Electron Dissipation Research Articles

Tailoring localized surface plasmons in Ag–Al alloys’ nanoparticles

Optical absorption properties of Ag–Al alloys’ nanoparticle (NPs) of radius of 10 nm embedded in silica for Al or Ag molar concentration in the range of 0.0–0.25 have been studied theoretically using a simple model. In case of monometal (Ag and Al) NPs, one localized surface plasmon resonance (LSPR) band, for each, (at ∼402 nm for Ag and ∼217 nm for Al) has been observed as expected from the corresponding real parts of dielectric constants. For alloys, the LSPR peak positions have been found to appear at wavelength positions in between the LSPR peaks corresponding to Ag and Al NPs confirming the Ag–Al alloys formation. The formation of these alloys in nano-phase was possible because of nearly same lattice parameters of Ag and Al. A blueshift (up to ∼54 nm, from visible to UV regions) of the LSPR peak for Ag1-xAlx alloy and a redshift (up to ∼20 nm, from deep–UV to UV regions) of the LSPR peak for Al1-xAgx alloy have been seen with increase in Al and Ag concentration, respectively. Additionally, for both the alloys the width of SPR bands have been found to increase linearly with increase in Al or Ag concentration due to electron dissipation caused mainly by electron scattering on impurity-induced defects with increase in impurity concentration in alloy NPs. Appearance of a broad LSPR (gap resonance) band is observed in addition with the usual LSPR for Ag1-xAlx due to the grain boundaries within the alloy NPs.

Energy transfer and electron energization in collisionless magnetic reconnection for different guide-field intensities

Electron dynamics and energization are one of the key components of magnetic field dissipation in collisionless reconnection. In 2D numerical simulations of magnetic reconnection, the main mechanism that limits the current density and provides an effective dissipation is most probably the electron pressure tensor term, which has been shown to break the frozen-in condition at the x-point. In addition, the electron-meandering-orbit scale controls the width of the electron dissipation region, where the electron temperature has been observed to increase both in recent Magnetospheric Multiple-Scale (MMS) observations and in laboratory experiments, such as the Magnetic Reconnection Experiment (MRX). By means of two-dimensional full-particle simulations in an open system, we investigate how the energy conversion and particle energization depend on the guide field intensity. We study the energy transfer from the magnetic field to the plasma in the vicinity of the x-point and close downstream regions, and E·J and the threshold guide field separating two regimes where either the parallel component, E||J||, or the perpendicular component, E⊥·J⊥, dominate the energy transfer, confirming recent MRX results and also consistent with MMS observations. We calculate the energy partition between fields and kinetic and thermal energies of different species, from electron to ion scales, showing that there is no significant variation for different guide field configurations. Finally, we study possible mechanisms for electron perpendicular heating by examining electron distribution functions and self-consistently evolved particle orbits in high guide field configurations.

Effects of HF frequency on plasma characteristics in dual-frequency helium discharge at atmospheric pressure by fluid modeling

On the basis of the fluid theory and the drift–diffusion approximation, a numerical model for dual-frequency atmospheric pressure helium discharge is established, in order to investigate the effects of the high frequency source (HF) on the characteristics of dual-frequency atmospheric pressure helium discharge. The numerical results showed that the electron heating rate increases with enhancing HF frequency, as well as the particles densities, electron dissipation rate, current density, net electron generation and bulk plasma region. Moreover, it is also observed that the efficient electron heating region moves when the HF frequency has been changed. The plasma parameters are not linear change with the HF frequency linearly increasing.

Observations of the Electron Jet Generated by Secondary Reconnection in the Terrestrial Magnetotail

Abstract We report in situ observations of an electron jet generated by secondary reconnection within the outflow region of primary reconnection in the terrestrial magnetotail by the Magnetospheric Multiscale (MMS) mission. The MMS spacecraft first passed through the primary X-line and then crossed the electron jet in the outflow of primary reconnection. There are a series of small-scale flux ropes in the secondary reconnection region. Decoupling from the magnetic field for both ions and electrons, an intense out-of-plane current, unambiguous Hall currents, and a Hall electromagnetic field appear in the electron jet. Strong electron dissipation ( ), a nonzero electric field in the electron frame ( ), and electron crescent-like shaped distributions are detected in the center of the electron jet, implying that MMS spacecraft were likely passing through the electron diffusion region. The significant electron dissipation indicates that the electrons can be accelerated in the electron jet and the electron jet may be another important electron acceleration channel along with the electron diffusion region.

Kinetic Dissipation Around a Dipolarization Front

AbstractKinetic aspects of energy conversion and dissipation near a dipolarization front (DF) in the magnetotail are considered using fully kinetic 3‐D particle‐in‐cell simulations. The energy conversion is described in terms of the pressure dilatation, as well as the double contraction of deviatoric pressure tensor and traceless strain rate tensor, also known as the Pi‐D parameter in turbulence studies. It is shown that in contrast to the fluid dissipation measure, the Joule heating rate, which cannot distinguish between ion and electron dissipation and reveals deep negative dips at the DF, the Pi‐D parameters, as kinetic analogs of the Joule heating rate, are largely positive and drastically different for ions and electrons. Further analysis of these parameters suggests that ions are heated at and ahead of the DF due to their reflection from the front, while electrons are heated at and behind the DF due to the long‐wavelength lower‐hybrid drift instability.

Study on Effect of Neutral Gas Pressure on Plasma Characteristics in Capacitive RF Argon Glow Discharges at Low Pressure by Fluid Modeling

For capacitively coupled radio frequency argon glow discharges at low pressure, based on the drift-diffusive approximation, a 1-D fluid model of the plasmas is established. The electrons and ions continuity equations and Poisson equation are simultaneously solved until a periodic steady state is achieved. Our results show that the electrons are accelerated by strong electric fields in the sheaths, resulting in more ohmic heating. These strong fields may be caused by low electron convection in bulk plasma region even when gas pressure increases due to the sheath thickness and the electrons’ temperatures all decrease. Thus, a local peak of the dissipation near the boundary between the sheath regions and the bulk plasma region is observed due to a higher local electron temperature in the sheath regions. Subsequently, the peak of the net power absorption grows in the bulk plasma region, since the electron thermal convection and dissipation contribute significantly and the electron density reaches the maximum. Finally, the ionization and excitation rates are clearly enhanced in the sheath regions compared to the bulk plasma region due to electron heating that has small changes in the bulk plasma region.

Synchrotron and collisional damping effects on runaway electron distributions

Validated models of runaway electron (RE) dissipation are required to confidently approve safe ITER Q=10 operation. DIII-D experiments using quiescent REs are exploring the importance of synchrotron and collisional damping terms to RE dissipation. New time and energy-resolved measurements of RE bremsstrahlung hard X-ray (HXR) emission reveal stark differences between high and low energy REs as damping terms are varied. Previously reported anomalously high RE dissipation only applies to low energy REs. At high energy (where synchrotron effects are strongest) low synchrotron damping cases reach higher peak RE energy despite weaker particle confinement. Low-energy RE decay is observed concurrently with high-energy RE growth. RE dissipation models predict bump-on-tail distributions whose properties depend on the damping terms. Measured HXR spectra are very broad, as expected for bump-on-tail distributions.

Momentum transport and nonlocality in heat-flux-driven magnetic reconnection in high-energy-density plasmas.

Recent theory has demonstrated a novel physics regime for magnetic reconnection in high-energy-density plasmas where the magnetic field is advected by heat flux via the Nernst effect. Here we elucidate the physics of the electron dissipation layer in this regime. Through fully kinetic simulation and a generalized Ohm's law derived from first principles, we show that momentum transport due to a nonlocal effect, the heat-flux-viscosity, provides the dissipation mechanism for magnetic reconnection. Scaling analysis, and simulations show that the reconnection process comprises a magnetic field compression stage and quasisteady reconnection stage, and the characteristic width of the current sheet in this regime is several electron mean-free paths. These results show the important interplay between nonlocal transport effects and generation of anisotropic components to the distribution function.

(Invited) Nanophotonic Structures for Solar Light Harvesting and the Generation of Hot Plasmonic Electrons

Future nanophotonic architectures for optical energy conversion and photocatalysis will require nanostructures with roles that work together cooperatively, such as structures with a large absorption cross section combined with features that enable guiding, propagating, and converting energy. Ultrafast optical responses in nanostructures are potentially important for modulating energy flow and also for characterizing and minimizing energy dissipation pathways in nanophotonic structures. Also, ultrafast extraction of hot plasmonic electrons from metal nanostructures is an important opportunity for increasing efficiency of solar energy conversion and photocatalysis. Efforts to greatly improve the efficiency of hot electron generation through plasmonic metamaterials, and characterization of hot electron dissipation pathways are described. I further describe efforts to induced long-range energy propagation in nanostructures. Nanostructured materials inspired by natural photosynthetic membranes represent an opportunity for efficient and directional energy transport. In natural photosystems, light harvesting complexes can transport excitons to the reaction center core with near unity conversion of absorbed photons to separated charge. A key to this process is the high rate of exciton hopping and the directionality of exciton flow due to an energy level waterfall effect of excitons in complexes as the reaction center core is approached. Efforts to induce similar behavior in biomimetically inspired nanostructures are described. Finally efforts to image exciton flow in light harvesting nanoscale materials are introduced. Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

The Martian dust devil electron avalanche: Laboratory measurements of the E-field fortifying effects of dust-electron absorption

Analogous to terrestrial dust devils, charged dust in Mars dust devils should become vertically stratified in the convective features, creating large scale E-fields. This E-field in a Martian-like atmosphere has been shown to stimulate the development of a Townsend discharge (electron avalanche) that acts to dissipate charge in regions where charge build-up occurs. While the stratification of the charged dust is a source of the electrical energy, the uncharged particulates in the dust population may absorb a portion of these avalanching electrons, thereby inhibiting dissipation and leading to the development of anomalously large E-field values. We performed a laboratory study that does indeed show the presence of enhanced E-field strengths between an anode and cathode when dust-absorbing filaments (acting as particulates) are placed in the avalanching electron flow. Further, the E-field threshold condition to create an impulsive spark discharge increases to larger values as more filaments are placed between the anode and cathode. We conclude that the spatially separated charged dust creates the charge centers and E-fields in a dust devil, but the under-charged portion of the population acts to reduce Townsend electron dissipation currents, further fortifying the development of larger-than-expected E-fields.

Magnetic reconnection in the high-energy density regime

Magnetic reconnection (MR) in the high-energy-density (HED) regime is comprehensively investigated by particle-in-cell simulations. In the HED regime, the MR process is driven by two colliding magnetized plasma bubbles produced by intense lasers, where there are ambiguities in distinguishing the actual MR consequences from the pure bubble squeezing effects, because both lead to similar behaviors. After discerning these similarities, such as two-fluid effects, plasma heating and jetting, we establish the direct relevance between the actual consequences of MR with the electron dissipation region, magnetic energy conversion and the relaxation of plasma density gradient. With additional discussions of the 3D effects and relativistic features, the results will guide the future HED experiments for MR and other laboratory astrophysics studies.

Particle-in-cell Simulations of Electron and Ion Dissipation by Whistler Turbulence: Variations with Electron β

Abstract Two ensembles of three-dimensional particle-in-cell (PIC) simulations of the forward cascade of decaying whistler turbulence have been carried out on a model of collisionless, homogeneous, magnetized plasma with parameters similar to those of the solar wind near Earth. Initial, relatively isotropic, narrowband spectra of relatively long wavelength modes cascade to anisotropic, broadband spectra of magnetic fluctuations at shorter wavelengths. Electron and ion dissipation rates are computed as functions of the initial electron beta, β e, over the range 0.1 ≤ β e ≤ 5.0, where this quantity is varied by changes in the background magnetic field magnitude B o. Ensemble One holds the value of the dimensionless initial magnetic fluctuation energy density ϵ o ≡ Σk constant; Ensemble Two follows solar wind observations, imposing the initial condition ϵ o = 0.20 β e. In both ensembles, the maximum dissipation rate of the electrons, Q e, and the maximum dissipation rate of the ions, Q i, satisfy Q e ≫ Q i. In Ensemble One, both dissipation rates scale approximately as , whereas over 0.1 ≤ β e ≤ 1.0 in Ensemble Two, Q e is approximately constant while Q i scales approximately as . These results, when combined with conclusions from earlier PIC simulations, suggest that sufficiently long wavelength and sufficiently large-amplitude magnetosonic-whistler turbulence at sufficiently large β e may heat ions more rapidly than electrons.

A simple model for electron dissipation in trapped ion turbulence

Trapped ion resonance-driven turbulence is investigated in the presence of electron dissipation in a simplified tokamak geometry. A reduced gyrokinetic bounce-averaged model for trapped ions is adopted. Electron dissipation is modeled by a simple phase-shift δ between density and electric potential perturbations. The linear eigenfunction features a peak at the resonant energy, which becomes stronger with increasing electron dissipation. Accurately resolving this narrow peak in numerical simulation of the initial-value problem yields a stringent lower bound on the number of grid points in the energy space. Further, the radial particle flux is investigated in the presence of electron dissipation, including kinetic effects. When the density gradient is higher than the temperature gradient, and the phase-shift is finite but moderate (δ≈0.02), the particle flux peaks at an order-of-magnitude above the gyro-Bohm estimate. Slight particle pinch is observed for δ<0.003.

Dispersive and dissipative nonlinear structures in degenerate Fermi–Dirac Pauli quantum plasma

We study the interplay between dispersion due to the electron degeneracy parameter and dissipation caused by plasma resistivity, in degenerate Fermi–Dirac Pauli quantum plasma. Considering relativistic degeneracy pressure for electrons, we investigate both arbitrary and small amplitude nonlinear structures. The corresponding trajectories are also plotted in the phase plane. The linear analysis for the dispersion relation yields interesting features. The present work is anticipated to be of physical relevance in the study of compact magnetized astrophysical objects like white dwarfs.

Characterization of magnetic reconnection in the high-energy-density regime.

The dynamics of magnetic reconnection (MR) in the high-energy-density (HED) regime, where the plasma inflow is strongly driven and the thermal pressure is larger than the magnetic pressure (β>1), is reexamined theoretically and by particle-in-cell simulations. Interactions of two colliding laser-produced plasma bubbles with self-generated poloidal magnetic fields of, respectively, antiparallel and parallel field lines are considered. Through comparison, it is found that the quadrupole magnetic field, bipolar poloidal electric field, plasma heating, and even the out-of-plane electric field can appear in both cases due to the mere plasma bubble collision, which may not be individually recognized as evidences of MR in the HED regime separately. The Lorentz-invariant scalar quantity D(e) ≃ γ(e)j · (E + v(e) × B) (γ(e) = [1-(v(e)/c)(2)](-1/2)) in the electron dissipation region is proposed as the key sign of MR occurrence in this regime.

Numerical Simulation of the Characteristics of Electrons in Bar-plate DC Negative Corona Discharge Based on a Plasma Chemical Model

In order to explore the characteristics of electrons in DC negative corona discharge, an improved plasma chemical model is presented for the simulation of bar-plate DC corona discharge in dry air. The model is based on plasma hydrodynamics and chemical models in which 12 species are considered. In addition, the photoionization and secondary electron emission effect are also incorporated within the model as well. Based on this model, electron mean energy distribution (EMED), electron density distribution (EDD), generation and dissipation rates of electron at 6 typical time points during a pulse are discussed emphatically. The obtained results show that, the maximum of electron mean energy (EME) appears in field ionization layer which moves towards the anode as time progresses, and its value decreases gradually. Within a pulse process, the electron density (ED) in cathode sheath almost keeps 0, and the maximum of ED appears in the outer layer of the cathode sheath. Among all reactions, R1 and R2 are regarded as the main process of electron proliferation, and R22 plays a dominant role in the dissipation process of electron. The obtained results will provide valuable insights to the physical mechanism of negative corona discharge in air.

THIN CURRENT SHEETS AND ASSOCIATED ELECTRON HEATING IN TURBULENT SPACE PLASMA

Intermittent structures, such as thin current sheets, are abundant in turbulent plasmas. Numerical simulations indicate that such current sheets are important sites of energy dissipation and particle heating occurring at kinetic scales. However, direct evidence of dissipation and associated heating within current sheets is scarce. Here, we show a new statistical study of local electron heating within proton-scale current sheets by using high-resolution spacecraft data. Current sheets are detected using the Partial Variance of Increments (PVI) method which identifies regions of strong intermittency. We find that strong electron heating occurs in high PVI (>3) current sheets while no significant heating occurs in low PVI cases ( 5) show the strongest heating and most of the time are consistent with ongoing magnetic reconnection. This suggests that reconnection is important for electron heating and dissipation at kinetic scales in turbulent plasmas.

Directed synthesis of bio-inorganic vanadium oxide composites using genetically modified filamentous phage

The growth of crystalline vanadium oxide using a filamentous bacteriophage template was investigated using sequential incubation in a V2O5 precursor. Using the genetic modification of the bacteriophage, we displayed two cysteines that constrained the RSTB-1 peptide on the major coat protein P8, resulting in vanadium oxide crystallization. The phage-driven vanadium oxide crystals with different topologies, microstructures, photodegradation and vanadium oxide composites were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), quartz microbalance and dissipation (QCM-D) and X-ray photoelectron spectroscopy (XPS). Non-specific electrostatic attraction between a wild-type phage (wt-phage) and vanadium cations in the V2O5 precursor caused phage agglomeration and fiber formation along the length of the viral scaffold. As a result, the addition of recombinant phage (re-phage) in V2O5 precursors formed heterogeneous structures, which led to efficient condensation of vanadium oxide crystal formation in lines, shown by QCM-D analysis. Furthermore, re-phage/VxOx composites showed significantly enhanced photodegradation activities compared with the synthesized wt-phage-V2O5 composite under illumination. This study demonstrates that peptide-mediated vanadium oxide mineralization is governed by a complicated interplay of peptide sequence, local structure, kinetics and the presence of a mineralizing aid, such as the two cysteine-constrained peptides on the phage surface, and has potential for use in nanotechnology applications.

Electron dynamics of solvated titanium hydroxide

Increasing interest in the photocatalytic activity of TiO2 has led to the consideration of using TiO2 nanoparticles in renewable energy generation. In order to better understand the electronic properties of nanoscale TiO2 structures, it is important to understand fundamental components that serve as synthesis precursors of TiO2 nanomaterials. Knowing the properties of basic building blocks of TiO2 materials, such as titanium hydroxide, Ti(OH)4, helps in a better understanding of the properties of larger scale structures. Titanium hydroxide is simulated in an explicit aqueous environment by density functional theory with hybrid functionals to model realistic conditions and to gain insight on mechanisms of how the charge carriers relax and recombine. Computed on-the-fly non-adiabatic couplings are determined from the first principles molecular dynamics calculations and allow the application of reduced density matrix formalism to calculate electron dynamics and dissipation times. An ultrafast charge transfer between ligand and metal is facilitated by energy dissipation via non-radiative relaxation occurring within a stepwise cascade thermalisation mechanism. The results show the possibility for design of promising and efficient materials for solar driven water splitting reactions.

Ion temperature gradient driven turbulence with strong trapped ion resonance

A theory to describe basic characterization of ion temperature gradient driven turbulence with strong trapped ion resonance is presented. The role of trapped ion granulations, clusters of trapped ions correlated by precession resonance, is the focus. Microscopically, the presence of trapped ion granulations leads to a sharp (logarithmic) divergence of two point phase space density correlation at small scales. Macroscopically, trapped ion granulations excite potential fluctuations that do not satisfy dispersion relation and so broaden frequency spectrum. The line width from emission due only to trapped ion granulations is calculated. The result shows that the line width depends on ion free energy and electron dissipation, which implies that non-adiabatic electrons are essential to recover non-trivial dynamics of trapped ion granulations. Relevant testable predictions are summarized.