump-out effect of the ECW. The density fluctuations are modulated by a supersonic molecular beam injection pulse and the modulation amplitude decreases with increasing electron density at the same ECW injection power and decreasing ECW power at the same electron density, respectively. Analysis revealed that a critical value of electron temperature gradient (ETG) triggers the density fluctuations, and the intensity of the relative density fluctuations is positively correlated with the ETG and approximately inversely proportional to the effective collision frequency. With plasma parameters similar to those of EXL-50 experiments, the HD7 code simulations demonstrate that trapped electron mode (TEM) turbulence can be excited by ETG higher than the critical value observed in the experiment. In addition, the dependence of the mode growth rate (supposed to be proportional to the saturation level of fluctuations in quasi-linear theory) and the measured intensity of the density fluctuations is comparable. The simulated outward particle flux integrated over the poloidal wave number spectrum is significant and proportional to ETG. These observations demonstrate that the density fluctuation is TEM turbulence, which is driven by ETG and induces particle pump-out when the electron density/effective electron collision frequency is low. The potential relevance of this work with the controls of plasma profiles, impurities, helium ash, and heat transport in future reactors of similar low effective collision frequency is also discussed.

Oscillating Solitons and ac Josephson Effect in Ferromagnetic Bose-Bose Mixtures.

Close to the demixing transition, the degree of freedom associated with relative density fluctuations of a two-component Bose-Einstein condensate is described by a nondissipative Landau-Lifshitz equation. In the quasi-one-dimensional weakly immiscible case, this mapping surprisingly predicts that a dark-bright soliton should oscillate when subject to a constant force favoring separation of the two components. We propose a realistic experimental implementation of this phenomenon which we interpret as a spin-Josephson effect in the presence of a movable barrier.

Neutron density fluctuation and neutron–proton correlation from AMPT model

Using the multiphase transport (AMPT) model, we study the relative neutron density fluctuation and neutron-proton correlation in matter produced by Au+Au collisions at $\sqrt{s_\text{NN}} = $7.7-200 GeV. The rapidity, centrality, and energy dependence of these two observations are also discussed. The light nuclei yield ratio of proton, deuteron, and triton $N_tN_p/N_d^2$ calculated directly from the relative neutron density fluctuation and neutron-proton correlation, decreases with rapidity coverage and increases with collision centrality. Our study also found that the ratio does not exhibit any non-monotonic behavior in collision energy dependence. Since there is no first-order phase transition or critical physics in the AMPT model, our work provides a reference for extracting the relative neutron density fluctuation from light nuclei production in experiments.

New insights on divertor parallel flows,E × B drifts, and fluctuations from in situ, two-dimensional probe measurement in the Tokamak à Configuration Variable

In situ, two-dimensional (2D) Langmuir probe measurements across a large part of the TCV outer divertor are reported in L-mode discharges with and without divertor baffles. This provides detailed insights into time averaged profiles, particle fluxes, and fluctuation behavior in different divertor regimes. The presence of the baffles is shown to substantially increase the divertor neutral pressure for a given upstream density and to facilitate the access to detachment, an effect that increases with plasma current. The detailed, 2D probe measurements allow for a divertor particle balance, including ion flux contributions from parallel flows and E × B drifts. The poloidal flux contribution from the latter is often comparable or even larger than the former, and the divertor parallel flow direction reverses in some conditions, pointing away from the target. In most conditions, the integrated particle flux at the outer target can be predominantly ascribed to ionization along the outer divertor leg, consistent with a closed-box approximation of the divertor. The exception is a strongly detached divertor, achieved here only with baffles, where the total poloidal ion flux even decreases towards the outer target, indicative of significant plasma recombination. The most striking observation from relative density fluctuation measurements along the outer divertor leg is the transition from poloidally uniform fluctuation levels in attached conditions to fluctuations strongly peaking near the X-point when approaching detachment.

Superfluid density and collective modes of fermion superfluid in dice lattice

The superfluid properties of attractive Hubbard model in dice lattice are investigated. It is found that three superfluid order parameters increase as the interaction increases. When the filling factor falls into the flat band, due to the infinite large density of states, the resultant superfluid order parameters are proportional to interaction strength, which is in striking contrast with the exponentially small counterparts in usual superfluid (or superconductor). When the interaction is weak, and the filling factor is near the bottom of the lowest band (or the top of highest band), the superfluid density is determined by the effective mass of the lowest (or highest) single-particle band. When the interaction is strong and filling factor is small, the superfluid density is inversely proportional to interaction strength, which is related to effective mass of tightly bound pairs. In the strong interaction limit and finite filling, the asymptotic behaviors of superfluid density can be captured by a parabolic function of filling factor. Furthermore, when the filling is in flat band, the superfluid density shows a logarithmic singularity as the interaction approaches zero. In addition, there exist three undamped collective modes for strong interactions. The lowest excitation is gapless phonon, which is characterized by the total density oscillations. The two others are gapped Leggett modes, which correspond relative density fluctuations between sublattices. The collective modes are also reflected in the two-particle spectral functions by sharp peaks. Furthermore, it is found that the two-particle spectral functions satisfy an exact sum-rule, which is directly related to the filling factor (or density of particle). The sum-rule of the spectral functions may be useful to distinguish between the hole-doped and particle-doped superfluid (superconductor) in experiments.

Effect of edge turbulent transport on scrape-off layer width on HL-2A tokamak

Effect of edge turbulent transport on scrape-off layer (SOL) width has been investigated in Ohmically heated L-mode plasma under limiter configurations on HL-2A tokamak. It has been found that SOL width is doubled when plasma current decreases about 20%. With larger plasma current, E × B shear is stronger and has greater suppression effect on edge turbulent transport. SOL width is larger when power of relative density fluctuation level in the edge region is larger. It is concluded that edge turbulent transport plays a significant role on SOL width. These experimental findings may provide a better understanding and controlling of power exhaust for present and future fusion devices.

Revival of Charge Density Waves and Charge Density Fluctuations in Cuprate High-Temperature Superconductors

I present here a short memory of my scientific contacts with K.A. Müller starting from the Interlaken Conference (1988), Erice (1992 and 1993), and Cottbus (1994) on the initial studies on phase separation (PS) and charge inhomogeneity in cuprates carried out against the view of the majority of the scientific community at that time. Going over the years and passing through the charge density wave (CDW) instability of the correlated Fermi liquid (FL) and to the consequences of charge density fluctuations (CDFs), I end with a presentation of my current research activity on CDWs and the related two-dimensional charge density fluctuations (2D-CDFs). A scenario follows of the physics of cuprates, which includes the solution of the decades-long problem of the strange metal (SM) state.

Local relative density fluctuation in the early stage of a sintered glass sphere compact determined by X-ray computed tomography

The relative density of a sintered glass-powder compact varies at the local level due to its heterogeneous nature, which makes its characterization essential. In this work, high-resolution X-ray computed tomography (XCT) was used to quantitatively characterize the local relative density fluctuation of a sintered glass compact in its initial stage. The degree of heterogeneity of the microstructure was calculated taking into account the coarseness parameter. With a basis of 23,500 particles scanned in the representative volume element (RVE), the local environment was subdivided into three different groups of particles, represented by a cubic cell with a volume equivalent to that of approximately 600, 80 and 10 spherical particles with a diameter of 80 μm. Independent of the particle group analysed, we found that the maximum of the local relative density distributions, which could be described by a normal distribution function, always occurs around the relative density of the RVE. However, the fluctuation of the data showed a considerable change regarding the width, which increases as the volume of the analysed group of particles decreases, suggesting that particle packing is more heterogeneous on a small scale. In particular, we observed that the heterogeneity of the microstructure for the RVE under consideration becomes pronounced for groups of approximately 10 to 80 particles.

Two-temperature effects in Hall-MHD simulations of the HIT-SI experiment

A two-temperature Hall-magnetohydrodynamic (Hall-MHD) model, which evolves the electron and ion temperatures separately, is implemented in the PSI-Tet 3D MHD code and used to model plasma dynamics in the Helicity Injected Torus–Steady Inductive (HIT-SI) experiment. The two-temperature model is utilized for HIT-SI simulations in both the PSI-Tet and NIMROD codes at a number of different injector frequencies in the 14.5–68.5 kHz range. At all frequencies, the NIMROD two-temperature model results in increased toroidal current, lower chord-averaged density, higher average temperatures, outward radial shift of the current centroid, and axial symmetrization of the current centroid, relative to the single-temperature NIMROD simulations. The two-temperature PSI-Tet model illustrates similar trends, but at high frequency operation, it exhibits lower electron temperature, smaller toroidal current, and decreased axial symmetrization with respect to the single-temperature PSI-Tet model. With all models, average temperatures and toroidal currents increase with the injector frequency. Power balance and heat fluxes to the wall are calculated for the two-temperature PSI-Tet model and illustrate considerable viscous and compressive heating, particularly at high injector frequency. Parameter scans are also presented for artificial diffusivity, wall temperature, and density. Both artificial diffusivity and the density boundary condition significantly modify the plasma density profiles, leading to larger average temperatures, toroidal current, and relative density fluctuations at low densities. A low density simulation achieves sufficiently high current gain (G &amp;gt; 5) to generate significant volumes of closed flux lasting 1–2 injector periods.

Density Fluctuations in the Solar Wind Based on Type III Radio Bursts Observed by Parker Solar Probe

Abstract Radio waves are strongly scattered in the solar wind, so that their apparent sources seem to be considerably larger and shifted than the actual ones. Since the scattering depends on the spectrum of density turbulence, a better understanding of the radio wave propagation provides indirect information on the relative density fluctuations, , at the effective turbulence scale length. Here, we analyzed 30 type III bursts detected by Parker Solar Probe (PSP). For the first time, we retrieved type III burst decay times, , between 1 and 10 MHz thanks to an unparalleled temporal resolution of PSP. We observed a significant deviation in a power-law slope for frequencies above 1 MHz when compared to previous measurements below 1 MHz by the twin-spacecraft Solar TErrestrial RElations Observatory (STEREO) mission. We note that altitudes of radio bursts generated at 1 MHz roughly coincide with an expected location of the Alfvén point, where the solar wind becomes super-Alfvénic. By comparing PSP observations and Monte Carlo simulations, we predict relative density fluctuations, ϵ, at the effective turbulence scale length at radial distances between 2.5 and 14 to range from 0.22 to 0.09. Finally, we calculated relative density fluctuations, ϵ, measured in situ by PSP at a radial distance from the Sun of 35.7 during perihelion #1, and perihelion #2 to be 0.07 and 0.06, respectively. It is in a very good agreement with previous STEREO predictions ( ) obtained by remote measurements of radio sources generated at this radial distance.

Quantum read-out for cold atomic quantum simulators

Quantum simulators allow to explore static and dynamical properties of otherwise intractable quantum many-body systems. In many instances, however, the read-out limits such quantum simulations. In this work, we introduce an innovative experimental read-out exploiting coherent non-interacting dynamics. Specifically, we present a tomographic recovery method allowing to indirectly measure the second moments of the relative density fluctuations between two one-dimensional superfluids, which until now eluded direct measurements. Applying methods from signal processing, we show that we can reconstruct the relative density fluctuations from non-equilibrium data of the relative phase fluctuations. We employ the method to investigate equilibrium states, the dynamics of phonon occupation numbers and even to predict recurrences. The method opens a new window for quantum simulations with one-dimensional superfluids, enabling a deeper analysis of their equilibration and thermalization dynamics.

Probing QCD critical fluctuations from intermittency analysis in relativistic heavy-ion collisions

It is shown that intermittency, a self-similar correlation with respect to the size of the phase space volume, is sensitive to critical density fluctuations of baryon numbers in a system belonging to the three-dimensional (3D) Ising universality class. The relation between intermittency index and relative baryon density fluctuation is obtained. We thus suggest that measuring the intermittency in relativistic heavy-ion collisions could be used as a good probe of density fluctuations associated with the QCD critical phenomena. From recent preliminary results on neutron density fluctuations in central Au + Au collisions at sNN=7.7,11.5,19.6,27,39,62.4 and 200 GeV at RHIC/STAR, the collision energy dependence of intermittency index is extracted and shows a non-monotonic behavior with a peak at around 20 - 27 GeV, indicating that the strength of intermittency becomes the largest in this energy region. The transport UrQMD model without implementing critical physics cannot describe the observed behavior.

The production of triton and reconstruction of [formula omitted] with the Heavy Flavor Tracker in Au+Au collisions at STAR

The transverse momentum spectra of triton with STAR Beam Energy Scan data from Au+Au collisions at sNN=7.7,11.5,14.5,19.6,27,39,62.4,200GeV in the centrality classes 0 – 10%, 10 – 20%, 20 – 40% and 40 – 80% are reported. The centrality dependence of the coalescence parameter B3 of triton and the collision energy dependence of the relative neutron density fluctuation are then calculated. The results indicate that the B3 decreases from peripheral to central collisions and the neutron density fluctuation shows a non-monotonic collision energy dependence with a peak around collision energy of 20-30 GeV. In addition, we report details about the reconstruction method of HΛ3 via HΛ3→3He+π− and HΛ3→d+p+π− using high statistics data in Au+Au collisions at sNN=200GeV collected in 2014 and 2016 with the Heavy Flavor Tracker.

Dependence of density fluctuations on shape and collisionality in positive- and negative-triangularity tokamak plasmas

The effects of plasma shaping, in particular of the triangularity δ, on plasma turbulence, in terms of relative density fluctuations, have been studied in the TCV tokamak. It has been found that for inner wall limited L-mode plasmas, negative triangularity leads to a substantial reduction of turbulence amplitude, as well as of the spectral index and correlation length, consistent with the beneficial effect on energy confinement. Crucially, this reduction extends deep in the core, where the local triangularity becomes vanishingly small. A stabilizing effect of effective collisionality on trapped electron mode turbulence was also observed. These observations are consistent with previous experimental results on the effects of triangularity and collisionality on electron heat transport, as well as with global gyrokinetic GENE simulation results.

Non-Oberbeck–Boussinesq zonal flow generation

Novel mechanisms for zonal flow (ZF) generation for both large relative density fluctuations and background density gradients are presented. In this non-Oberbeck–Boussinesq (NOB) regime ZFs are driven by the Favre stress, the large fluctuation extension of the Reynolds stress, and by background density gradient and radial particle flux dominated terms. Simulations of a nonlinear full-F gyro-fluid model confirm the predicted mechanism for radial ZF propagation and show the significance of the NOB ZF terms for either large relative density fluctuation levels or steep background density gradients.

Sinter structure analysis of titanium structures fabricated via binder jetting additive manufacturing

To facilitate functional part production in metal binder jetting additive manufacturing, the relationship between materials, process and sintering needs to be understood. This work relates sintering theory with process outcomes. For this, commercially pure titanium was deployed to study the effect of powder size distributions on green and sintered part qualities (bulk density, relative density, particle size, pore size, sinter neck size). The powders were uni- and bi-modal blends of 0–45 μm, 45–106 μm, and 106–150 μm. Computed tomography analysis was used to evaluate non-densifying (1000 °C) and densifying (1400 °C) sintering regimes. For green parts, the relative density and powder size distribution along the build direction followed a periodic fluctuation equivalent to the 150 μm layer thickness. The relative density fluctuation range was higher (±20%) for bi-modal blends with 0–45 μm, compared to all other blends (±8%) due to powder segregation. For non-densifying sintering, parts with 0–45 μm blends displayed both densifying and non-densifying behavior. For densifying sintering, powders containing 0–45 μm blends surpassed the 70% density threshold expected for this sintering regime. Overall, the finer particles improved bulk density of sintered parts, at the expense of higher levels of shrinkage and density anisotropy along the build direction.