Interaction Of Fast Electrons Research Articles

Probing quantum optical excitations with fast electrons

Probing optical excitations with nanometer resolution is important for understanding their dynamics and interactions down to the atomic scale. Electron microscopes currently offer the unparalleled ability of rendering spatially-resolved electron spectra with combined meV and sub-nm resolution, while the use of ultrafast optical pulses enables fs temporal resolution and exposure of the electrons to ultraintense confined optical fields. Here, we theoretically investigate fundamental aspects of the interaction of fast electrons with localized optical modes that are made possible by these advances. We use a quantum-optics description of the optical field to predict that the resulting electron spectra strongly depend on the statistics of the sample excitations (bosonic or fermionic) and their population (Fock, coherent, or thermal), whose autocorrelation functions are directly retrieved from the ratios of electron gain intensities. We further explore feasible experimental scenarios to probe the quantum characteristics of the sampled excitations and their populations.

Dynamics of Fast Electrons in an Inhomogeneous Plasma with Plasma Beam Instability

The results of numerical simulation of the one-dimensional propagation of fast electrons in a plasma with Langmuir turbulence are presented. The resonant interaction of fast electrons with Langmuir waves and Coulomb collisions with particles of the background plasma are taken into account. It is shown that an increase in plasma density along the direction of the propagation of the electron beam leads to the transformation of the Langmuir turbulence spectrum into a region of high phase velocities and “spreading” of the velocity distribution function of accelerated electrons. For a Gaussian electron beam with an energy of 30 keV at its peak and a characteristic scale of an increase in plasma density of 107 cm, the electron spectrum expands; some of the electrons are accelerated to 1.3 of their initial velocity. In addition, a narrow (~105 cm) layer with an increased turbulence level forms in the plasma. As the electrons move deeper into the plasma, Coulomb collisions restore the inverse over velocity part of the electron distribution function, which leads to the formation of a turbulent layer at a distance from the plasma boundary of ~107 cm–108 cm; however, the energy density of Langmuir turbulence in this layer is ~(10–6–10–7) of the plasma thermal energy.

Vibrational electron energy loss spectroscopy in truncated dielectric slabs

Specially designed instrumentation for electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope makes it possible to probe very low-loss excitations in matter with a focused electron beam. Here we study the nanoscale interaction of fast electrons with optical phonon modes in silica. In particular, we analyze the spatial dependence of EEL spectra in two geometrical arrangements: a free-standing truncated slab of silica and a slab with a junction between silica and silicon. In both cases, we identify different loss channels, involving polaritonic and nonpolaritonic contributions to the total electron energy loss, and we obtain the corresponding energy-filtered maps. Furthermore, we present a comparison of the theoretical simulations for a silica-silicon junction with experimental results, and we discuss the spatial resolution attainable from the energy-filtered map considering optical phonon excitations in a conventional experimental arrangement.

Modeling the process of interaction of 10 keV electrons with a plane dielectric surface

Abstract The effect of guiding of charged particles by dielectric channels is of noticeable interest at the present time. The phenomenon is widely studied experimentally and theoretically but some points still need to be clarified. A previously developed model of interaction of fast electrons with dielectric surface at grazing incidence is used to study the independence of electron deflection on the value of electron beam current. The calculations were performed assuming a smooth dependence of the surface conductivity on the beam current in the 40–3000 nA range.

Experimental Setup for Studying the Processes Occurring During Interaction of Fast Electrons with Matter

Experimental Setup for Studying the Processes Occurring During Interaction of Fast Electrons with Matter

Probing low-energy hyperbolic polaritons in van der Waals crystals with an electron microscope

Van der Waals materials exhibit intriguing structural, electronic, and photonic properties. Electron energy loss spectroscopy within scanning transmission electron microscopy allows for nanoscale mapping of such properties. However, its detection is typically limited to energy losses in the eV range—too large for probing low-energy excitations such as phonons or mid-infrared plasmons. Here, we adapt a conventional instrument to probe energy loss down to 100 meV, and map phononic states in hexagonal boron nitride, a representative van der Waals material. The boron nitride spectra depend on the flake thickness and on the distance of the electron beam to the flake edges. To explain these observations, we developed a classical response theory that describes the interaction of fast electrons with (anisotropic) van der Waals slabs, revealing that the electron energy loss is dominated by excitation of hyperbolic phonon polaritons, and not of bulk phonons as often reported. Thus, our work is of fundamental importance for interpreting future low-energy loss spectra of van der Waals materials.

Secondary Electron Imaging of Light at the Nanoscale.

The interaction of fast electrons with metal atoms may lead to optical excitations. This exciting phenomenon forms the basis for the most powerful inspection methods in nanotechnology, such as cathodoluminescence and electron-energy loss spectroscopy. However, direct nanoimaging of light based on electrons is yet to be introduced. Here, we experimentally demonstrate simultaneous excitation and nanoimaging of optical signals using unmodified scanning electron microscope. We use high-energy electron beam for plasmon excitation and rapidly image the optical near fields using the emitted secondary electrons. We analyze dipole nanoantennas coupled with channel nanoplasmonic waveguides and observe both surface plasmons and surface plasmon polaritons with spatial resolution of 25 nm. Our experimental results are confirmed by rigorous numerical calculations based on full-wave solution of Maxwell's equations, showing high correlation between optical near fields and secondary electrons images. This demonstration of optical near-field mapping using direct electron imaging provides essential insights to the exciting relations between electrons plasmons and photons, paving the way toward secondary electron-based plasmon analysis at the nanoscale.

Current self-limitation in a transverse nanosecond discharge with a slotted cathode

A high-voltage transverse pulsed nanosecond discharge with a slotted hollow cathode was found to be a source of high-energy (few kV) ribbon electron beams. Conditions for the formation and extinction of electron beams were experimentally studied in discharges in helium at pressures of 1–100 Torr. It was found that interaction of fast electrons with a non-uniform electric field near the slotted cathode led to limitation of the magnitude of the discharge current. A physical model was developed to describe the discharge current self-limitation that was in satisfactory agreement with the experimental results. Some technical solutions that are expected to increase the upper current limits in transverse nanosecond discharge are discussed.

The characteristics of the grazing interaction of fast electrons with dielectric surfaces

Experimental data on the mechanism of the grazing interaction of fast electron beams with planar dielectric surfaces of various materials are analyzed. New experimental results regarding the grazing interaction between electrons with an energy of 10 keV and metalized structured surfaces are reported. The difference between the processes of the interaction of fast electron beams with planar and structured surfaces is demonstrated.

Enhancement Effects of Transition and Vavilov-Cherenkov Radiation Mechanisms Under Grazing Interaction of Fast Electrons With a Thick Substrate Applied by Thin Layer

The paper presents the results of a theoretical study and a mathematical model of radiation processes occurred during the grazing interaction of fast electrons with semi-infinite targets applied on a thin amorphous layer. The developed model considers Vavilov-Cherenkov and transition radiation mechanisms and predicts the possibility to enhance the angular radiation density under grazing incidence of fast electrons on the layer. The characteristics of possible extreme vacuum ultraviolet and soft X-ray sources are estimated.

Smith-Purcell radiation emission in aperiodic arrays

We study the Smith-Purcell light emission produced by electrons moving parallel to linear aperiodic particle arrays. This constitutes a generalization of this type of phenomenon from periodic to aperiodic structures. As in the periodic case, the emission is found to exhibit intense features in its angular and frequency distributions, associated with the condition of constructive interference between the contributions arising from different particles in the array. This condition can also be expressed in terms of momentum conservation involving reciprocal wave-vector transfers from the array. We consider two examples of quasiperiodic and hyperuniform aperiodic arrays that allow us to illustrate this idea. Our study provides insight into the interaction of fast electrons with aperiodic arrays characterized by strong features in reciprocal space, which dominate the electron-array coupling.

Solutions to mitigate heat loads due to electrons on sensitive components of ITER HNB beamlines

The operation of neutral beam injectors for plasma heating and current drive in a fusion device provides challenges in the thermal management of beamline components. Sensitive components such as the cryogenic pumps at beamline periphery shall be protected from the heat flux due to stray electrons. These are emitted by the negative ion accelerator or generated along the beamline by interaction of fast electrons, ions or atoms with background gas and surfaces. In this article the case of the ITER Heating Neutral Beam (HNB) and its test facility MITICA is discussed, for which the beam parameters and the required pulse length of one hour is a major leap forward with respect to the present experience with neutral beam systems. The engineering solutions adopted for effective cryopump protection against the heat load from electrons are described. The use of three-dimensional numerical simulations of particle trajectories in the complex geometry of the beamline was needed for the quantitative estimations of the heat loads. The presented solutions were optimized to minimize the impact on gas pumping and on the functionality of other components.

The orientation effects during the grazing interaction of fast electrons with structured surfaces

Background/Objectives: The paper presents the experimental results of a study of the processes of grazing interaction of fast charged particles with structured surfaces. Methods/Statistical Analysis: The experiments performed to study the radiation arising when the electrons pass along a structural surface are carried within a small angle region. According to the results presented in Figure 4 the maximum of electrons lifting can be observed at angles close to 1°-2°. So it will be necessary to use different grounding schemes to improve the yield efficiency. This can be achieved when both sides are grounded. Findings: The contribution of the structure of the target surface to the scattering of the incidence primary electrons was observed for different geometries for the interaction process of a 10keV electron beam with a diffraction grating. The experimental results demonstrate the possibility of increasing the total time of the interaction between the electron beam and the target, which opens the possibility to increase the effectiveness of radiation sources, based on diffraction mechanisms. Applications/Improvements: The effect of contactless transmission of electrons through dielectric channels causes great attention because of the potential use of simple and independent systems for electron beams formation.

The dynamics of the interaction of fast electrons with dielectric surfaces at grazing incidence

For the first time a classical model was developed to describe quantitatively the dynamics of charge distribution formation on a planar dielectric surface during its irradiation by fast electrons at a grazing incidence. The dynamic effects of the mutual influence of the beam and the dielectric surface charge predicted by the developed model are in a good agreement with the performed experiments. The proposed model can be used to describe time dependent processes of the electron beam–dielectric surface interaction, charging up and discharging for dielectric channels of more complex shapes. • The first model to describe the guiding process of a 10-keV electron beam by dielectric surface was developed. • The model can describe time processes during electron–surface interaction. • The model takes into account secondary electron emission from the surface. • The function of charge up dependence on angle of incidence is proposed. • The mechanism of charge leakage is proposed for one-dimensional model.

Electron energy losses and cathodoluminescence from complex plasmonic nanostructures: spectra, maps and radiation patterns from a generalized field propagator

We present a unified framework for the description of the interaction of fast electrons with complex nanostructures based on the Green dyadic method. We show that the computation of a generalized field propagator yields the electron energy losses and cathodoluminescence of nano-objects of arbitrary morphologies embedded in complex dielectric media. Spectra and maps for both penetrating and non-penetrating electron trajectories are provided. This numerical approach can be extended to describe complex experiments involving fast electrons and optically excited nanostructures.

Environmental Transmission Electron Microscopy in an Aberration-Corrected Environment

The increasing use of environmental transmission electron microscopy (ETEM) in materials science provides exciting new possibilities for investigating chemical reactions and understanding both the interaction of fast electrons with gas molecules and the effect of the presence of gas on high-resolution imaging. A gaseous atmosphere in the pole-piece gap of the objective lens of the microscope alters both the incoming electron wave prior to interaction with the sample and the outgoing wave below the sample. Whereas conventional TEM samples are usually thin (below 100 nm), the gas in the environmental cell fills the entire gap between the pole pieces and is thus not spatially localized. By using an FEI Titan environmental transmission electron microscope equipped with a monochromator and an aberration corrector on the objective lens, we have investigated the effects on imaging and spectroscopy caused by the presence of the gas.

Optical Emission from the Interaction of Fast Electrons with Metallic Films Containing a Circular Aperture: A Study of Radiative Decoherence of Fast Electrons

Light emission resulting from the interaction of swift electrons with a distant material is shown to produce an unexpectedly large fraction of decoherence in the moving charges. The decoherence probability diverges for an electron passing through a hole drilled in a perfectly conducting metal film, regardless of the size of the opening. This divergence, which is logarithmic in the ratio of film radius to aperture radius, originates in an infrared catastrophe that differs from other sources of decoherence (e.g., bremsstrahlung radiation). Our results provide new avenues for controlling and assessing the role of coherence during electron acceleration (for example, in transmission electron microscopes) and for exploiting partial quantum interference of fast electrons.

Interaction of fast electrons and gamma-quanta with radiation protection ferric oxide composites

Physical processes of fast electron and gamma-radiation beam interaction with radiation protection ferric oxide composite materials are investigated together with dependences of energy (flux) accumulation and transmission coefficients on the gamma-radiation energy and thickness of a protective screen. Integral characteristics of radiation protection materials are calculated.

Evidence for Resonant Transition Radiation in Decimetric Continuum Solar Bursts

We investigate the properties of radio bursts observed in the 1–18 GHz frequency range that display two distinct spectral components peaking at decimetric (dm) and centimetric (cm) ranges. Thedmemission is relatively smooth in both frequency and time, with timescales comparable to those of the cm component. The two spectral components display specific correlations in their temporal and spectral behavior. Through detailed analysis, we find the following: (1) A large ratio of plasma frequency to gyrofrequency is a characteristic of all of these events. (2) The dm component generally displays a high degree of o-mode polarization. (3) The dm component is produced by an incoherent emission mechanism. (4) The dm and cm components are generated by the same electron distribution. (5)Thecharacteristicenergyofthefastelectronsproducingthedmcontinuumissignificantlylowerthantheenergy of the electrons generating the microwave gyrosynchrotron component. (6) The spectral shape of the dm component is not well correlated with the fast electron distribution over energy. These findings cannot easily be explainedwithstandardmechanismsfordmsolarburstemissionbutagreewellwiththepredictionsofthetheoryof resonant transition radiation. We conclude that these two-component bursts represent a distinctive subclass of events, comprising about 10% of all bursts, with the dm continuum component generated most probably by resonant transition radiation produced by interaction of fast electrons with small-scale inhomogeneities of the background plasma. We discuss new possibilities for flaring plasma diagnostics using these two-component bursts.

Irradiação de polímeros com feixe de elétrons: caracterização de íons positivos através da técnica de tempo-de-vôo

With the aim of studying the interaction of fast electrons with solid surfaces we have developed an experimental set-up based on electron stimulated desorption (ESD) coupled to time-of-flight (TOF) mass spectrometry. Poly(methyl methacrylate) and poly(vynil chloride) samples have been irradiated by a pulsed electron beam of 1.2 keV and 0.18 µs FWHM. The results show that H+ is the main ionic species to desorb after electron bombardment. In addition, other ionic fragments were also observed and assigned. These results show the potentiality of this technique in the study of ESD of polymers.