Initial Electron Velocity Research Articles

Signature of Single Binary Encounter in Intermediate Energy He2+—Ar Collisions**Supported by the National Basic Research Program of China under Grant No 2010CB832902, the National Natural Science Foundation of China under Grants Nos U1332128 and 11274317, 10979007 and 11004202, and the Program of One Hundred Talents of the Chinese Academy of Sciences.

We experimentally observe the signature of electron emission resulting from a single binary encounter mechanism in the intermediate collision energy regime of 30 keV/u He2+ on argon. Electron emission spectra in the transfer ionization are obtained and compared with classical calculations from a two-step model considering the initial electron velocity and re-scattering of the binary encounter electron in the recoil potential. Although the present reaction is actually a four-body problem, the model starting from a binary encounter gives out surprisingly good agreement with the experimental data. Our studies show that orbital velocities of the electron affect the emission patterns of ionized electrons significantly.

Double-Gated, Spindt-Type Field Emitter With Improved Electron Beam Extraction

We have developed a highly sensitive, compact image sensor that comprises a field emitter array (FEA) and a high-gain avalanche-rushing amorphous photoconductor target, with the ultimate aim of developing an ultrahighly sensitive, compact, high-definition television camera. Double-gated FEs have an advantage of having a compact electron beam focusing system; however, image intensities reproduced by the sensor were nonuniform due to low electron beam current. Furthermore, the simulated electron beam current disagreed with the measured current. The electron beam current characteristics of two types of double-gated, Spindt-type FEs (both with improved electron beam current extraction) are discussed for possible use within the sensor: convex-structured and volcano-structured. A highly accurate simulation model of the image sensor using a double-gated, Spindt-type FE has been examined; the simulated electron beam currents extracted from the double-gated, Spindt-type FE are in agreement with the measured electron beam currents when the initial electron velocity is assumed, thus suggesting that the simulated anode current–anode voltage characteristic conforms to the measured one. For example, the electron beam currents extracted from the convex-structured and volcano-structured FEs when the focusing electrode is placed 0.2 $\mu \text{m}$ below the gate electrode opening at a focusing electrode voltage of 15 V are, respectively, 1.8 and 1.9 times larger than that extracted from the previously used FEs when the focusing electrode is stacked 1.5 $\mu \text{m}$ above the gate electrode. The results show potential for reducing the degradation of the uniformity of the reproduced image’s intensity, and show that the highly accurate simulation model of the sensor is valid to design the double-gated FEAs for the sensor.

Origin of the ring-shaped emission pattern observed from the field emission of ZnO nanowire: role of adsorbates and electron initial velocity

A ring-shaped pattern was observed in the field emission from ZnO nanowires. These ring patterns are different from those previously reported from metallic or carbon nanotube emitters, which occur under high current density. The influences of the applied field and anode to emitter distance on the ring-shaped pattern have been investigated. On the basis of the experiment results, an edge emission model induced by the field desorption has been proposed. Furthermore, it is found that an initial velocity is essential to obtain the desired pattern size. The initial velocity of emitting electron is estimated by using field induced hot electron emission model. Simulation results from the model are consistent with the experimental results.

Amplification of an electromagnetic field at the scattering of the nonrelativistic electron by an ion in the external field of medium intensity for an arbitrary angle of the initial electron

We theoretically study the gain coefficient for a electromagnetic field, in the scattering of nonrelativistic electrons by ions in a elliptically polarized light wave. We obtain a simple analytical expression for a field amplification constant in logarithmic approach to an arbitrary angle of the initial electron. The formula supplements and extends the domain of applicability of the known Marcuse formula for the linear polarization in the presence of a weak field. It is demonstrated that the maximum gain is reached when the initial electron velocity directs along the major semi-axis of the polarization ellipse. In the range of optical frequencies, the gain coefficient of the laser radiation can be significant for relatively high powers of electron beams. Obtained results may be experimentally verified, for example, by the scientific facilities at the SLAC National Accelerator Laboratory and FAIR (Facility for Antiproton and Ion Research, Darmstadt, Germany).

Time evolution of electron flow in a model diode: Non-perturbative analysis

Using a combination of Eulerian and Lagrangian variables we study the time evolution of the electron flow from a no-current state to a final state with the stationary current in a planar one-dimensional diode. The electrons can be injected externally or generated by the cathode via field emission governed by a current-field law. The initial zero current regime is replaced suddenly by injection or, in the case of field emission, by jumping the anode voltage from zero to a constant positive value. The case of equipotential electrodes and fixed injection is studied along with a positive anode potential. When the current is fixed externally, the approach to the stationary state goes without oscillations if the initial electron velocity is high enough and the anode can absorb the injected flow. Otherwise the accumulated space charge creates a potential barrier which reflects the flow and leads to its oscillations, but our method of analysis is invalid in such conditions. In the field emission case the flow goes to its stationary state through a train of decaying oscillations whose period is of the order of the electron transit time, in agreement with earlier studies based on perturbation techniques. Our approximate method does not permit very high cathode emissivity although the method works when the stationary current density is only about 10% smaller than the Child-Langmuir limit.

Particle in cell simulation of relativistic Buneman instability in a current-carrying plasma

Non-linear evolution of the relativistic Buneman instability in a current-carrying plasma is investigated by a particle in cell simulation. These simulations show that as the time progresses, some electrons are trapped in phase space holes and thus counter-streaming and plateau can be formed. Moreover, the electron and ion density profiles indicate a periodic pattern of the density steepening. This density distribution is similar to the generation of the grating-like patterns which strongly depends on the initial electron velocity and saturation time. It is also shown that the electric field profile has the sawtooth form; charged particles can be accelerated by this field. Finally, it is found that increasing the electron velocity increases the saturation time and consequently the growth rate decreases which is in good agreement with the result obtained by the fluid model.

Effect of Electron Initial Longitudinal Velocity on Low-Energy Structure in Above-Threshold Ionization Spectra

Using a semiclassical model, we investigate the effect of the initial longitudinal velocity of the tunnel-ionized electron on low-energy structure (LES) in above-threshold ionization (ATI) spectra. Our analysis shows that the effect (reduction or enhancement) of the initial longitudinal velocity pz0 on the LES is dependent on the direction of the initial longitudinal velocity. For the initial longitudinal velocity along the direction of the laser electric field at the moment of tunneling (i.e., pz0 > 0), the Coulomb interaction between the photoelectron and ion core is enhanced, while for the initial longitudinal velocity in the opposite direction (i.e., pz0 < 0), the Coulomb effect is reduced. Our work provides solid evidence that the initial electron longitudinal velocity has considerable effect on the photoelectron dynamics in the ATI process.

Can accurate investigation of phase stability explain the experimental results of multipactor phenomenon?

The simple two-point theory cannot explain the results of known experimental data of the multipactor (MP) discharge for two-parallel plate. Here the concept of accurate phase stability for resonance and regions not satisfying the resonance condition are presented. In this order the results of numerical simulation and improved theoretical calculation are stated. The results show that the most important thing that can result in a MP discharge is the behaviour of stability factor of the electron during its flights. It seems that introduction of ``main condition'' of phase stability can help to understand the experimental results. In numerical simulation a fixed value for the initial velocity of secondary electron is considered while the random initial rf phase is assumed.

Non-sequential double ionization of argon by elliptically polarized laser pulses

With a classical ensemble model, we investigated non-sequential double ionization (NSDI) of argon by elliptically polarized laser pulses. The results show that the correlation behaviors of two electrons depend strongly on the laser intensity. At relatively high laser intensity, the momentum spectra of two electrons along the long axis of the laser polarization plane are mainly distributed in the first and third quadrants and display V-like structures. However, at relatively low laser intensity, the momentum spectra of two electrons are mainly distributed in the second and fourth quadrants. By back analyzing the classical trajectories of NSDI, we find that all of the successful NSDI events still come from recollision in the cases of elliptically polarized laser pulses, and the final-state electron repulsion plays a decisive role for the V-like structure along the long axis of the laser polarization plane. In addition, we find that the initial velocity of the first electron at ionization along the short axis of the laser polarization plane are essential for the recollision, and the time delay between the first ionization and recollision depends on the ellipticity strongly.

Nonlinear relativistic single-electron Thomson scattering power spectrum for incoming laser of arbitrary intensity

The classical nonlinear incoherent Thomson scattering power spectrum from a single relativistic electron with incoming laser radiation of any intensity, investigated numerically by the present authors in a previous publication, displayed both an approximate quadratic behavior in frequency and a redshift of the power spectrum for high intensity incoming radiation. The present work is devoted to justify, in a more general setup, those numerical findings. Those justifications are reinforced by extending suitably analytical approaches, as developed by other authors. Moreover, our analytical treatment exhibits differences between the Doppler-like frequencies for linear and circular polarization of the incoming radiation. Those differences depend nonlinearly on the laser intensity and on the electron initial velocity and do not appear to have been displayed by previous authors. Those Doppler-like frequencies and their differences are validated by new Monte Carlo computations beyond our previuos ones and reported here.

Effect of space charge and the initial electron velocity on the potential distribution in a plane diode

The problem of the potential distribution in a plane vacuum diode is solved for arbitrary values of the space charge density and the initial electron velocity. The resultant solution makes it possible to study the conditions for the potential minimum appearing near the cathode, to determine the position and depth of this minimum, to derive analytic formulas for the potential distribution in the entire space between the cathode and the anode, and to analyze the problem of an infinitely high space charge density.

Periodicity property of relativistic Thomson scattering with application to exact calculations of angular and spectral distributions of the scattered field

We prove that the analytical expression of the intensity of the relativistic Thomson scattered field for a system composed of an electron interacting with a plane electromagnetic field can be written in the form of a composite periodic function of only one variable, that is, the phase of the incident field. This property is proved without using any approximation in the most general case in which the field is elliptically polarized, the initial phase of the incident field and the initial velocity of the electron are taken into consideration, and the direction in which the radiation is scattered is arbitrary. This property leads to an exact method for calculating the angular and spectral distributions of the scattered field, which reveals a series of physical details of these distributions, such as their dependence on the components of the initial electron velocity. Since the phase of the field is a relativistic invariant, it follows that the periodicity property is also valid when the analysis is made in the inertial system in which the initial velocity of the electron is zero in the case of interactions between very intense electromagnetic fields and relativistic electrons. Consequently, the calculation method can be used for the evaluation of properties of backscattered hard radiations generated by this type of interaction. The theoretical evaluations presented in this paper are in good agreement with the experimental data from literature.

Effect of the electron initial longitudinal velocity on the nonsequential double-ionization process

The effect of initial longitudinal velocity of the first tunneling electron on the nonsequential double-ionization (NSDI) process is studied using a semiclassical model. Our analysis shows that nonzero initial longitudinal velocity affects two distinct typical trajectories [single-return collision (SRC) and multiple-return collision (MRC)] in the NSDI process in different ways: for SRC, the yield in the center part of the correlation momentum distribution is reduced; for MRC, the tunneling time of the first electron is apparently delayed. In comparison with the case of zero initial longitudinal velocity, the double ionization is suppressed for a long wavelength, and a better agreement with the experimentally observed electron correlation distribution is achieved when the nonzero initial longitudinal velocity is taken into account.

The Influence of Initial Electron Velocities Distribution on the Energy Spectra of the Spent Electron Beam in Gyrotron

The results of numerical analysis of the electron beam parameters influencing on the efficiency of energy recovery in gyrotrons are given. The weak dependence of minimal electron beam energy in collector region versus velocity distribution function is demonstrated. This fact may simplify the electron-optical system design of modern gyrotrons.

Electron acceleration in vacuum induced by a tightly focused chirped laser pulse

Electron acceleration in vacuum induced by a tightly focused chirped laser pulse has been studied. For a fixed laser output power, the tightly focused chirped laser pulse can accelerate electrons to much higher energies. Focusing laser down to the order of wavelength requires inclusion of terms of third order at least in the diffraction angle ε in the description of the associated field. Retained electron energy depends strongly on frequency chirp parameter and initial position of the electron. Besides, retained energy increases with laser intensity, pulse duration, and initial velocity of electron, and varies periodically with laser constant phase.

On the phase stability in two-sided multipactor

An attempt is made to take a different look on the concept of phase stability effect of secondary emissions and discharge phenomenon. In the present work, using the concept of phase focusing, we obtain a new equation for the stability condition. Using this equation we show the existence of an unknown region of the stable trajectories. In fact, we show that the resonance boundary neighborhood in which multipactor (MP) could occur is more extended than previously thought and regardless of closeness to the stable resonance trajectories, some regions cannot take part in MP resonance discharge. A numerical simulation, which assumes a fixed value for the initial velocity of secondary electrons, is used to show MP zones in the planar gap. Even though the velocity spread is ignored, the simulation shows the existence of hybrid resonance modes.

Hybrid planar free-electron maser in the magnetoresonance regime

We study the operation regime of a hybrid planar free-electron maser (FEM) amplifier near the magnetoresonant value of the uniform longitudinal (guide) magnetic field. Using analytical expressions for individual test electron trajectories and normal frequencies of their three-dimensional oscillations in the magnetostatic field of the hybrid planar FEM, an analytical condition of chaotization of motion is established and shown to be given by the Chirikov resonance-overlap criterion applied to the normal undulator and cyclotron frequencies with respect to the coupling induced by the undulator magnetic field. It is also shown analytically that, in spite of the well-known drop for the exact magnetoresonance, the gain attains its maximal value in the zone of regular dynamics slightly above the magnetoresonant value of the guide magnetic field. Under the condition of undulator resonance, it is practically independent of the amplitude of the undulator magnetic field and the wavelength of amplified signal. To account for space-charge effects, we propose a theoretical model of a weakly relativistic FEM, which accommodates not only potential but also rotational parts of the nonradiated electromagnetic field of a moving charged particle. It turns out that the rotational part of nonradiated field diminishes the defocusing influence of the potential part on the beam bunching. Numeric simulation of the nonlinear stage of amplification is fulfilled, taking into consideration adiabatic entrance of the electron beam to the interaction region and initial electron velocity spread. We find that nonradiated field and initial electron velocity spread do not influence essentially the efficiency of hybrid planar FEM amplification if parameters of the beam-microwave interaction correspond to the operational regime in the zone of regular dynamics near the magnetoresonance.

Simulations of multipactor thresholds in shielded microstrip lines

A particular software ‘MuSLi’ has been developed and applied for simulations of the multipactor effect in shielded microstrip lines with cross-sections that are partially filled with dielectric material. The software combines an electromagnetic field solver, determining the electric field structure in the microstrip line, and a Monte Carlo algorithm, calculating the corresponding electron trajectories taking into account a spread of the electron initial velocity and different secondary emission properties of the metal and dielectric surfaces. The simulations were carried out for a number of structures of interest for communication systems, currently being discussed for application in coming space missions. The microstrip line system is shown to be strongly resistant to multipactor growth and the main reason for this is identified as the ponderomotive (or Miller) force, which governs the average electron motion in non-uniform rf fields and which tends to push the electrons out of regions with strong rf fields.

Amplification of electromagnetic field in the course of the nonrelativistic electron scattering by ion in the presence of the field of the medium-intensity elliptically polarized light wave

The amplification factor of the electromagnetic field is theoretically studied for the scattering of nonrelativistic electrons by ions in the presence of the field of the elliptically polarized electromagnetic wave. A simple analytical formula for the gain is derived for the medium-intensity range. The formula supplements and extends the domain of applicability of the known Marcuse formula for the linear polarization in the presence of a weak field. It is demonstrated that the maximum gain is reached when the initial electron velocities belong to the polarization plane of the electromagnetic wave. In the range of optical frequencies, the amplification factor of the laser radiation can be significant for relatively high powers of electron beams.

Electron beam in the presence of a periodic magnetic field with allowance for the thermal velocities of electrons

Equations are derived to study the effect of the initial transverse thermal velocities of electrons on the configuration of an intense electron beam in an electron-optical system with a significant divergence for the case of the beam focusing by a periodic magnetic field. It is demonstrated that the minimization of the effect of thermal velocities of electrons in the electron-optical system with the given level of the periodic focusing field and the cathode radius and temperature and the limiting compression of the beam in the presence of such a field is reached with the shaping systems in which an optimal magnetic flux passes through the cathode and the flux depends on the parameters of the thermal beam and the focusing field.