Electron Pressure Research Articles

Influence of radiative cooling effect on the plasma filamentations in the interaction of high-power laser with planar targets

Interaction of high-power laser with planar target is studied by using radiation-hydrodynamics simulation. When the laser interacts with the tungsten planar target, the laser energy deposition is uneven due to thermal filamentation instability and other reasons, and density fluctuations will appear in the front of the plasma, resulting in obvious plasma filamentation in the later stage. The researches of four materials, i.e. aluminum, copper, tungsten and gold, show that in the high-<i>Z</i> material tungsten and gold, due to the strong radiative cooling effect, the filamentation phenomena of the density distribution, electron temperature distribution and pressure distribution obviously occur. The order of magnitude of filamentous plasma density is different from that of the surrounding plasma. The filamentation phenomenon is closely related to the non-uniform energy deposition of the laser and the radiative cooling effect, although the ray beam will cause inhomogeneity of the laser irradiation to a certain extent, this is not the main reason for the filamentation phenomenon observed in this paper. Owing to refraction, reflection and the thermal filamentation instability when the laser is transmitted in the ablation plasma, the laser energy is deposited unevenly, which generates instability seeds in the early stage of plasma formation. The radiative cooling effect then amplifies this instability seeds, creating a radiative cooling instability that eventually results in a filamentous distribution of physical quantities such as plasma density, temperature, and pressure. This filamentation phenomenon destroys the uniformity of the plasma to a certain extent, and lays the seeds for the growth of fluid instability, which will seriously affect fusion-related research. It is shown that radiative cooling is crucial to the filamentation phenomenon, which causes uneven distribution of the plasma pressure during the evolution of the plasma, thereby affecting its transverse motion and enhancing the density fluctuation. After the laser irradiation ends, the density fluctuation gradually develops into filamentations. We also find that the clear filamentation occurs only for high-<i>Z</i> materials like tungsten and gold, but not for the moderate-<i>Z</i> materials like aluminum and copper. This can be attributed to the fact that radiative cooling is stronger for the high-<i>Z</i> materials. Studying the filamentation effect in laser-irradiated planar targets can contribute to understanding the instability in laser plasma, and then suppressing this instability and improving the gain of fusion. The results here can thus be of reference significance to the research of laser fusion, laboratory astrophysics, and other applications of intense-laserdriving.

Investigation of the homogeneity of energy conversion processes at dipolarization fronts from MMS measurements

We report on six dipolarization fronts (DFs) embedded in fast earthward flows detected by the Magnetospheric Multiscale mission during a substorm event on 23 July 2017. We analyzed Ohm's law for each event and found that ions are mostly decoupled from the magnetic field by Hall fields. However, the electron pressure gradient term is also contributing to the ion decoupling and likely responsible for an electron decoupling at DF. We also analyzed the energy conversion process and found that the energy in the spacecraft frame is transferred from the electromagnetic field to the plasma (J·E>0) ahead or at the DF, whereas it is the opposite (J·E<0) behind the front. This reversal is mainly due to a local reversal of the cross-tail current indicating a substructure of the DF. In the fluid frame, we found that the energy is mostly transferred from the plasma to the electromagnetic field (J·E′<0) and should contribute to the deceleration of the fast flow. However, we show that the energy conversion process is not homogeneous at the electron scales due to electric field fluctuations likely related to lower-hybrid drift waves. Our results suggest that the role of DF in the global energy cycle of the magnetosphere still deserves more investigation. In particular, statistical studies on DF are required to be carried out with caution due to these electron scale substructures.

EVALUATION OF RECONNECTION PARAMETERS FOR COLLISIONLESS DISPENSATION IN SOLAR CORONA

In the present paper, we have obtained an analytical solution for the thickness of current sheet in the dissipation region of the solar corona, reconnection time and reconnection rate for collisionless magnetic reconnection occuring in the solar corona. For our analysis we have used the length scale of the electron dissipation region based on field reversals. The reconnection electric field and component of the electron pressure tensor used in the Sweet-Parker model have been utilised to finally determine the exact numerical value of the thickness of current sheet, reconnection time and reconnection rate. It is shown that the thickness of the current sheet in the collisionless reconnection model is directly proportional to the electron mass and inversely proportional to the length scale of the electron dissipation region. Also, it is shown that the reconnection time is inversely proportional to the cube of the magnetic field and directly proportional to length scale, whereas reconnection rate is inversely proportional to the length scale of the electron dissipation region.

Electron magnetosonic waves and sub-ion magnetic holes in the magnetotail plasma

Recent observations of hot plasma in Earth's magnetotail and magnetosheath demonstrate nonlinear sub-ion scale magnetic holes, spatially localized dips of magnetic field intensity. These structures resemble magnetosonic solitary waves, but do not include any significant density perturbations. Instead, the magnetic field depression is balanced by hot electron pressure. This study aims to investigate linear modes of hot plasma with two electron components, hot and cold, to find a mode sharing properties (propagation direction, velocity, polarization and density/temperature perturbations) of observed sub-ion scale magnetic holes. The linear analysis of three-component plasma (ions, hot and cold electrons) shows an electromagnetic mode with slow, nearly cross field propagation and absence of density perturbations. Compressional perturbations of the magnetic field in this mode are balanced by hot electron pressure perturbations. This mode resembles the electron acoustic mode with cross field propagation in hot plasma with a significant magnetic field perturbation and can be called electron magnetosonic mode. We discuss properties of this mode in comparison with spacecraft observations.

A deformable CNN-based triplet model for fine-grained sketch-based image retrieval

With the popularity of electronic touch-screen and pressure sensing devices, fine-grained sketch based image retrieval (FG-SBIR) has become a research hotspot. In this paper, we stress the core problems of FG-SBIR: a. how to reduce the difference between the non-homogenous of heterogeneous media, and b. how to improve the distinguishability of sketch features. Specifically, a sketch generation model is first proposed to replace the conventional pre-processing of roughly extracting image edges, moreover, this model can alleviate the dilemma of sketch data scarcity. We then construct a novel FG-SBIR model which takes advantage of deformable convolutional neural network while taking into consideration of semantic attributes together. In addition, we build a fine-grained clothing sketch-image dataset, which has rich attribute annotations, for the first time. Extensive experiments exhibit that our proposed model achieves a better performance in improving the retrieval accuracy over the state-of-the-art baselines.

Parker Solar Probe Enters the Magnetically Dominated Solar Corona.

The high temperatures and strong magnetic fields of the solar corona form streams of solar wind that expand through the Solar System into interstellar space. At 09:33 UT on 28 April 2021 Parker Solar Probe entered the magnetized atmosphere of the Sun 13million km above the photosphere, crossing below the Alfvén critical surface for five hours into plasma in casual contact with the Sun with an Alfvén Mach number of 0.79 and magnetic pressure dominating both ion and electron pressure. The spectrum of turbulence below the Alfvén critical surface is reported. Magnetic mapping suggests the region was a steady flow emerging on rapidly expanding coronal magnetic field lines lying above a pseudostreamer. The sub-Alfvénic nature of the flow may be due to suppressed magnetic reconnection at the base of the pseudostreamer, as evidenced by unusually low densities in this region and the magnetic mapping.

Borohydride and metallic copper as a robust dehalogenation system: Selectivity assessment and system optimization

Hydrodechlorination (HDC) using noble-metal catalysts in the presence of H-donors is a promising tool for the treatment of water contaminated by halogenated organic compounds (HOCs). Cu is an attractive alternative catalyst to noble metals since it is cheaper than Pd, Rh, or Pt and more stable against deactivation. Cu with borohydride (BH4−) as reductant (copper-borohydride reduction system; CBRS) was applied here for the treatment of saturated aliphatic HOCs. The HDC ability of CBRS was evaluated based upon product selectivities during reduction of CCl3–R compounds (R = H, F, Cl, Br, and CH3). For CHCl3, CH2Cl2, and CHCl2–CH3, the dechlorination reaction proceeds predominantly via α-elimination with initial product selectivities to CH4 and C2H6 of 84–85 mol-% and 70–72 mol-%. For CCl4, CBrCl3, CFCl3, and CCl3–CH3, stepwise hydrogenolysis dominates. CH2Cl–R compounds are formed as recalcitrant intermediates with initial selectivities of 50–72 mol-%, whereas CH4 and C2H6 are minor products with 16–35 mol-% and 30–35 mol-%. The effect of reaction conditions on product selectivities were investigated for CHCl3 as target. Solution composition, variation of reducing agents (BH4−, H* from H2) and increase of electron pressure (electric potential at Cu electrode and Fe0 as support) did not have marked influence on the selectivities (ratio of CH4 : CH2Cl2). Product selectivities for reduction of CCl3–R compounds were found to be substrate-specific rather than reductant-specific. Since the formation of halogenated by-products could not be avoided, transformation via a second reduction step was optimized by higher catalyst dose, addition of Ag, and vitamin B12 to the CBRS. Comparison between Pd and Cu based on costs, catalyst activities, selectivities, metal stability, and fate of halogenated by-products shows that the CBRS is a potent alternative to conventional HDC catalysts and can be recommended as ‘agent of choice’ for treatment of α-substituted haloalkanes in heavily contaminated waters.

Effects of Electron Precipitation on E‐Region Instabilities: Theoretical Analysis

AbstractDuring periods of strong geomagnetic activity, intense currents and electric fields originating in the magnetosphere inundate the high‐latitude E‐region ionosphere. These strong electric fields drive plasma instabilities, including the Farley‐Buneman instability (FBI). These instabilities give rise to small‐scale plasma turbulence that modifies the large‐scale ionospheric conductance that, in turn, affects the evolution of the magnetosphere‐ionosphere‐thermosphere system. Also, during geomagnetic storms, high‐energy precipitating electrons, ≳5 keV, frequently penetrate down to the same regions where the intense currents and electric fields exist. This research examines the effects of precipitating electrons on the generation of the FBI and shows that, under many common conditions, it can easily suppress the FBI in a predictable manner. We demonstrate that the plasma pressure of superthermal electrons may significantly exceed the regular plasma pressure of the cold ionospheric plasma. This effect will increase the FBI threshold and suppress the instability in auroral regions. However, our detailed theoretical analysis shows that the effect of the superthermal precipitating electrons on the FBI threshold is much stronger than the pressure effect. The energy dependence of the electron‐N2 collision frequency can greatly enhance the effect of this additional pressure, further suppressing the FBI, even at a moderate precipitation level. Therefore, we expect that precipitation will exert an additional significant feedback on the magnetosphere by preventing the elevated conductivity caused by FBI driven turbulence. Both the turbulence‐enhanced conductivities and this suppression should be taken into account in global modeling of the magnetosphere‐ionosphere coupling.

The fast transit-time limit of magnetic pumping with trapped electrons

Recently, the energization of superthermal electrons at the Earth's bow shock was found to be consistent with a new magnetic pumping model derived in the limit where the electron transit time is much shorter than any time scale governing the evolution of the magnetic fields. The new model breaks with the common approach of integrating the kinetic equations along unperturbed orbits. Rather, the fast transit-time limit allows the electron dynamics to be characterized by adiabatic invariants (action variables) accurately capturing the nonlinear effects of electrons becoming trapped in magnetic perturbations. Without trapping, fast parallel streaming along magnetic field lines causes the electron pressure to be isotropized and homogeneous along the magnetic field lines. In contrast, trapping permits spatially varying pressure anisotropy to form along the magnetic field lines, and through a Fermi process this pressure anisotropy in turn becomes the main ingredient that renders magnetic pumping efficient for energizing superthermal electrons. We here present a detailed mathematical derivation of the model.

Global Ambipolar Potentials and Electric Fields at Mars Inferred From MAVEN Observations

AbstractThe motion of charged particles is governed by electromagnetic forces at high altitudes at Mars and thus the characterization of electrostatic potential and electric fields is important for understanding ion escape at Mars. In this study, we utilize measurements from the Mars Atmosphere and Volatile EvolutioN mission to derive electrostatic potentials above the collisional atmosphere at Mars. We find averaged potentials to be up to ∼100 V in the magnetosheath and down to ∼−70 V in the tail, with respect to the upstream. We then derive electric fields based on averaged potential maps, ranging V/km. These data‐derived electric fields are in good agreement with ambipolar electric fields from a multi‐fluid magnetohydrodynamic (MHD) model. MHD results also reveal that these large electric fields mainly originate from the electron pressure gradient in the magnetosheath and in the transition region from the hot solar wind flow to the cold ionospheric flow. This work provides the first data‐based characterization of global ambipolar electric fields at Mars (outside of the main ionosphere).

Large‐Scale Parallel Electric Field Colocated in an Extended Electron Diffusion Region During the Magnetosheath Magnetic Reconnection

AbstractIn this study, we report observations from the Magnetospheric Multiscale (MMS) mission of large‐scale parallel electric fields in a magnetosheath reconnecting current sheet. The four MMS satellites successively crossed an electron jet embedded in a broad current sheet and detected a unipolar being colocated with the jet. The strong electron pressure anisotropy inside the broad current sheet suggests that the jet could be generated by the firehose instability. The observations show that the large‐amplitude can fill the entire electron diffusion region (EDR) in the magnetosheath reconnection and thus dominates electron acceleration therein, dramatically different from EDRs in the magnetotail and magnetopause where large‐amplitude unipolar was rarely detected. The appearance of this could be due to the strong guide field which is common in the magnetosheath.

Built-in Chamber-Coupled Electronic Pressure Control of Ambient Air as the Carrier Gas for a Cylinder-Free Portable Gas Chromatograph

Built-in Chamber-Coupled Electronic Pressure Control of Ambient Air as the Carrier Gas for a Cylinder-Free Portable Gas Chromatograph

Reconstruction of the Electron Diffusion Region With Inertia and Compressibility Effects.

A method based on electron magnetohydrodynamics (EMHD) for the reconstruction of steady, two‐dimensional plasma and magnetic field structures from data taken by a single spacecraft, first developed by Sonnerup et al. (2016), https://doi.org/10.1002/2016ja022430, is extended to accommodate inhomogeneity of the electron density and temperature, electron inertia effects, and guide magnetic field in and around the electron diffusion region (EDR), the central part of the magnetic reconnection region. The new method assumes that the electron density and temperature are constant along, but may vary across, the magnetic field lines. We present two models for the reconstruction of electron streamlines, one of which is not constrained by any specific formula for the electron pressure tensor term in the generalized Ohm's law that is responsible for electron unmagnetization in the EDR, and the other is a modification of the original model to include the inertia and compressibility effects. Benchmark tests using data from fully kinetic simulations show that our new method is applicable to both antiparallel and guide‐field (component) reconnection, and the electron velocity field can be better reconstructed by including the inertia effects. The new EMHD reconstruction technique has been applied to an EDR of magnetotail reconnection encountered by the Magnetospheric Multiscale spacecraft on 11 July 2017, reported by Torbert et al. (2018), https://doi.org/10.1126/science.aat2998 and reconstructed with the original inertia‐less version by Hasegawa et al. (2019), https://doi.org/10.1029/2018ja026051, which demonstrates that the new method better performs in recovering the electric field and electron streamlines than the original version.

On Ohm’s law in reduced plasma fluid models

Drift-reduced MHD models are widely used to study magnetised plasma phenomena, in particular for magnetically confined fusion applications, as well as in solar and astrophysical research. This letter discusses the choice of Ohm’s law in these models, the resulting dispersion relations for the dynamics parallel to the magnetic field, and the implications for numerical simulations. We find that if electron pressure is included in Ohm’s law, then both electromagnetic and finite electron mass effects must also be included in order to obtain physical dispersion relations. A simple modification to the plasma vorticity is also found which improves handling of low density regions, of particular relevance to the simulation of the boundary region of magnetised plasmas.

Assessment of Sensory Thresholds in Dogs using Mechanical and Hot Thermal Quantitative Sensory Testing.

Quantitative sensory testing (QST) is used to evaluate the function of the somatosensory system in dogs by assessing the response to applied mechanical and thermal stimuli. QST is used to determine normal dogs' sensory thresholds and evaluate alterations in peripheral and central sensory pathways caused by various disease states, including osteoarthritis, spinal cord injury, and cranial cruciate ligament rupture. Mechanical sensory thresholds are measured by electronic von Frey anesthesiometers and pressure algometers. They are determined as the force at which the dog exhibits a response indicating conscious stimulus perception. Hot thermal sensory thresholds are the latency to respond to a fixed or ramped temperature stimulus applied by a contact thermode. Following a consistent protocol for performing QST and paying attention to details of the testing environment, procedure, and individual study subjects are critical for obtaining accurate QST results for dogs. Protocols for the standardized collection of QST data in dogs have not been described in detail. QST should be performed in a quiet, distraction-free environment that is comfortable for the dog, the QST operator, and the handler. Ensuring that the dog is calm, relaxed, and properly positioned for each measurement helps produce reliable, consistent responses to the stimuli and makes the testing process more manageable. The QST operator and handler should be familiar and comfortable with handling dogs and interpreting dogs' behavioral responses to potentially painful stimuli to determine the endpoint of testing, reduce stress, and maintain safety during the testing process.

Hamiltonian kinetic-Hall magnetohydrodynamics with fluid and kinetic ions in the current and pressure coupling schemes

We present two generalized hybrid kinetic-Hall magnetohydrodynamics (MHD) models describing the interaction of a two-fluid bulk plasma, which consists of thermal ions and electrons, with energetic, suprathermal ion populations described by Vlasov dynamics. The dynamics of the thermal components are governed by standard fluid equations in the Hall MHD limit with the electron momentum equation providing an Ohm's law with Hall and electron pressure terms involving a gyrotropic electron pressure tensor. The coupling of the bulk, low-energy plasma with the energetic particle dynamics is accomplished through the current density (current coupling scheme; CCS) and the ion pressure tensor appearing in the momentum equation (pressure coupling scheme; PCS) in the first and the second model, respectively. The CCS is a generalization of two well-known models, because in the limit of vanishing energetic and thermal ion densities, we recover the standard Hall MHD and the hybrid kinetic-ions/fluid-electron model, respectively. This provides us with the capability to study in a continuous manner, the global impact of the energetic particles in a regime extending from vanishing to dominant energetic particle densities. The noncanonical Hamiltonian structures of the CCS and PCS, which can be exploited to study equilibrium and stability properties through the energy-Casimir variational principle, are identified. As a first application here, we derive a generalized Hall MHD Grad–Shafranov–Bernoulli system for translationally symmetric equilibria with anisotropic electron pressure and kinetic effects owing to the presence of energetic particles using the PCS.

Initial characterization of electron temperature and density profiles in PEGASUS spherical tokamak discharges driven solely by local helicity injection

Local helicity injection (LHI) is a non-solenoidal startup technique that utilizes electron current injectors at the plasma edge to initiate tokamak discharges. Viable non-solenoidal startup techniques require high central Te to combat resistive losses and enhance coupling to auxiliary methods of current drive/heating. Thomson scattering measurements of LHI discharges in Pegasus showed peaked Te profiles at Ip∼0.15 MA and Bt∼0.15 T with Te,0∼100– 150 eV. These results are similar to Te profiles observed with Ohmic induction. At lower levels of Bt, LHI Te profiles were hollow with Te,0∼40 eV and Te,max≤120 eV depending upon the helicity input. Regardless of the Bt level and helicity input, the electron pressure profiles were flat/peaked with hollow JR profiles. Equilibrium reconstructions and measurements of core absolute extreme ultraviolet radiation suggest the hollow Te profiles are a result of very low resistive heating power in the core due to the edge-localized nature of LHI and low-Z line radiation losses. Estimates of Zeff from the plasma conductivity indicate averaged values of ∼1 or ∼3 assuming neoclassical or Spitzer conductivity, respectively. When auxiliary heating power from magnetic reconnection is considered, this observed LHI performance is comparable to expectations from a linear Ohmic confinement scaling estimate and a collisional stochastic confinement scaling estimate of the core plasma region.

Biomechanical consequences of tibial tubercle osteotomy in a 3D-printed model of patellofemoral dysplasia (207)

Biomechanical consequences of tibial tubercle osteotomy in a 3D-printed model of patellofemoral dysplasia (207)

Structural instability of transition metals upon ultrafast laser irradiation

Precision in laser material structuring is critically defined by the energy flow during the irradiation process, particularly in ultrafast regimes. Alternative to thermal evolutions, nonequilibrium electronic excitation can exercise a direct influence on the atomic bonding delivering a potentially rapid destructuring process. In this context, the atomic disordering of transition metals (Cr, Ni, and Ti) induced by nonequilibrium electronic excitation typical for ultrafast laser processing is studied with an emphasis on the role of the $d$-band filling and crystalline structure. The density functional theory is used to obtain, from first principles, structural stability criteria and nonthermal disordering pathways on timescales shorter than picosecond electron-phonon dynamics. We show that the hot electrons distort charge distribution in the cold crystalline arrangement and increase the entropy of the system, thus driving the mechanical expansion. The calculated nonequilibrium free-energy potentials indicate that a solid destabilization is possible when electron temperature reaches a universal value of around 2 eV for all considered metals. Under a uniaxial lattice relaxation expected in the laser ablation of surface layers, the charge redistribution and loss of lattice stability is shown to be oriented along specific crystal directions. Moreover, we show that the interatomic potential destabilization is energetically more favorable for Ni than for Cr and Ti. Lattice dynamics is affected by space-charge redistribution induced by Fermi smearing associated with anisotropic expansion due to electron pressure gradients. This ultrafast destructuring mechanism is shown to be a general feature of partially filled $d$-band transition metals.

Constraining reionization with the first measurement of the cross-correlation between the CMB optical-depth fluctuations and the Compton y -map

We propose a new reionization probe that uses cosmic microwave background (CMB) observations; the cross-correlation between fluctuations in the CMB optical depth which probes the integrated electron density, $\delta\tau$, and the Compton $y$-map which probes the integrated electron pressure. This cross-correlation is much less contaminated than the $y$-map power spectrum by late-time cluster contributions. In addition, this cross-correlation can constrain the temperature of ionized bubbles while the optical-depth fluctuations and kinetic SZ effect can not. We measure this new observable using a Planck $y$-map as well as a map of optical-depth fluctuations that we reconstruct from Planck CMB temperature data. We use our measurements to derive a first CMB-only upper limit on the temperature inside ionized bubbles, $T_{\rm b}\lesssim 7.0\times10^5\,$K ($2\,\sigma$). We also present future forecasts, assuming a fiducial model with characteristic reionization bubble size $R_{\rm b}=5\,$Mpc and $T_{\rm b}=5\times10^4\,$K. The signal-to-noise ratio of the fiducial cross-correlation using a signal dominated PICO-like $y$-map becomes $\simeq7$ with CMB-S4 $\delta\tau$ and $\simeq13$ with CMB-HD $\delta\tau$. For the fiducial model, we predict that the CMB-HD $-$ PICO cross-correlation should achieve an accurate measurement of the reionization parameters; $T_{\rm b}\simeq 49800^{+4500}_{-5100}\,$K and $R_{\rm b}\simeq 5.09^{+0.66}_{-0.79}\,$Mpc. Since the power spectrum of the electron density fluctuations is constrained by the $\delta\tau$ auto spectrum, the temperature constraints should be only weakly model-dependent on the details of the electron distributions and should be statistically representative of the temperature in ionized bubbles during reionization. This cross-correlation could, therefore, become an important observable for future CMB experiments.