Decay Length Research Articles

Structure of the electron temperature profile around the separatrix

The temperature decay length () in the near scrape-off layer (SOL) reflects the ratio between the transport parallel to the magnetic field lines via Spitzer–Härm electron conduction and the perpendicular mechanisms involving neoclassical and anomalous transport. The implementation of the thermal helium beam diagnostic on ASDEX Upgrade has enabled an excellent spatiotemporal study of the structure of the electron temperature profile () around the separatrix and the derivation of the near SOL decay lengths. From the analysis of the profile structure of attached H- and L-mode discharges, a self-consistent correspondence between the extrema of the electron temperature curvature profile () and the position of the separatrix () is revealed. A 1.5 D model for the power balance from closed to open field lines, including the escaping heat flux from the confined region and the parallel losses to the divertor, as well as results from the plasma edge code GRILLIX, support these experimental results. The evaluation shows that the temperature decay length is not constant over the plasma edge and SOL region, so that the absolute value of strongly depends on the analyzed region. Independent of the exact location and in agreement with edge Thomson scattering evaluations, the decay lengths show the known inverse relation to the plasma current ().

Self-consistent multi-component simulation of plasma turbulence and neutrals in detached conditions

Simulations of high-density deuterium plasmas in a lower single-null magnetic configuration based on a TCV discharge are presented. We evolve the dynamics of three charged species (electrons, D+ and D2+ ), interacting with two neutrals species ( D and D2 ) through ionization, charge-exchange, recombination and molecular dissociation processes. The plasma is modelled by using the drift-reduced fluid Braginskii equations, while the neutral dynamics is described by a kinetic model. To control the divertor conditions, a D2 puffing is used and the effect of increasing the puffing strength is investigated. The increase in fuelling leads to an increase of density in the scrape-off layer and a decrease of the plasma temperature. At the same time, the particle and heat fluxes to the divertor target decrease and the detachment of the inner target is observed. The analysis of particle and transport balance in the divertor volume shows that the decrease of the particle flux is caused by a decrease of the local neutral ionization together with a decrease of the parallel velocity, caused by the lower plasma temperature and the increase in momentum losses. The relative importance of the different collision terms is assessed, showing the crucial role of molecular interactions, as they are responsible for increasing the atomic neutral density and temperature, since most of the D neutrals are produced by molecular activated recombination and D2 dissociation. The presence of strong electric fields in high-density plasmas is also shown, revealing the role of the E × B drift in setting the asymmetry between the divertor targets. Simulation results are in agreement with experimental observations of increased density decay length, attributed to a decrease of parallel transport, together with an increase of plasma blob size and radial velocity.

Conception of a compact flow boiling loop for the International Space Station- First results in parabolic flights

The design of a pipe flow boiling experiment for the International Space Station is proposed, taking into account typical weight, power consumption and size constraints. The effect of singularities such as elbows upstream the test section is investigated. Velocity profiles downstream two elbows, measured by Particle Image Velocimetry are in good agreement with numerical simulation and allow to determine a specific distance (decay length) downstream the elbows for which the velocity profile recover its axisymmetry. From these results a breadboard is designed and tested in parabolic flights. Care has been taken to generate boiling downstream the decay length. Two-phase bubbly flow is observed with 2 perpendicular high-speed cameras in the test section and a symmetry of the bubble distribution in the pipe is verified for different gravity conditions when the bubbles are created after the decay length.

Stationary axial model of the Hall thruster plasma discharge: electron azimuthal inertia and far plume effects

One-dimensional axial models of the plasma discharge of a Hall thruster provide a valuable picture of its physical behavior with a small computational effort. Therefore, they are very suitable for quick parametric analyses or as a support tool for analyzing the impact of modeling decisions. This paper extends a well-known drift-diffusion stationary, quasineutral model by adding electron azimuthal inertia (EAI), a nonzero thickness cathode layer, and the far-plume region where electrons demagnetize and cool down. The EAI dominates on the far plume and affects positively to thrust. For a small ion backstreaming current, EAI modifies much the electron velocities and density near the anode, but has no discernible effect on the electron cross-field transport. Electron axial inertia and azimuthal gyrovisosity are estimated. The thick cathode layer connects quasineutrally the near and far plumes but the coupling between these two regions is weak. The far plume region is sensitive to the decay length of the magnetic field, the downstream boundary conditions on the electron currents, and the stray electric currents.

Kinematic analyses of wave–packet structures in non-isothermal jet flows: Effects of length scales

A kinematic wave–packet sound-source model is developed for non-isothermal jets based on large eddy simulation results of subsonic jets at temperature ratios 0.86, 1.0, and 2.7. To find the suitable variable for the sound-source model, coherent structures in these jets are extracted by leading modes of the proper orthogonal decomposition (POD), and they are classified according to spatial–temporal features. To extend the model, an approach is proposed to represent the growth and decay length scales separately by a single continuous function. Applying such function, the acoustic affections are discussed for the variable length scales of amplitude envelope, L, and coherence, Lc. The results show that the jet temperature desynchronizes the leading POD modes of radial velocity, pressure, and density, and the jet temperature changes the density mode from radial puffs into stripes or ridges. The axisymmetric component of the pressure clearly presents a train of radiant waves, as captured by its leading spectral-POD mode at the peak radiation frequency. Therefore, this pressure component is employed for modeling. In the wave-number domain, the elongation of L stretches the cross-spectral density (CSD) of the source signal, denoted by CSD(k1, k2), along the k1- and k2-axes; the decay of Lc stretches the CSD along the diagonal of the axes. Both of them tend to spread the CSD into a radiant region near the origin point, so as to enhance the radiation. The radiation seems insensitive to the variation of the L, as it only slightly distorts the CSD in the radiant region.

Top Secrets: long-lived ALPs in top production

We investigate the discovery potential for long-lived particles produced in association with a top-antitop quark pair at the (High-Luminosity) LHC. Compared to inclusive searches for a displaced vertex, top-associated signals offer new trigger options and an extra handle to suppress background. We design a search strategy for a displaced di-muon vertex in the tracking detectors, in association with a reconstructed top-antitop pair. For axion-like particles with masses above the di-muon threshold, we find that the (High-Luminosity) LHC can probe effective top-quark couplings as small as |ctt|/fa = 0.03 (0.002)/TeV and proper decay lengths as long as 20 (300) m, assuming a cross section of 1 fb, with data corresponding to an integrated luminosity of 150 fb−1 (3 ab−1). Our predictions suggest that searches for top-associated displaced di-muons will explore new terrain in the current sensitivity gap between searches for prompt di-muons and missing energy.

Infrared detection of tungsten cracking on actively cooled ITER-like component during high power experiment in WEST

The consequences of tungsten (W) damaging processes, such as cracking and melting, on divertor lifetime and plasma operation are high priority issues for ITER. A sustained melting experiment was conducted in WEST using a 2 mm deep groove geometry on the upstream mono-block (MB) to overexpose the sharp leading edge (LE) of the downstream MB. W-cracking has been evidenced for the first time with a very high spatial resolution infrared camera before tungsten melting was reached. These cracks develop when the monoblock temperature is about 2600 °C, thus higher than both ductile to brittle transition and softening threshold of tungsten, suggesting that these cracks are different from the ones observed in previous campaigns where brittle failure was involved, because of transient events on cold monoblock. Post-exposure analyses have been performed on the damaged monoblock, highlighting 12 main cracks on the LE, with a width varying from 33 µm to 77 µm, and an average spacing of 0.45 mm. Parallel heat flux about 90 MW/m2 has been derived from infrared temperature measurements, with a heat flux decay length on the target of 4 mm. The T-REX modelling code suggest here that with these thermal inputs, a crack can initiates due to thermal cycling without disruption, with a ductile failure, under 1 to 5 cycles for a tungsten DBTT varying from 400 °C to 500 °C.

Thermal near-field scattering characteristics for dielectric materials

In this study, we passively analyzed the near-field characteristics of thermally excited evanescent waves, which are radiation waves generated by the local dynamics of materials, including electron motions and lattice vibrations. The thermally excited evanescent waves on aluminium nitride (AlN) and gallium nitride (GaN) were measured using passive spectroscopic scattering-type scanning near-field optical microscopy (s-SNOM) in the wavelength ranges of 10.5–12.2 μm and 14.0–15.0 μm, which include the surface phonon-polariton (SPhP) wavelength of the studied dielectrics. We determined the unique decay characteristics of AlN and GaN, indicating a ten-fold increase in the probe area contributing to the scattering of waves near the SPhP wavelength compared to that in other wavelength ranges. The extended probe area correlated with the polariton decay lengths, indicating that the non-enhanced polaritons around K ~ ω/c were dominant in the scattered waves near the SPhP wavelength. In addition to the conventional passive detection mechanisms for metals, the proposed detection scheme will be a versatile passive detection model in the near future.

Numerical studies of plasma edge in W7-X with 3D FINDIF code

Abstract Modelling of the plasma transport for inherently three-dimensional (3D) problems as in stellarators requires dedicated complex codes. FINDIF is a 3D multifluid plasma edge transport code that has been previously successfully used for the analysis of energy transport in the TEXTOR-DED tokamak [1], where 3D perturbations led to an ergodic structure of field lines in the plasma edge. The ongoing efforts to apply it meaningfully to Wendelstein 7-X (W7-X) plasma problems resulted in advancements in the main model and accompanying tools for mesh generation and post-processing. In order to verify the applicability of the code and to compare with the reported simulation (EMC3-EIRENE) and experimental (OP1.1) results, a series of simulations for varying plasma density, temperature and anomalous transport coefficients as well as for fixed input power were performed. The connection length pattern of FINDIF traced magnetic field lines on the limiter was reproduced and its impact on heat loads was confirmed. An increase in the peak heat load on the limiter with a rise in plasma density, temperature and anomalous plasma transport coefficients was observed. The decay lengths of density, electron temperature and heat flux did not change with density, and were decreasing with temperature and increasing with anomalous plasma transport coefficient, which was compared to the simple scrape-off layer (SOL) model.

Scanning Photocurrent Microscopy in Single Crystal Multidimensional Hybrid Lead Bromide Perovskites.

We investigated solution-grown single crystals of multidimensional 2D-3D hybrid lead bromide perovskites using spatially resolved photocurrent and photoluminescence. Scanning photocurrent microscopy (SPCM) measurements where the electrodes consisted of a dip probe contact and a back contact. The crystals revealed significant differences between 3D and multidimensional 2D-3D perovskites under biased detection, not only in terms of photocarrier decay length values but also in the spatial dynamics across the crystal. In general, the photocurrent maps indicate that the closer the border proximity, the shorter the effective decay length, thus suggesting a determinant role of the border recombination centers in monocrystalline samples. In this case, multidimensional 2D-3D perovskites exhibited a simple fitting model consisting of a single exponential, while 3D perovskites demonstrated two distinct charge carrier migration dynamics within the crystal: fast and slow. Although the first one matches that of the 2D-3D perovskite, the long decay of the 3D sample exhibits a value two orders of magnitude larger. This difference could be attributed to the presence of interlayer screening and a larger exciton binding energy of the multidimensional 2D-3D perovskites with respect to their 3D counterparts.

Solvent Effects on Structure and Screening in Confined Electrolytes.

Using classical density functional theory, we investigate the influence of solvent on the structure and ionic screening of electrolytes under slit confinement and in contact with a reservoir. We consider a symmetric electrolyte with implicit and explicit solvent models and find that spatially resolving solvent molecules is essential for the ion structure at confining walls, excess ion adsorption, and the pressure exerted on the walls. Despite this, we observe only moderate differences in the period of oscillations of the pressure with the slit width and virtually coinciding decay lengths as functions of the scaling variable σ_{ion}/λ_{D}, where σ_{ion} is the ion diameter and λ_{D} the Debye length. Moreover, in the electrostatic-dominated regime, this scaling behavior is practically independent of the relative permittivity and its dependence on the ion concentration. In contrast, the crossover to the hard-core-dominated regime depends sensitively on all three factors.

Photonic nano-jet generated by suspended core fiber

The photonic nano-jet (PNJ) generated by the suspended core fiber (SCF) is proposed and experimentally demonstrated. Gaussian light irradiates the suspended core fiber, the PNJ is formed at the shadow-side surface of the suspended core. The measured results show that the full-width half-maximum (FWHM) of the PNJ is smaller than the incident light wavelength, and the propagation distance is several times that of the light wavelength. The experimental results are in a good agreement with the theoretical calculations. As the incident wavelength or the fiber core diameter decreases, the FWHM and decay length of the PNJ are significantly reduced. The FWHM of 0.73λ is achieved. The calculation results show that the SCF-based PNJ can achieve non-contact capture of a single nanoparticle. Suspended core fiber is easy to fabricate, simple and convenient to operate experimentally, and can produce a more stable PNJ that is less contaminated. Fiber-based PNJs offer new applications in particle capture and biophotonic sensing.

Results on light (anti)hypernuclei production with ALICE at the LHC

The high collision energies reached at the LHC lead to significant production yields of light (anti)hypernuclei in proton–proton (pp), proton–lead (p–Pb) and, in particular, Pb–Pb collisions. The lightest known hypernucleus is the hypertriton, which is a bound state of a proton, a neutron, and a Λ hyperon. It decays weakly with a decay length of a few centimeters. The excellent tracking and particle identification capabilities of the ALICE detector, exploiting the energy loss measurement of the Time Projection Chamber (TPC) and using the Inner Tracking System (ITS) to distinguish between primary and secondary (decay) vertices, allow for the determination of the hypertriton yield across different collision systems, its lifetime, and its binding energy. The latest hypertriton lifetime measurement in Pb–Pb collisions performed in the 2-body decay channel will be presented. This measurement contributes to the solution of the hypertriton lifetime puzzle. In addition, the hypertriton production in different collision systems and at different energies will be compared to model predictions. Due to its low binding energy, and hence to its large size, the hypertriton is the ideal candidate to distinguish between statistical hadronization and coalescence models. With the precision of the presented yield measurements some variants of the aforementioned models can be excluded.

A sheath correction method for electron density measurements with the microwave resonant curling probe

A correction method accounting for plasma sheath effects that appear when performing an electron density measurement with a microwave resonant probe immersed in plasma is described. The diagnostic is the novel curling probe (CP) that has already shown promising capabilities in various plasma sources. The correction method is based on the evaluation of two effects relative to the resonant probe operation and its interface with the plasma. First, the characteristic decay length of the electromagnetic field emitted by the CP, which defines the probed volume, has been characterized for the different harmonic resonance modes. Second, a semi-analytical model has been adapted to describe the plasma structure forming near the probe, which quantitatively describes the electron density profiles across the sheath structure. The correction method is then developed uniquely from numerical simulations and is independent of other diagnostics. Experimental results inside two plasma sources, an inductively coupled plasma and an electron cyclotron resonance plasma thruster, are presented and discussed. The validity of the method is assessed (i) by comparing CP corrected densities with Langmuir probe results, (ii) by varying the probe orientation to the expanding plasma flow, and (iii) by using two harmonics of the probe. The method is shown to significantly improve the accuracy of electron density measurements. Possible improvements to the method are also discussed.

Probing Chern number by opacity and topological phase transition by a nonlocal Chern marker

In 2D semiconductors and insulators, the Chern number of the valence band Bloch state is an important quantity that has been linked to various material properties, such as the topological order. We elaborate that the opacity of 2D materials to circularly polarized light over a wide range of frequencies, measured in units of the fine structure constant, can be used to extract a spectral function that frequency-integrates to the Chern number, offering a simple optical experiment to measure it. This method is subsequently generalized to finite temperature and locally on every lattice site by a linear response theory, which helps to extract the Chern marker that maps the Chern number to lattice sites. The long range response in our theory corresponds to a Chern correlator that acts like the internal fluctuation of the Chern marker, and is found to be enhanced in the topologically nontrivial phase. Finally, from the Fourier transform of the valence band Berry curvature, a nonlocal Chern marker is further introduced, whose decay length diverges at topological phase transitions and therefore serves as a faithful indicator of the transitions, and moreover can be interpreted as a Wannier state correlation function. The concepts discussed in this work explore multi-faceted aspects of topology and should help address the impact of system inhomogeneities.

First Analysis of Jupiter in Gamma Rays and a New Search for Dark Matter.

We present the first dedicated γ-ray analysis of Jupiter, using 12years of data from the Fermi Telescope. We find no robust evidence of γ-ray emission, and set upper limits of ∼10^{-9} GeV cm^{-2} s^{-1} on the Jovian γ-ray flux. We point out that Jupiter is an advantageous dark matter (DM) target due to its large surface area (compared with other solar system planets), and cool core temperature (compared with the Sun). These properties allow Jupiter to both capture and retain lighter DM, providing a complementary probe of sub-GeV DM. We therefore identify and perform a new search for DM-sourced γ-rays in Jupiter, where DM annihilates to long-lived particles, which can escape the Jovian surface and decay into γ rays. We consequently constrain DM-proton scattering cross sections as low as about 10^{-40} cm^{2}, showing Jupiter is up to 10 orders of magnitude more sensitive than direct detection. This sensitivity is reached under the assumption that the mediator decay length is sufficient to escape Jupiter, and the equilibrium between DM capture and annihilation; sensitivities can be lower depending on the DM model. Our work motivates follow-up studies with upcoming MeV telescopes such as AMEGO and e-ASTROGAM.

Time-domain observation of ballistic orbital-angular-momentum currents with giant relaxation length in tungsten

The emerging field of orbitronics exploits the electron orbital momentum L. Compared to spin-polarized electrons, L may allow the transfer of magnetic information with considerably higher density over longer distances in more materials. However, direct experimental observation of L currents, their extended propagation lengths and their conversion into charge currents has remained challenging. Here, we optically trigger ultrafast angular-momentum transport in Ni|W|SiO2 thin-film stacks. The resulting terahertz charge-current bursts exhibit a marked delay and width that grow linearly with the W thickness. We consistently ascribe these observations to a ballistic L current from Ni through W with a giant decay length (~80 nm) and low velocity (~0.1 nm fs−1). At the W/SiO2 interface, the L flow is efficiently converted into a charge current by the inverse orbital Rashba–Edelstein effect, consistent with ab initio calculations. Our findings establish orbitronic materials with long-distance ballistic L transport as possible candidates for future ultrafast devices and an approach to discriminate Hall-like and Rashba–Edelstein-like conversion processes.

Photoemission Electron Microscopy of Blue and UV Surface Plasmons on Nanostructured Aluminum Films

We image surface plasmons on aluminum surfaces in the ultraviolet and blue spectral range with photoemission electron microscopy (PEEM). Plasmon excitation is mediated by light incident on straight aluminum ridges with subwavelength cross section on flat aluminum films. We observe interference patterns due to light/plasmon interaction, with distinct periodicity and orientation depending on the light in-plane incidence angle with respect to the ridge orientation. From the interference patterns, we deduce the surface plasmon propagation directions and decay lengths. The latter are remarkably high with 2.8 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}m and 8.4 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}m for excitation wavelengths of 273 nm and 410 nm, respectively. Our results extend PEEM imaging of surface plasmons into the ultraviolet region.

Fine-tuning ionic transport through hybrid soft nanochannels: The role of polyelectrolyte charge density distribution

This study investigates a hybrid nanochannel consisting of two cylindrical and conical parts coated with a soft layer exhibiting either of four different spatial distribution functions: constant (type I), exponential (type II), sigmoidal (type III), and soft-step (type IV). The Poisson–Nernst–Planck and Navier–Stokes equations are numerically solved using the finite element method under steady-state conditions. The research focuses on the modification of behavior and enhancement of performance in nanochannels inspired by nature. Considering the spatial variation in charge density distribution and the limited understanding of ion transport mechanisms, this study highlights the importance of modeling tools in advancing this field. The findings contribute to the development of effective strategies for controlling and manipulating the behavior of charged nanochannels. The results demonstrate that changing the decay length from 0.2 to 1 at a concentration of 1 mM leads to an increase in the rectification factor for type II up to 6.129, i.e., 5.7 times. Furthermore, varying NPEL/NA from 25 to 100 mol m−3 at Vapp=+1 V results in ionic selectivity of 0.9072, 0.2009, 0.1543, and 0.9031 for functions of type I to type IV, respectively. These findings not only enhance our understanding of ion transport mechanisms in hybrid nanochannels but also suggest that manipulating the charge density of the soft layer enables the production of intelligent nanochannels with applications in separation, diagnostics, and sensing.

Light Scalars at FASER

FASER, the ForwArd Search ExpeRiment, is a currently operating experiment at the Large Hadron Collider (LHC) that can detect light long-lived particles produced in the forward region of the LHC interacting point. In this paper, we study the prospect of detecting light CP-even and CP-odd scalars at FASER and FASER 2. Considering a model-independent framework describing the most general interactions between a CP-even or CP-odd scalar and SM particles using the notation of coupling modifiers in the effective Lagrangian, we develop the general formalism for the scalar production and decay. We then analyze the FASER and FASER 2 reaches of light scalars in the large tan β region of the Type-I two Higgs double model as a case study, in which light scalars with relatively long lifetime could be accommodated. In the two benchmark scenarios we considered, the light (pseudo)scalar decay length varies in (10−8, 105) meters. Both FASER and FASER 2 can probe a large part of the parameter space in the large tan β region up to 107, extending beyond the constraints of the other existing experiments.