Untrapped Electrons Research Articles

Modeling the energetic electron fluxes in the inner radiation belt based on a drift-source model

The Macau Science Satellite-1 (MSS-1), designed by the Macao University of Science and Technology and the National Space Science Center (NSSC) of China, is equipped to detect the fine structure of the magnetic field over the South Atlantic Anomaly (SAA) region, monitoring geomagnetic field variations, and obtaining the energetic electron spectrum distributions in the Earth’s inner radiation belt. In this study, we simulate the distributions of trapped, quasi-trapped, and untrapped electrons along the orbit of MSS-1 based on a drift-source model. The simulation results show that the particle detector with 90° looking direction can observe trapped electrons in the SAA region, untrapped electrons in the regions conjugated with the SAA region at the north hemisphere, and quasi-trapped electrons in all other regions. In contrast, the detectors with <60° looking directions can measure only untrapped electrons. Generally, quasi-trapped electron fluxes accumulate along the drift trajectory and are due primarily to CRAND, until reaching the SAA region where quasi-trapped electrons are all lost into the atmosphere.

Excitation of long electrostatic solitary waves in ion beam neutralization process

Unusually long electrostatic solitary waves (ESWs) are discovered in a particle-in-cell simulation study of the process of ion beam neutralization by electron emission from a filament. These ESWs are long because the density perturbation responses to the potential wells created by the ESWs are very small. The density perturbation is small because the trapped (positive) and untrapped (negative) electron density perturbations nearly compensate each other because of a non-Maxwellian electron velocity distribution in the beam.

Photodoping and Fast Charge Extraction in Ionic Carbon Nitride Photoanodes

AbstractIonic carbon nitrides based on poly(heptazine imides) (PHI) represent a vigorously studied class of materials with possible applications in photocatalysis and energy storage. Herein, for the first time, the photogenerated charge dynamics in highly stable and binder‐free PHI photoanodes using in operando transient photocurrents and spectroelectrochemical photoinduced absorption measurements is studied. It is discovered that light‐induced accumulation of long‐lived trapped electrons within the PHI film leads to effective photodoping of the PHI film, resulting in a significant improvement of photocurrent response due to more efficient electron transport. While photodoping is previously reported for various semiconductors, it has not been shown before for carbon nitride materials. Furthermore, it is found that the extraction kinetics of untrapped electrons are remarkably fast in these PHI photoanodes, with electron extraction times (ms) comparable to those measured for commonly employed metal oxide semiconductors. These results shed light on the excellent performance of PHI photoanodes in alcohol photoreforming, including very negative photocurrent onset, outstanding fill factor, and the possibility to operate under zero‐bias conditions. More generally, the here reported photodoping effect and fast electron extraction in PHI photoanodes establish a strong rationale for the use of PHI films in various applications, such as bias‐free photoelectrochemistry or photobatteries.

Effects of 5 MeV Proton Irradiation on 1200 V 4H-SiC VDMOSFETs ON-State Characteristics

The effects of 5 MeV proton irradiation on ON-state characteristics of 1200 V 4H-SiC VDMOSFETs are investigated in this paper, and related mechanisms have been revealed by the analysis of their test structure of ohmic contacts, lateral nMOSFETs and MOS capacitors simultaneously fabricated on same wafer. The results show that the threshold voltage (V TH ) decreases obviously with increasing irradiation doses because the dominant hole traps induced by nitrogen passivation might capture more holes produced by the ionization effect of proton irradiation, resulting in the generation of net positive charges nearby the SiO 2 /4H-SiC interface. The interface trap density (Dit) extracted by subthreshold swing increases but field effective mobility (μ FE ) is improved with increasing irradiation doses. The un-trapped near interface electron traps (NIETs) are reduced by the ionization effect of proton irradiation, which could be beneficial to the improvement of μ FE . And after the maximum irradiation dose of 1 × 10 14 p/cm 2 the VDMOSFET fails in the ON-state capability. Before it fails, the ON-state resistivity (R ON ) firstly decreases and then increases with the increase of irradiation dose. The deterioration of the resistance of lightly doped drift region (R D ) and the resistance of JFET region (R JFET ) caused by the displacement effect of proton irradiation covers up the improvement of the channel resistance (R CH ) with increasing irradiation doses, finally leading to the failure of the VDMOSFET at the maximum proton irradiation dose.

Three dimensional ion-acoustic rogons in quantized anisotropic magnetoplasmas with trapped/untrapped electrons.

Three-dimensional (3D) modulational instability (MI) and ion-acoustic (IA) envelopes are studied in a quantized degenerate magnetoplasma, whose constituents are the trapped/untrapped electrons and anisotropic positive ions. By using quantum hydrodynamic equations and the multiscale reductive perturbation technique, a 3D nonlinear Schrödinger equation is derived to account for electron quantization and ion pressure anisotrophy effects. The potential excitations are shown stable (unstable) against the perturbations for Kc<0(Kc>0), where Kc is a critical parameter that accounts for the longitudinal (transverse) dispersion(s) and nonlinearity effects. Numerically, the nonlinear evolution of IA wavepackets into a 3D MI may be revealed in the ranges of low and high frequencies 0<ω≤0.05 and 0.75≤ω≤1.1. The quantizing magnetic field reduces (enhances) the group speed (wave frequency) of IA excitations, concentrating the wave energy to favor the modulational instability. Finite electronic temperature (viz.,Te≤10keV) enhances the untrapped electrons and significantly widens the instability domain Kc>0. The ionic pressure anisotropy increases the wave frequency (ω), piles up the harmonics under Kc>0, and give rise to modulational instability. The quantized magnetic field and anisotropic pressure reduce the amplitude and spatial extension of the IA rogons. This study is important for understanding the 3D MI and unstable excitations in degenerate plasmas, relevant to white dwarfs, neutron stars, and high-energy density experiments, where strong magnetic field quantizes the dynamics of trapped/untrapped electrons.

Modulational instability and ion-acoustic envelopes in dense plasmas with trapped/untrapped electrons

The linear and nonlinear properties of ion-acoustic rogons and associated modulational instability (MI) are studied in an unmagnetized dense electron-ion plasma, containing degenerate trapped/untrapped electrons and classical adiabatic ions. Solving the quantum hydrodynamic equations by using the standard multiscale reductive perturbation technique, a nonlinear Schrödinger equation is derived, which admits potential envelopes to be stable (unstable) against the perturbations for PQ &amp;lt; 0 (PQ&amp;gt;0). Here, P and Q are the dispersion and nonlinearity coefficients, respectively. It is numerically shown that for the vanishing ionic temperature ratio (σ=0), the parametric regime at perturbation wavelengths λ≥2.5λeff (λ≤2.5λeff) is always modulationally stable (unstable); here, λeff is the effective screening length. Moreover, the finite ionic temperature (Ti≤10 eV) restores the modulational stability at relatively short wavelengths by confining MI within the perturbation range 4.5λeff≤λ≤1.3λeff. The parameter Θ(=Te/22μ) shows the impact of the untrapped electrons which not only enhances (reduces) the angular frequency (group speed) of the envelope but also piles up the wave crests (energy) to produce the MI. Furthermore, due to degenerate trapped electrons, the instability domain gets widened in the limit PQ &amp;gt; 0 and leads to the onset of MI and unstable excitations. The study has important results for understanding the mechanism of MI and unstable modes in the context of astrophysical environments (white dwarfs, neutron stars, etc.,) and high density experiments.

Quantifying the Contribution of Microbursts to Global Electron Loss in the Radiation Belts

AbstractWe use the Solar, Anomalous, and Magnetospheric Particle Explorer to explore the relationship between microbursts and global flux decay of electrons from the outer Van Allen belt during the recovery phase of geomagnetic storms. We investigate the correlation between microbursts and global electron loss in each of the quasi‐trapped (drift loss cone), stably trapped, and untrapped electron (bounce loss cone) populations. For the quasi‐trapped electrons, we separately classify the storms as driven by coronal mass ejections or corotating interaction regions and explore their connection to microburst loss. We find that the decay lifetime of electron fluxes, that is, e‐folding times of macroscopic fluxes in the recovery phase is correlated with strong microburst activity. That is, when the microburst activity is high, global flux decay times are short, and vice versa, suggesting a cross‐scale coupling between microloss and macroloss phenomena. Furthermore, we find that the microburst to global loss coupling is predominant in the quasi‐trapped population of radiation belt electrons while having negligible influence on the untrapped and stably trapped populations. We find that microburst activity during storms driven by coronal mass ejections is coupled more strongly with global flux decay as compared with corotating interaction regions. In addition, we find that distance from the plasmapause is likely a better indicator of microburst location than L‐shell, with most microbursts occurring ~0.5–2.0 L from the model plasmapause location.

Electron Dynamics in Magnetosheath Mirror‐Mode Structures

AbstractMirror‐mode structures are widely observed in space plasma environments. Although plasma features within the structures have been extensively investigated in theoretical models and numerical simulations, relatively few observational studies have been made, due to a lack of high‐cadence measurements of particle distributions in previous space missions. In this work, electron dynamics associated with mirror‐mode structures are studied based on Magnetospheric Multiscale observations of electron pitch angle distributions. We define mirror‐mode peaks/troughs as the region where the magnetic field strength is greater/smaller than the mean field. The observations show that most electrons are trapped inside the mirror‐mode troughs and display a donut‐like pitch angle distribution configuration. Besides the trapped electrons in mirror‐mode troughs, we find that electrons are also trapped between ambient mirror‐mode peaks and coexisting untrapped electrons within the mirror‐mode structure. Analysis shows that the observed donut‐like electron distributions are the result of betatron cooling and the spatial dependence of electron pitch angles within the structure.

Electron Distributions in Kinetic Scale Field Line Resonances: A Comparison of Simulations and Observations

AbstractObservations in kinetic scale field line resonances, or eigenmodes of the geomagnetic field, reveal highly field‐aligned plateaued electron distributions. By combining observations from the Van Allen Probes and Cluster spacecraft with a hybrid kinetic gyrofluid simulation we show how these distributions arise from the nonlocal self‐consistent interaction of electrons with the wavefield. This interaction is manifested as electron trapping in the standing wave potential. The process operates along most of the field line and qualitatively accounts for electron observations near the equatorial plane and at higher latitudes. In conjunction with the highly field‐aligned plateaus, loss cone features are also evident, which result from the action of the upward‐directed wave parallel electric field on the untrapped electron populations.

Electron holes in the outer radiation belt: Characteristics and their role in electron energization

AbstractVan Allen Probes have detected electron holes (EHs) around injection fronts in the outer radiation belt. Presumably generated near equator, EHs propagate to higher latitudes potentially resulting in energization of electrons trapped within EHs. This process has been recently shown to provide electrons with energies up to several tens of keV and requires EH propagation up to rather high latitudes. We have analyzed more than 100 EHs observed around a particular injection to determine their kinetic structure and potential energy sources supporting the energization of trapped electrons. EHs propagate with velocities from 1000 to 20,000 km/s (a few times larger than the thermal velocity of the coldest background electron population). The parallel scale of observed EHs is from 0.3 to 3 km that is of the order of hundred Debye lengths. The perpendicular to parallel scale ratio is larger than one in a qualitative agreement with the theoretical scaling relation. The amplitudes of EH electrostatic potentials are generally below 100 V. We determine the properties of the electron population trapped within EHs by making use of the Bernstein‐Green‐Kruskal analysis and via analysis of EH magnetic field signatures. The density of the trapped electron population is on average 20% of the background electron density. The perpendicular temperature of the trapped population is on average 300 eV and is larger for faster EHs. We show that energy losses of untrapped electrons scattered by EHs in the inhomogeneous background magnetic field may balance the energization of trapped electrons.

Electron holes in inhomogeneous magnetic field: Electron heating and electron hole evolution

Electron holes are electrostatic non-linear structures widely observed in the space plasma. In the present paper, we analyze the process of energy exchange between electrons trapped within electron hole, untrapped electrons, and an electron hole propagating in a weakly inhomogeneous magnetic field. We show that as the electron hole propagates into the region with stronger magnetic field, trapped electrons are heated due to the conservation of the first adiabatic invariant. At the same time, the electron hole amplitude may increase or decrease in dependence on properties of distribution functions of trapped and untrapped resonant electrons. The energy gain of trapped electrons is due to the energy losses of untrapped electrons and/or decrease of the electron hole energy. We stress that taking into account the energy exchange with untrapped electrons increases the lifetime of electron holes in inhomogeneous magnetic field. We illustrate the suggested mechanism for small-amplitude Schamel's [Phys. Scr. T2, 228–237 (1982)] electron holes and show that during propagation along a positive magnetic field gradient their amplitude should grow. Neglect of the energy exchange with untrapped electrons would result in the electron hole dissipation with only modest heating factor of trapped electrons. The suggested mechanism may contribute to generation of suprathermal electron fluxes in the space plasma.

Comment on “Two-dimensional positive column structure in a discharge tube with radius discontinuity” [Phys. Plasmas 21, 113503 (2014)

Zobnin et al. have published a paper [Phys. Plasmas, 21, 113503 (2014)] on a topic of discharge physics in the presence of a sharp change in cylindrical discharge geometry. In the comment it is pointed out that for untrapped electrons a full kinetic equation, which includes dependences on spatial coordinates and energies, has to be used for the electron velocity distribution function determination. It is also unclear what probe theories Zobnin et al. have used in their paper for the calculation of electron current to the discharge tube wall.

Energetic electron transport in the presence of magnetic perturbations in magnetically confined plasmas

The transport of energetic electrons is sensitive to magnetic perturbations. By using three-dimensional numerical simulation of test particle drift orbits we show that the transport of untrapped electrons through an open region with magnetic perturbations cannot be described by a diffusive process. Based on our test particle simulations, we propose a model that leads to an exponential loss of particles.

Observation of warm, higher energy electrons transiting a double layer in a helicon plasma

Measurements of an inductive RF helicon argon plasma double layer with two temperature electron distributions including a fast (&amp;gt;80 eV) tail are observed at 0.17 mTorr Ar pressure. The fast, untrapped electrons observed downstream of the double layer have a higher temperature (13 eV) than the trapped (Te = 4 eV) electrons. The reduction of plasma potential and density observed in the double layer region would require an upstream temperature ten times the measured 4 eV if occurring via Boltzmann ambipolar expansion. The experimental observation in Madison helicon experiment indicates that fast electrons with substantial density fractions can be created at low helicon operating pressures.

Magnetospheric convection and magnetopause shadowing effects in ULF wave‐driven energetic electron transport

Magnetospheric radiation belt electron transport in the presence of ULF waves and a convection electric field is investigated using a model that includes a nightside plasma sheet source and electron losses by magnetopause shadowing. Narrow‐band ULF waves launched from a prescribed dayside magnetopause source are shown to interact with trapped and untrapped equatorially mirroring electrons within the magnetosphere. For magnetic moments less than 8 keV/nT and a strong convection electric field (in the order of 5 mV/m), we find that a limb of untrapped plasma sheet electrons extending across the dayside magnetosphere into the afternoon sector provides a phase space density (PSD) source for injection to lower L‐shells by ULF waves, causing a rapid enhancement in PSD. However, the same ULF wave activity gives rise to a rapid dropout in PSD for electrons with a higher magnetic moment or in the presence of a weaker convection electric field, since in this case the plasma sheet electrons escape through the magnetopause before they reach the afternoon sector. In their place, a lack of PSD, or PSD “holes” can be periodically injected from the magnetopause to lower L‐shells by ULF waves, leading to the rapid depletion in average PSD. In each case, the magnitude and extent in L‐shell of the PSD enhancement or depletion is strongly dependent on the amplitude of ULF waves in the afternoon sector, and is significantly augmented by the overlap of drift‐resonant islands and an associated stochastic transport layer.

Wall-Plug Efficiency and Beam Dynamics in Free-Electron Lasers Using Energy Recovery Linacs

In a high average power free-electron laser (FEL) the wall-plug efficiency is of critical importance in determining the size, complexity, and cost of the overall system. The wall-plug efficiency for the FEL oscillator and amplifier (uniform and tapered wiggler) is strongly dependent on the energy recovery process. A theoretical model for electron beam dynamics in the energy recovery linac is derived and applied to the acceleration and deceleration of nano-Coulomb electron bunches for a tapered FEL amplifier. For the tapered amplifier, the spent electron beam exiting the wiggler consists of trapped and untrapped electrons. Decelerating these two populations using different phases of the radio-frequency wave in the recovery process enhances wall-plug efficiency. For the parameters considered here, the wall-plug efficiency for the tapered amplifier can be ~10% using this approach.

Ion acoustic solitary waves in magneto-rotating plasmas

Propagation of an ion acoustic wave (IAW) in a magnetized electron–ion plasma, which is rotating around an axis at an angle θ with the direction of magnetic field, is studied by incorporating the effects of trapped and untrapped electron distributions. Employing the perturbation scheme, Korteweg–deVries and Schamel's modified KdV equations are derived for the small angle θ which may support the nonlinear IAW on a slow time scale of ion motion. The amplitude and width of the solitary wave in both cases (trapped and untrapped electrons) have been discussed with the effects of oblique rotation and external magnetic field. It is shown that the nonlinear effects considerably influence the propagation of waves in rotating plasmas.

Charge Carrier Resolved Relaxation of the First Excitonic State in CdSe Quantum Dots Probed with Near-Infrared Transient Absorption Spectroscopy

This manuscript describes a global regression analysis of near-infrared (NIR, 900-1300 nm) transient absorptions (TA) of colloidal CdSe quantum dots (QDs) photoexcited to their first (1S(e)1S(3/2)) excitonic state. Near-IR TA spectroscopy facilitates charge carrier-resolved analysis of excitonic decay of QDs because signals in the NIR are due exclusively to absorptions of photoexcited electrons and holes, as probe energies in this region are not high enough to induce absorptions across the optical bandgap that crowd the visible TA spectra. The response of each observed component of the excitonic decay to the presence of a hole-trapping ligand (1-octanethiol) and an electron-accepting ligand (1,4-benzoquinone), and comparison of time constants to those for recovery of the ground state bleaching feature in the visible TA spectrum, allow for the assignment of the components to (i) a 1.6 ps hole trapping process, (ii) 19 ps and 274 ps surface-mediated electron trapping processes, and (iii) a ∼5 ns recombination of untrapped electrons.

Sluggish response of untrapped electrons and global electrostatic micro-instabilities in a tokamak

Electrons whether adiabatic or non-adiabatic play important role in determining the stability properties of a global electrostatic mode. In the present analysis, the effect of non-adiabaticity of electrons with all its kinetic effects is investigated for ion temperature gradient driven mode in the presence of fast ions using a linear gyro-kinetic code EM-GLOGYSTO-F.

Bounce-averaged diffusion coefficients for superluminous wave modes in the magnetosphere

In this study, we present a first evaluation of bounce-averaged diffusion coefficients of superluminous wave (R-X and L-O) modes at two locations: L = 6.5 and 4.5. We show that, analogous to the local diffusion coefficients, bounce-averaged momentum diffusion coefficients play a dominant role for pitch angles α above a critical angle α c , namely, 〈 D pp 〉 > | 〈 D p α 〉 | > 〈 D α α 〉 . We also demonstrate that for a Gaussian distribution of wave normal angle θ ( X = tan θ ), diffusion coefficients are sensitively dependent on the peak X m . As X m increases, 〈 D pp 〉 and 〈 D α α 〉 are found to increase in cases of interest: X m = tan 25 ∘ , tan 45 ∘ and tan 65 ∘ . We have estimated wave amplitudes for particular stochastic acceleration timescale for 1 MeV electrons and found that the required wave amplitudes B te ∼ 86 to ∼ 170 pT for a timescale τ = 1 day . These results indicate that superluminous wave modes may have a significant potential for both stochastic acceleration of trapped electrons (with higher pitch angles) and loss process of untrapped electrons (with smaller pitch angles).