Initial Electron Distribution Research Articles

Control of wire heating with resistively guided fast electrons through an inverse conical taper

The heating of a solid wire embedded in a solid substrate (of lower Z material) with relativistic electrons generated by ultra-intense laser irradiation is considered. Previously, it has been noted that the initial angular distribution of the fast electrons is a highly important factor in the efficacy of the heating [Robinson et al., Phys. Plasmas 20, 122701 (2013)]. We show that, using 3D numerical simulations, the addition of an inverse conical taper at the front of wire can considerably improve the heating of the wire due to the reduction of angular spread of the fast electrons which is caused by transport through the inverse conical taper [Robinson et al., “Guiding of laser-generated fast electrons by exploiting the resistivity-gradients around a conical guide element,” Plasma Phys. Controlled Fusion (to be published)].

Comparison of Excitation of Acoustic-Electromagnetic Wave in Piezoelecric Crystal and Crystal with Potential of Deformation

In this article, the comparison of excitation in high frequencies of acoustic-electromagnetic wave in piezoelecric crystal and crystal with potential of deformation GaAs is investigating. Possible mechanisms of coupling different hybrid waves are the piezoeffect and the deformation potential. As a model it is analyzing a film of crystal places between two symmetrical substrates with the other materials without an acoustic contact. This film includes 2D electron gas with a high negative differential conductivity and uniform initial distribution of electrons. The hybrid acoustic-electromagnetic wave and hybrid space charge wave interact. Amplification of space charge wave takes place due to negative differential conductivity in GaAs. This amplification of space charge waves is causing the amplification of acoustic-electromagnetic wave. It is to show that the symmetric modes, emerging as transverse ones, interact more effectively with the space charge waves. Another important result is the following: at the frequencies f ≈ 10 GHz, the excitation efficiency of acoustic-electromagnetic wave with transverse displacement due to piezoeffect is more effective, but at higher frequencies, the deformation potential is dominating.

Dissipative Floquet topological systems

Motivated by recent pump-probe spectroscopies, we study the effect of phonon dissipation and potential cooling on the nonequilibrium distribution function in a Floquet topological state. To this end, we apply a Floquet kinetic equation approach to study two dimensional Dirac fermions irradiated by a circularly polarized laser, a system which is predicted to be in a laser induced quantum Hall state. We find that the initial electron distribution shows an anisotropy with momentum dependent spin textures whose properties are controlled by the switching-on protocol of the laser. The phonons then smoothen this out leading to a non-trivial isotropic nonequilibrium distribution which has no memory of the initial state and initial switch-on protocol, and yet is distinct from a thermal state. An analytical expression for the distribution at the Dirac point is obtained that is relevant for observing quantized transport.

Spatiotemporal evolution of a thin plasma foil with Kappa distribution

AbstractThe one-dimensional behavior of a thin plasma foil heated by laser is studied, emphasizing on the fully kinetic effects associated with initial energetic electrons using a relativistic kinetic 1D3V Particle-In-Cell code. For this purpose, the generalized Lorentzian (Kappa) function inclusive the high energy tail is employed for initial electron distribution. The presence of the initially high-energy electrons leads to a different ion energy spectrum than the initially Maxwellian distribution. It is shown for the smaller Kappa parameter k where the high energy tail of the electron distribution function becomes more significant, the electron cooling rate increases. Moreover, the spatiotemporal evolution of electric field is strongly affected by the initial super-thermal electrons.

The global precipitation of magnetospheric electrons into Titan’s upper atmosphere

We couple a two-stream electron transport model to a three-dimensional model of Titan’s plasma interaction to calculate the global precipitation of magnetospheric electrons into Titan’s atmosphere. We describe energy deposition rates from eleven simulations that vary the following parameters: relative orientation of the solar and magnetospheric ram directions, initial electron distribution, electron bounce times in Saturn’s magnetosphere, and whether we account for magnetic mirroring. Most of the energy from auroral electrons is deposited on the magnetospheric wake-side of Titan’s thermosphere, with peak rates between 25 and 35eVcm−3s−1, and the least amount of energy is deposited on the magnetospheric ram-side. We calculate globally averaged peak energy deposition rates of ∼13eVcm−3s−1 near 1200km altitude, ∼1eVcm−3s−1 near 1200km altitude, and ∼10eVcm−3s−1 near 1350km altitude for electron distributions characteristic of Saturn’s plasma sheet, lobe, and magnetosheath, respectively. Globally averaged energy deposition rates are decreased by ∼70% when we assume that the electron bounce times are a factor of 10 shorter because the thermalization of magnetospheric electrons in Titan’s atmosphere erodes Saturn’s flux tubes over time scales comparable to the time it takes for electrons to bounce in Saturn’s magnetosphere. Magnetic mirroring further reduces the globally averaged energy deposition rates by ∼30% to 70%. The total power deposited in Titan’s thermosphere by magnetospheric electrons varies between 0.13 and 1.5×108W for the eleven simulations analyzed, which is about an order of magnitude smaller than the power deposited by solar EUV (∼109W for λ<800Å) during the 2007 to 2009 solar minimum.

Comparative study ofH(2s)andH(2p0)ionization dynamics in the over-barrier regime

We investigate theoretically the ionization dynamics of $\text{H}(2s)$ and $\text{H}(2p$${}_{0})$ by short midinfrared laser pulses in the over-barrier ionization (OBI) regime. Through the numerical solution to the time-dependent Schr\odinger equation, we calculate the differential and angular distributions of photoelectrons as well as the total ionization probability. In a wide range of laser intensities and wavelengths, significant differences are found between the $\text{H}(2s)$ and $\text{H}(2p$${}_{0})$ states in both the differential and total ionization yields. Analysis of the excitation dynamics reveals that both the low-lying excited states and the Rydberg states are significantly populated for a 1200-nm laser. As the laser wavelength gradually increases to 2100 nm, the population on the low-lying excited state drastically decreases. We observe the population of the Rydberg states oscillates as a function of the laser peak intensity, which gradually disappears when the Keldysh parameter $\ensuremath{\gamma}$ decreases. As the laser intensity increases further, the total ionization probability and the population of the Rydberg states stay almost the same, which is related to the partial stabilization phenomena recently confirmed in experiments. In the deep OBI regime, we find that this kind of ionization suppression is more obvious in the 2$s$ state than in the 2$p$${}_{0}$ state. We also compare the transverse momentum distributions of the electrons ionized from the two states and find the effect of the Coulomb potential is stronger in the 2$p$${}_{0}$ state than that in the 2$s$ state, which is due to the different initial electron distributions.

A simulation study of the propagation of whistler-mode chorus in the Earth’s inner magnetosphere

We study the propagation of whistler-mode chorus in the magnetosphere by a spatially two-dimensional simulation code in the dipole coordinates. We set the simulation system so as to assume the outside of the plasmapause, corresponding to the radial distance from 3.9 to 4.1 RE in the equatorial plane and the latitudinal range from −15° to +15°, where RE is the Earth’s radius. We assume a model chorus element propagating northward from the magnetic equator of the field line at L=4 with a rising tone from 0.2 to 0.7 Ωe 0 in the time scale of 5,000 , where Ωe 0 is the electron gyrofrequency at the magnetic equator. For the initial density distribution of cold electrons, we assume three types of initial conditions in the outside of the plasmapause: without a duct (run 1), a density enhancement duct (run 2), and a density decrease duct (run 3). In run 1, the simulation result reveals that whistler-mode waves of the different wave frequencies propagate in the different ray path in the region away from the magnetic equator. In runs 2 and 3, the model chorus element propagates inside the assumed duct with changing wave normal angle. The simulation results show the different propagation properties of the chorus element in runs 2 and 3 and reveal that resultant wave spectra observed along the field line are different between the density enhancement and density decrease duct cases. The spectral modification of chorus by the propagation effect should play a significant role in the interactions between chorus and energetic electrons in the magnetosphere, particularly in the region away from the equator. The present study clarifies that the variation of propagation properties of chorus should be taken into account for the thorough understanding of resonant interactions of chorus with energetic electrons in the inner magnetosphere.

Dynamics of Excess Electronic Charge in Aliphatic Ionic Liquids Containing the Bis(trifluoromethylsulfonyl)amide Anion

In a recent article (J. Am. Chem. Soc. 2011, 133, 20186) we investigated the initial spatial distribution of dry excess electrons in a series of room-temperature ionic liquids (RTILs). Perhaps unexpectedly, we found that in some alkylammonium-based systems the excess negative charge resided on anions and not on the positive cations. Following on these results, in the current paper we describe the time evolution of an excess electronic charge introduced in alkylammonium- and pyrrolidinium-based ionic liquids coupled with the bis(trifluoromethylsulfonyl)amide ([Tf2N(-)]) anion. We find that on a 50 fs time scale an initially delocalized excess electron localizes on a single [Tf2N(-)] anion which begins a fragmentation process. Low-energy transitions have a very different physical origin on the several femtoseconds time scale when compared to what occurs on the picosecond time scale. At time zero, these are intraband transitions of the excess electron. However after 40 fs when the excess electronic charge localizes on a single anion, these transitions disappear, and the spectrum is dominated by electron-transfer transitions between the fragments of the doubly charged breaking anion.

Relativistic electron acceleration by oblique whistler waves

Test-particle simulations of electrons interacting with finite-amplitude, obliquely propagating whistler waves are carried out in order to investigate the acceleration of relativistic electrons by these waves. According to the present findings, an efficient acceleration of relativistic electrons requires a narrow range of oblique propagation angles, close to the whistler resonance cone angle, when the wave amplitude is held constant at relatively low value. For a constant wave propagation angle, it is found that a range of oblique whistler wave amplitudes permits the acceleration of relativistic electrons to O(MeV) energies. An initial distribution of test electrons is shown to form a power-law distribution when plotted in energy space. It is also found that the acceleration is largely uniform in electron pitch-angle space.

Modeling of prebreakdown stage of gaseous discharge

A mathematical model of electron cascade transfer is presented. It is developed for studying the prebreakdown stage of gaseous discharge based on the kinetic equation for electrons in a self-consistent electromagnetic field. An elastic scattering of electrons, excitation of molecules by electron impact, and impact ionization are taken into account. A natural ionization background is used as the initial distribution of free electrons. The results of the computational experiment are compared to the data of measurements of drift velocity and the average energy of electrons of discharge, as well as the Townsend coefficient in a flat charged condensor filled with diluted xenon. The comparison showed that the model is not in contrast with the experiment. In order to contract the volume of the discharge’s full current calculation, a transport approximation is built. It is based on the averaging of angular scattering. The applicability of the transport approximation for modeling the full current of the capillary discharge is studied.

On the breaking of a plasma wave in a thermal plasma. I. The structure of the density singularity

The structure of the singularity that is formed in a relativistically large amplitude plasma wave close to the wave breaking limit is found by using a simple waterbag electron distribution function. The electron density distribution in the breaking wave has a typical “peakon” form. The maximum value of the electric field in a thermal breaking plasma is obtained and compared to the cold plasma limit. The results of computer simulations for different initial electron distribution functions are in agreement with the theoretical conclusions. The after-wavebreak regime is then examined, and a semi-analytical model of the density evolution is constructed. Finally the results of two dimensional particle in cell simulations for different initial electron distribution functions are compared, and the role of thermal effects in enhancing particle injection is noted.

Uncertainty and sensitivity analysis of the effect of the mean energy and FWHM of the initial electron fluence on the Bremsstrahlung photon spectra of linear accelerators

A calculation of the correct dose in radiation therapy requires an accurate description of the radiation source because uncertainties in characterization of the linac photon spectrum are propagated through the dose calculations. Unfortunately, detailed knowledge of the initial electron beam parameters is not readily available, and many researchers adjust the initial electron fluence values by trial-and-error methods. The main goal of this work was to develop a methodology to characterize the fluence of initial electrons before they hit the tungsten target of an Elekta Precise medical linear accelerator. To this end, we used a Monte Carlo technique to analyze the influence of the characteristics of the initial electron beam on the distribution of absorbed dose from a 6MV linac photon beam in a water phantom. The technique is based on calculations with Software for Uncertainty and Sensitivity Analysis (SUSA) and Monte Carlo simulations with the MCNP5 transport code. The free parameters used in the SUSA calculations were the mean energy and full-width-at-half-maximum (FWHM) of the initial electron distribution. A total of 93 combinations of these parameters gave initial electron fluence configurations. The electron spectra thus obtained were used in a simulation of the electron transport through the target of the linear accelerator, which produced different photon (Bremsstrahlung) spectra. The simulated photon spectra were compared with the 6-MV photon spectrum provided by the linac manufacturer (Elekta). This comparison revealed how the mean energy and FWHM of the initial electron fluence affect the spectrum of the generated photons. This study has made it possible to fine-tune the examined electron beam parameters to obtain the resulted absorbed doses with acceptable accuracy (error <1%).

Strongly interacting photons in asymmetric quantum well via resonant tunneling

We propose an asymmetric quantum well structure to realize strong interaction between two slow optical pulses. The essential idea is the combination of the advantages of inverted-Y type scheme and resonant tunneling. We analytically demonstrate that giant cross-Kerr nonlinearity can be achieved with vanishing absorptions. Owing to resonant tunneling, the contributions of the probe and signal cross-Kerr nonlinearities to total nonlinear phase shift vary from destructive to constrictive, leading to nonlinear phase shift on order of π at low light level. In this structure, the scheme is inherent symmetric for the probe and signal pulses. Consequently, the condition of group velocity matching can be fulfilled with appropriate initial electron distribution.

FORMATION OF VERY HARD GAMMA-RAY SPECTRA OF BLAZARS IN LEPTONIC MODELS

The very high energy (VHE) $\gamma$-ray spectra of some TeV Blazars, after being corrected for absorption in the extragalactic background light (EBL), appear unusually hard, which poses challenges to conventional acceleration and emission models. We investigate the parameter space that allows the production of such hard TeV spectra within time-dependent leptonic models, both for synchrotron self-Compton (SSC) and external Compton (EC) scenarios. In the context of interpretation of very hard $\gamma$-ray spectra, time-dependent considerations become crucial because even extremely hard, initial electron distributions can be significantly deformed due to radiative energy losses. We show that very steep VHE spectra can be avoided if adiabatic losses are taken into account. Another way to keep extremely hard electron distributions in the presence of radiative losses, is to assume stochastic acceleration models that naturally lead to steady-state relativistic, Maxwellian-type particle distributions. We demonstrate that in either case leptonic models can reproduce TeV spectra as hard as $E_{\gamma} dN/dE_{\gamma} \propto E_{\gamma}$. Unfortunately this limits, to a large extend, the potential of extracting EBL from $\gamma$-ray observations of blazars.

Equilibration of integer quantum Hall edge states

We study equilibration of quantum Hall edge states at integer filling factors, motivated by experiments involving point contacts at finite bias. Idealizing the experimental situation and extending the notion of a quantum quench, we consider time evolution from an initial nonequilibrium state in a translationally invariant system. We show that electron interactions bring the system into a steady state at long times. Strikingly, this state is not a thermal one: Its properties depend on the full functional form of the initial electron distribution and not simply on the initial energy density. Further, we demonstrate that measurements of the tunneling density of states at long times can yield either an overestimate or an underestimate of the energy density, depending on details of the analysis, and discuss this finding in connection with an apparent energy loss observed experimentally. More specifically, we treat several separate cases: for filling factor $\ensuremath{\nu}=1$ we discuss relaxation due to finite-range or Coulomb interactions between electrons in the same channel, and for filling factor $\ensuremath{\nu}=2$ we examine relaxation due to contact interactions between electrons in different channels. In both instances we calculate analytically the long-time asymptotics of the single-particle correlation function. These results are supported by an exact solution at arbitrary time for the problem of relaxation at $\ensuremath{\nu}=2$ from an initial state in which the two channels have electron distributions that are both thermal but with unequal temperatures, for which we also examine the tunneling density of states.

Turbulence and coherent structures in non-neutral plasmas

Electron plasmas confined in Penning-Malmberg traps evolve as near-ideal two-dimensional (2D) fluids, allowing a quantitative study of shear flow instabilities, vortex formation, turbulence, and self-organization. At first, an overview of the main physical properties of non-neutral plasmas is briefly presented, and the characteristics and mode of operation of their confinement devices are described. The 2D model for the transverse plasma dynamics is explicitly derived, and its main features and limits of validity are critically discussed. The attention is then focused on the free relaxation of 2D turbulence, with a short review of recent experiments and theoretical analyses. Experimental results on the dynamics of the 2D turbulence in pure electron plasmas obtained in the Penning-Malmberg trap ELTRAP are reported. Different initial electron distributions, leading to different relaxed states, are considered. The dynamics is investigated by means of the Proper Orthogonal Decomposition technique. The analysis enables to identify the coherent structures which give the dominant contribution to the plasma turbulent evolution. Finally, the scaling properties of 2D turbulence are analyzed, showing that intermittency increases as the turbulence evolution proceeds, due to the development of strong fluctuations which give rise to non-Gaussian tails in the probability distribution functions of vorticity increments.

Streak investigations of the initial phase of a subnanosecond pulsed electrical breakdown of high-pressure gas gaps

The initiation and the development of a breakdown of highly overvoltaged high-pressure (from 4 to 40 atm) gas gaps by voltage pulses having the risetime of 1 ns or shorter are studied experimentally and in theoretical terms. The study revealed that ionization processes leading to the breakdown start in the gas volume and not from the surface of the electrodes. The gap flashover is followed by ionization wave processes initiating in the gas volume and playing the decisive role at the first phase of the breakdown. The dynamics of the ionization waves strongly depends on the initial distribution of free electrons over the gas gap. The distribution of ionization waves is analysed when the initial electrons are distributed uniformly and nonuniformly over the gap. The calculation results are in qualitative agreement with the relevant experimental data. It is shown that the propagation of the ionization waves at the initial stage of subnanosecond pulsed electrical breakdown of gas leads to a redistribution of the electric field in the discharge gap and a region of a strong field, whose intensity is sufficient for the onset of emission processes and the generation of a short beam of fast electrons near the cathode, is formed at the cathode for a very short (up to 100 ps) time.

Transient Bloch oscillation with the symmetry-governed phase in semiconductor superlattices

We have investigated the time evolution of electron transport in the lowest miniband of semiconductor superlattices for various initial electron distributions created by femtosecond laser pulses. It is found that the terahertz wave forms emitted by the transient motion of Bloch oscillating electrons are insensitive to the pump photon energy and that the current starts from its maximum as damped $\text{cos}\text{ }{\ensuremath{\omega}}_{B}t$ (${\ensuremath{\omega}}_{B}$: Bloch frequency), in contrast to the expectation from semiclassical miniband transport. We show that the translationally symmetric electron distribution on the Wannier-Stark ladder is essential for the observed unique oscillation phase.

Monte Carlo Study of the Single-Surface Multipactor Electron Discharge on a Dielectric

This paper presents a Monte Carlo model to investigate the single-surface multipactor discharge on a dielectric surface in the presence of RF and dc electric fields. By employing a novel method in the numerical implementation of the secondary-electron emission, the susceptibility diagram is constructed, and beam loading and its power absorption by the multipactor discharge are examined. Meanwhile, the temporal evolution of the multipactor is also studied. The simulation results show clearly that a steady-state multipactor discharge can be built up from a very low density initial electron distribution, and an oscillatory steady state can be achieved when the positive charge, which is left by the emission of secondary electrons, is capable to build a large-enough dc electric field. During the saturation state, the normal electric field and the number of electrons in flight oscillate at twice the RF frequency. The average power absorbed by the multipactor, strongly depending on material parameters, is on the order of 1% incident power or less. Based on this model, several useful guidelines to prevent or extinguish the multipactor are presented.

Ideal distortion-less bending of a focused non-paraxial electron beam

In specific applications, where electron beam is required to be bent after passing through the focusing optics, the beam spot on the target is distorted due to asymmetry introduced by the bending. For complete elimination of the above distortion, a circularly asymmetric initial velocity distribution of electrons that move in a radially decreasing magnetic field (i.e. index of field gradient=1) is proposed. The expression is quantitatively exact for beams with any angle of bending even when paraxial condition is not satisfied i.e. beam cross-sectional diameter is comparable with the radius of curvature of bending. Modified expression for the case, when kinetic energy of the electrons increases due to time-varying field during its motion through the bending magnetic field is also given as a result of which complexity of the magnetic field design (for strong focusing) can be avoided. Finally, a numerical model of electron gun is utilized to compare the results of proposed model.